AN EVALUATION OF CUCURBITS AND ORNAMENTALS FOR SUSCEPTIBILITY
TO PHYTOPHTHORA SPP.
By
Tiffany Brice Enzenbacher

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
Plant Pathology
2011

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ABSTRACT
AN EVALUATION OF CUCURBITS AND ORNAMENTALS FOR SUSCEPTIBILITY
TO PHYTOPHTHORA SPP.
By
Tiffany Brice Enzenbacher
Phytophthora capsici Leonian is the causal agent of root, crown, and fruit rot and foliar
blight and is a limiting factor in the production of vegetable crops in Michigan, the United States,
and globally. Differences in symptom onset and disease severity of cucurbits to Phytophthora
rot were found in laboratory and greenhouse studies. Eight fruit types were evaluated with five
P. capsici isolates in a fruit experiment. All isolates incited disease with significant differences
found among fruit type and pathogen isolate. Six squash crops were evaluated with the same
five P. capsici isolates in seedling experiments. Crop type, pathogen isolate, or the crop
type*pathogen isolate interaction term were significant for symptom appearance and area under
the disease progress curve values.
Phytophthora capsici and P. tropicalis Aragaki & J. Y. Uchida, formerly conspecific
with P. capsici, also cause disease on tropical crops and ornamental plants. Seven Fabaceous
and Solanaceous ornamental genera and seven million bells (Calibrachoa x hybrida) cultivars
were evaluated for susceptibility to P. capsici or P. tropicalis in greenhouse studies. Million
bells and flowering tobacco (Nicotiana x sanderae) exhibited reduced mean volumes, and P.
capsici and P. tropicalis were recovered from tissue. Disease was observed on and pathogen
recovery was possible from squash (Cucurbita pepo, included as a P. capsici-susceptible
control), sweet pea (Lathyrus latifolius), and lupine (Lupinus polyphyllus). The potential host
range of P. capsici has expanded to flowering tobacco, lupine, million bells, and sweet pea. The
potential host range of P. tropicalis has expanded to flowering tobacco, squash, and sweet pea.

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To my parents,
Michael and Sheila Enzenbacher,
for your love and support
not only while obtaining my graduate degree,
but all the days of my life.

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ACKNOWLEDGEMENTS

I would like to thank all of the individuals who contributed to the completion of these
studies. I am especially grateful for the guidance of my major professor, Dr. Mary Hausbeck,
who facilitated this research and spent many hours discussing and reviewing my work. She has
given me abundant opportunities to cultivate my knowledge of plant pathology and to develop as
a professional. Thank you to my committee members, Drs. Jay Hao and Mathieu Ngouajio, for
encouragement and critical analysis of my work. Drs. Hausbeck, Hao, and Ngouajio have been
exceptional mentors and inspiring representations of researchers.
Many thanks to Brian Cortright for field help, Blair Harlan for greenhouse assistance, and
Sheila Linderman for document review and laboratory coordination; Adam Bloemers, Tara
Gallagher, and Justin Schmitt for laboratory and greenhouse aid; fellow laboratory members for
review of my work, especially Michael Meyer; Dr. Leah Granke for her seemingly limitless
solutions to my experimental design and data analyses questions; and Kateryna Ananyeva and
Andrew Worth for statistical advice.
I am very appreciative of the never-ending support offered by my family and friends.

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TABLE OF CONTENTS

LIST OF TABLES .......................................................................................................................vi
LIST OF FIGURES .....................................................................................................................viii
LITERATURE REVIEW
Introduction ........................................................................................................................1
Host Range and Disease Symptoms ..................................................................................4
The Pathogen .....................................................................................................................8
Disease Management .........................................................................................................13
CHAPTER I
Cucurbit Fruit and Seedling Response to Phytophthora capsici
Abstract ..............................................................................................................................21
Introduction ........................................................................................................................22
Materials and Methods .......................................................................................................25
Results ................................................................................................................................37
Discussion ..........................................................................................................................53
CHAPTER II
Ornamental Plant Susceptibility to Phytophthora capsici or P. tropicalis
Abstract ..............................................................................................................................61
Introduction ........................................................................................................................62
Materials and Methods .......................................................................................................66
Results ................................................................................................................................74
Discussion ..........................................................................................................................98
LITERATURE CITED ................................................................................................................106

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LIST OF TABLES

Table 1.1. Cucurbit fruit evaluated for susceptibility to fruit rot caused by Phytophthora
capsici ..........................................................................................................................................27
Table 1.2. Cucurbit seedlings evaluated in greenhouse experiments for susceptibility to root rot
caused by Phytophthora capsici ..................................................................................................33
Table 1.3. Area under the disease progress curve (AUDPC) values for experiment 1 evaluating
cucurbit seedling susceptibility to root rot caused by Phytophthora capsici...............................50
Table 1.4. Area under the disease progress curve (AUDPC) values for experiment 1 evaluating
acorn squash, pumpkin, semi-crookneck squash, and zucchini seedling susceptibility to root rot
caused by Phytophthora capsici isolates SF3, OP97, SP98, 12889, and 13351..........................50
Table 1.5. Area under the disease progress curve (AUDPC) values for experiment 2 evaluating
cucurbit cultivar seedling susceptibility to root rot caused by Phytophthora capsici isolates SF3,
OP97, SP98, 12889, and 13351 ...................................................................................................52
Table 2.1. Ornamental plants evaluated in experiment 1 for susceptibility to root rot caused by
Phytophthora capsici and P. tropicalis........................................................................................69
Table 2.2. Million bells (Calibrachoa x hybrida) cultivars evaluated in experiment 2 for
susceptibility to root rot caused by Phytophthora capsici and P. tropicalis ...............................70
Table 2.3. Disease incidence of ornamental plants evaluated in experiment 1 for susceptibility to
Phytophthora capsici isolates OP97, SP98, 12889, and 13351 and P. tropicalis isolate
13715............................................................................................................................................77
Table 2.4. Recovery of Phytophthora capsici isolates OP97, SP98, 12889, and 13351 and P.
tropicalis isolate 13715 from ornamental plants evaluated in experiment 1 for susceptibility to
root rot ..........................................................................................................................................78
Table 2.5. Disease incidence of million bells (Calibrachoa x hybrida) cultivars evaluated in
experiment 2 for susceptibility to Phytophthora capsici and P. tropicalis..................................79
Table 2.6. Recovery of Phytophthora capsici and P. tropicalis from million bells (Calibrachoa x
hybrida) cultivars evaluated in experiment 2 for susceptibility to root rot..................................79
Table 2.7. ANOVA result for mean plant volume for experiment 1 (runs 1 and 2) evaluating
ornamental plant susceptibility to root rot caused by Phytophthora capsici isolates OP97, SP98,
12889, and 13351 and P. tropicalis isolate 13715 .......................................................................82

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Table 2.8. ANOVA result for mean plant volume for experiment 1 (all runs combined)
evaluating ornamental plant susceptibility to root rot caused by Phytophthora capsici isolates
OP97, SP98, 12889, and 13351 and P. tropicalis isolate 13715 .................................................82
3

Table 2.9. Mean plant volume (cm ) of ornamental plants in experiment 1 (all runs combined)
evaluating susceptibility to root rot caused by Phytophthora capsici isolates OP97, SP98, 12889,
and 13351 and P. tropicalis isolate 13715 ...................................................................................84
Table 2.10. ANOVA for area under the plant growth curve (AUPGC) values and plant
type*pathogen isolate slices for experiment 1 (runs 1 and 2) evaluating ornamental plant
susceptibility to root rot caused by Phytophthora capsici isolates OP97, SP98, 12889, and 13351
and P. tropicalis isolate 13715.....................................................................................................87
Table 2.11. ANOVA for area under the plant growth curve (AUPGC) values and plant
type*pathogen isolate slices for experiment 1 (all runs combined) evaluating ornamental plant
susceptibility to root rot caused by Phytophthora capsici isolates OP97, SP98, 12889, and 13351
and P. tropicalis isolate 13715.....................................................................................................88
Table 2.12. Area under the plant growth curve (AUPGC) values of ornamental plants evaluated
in experiment 1 (runs 1 and 2) for susceptibility to root rot caused by Phytophthora capsici
isolates OP97, SP98, 12889, and 13351 and P. tropicalis isolate 13715 ....................................89
Table 2.13. Area under the plant growth curve (AUPGC) values of ornamental plants evaluated
in experiment 1 (all runs combined) for susceptibility to root rot caused by Phytophthora capsici
isolates OP97, SP98, 12889, and 13351 and P. tropicalis isolate 13715 ....................................90
Table 2.14. ANOVA result for mean plant volume and cultivar*days post inoculation (dpi)
slices of the three-way interaction term for experiment 2 evaluating million bells (Calibrachoa x
hybrida) cultivar susceptibility to root rot caused by Phytophthora capsici and P. tropicalis ...92
Table 2.15. ANOVA result for area under the plant growth curve (AUPGC) values and
cultivar*pathogen isolate slices for experiment 2 evaluating million bells (Calibrachoa x
hybrida) cultivars for susceptibility to root rot caused by Phytophthora capsici and P.
tropicalis ......................................................................................................................................96

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LIST OF FIGURES

Figure 1.1. Cucurbit fruit inoculation techniques used to evaluate fruit rot susceptibility to
inoculation with Phytophthora capsici. Fruit incubating in a clear chamber in the laboratory at
high humidity (A). Large fruit in individual clear bags to maintain humidity in the greenhouse
(B). Slicing cucumber fruit at 5 days post inoculation (dpi) (C). Note lesion (L) and pathogen
growth (PG) diameter. Large fruit inoculation technique displayed on a butternut squash fruit
(D). Butternut squash fruit at 8 dpi (E). For interpretation of the references to color in this and
all other figures, the reader is referred to the electronic version of this thesis. ...........................28
Figure 1.2. Disease severity rating scale used to evaluate seedling susceptibility to root rot
caused by Phytophthora capsici, where A, 0 = no visible symptoms; B, 1 = leaves slightly
wilted; C, 2 = foliar wilting with crown lesion present at soil line; D, 3 = plant partially
collapsed; E, 4 = plant completely collapsed; and F, 5 = death ..................................................35
Figure 1.3. Lesion and pathogen growth diameters (A) present on summer squash fruit following
inoculation with Phytophthora capsici isolates SF3, OP97, SP98, 12889, and 13351 (B). Disease
was evaluated at 3 and 5 days post inoculation (dpi) in fruit experiment. Bars with a letter in
common are not significantly different for each response variable within each dpi (Fisher‟s
protected LSD, P = 0.05). Disease was not observed on, nor was P. capsici was recovered from
control fruit, and fruit were not included in data analyses. ..........................................................38
Figure 1.4. Straightneck squash (A) and slicing cucumber fruit (B) at 5 days post inoculation
(dpi) and acorn squash fruit (C) at 8 dpi following inoculation with Phytophthora capsici isolates
SF3, OP97, SP98, 12889, and 13351. Note differences in isolate virulence within fruit types .40
Figure 1.5. Lesion and pathogen growth (A, B) diameters present on cucumber fruit following
inoculation with Phytophthora capsici isolates SF3, OP97, SP98, 12889, and 13351 (C).
Disease was evaluated at 3 and 5 days post inoculation (dpi) in fruit experiment. Bars with a
letter in common are not significantly different for each response variable within each dpi (A, C)
(Fisher‟s protected LSD, P = 0.05). Bars with a (lower case) letter in common are not
significantly different within each fruit type and (upper case) among isolates (B) (Fisher‟s
protected LSD, P = 0.05). Disease was not observed on, nor was P. capsici recovered from
control fruit, and fruit were not included in data analyses ...........................................................42
Figure 1.6. Lesion and pathogen growth diameters (A) present on winter squash fruit following
inoculation with Phytophthora capsici isolates SF3, OP97, SP98, 12889, and 13351 (B). Disease
was evaluated at 6 and 8 days post inoculation (dpi) in fruit experiment. Bars with a letter in
common are not significantly different for each response variable within each dpi (Fisher‟s
protected LSD, P = 0.05). Disease was not observed on, nor was P. capsici was recovered from
control fruit, and fruit were not included in data analyses ...........................................................46
Figure 1.7. Symptom appearance days post inoculation (dpi) of cucurbit seedlings evaluated in
experiment 1 for susceptibility to root rot caused by Phytophthora capsici isolates SF3, OP97,

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SP98, 12889, and 13351. Bars with a (lower case) letter in common are not significantly
different within each crop and (upper case) among isolates (Fisher‟s protected LSD, P = 0.05).
Disease was not observed on, nor was P. capsici recovered from control seedlings, and seedlings
were not included in data analyses...............................................................................................49
Figure 2.1. Aboveground root rot symptoms caused by Phytophthora capsici or P. tropicalis in
experiments 1 or 2. Stunting (A) and death of flowering tobacco (Nicotiana x sanderae) (B),
foliar and stem wilting of lupine (Lupinus polyphyllus) (C), uneven foliar wilting (D) and death
(E) of million bells (Calibrachoa x hybrida), crown lesions on straightneck squash (Cucurbita
pepo) (F, G), and chlorotic and necrotic foliage of sweet pea (Lathyrus latifolius) (H).............75
3

Figure 2.2. Mean plant volume (cm ) of ornamental plants at 42 days post inoculation evaluated
in experiment 1 (runs 1 and 2) for susceptibility to root rot caused by Phytophthora capsici
isolates OP97, SP98, 12889, and 13351 and P. tropicalis isolate 13715. Bars represent ±
standard error. MB = million bells..............................................................................................83
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Figure 2.3. Mean plant volume (cm ) of ornamental plants in evaluated in experiment 1 (all runs
combined) for susceptibility to root rot caused by Phytophthora capsici isolates OP97, SP98,
12889, and 13351 and P. tropicalis isolate at 7-day intervals from 14 to 42 days post inoculation
(dpi). Bars represent ± standard error .........................................................................................85
3

Figure 2.4. Mean plant volume (cm ) of million bells (Calibrachoa x hybrida) cultivars at 42
days post inoculation evaluated in experiment 2 for susceptibility to root rot caused by
Phytophthora capsici and P. tropicalis. Bars with a letter in common are not significantly
different within each cultivar (Fisher‟s protected LSD, P = 0.05) ..............................................94
Figure 2.5. Area under plant growth curve (AUPGC) values of million bells (Calibrachoa x
hybrida) cultivars evaluated in experiment 2 for susceptibility to root rot caused by Phytophthora
capsici and P. tropicalis. Plant volume measurements were placed into the area under the
disease progress curve (AUDPC) formula to assess stunting or death over time. Larger AUPGC
values signify greater plant volume. Bars with a letter in common are not significantly different
within each cultivar (Fisher‟s protected LSD, P = 0.05) .............................................................97

ix

LITERATURE REVIEW
INTRODUCTION

Phytophthora capsici Leonian is a fungal-like microorganism in the kingdom
Stramenopila, formerly Chromista, and in the class Oomycetes (Erwin and Ribeiro, 1996).
Although, P. capsici resembles true fungi in its growth habit and mode of nutrition, phylogenetic
analyses place it closer to algae than true fungi (Baldauf, 2000). Morphological and
physiological differences exist between oomycete pathogens and true fungi. P. capsici is
predominately diploid, has cell walls composed of cellulose and -glucans, and has non-septate
mycelia (coenocytic), unlike true fungi that are predominately haploid, have cells walls
composed of chitin, and have septate mycelia (Agrios, 1997; Erwin and Ribeiro, 1996).
Leonian (1922) first described P. capsici following a foliar blight outbreak on chile pepper
(Capsicum annuum L.) in 1918 in New Mexico. Since then, losses of susceptible crops due to
root, crown, and fruit rot and foliar blight have been documented worldwide (Erwin and Ribeiro,
1996; Zitter et al., 1996). In Europe, for example, it is the most devastating disease of pepper in
Northwest Spain (Silvar et al., 2006). Pepper is one of the chief vegetable crops in Korea, and
losses due to P. capsici occur frequently (Hwang and Kim, 1995). In the United States (U. S.),
up to 100% crop loss of processing pumpkins (Cucurbita moschata P.) have occurred in Illinois,
where 90% of the U. S. crop is produced (Babadoost, 2000; Tian and Babadoost, 2004).
Extensive economic losses have occurred in other major U. S. vegetable production states,
including Florida (Ploetz and Haynes, 2000), Georgia (Mathis et al., 1999), Michigan (Hausbeck
and Lamour, 2004), New Jersey, and North Carolina (Ristaino and Johnston, 1999).

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Michigan is ranked among the top ten states in the nation for fresh market bell pepper (C.
annuum), cucumber (Cucumis sativus L.), squash (Cucurbita spp.), tomato (Solanum
lycopersicon L.), and snap bean (Phaseolus vulgaris L.) harvested ha (USDA, 2011b). The
combined value of these crops, all of which are susceptible to P. capsici (Crossan et al., 1954;
Gevens et al., 2008; Kreutzer, 1937; Kreutzer et al., 1940; Tompkins and Tucker, 1941), in 2010
was $137 million with 22,460 harvested ha (USDA, 2011b). Specifically, Michigan was ranked
number one in 2010 for the harvested ha of pickling cucumber and second for the combination of
processing and fresh market squash (USDA, 2011b). Pickling cucumber and squash sales
accounted for $61.7 million in 2010 with 15,216 harvested ha (USDA, 2011b). The processing
cucurbit industry is especially at risk to P. capsici epidemics (Babadoost, 2004). For instance, a
farm in southern Michigan was unable to harvest $300,000 of pickling cucumbers and ended
future production of cucumbers at this site (Hausbeck and Lamour, 2004).
Recently, Fraser fir (Abies fraseri Pursh Poir.) was identified as a P. capsici host
(Quesada-Ocampo et al., 2009). This has significant implications for the Michigan nursery
industry, as Michigan is ranked ninth among the top ten states in the U. S. for nursery stock
production with $148 million in sales in 2006 (USDA, 2007). Other Phytophthora spp., such as
P. tropicalis Aragaki and J. Y. Uchida, are threats to the floriculture industry. Michigan also
places third in sales among the top five floricultural states for the production of annuals,
perennials, cut flowers, foliage plants, and propagative materials (USDA, 2011a). In 2010,
wholesale floricultural sales in Michigan were approximately $389 million, about 9.8% of the U.
S. total (USDA, 2011a).
Greenhouse and nursery production systems, where conditions are favorable for root rot
pathogens, such as P. tropicalis, require a combination of cultural and chemical strategies to

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lessen disease occurrence. Resistance has developed to commonly applied fungicides used to
combat P. capsici (Hausbeck and Lamour, 2004; Lamour and Hausbeck, 2000) and other
Phytophthora spp. (Ferrin and Kabashima, 1991; Hwang and Benson, 2005; Lamour et al.,
2003), and hence, integrated management techniques are necessary to keep disease under
control. Phytophthora capsici has an increasing host range and pathogen isolates have been
recovered in temperate, subtropical, and tropical climates (Zitter et al., 1996). Identification of
P. capsici has been challenging. Its morphology is variable and susceptible hosts and geographic
origins of isolates overlap those of P. tropicalis (Alizadeh and Tsao, 1985; Aragaki and Uchida,
2001; Lamour, 2009).

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HOST RANGE AND DISEASE SYMPTOMS

Phytophthora capsici causes disease on over 50 genera in 28 families of host plants (Farr
and Rossman, 2011). In Michigan, vegetables hosts of significant economic importance include
members of the Cucurbitaceae, Fabaceae, and Solanaceae families (Erwin and Ribeiro, 1996).
Besides the aforementioned crops, P. capsici has been found to cause disease on other major
crops, including watermelon (Citrullus lanatus [Thunb.] Matsum & Nakai) (Wiant and Tucker,
1940), cantaloupe (C. melo L.), honey dew (C. melo) (Drechsler, 1929), macadamia (Macadamia
spp.), eggplant (S. melongena L.), and cacao (Theobroma cacao L.) (Erwin and Ribeiro, 1996).
Recently, P. capsici has been found to incite disease on additional crops previously thought to be
non-susceptible, such as lima bean (Phaseolus lunatus L.) (Davidson et al., 2002). Additional
hosts that have been infected during greenhouse experiments include crops in the
Chenopodiaceae family (beet [Beta vulgaris L.], Swiss chard [B. vulgaris var. cicla], and spinach
[Spinacia oleracea L.]), and the weed species, Queen Anne‟s lace (Daucus carota L.) in the
Apiaceae family and velvetleaf (Abutilon theophrasti Medik.) in the Malvaceae family (Tian and
Babadoost, 2004). Carolina geranium (Geranium carolinianum L.), common purslane
(Portulaca oleracea L.) (Ploetz and Haynes, 2000; Ploetz et al., 2002), and American black
nightshade (Solanum americanum Mill.) are additional alternate weed hosts for pathogen
survival (French-Monar et al., 2006).
Pepper, pumpkin, and squash are highly susceptible to root and crown rot (Hausbeck and
Lamour, 2004). Diseased roots are often brown or black in color (Hausbeck and Lamour, 2004),
and affected crown tissue is brown, water-soaked, and soft (Zitter et al., 1996). Plants succumb
relatively quickly after initial symptoms are apparent, following a heavy rainfall or prolonged

4

irrigation period (Kreutzer et al., 1940). A decreased susceptibility to P. capsici with an increase
in plant age has been found in some crops, including pepper (Kim et al., 1989) and pumpkin (Lee
et al., 2001). Cucumber and tomato plants appear somewhat tolerant to P. capsici, and may
exhibit only limited root rot (Hausbeck and Lamour, 2004; Lamour, 2009). However, cucumber
fruit are extremely sensitive to Phytophthora rot (Hausbeck and Lamour, 2004).
Pathogen growth (sporangia and mycelia) is typically present on surfaces of infected fruit.
A layer of sporangia that appears as a white, powdery growth may be present on sunken,
discolored lesions or on the entire fruit surface (Zitter et al., 1996). Some crops have increased
tolerance as their fruit undergo exocarp maturation (Ando et al., 2009). Wounding has been
found to negate age-related resistance in cucumber fruit (Granke and Hausbeck, 2010b). Disease
symptoms and pathogen growth may also develop during postharvest storage (Hausbeck and
Lamour, 2004). Growers in Michigan have had entire loads of cucurbit fruit rejected due to
postharvest fruit rot (Hausbeck and Lamour, 2004).
There has been much debate about a separate species designation for the isolates
recovered from tropical hosts (Bowers et al., 2007; Zhang et al., 2004). Cacao isolates were
originally classified as P. palmivora Butler, which was later divided into four forms based on
sporangia morphology and pedicel length (Alizadeh and Tsao, 1985; Bowers et al., 2007).
Morphological form 4 (MF4) was subsequently considered to be P. capsici, and included isolates
from cacao and black pepper (Piper nigrum L.), among other tropical plants (Alizadeh and Tsao,
1985; Brasier and Griffin, 1979; Uchida and Aragaki, 1980). In 1991, isozyme analyses were
conducted and morphological characteristics were examined for almost 400 isolates falling into
twelve Phytophthora spp. (Oudemans and Coffey, 1991). The previously classified „P.
palmivora MF4‟ isolates were placed into three subgroups, CAP1, CAP2, and CAP3, with

5

isolates collected from tropical hosts falling into the CAP2 and CAP3 subgroups (Oudemans and
Coffey, 1991). As a follow-up study, a worldwide sampling of P. capsici isolates was done in
1995 in an effort to re-examine the species, resulting in two subgroups of P. capsici, CAPA and
CAPB (Mchau and Coffey, 1995). Most isolates previously designated as „P. palmivora MF4‟
fell into the CAPB subgroup (Mchau and Coffey, 1995).
In 2001, Aragaki and Uchida (2001) named a new Phytophthora taxon for the „P.
palmivora MF4‟ isolates, P. tropicalis, based on morphology, absence of or minimal growth at
35 C, and host range. However, there is no precise separation of P. capsici and P. tropicalis
based on these characteristics alone (Lamour, 2009). But, genetic and phylogenetic analyses
support the division of P. capsici and P. tropicalis into separate species (Donahoo and Lamour,
2008; Quesada-Ocampo et al., 2011; Zhang et al., 2004).
Phytophthora tropicalis causes disease on 14 genera in 12 families of host plants (Farr
and Rossman, 2011). Like P. capsici, it causes root rot of the Solanaceous vegetable, eggplant
(Aragaki and Uchida, 2001). It also causes disease on tropical crops, such as cacao and
macadamia, members of the Malvaceae and Proteaceae families, respectively (Aragaki and
Uchida, 2001; Erwin and Ribeiro, 1996). Other overlapping host genera of P. capsici and P.
tropicalis include the ornamental plants flamingo flower (Anthurium spp.), carnation (Dianthus
spp.), pothos (Epipremnum spp.) (Orlikowski et al., 2006; Wick and Dicklow, 2002), and
pincushion flower (Leucospermum spp.) (Aragaki and Uchida, 2001; Donahoo and Lamour,
2008; Oudemans and Coffey, 1991). Additional hosts of P. tropicalis grown in temperate
regions of the U. S. as annual ornamental plants, but are tropical in nature, include cuphea
(Cuphea ignea A. DC.) (Cacciola et al., 2006), cyclamen (Cyclamen persicum Mill.) (Gerlach
and Schubert, 2001), and vinca (Catharanthus roseus [L.] G. Don) (Hao et al., 2010), which are

6

susceptible to root rot, bulb rot, and stem and foliar blight, respectively. Temperate woody
plants are also susceptible to P. tropicalis. Japanese andromeda (Pieris japonica [Thunb.] D.
Don ex G. Don) and rhododendron (Rhododendron catawbiense Michx.) exhibit foliar necrosis
and dieback symptoms when infected by P. tropicalis (Hong et al., 2006).

7

THE PATHOGEN

Morphology and requirements for growth. Phytophthora capsici is heterothallic and
sexually reproduces by the pairing of two distinct mating or compatibility types (CT), A1 and
A2. The union of the male antheridia and the female oogonia results in the production of
oospores (Erwin and Ribeiro, 1996). When fertilization takes place, the antheridium attaches to
the oogonium, whereby the oogonial incept grows through the antheridial incept (Erwin and
Ribeiro, 1996). Sizes of oogonia fluctuate by host and are between 23 to 50 m (Erwin and
Ribeiro, 1996). Oogonia are spherical to sub-spherical in shape, plerotic, and hyline to brown in
color (Erwin and Ribeiro, 1996).
Crossing of P. capsici and P. tropicalis has been demonstrated in vitro with resulting
oospore progeny able to hybridize with P. capsici (Donahoo and Lamour, 2008).
Chlamydospores are rarely produced in cultures from cucurbits or pepper (Ristaino, 1990;
Tucker, 1931), but have been reported in some isolates (Alizadeh and Tsao, 1985).
Chlamydospore production is common of P. tropicalis isolates (Aragaki and Uchida, 2001).
The formation of P. capsici oospores is promoted by darkness and red fluorescent light
(650 m ) at 20 to 24 C (Harnish, 1965). Maximum P. capsici oospore germination is favored by
the use of clarified V8 juice (CV8) agar (V8 juice submitted to centrifugation for ten min at
4,340 g before media preparation) at 24 C in continuous darkness for 12 days with root exudates
(Hord and Ristaino, 1991). Oospores may successfully be formed on bean meal, rapeseed
extract, V8 juice broth or agar, or carrot broth (Erwin and Ribeiro, 1996).
Phytophthora capsici reproduces asexually though the production of sporangia and
zoospores. Sporangia are prominently papillate, but may be semipapillate having two to three

8

apices (Erwin and Ribeiro, 1996; Waterhouse, 1963). The sporangia are ellipsoid to pyriform in
shape (Waterhouse, 1963) and are variable in size at 32.8 to 65.8 x 17.4 to 38.7 m (Erwin and
Ribeiro, 1996). They are tapered at the base and deciduous with long pedicels that fluctuate
from 35 to 138 m produced on simple, sympodial, or umbellate sporangiophores (Erwin and
Ribeiro, 1996). They germinate by the formation of a germ tube(s), referred to as direct
germination, or by indirect germination by the release of zoospores, which are produced by the
differentiation of cytoplasm within sporangia (Ribeiro, 1983).
Sporangia development ensues when relative humidity approaches 100% (Ribeiro, 1983).
In laboratory studies, Granke and Hausbeck found that a relative humidity of 60 to 80% favored
sporangia production on pickling cucumber fruit (Granke and Hausbeck, 2010b). The optimal
temperature for sporangial development occurs at 25 C (Granke and Hausbeck, 2010b), whereas
no sporangia are produced at temperatures of 18 C or below and 35 C or above (Kreutzer and
Bryant, 1946). Copious production occurs under white light (700 ft. c.) and blue fluorescent
light (400 to 675 m ), and darkness or red fluorescent light (650 m ) are inhibitory (Harnish,
1965).
Indirect or direct germination is dependant upon temperature, matric potential of the soil,
and available food sources (MacDonald and Duniway, 1978; Mitchell and KannwischerMitchell, 1992). Indirect germination is favored by low temperatures (Ribeiro, 1983), and in
vitro, zoospore production and liberation may be induced by chilling P. capsici cultures at 10 C
for 15 to 30 min (Mitchell and Kannwischer-Mitchell, 1992). High amounts of sugars and amino
acids that promote direct germination, inhibit indirect germination (Ribeiro, 1983).
Zoospores are reniform and biflagellate, with flagella on the concave side (Hickman,
1970). Dissemination of zoospores occurs actively by means of flagella, but also passively by

9

zoospore movement in irrigation water or within waterways (Gevens et al., 2007). Zoospores
exhibit positive chemotaxis (Carlile, 1983). They detect a favorable stimulus from plant tissue
and swim towards it, with the maximum duration of swimming possibly determined by food
reserves present within the zoospore (Carile, 1983). After the zoospore detects a stimulus, its
normal spiral path changes to a haphazard course, with it becoming trapped in the stimulus zone,
later followed by encystment (loss of flagella and development of a cell wall) and germ tube
formation (Carlile, 1983). The root exudates significant to the attraction of zoospores include
amino acids and sugars (Hickman, 1970). Negative geotaxis may help keep zoospores near the
surface of the soil where roots are more likely to be (Carlile, 1983). Rapid attraction,
accumulation, and encystment take place just behind the root tip (Hickman, 1970).
Identification of P. capsici and P. tropicalis. The isolation and subsequent identification
of P. capsici by direct plating on agar media has historically been challenging (Papavizas et al.,
1981; Tsao and Ocana, 1969). Culturing for P. capsici while likewise inhibiting the growth of
Pythium spp., other oomycete pathogens, has been difficult (Tsao and Ocana, 1969). Media that
promotes adequate growth and amended with chemicals that possess antifungal and antibiotic
properties have been useful in culturing (Tsao, 1983). Amendments of fungicides such as
benomyl, nystatin, pentachloronitrobenzne (PCNB), and pimaricin and various antibiotics
including ampicillin, rifampicin, and vancomycin have been effective in the suppression of
undesirable fungi and bacteria (Masago et al., 1977; Tsao and Ocana, 1969).
Differentiating between isolates of P. capsici and P. tropicalis has been complicated, as
well. Separation of the species by morphology governs that P. tropicalis has sporangial
diameters of less than or equal 26.0 m, length to diameter sporangial ratios greater than or equal
to 1.8, and tapered sporangial bases when cultured on vegetable juice agar (VJA) at 24ºC under

10

2

continuous cool-white fluorescent lighting (ca 4700 ergs/s/cm ) (Aragaki and Uchida, 2001).
DNA sequence data has helped to provide a more clear distinction between the species rather
than solely relying on morphology or pathogenicity studies alone (Bowers et al., 2007; Donahoo
and Lamour, 2008; Zhang et. al, 2004). Recently, accessible online databases with reference
sequences have been created and are helpful in the identification of Phytophthora spp.
(Grünwald et al., 2011; Park et al., 2008).
Life cycle. Phytophthora capsici overwinters as oospores that may survive in the soil for
greater than five years (Hausbeck and Lamour, 2004). When conditions are favorable for
germination, oospores produce a germ tube to infect the host directly or give rise to sporangia,
which may penetrate plant tissue, or form zoospores. Zoospores also serve as inoculum for
infection. Sporangia are detached from the sporangiophores by means of capillary water
movement or hard, driving rain (Granke et al., 2009b) and may be moved down plot rows with
surface water from irrigation or rainfall (Ristaino and Johnston, 1999). Infection is favored by
rainy weather that saturates the soil for long time periods (Hausbeck and Lamour, 2004).
Phytophthora capsici is a polycyclic pathogen; asexual reproduction will continue throughout
the growing season and creation of oospores will result if conditions are favorable and both CT
are present (Schlub, 1983). Overall, dissemination of and infection by sporangia and zoospores
account for rapid disease development (Erwin and Ribeiro, 1996).
Pathogenicity and virulence. Phytophthora capsici isolates obtained from one genus or
family of host plants are pathogenic to others (Crossan et al., 1954; Tompkins and Tucker,
1941). For example, isolates recovered from Solanaceous hosts are pathogenic on
Cucurbitaceous hosts and vice versa (Ristaino, 1990; Tompkins and Tucker, 1941). Isolates
have been found to differ in virulence on the same crop. When isolates collected from pepper

11

and pumpkin were tested on eight Korean and Japanese pumpkin cultivars, different levels of
isolate virulence were observed (Lee et al., 2001). Evaluating Solanaceous crops with P. capsici
isolates have yielded similar results (Foster and Hausbeck, 2010; Kim and Hwang, 1992;
Ristaino, 1990). Significant differences in pathogen-host response were found in 12 Korean
pepper cultivars evaluated with 14 isolates from diverse global locations (Kim and Hwang,
1992). Likewise, Foster and Hausbeck (2010) screened 31 pepper lines for crown, root, and fruit
rot with four isolates obtained from Cucurbitaceous and Solanaceous hosts with comparable
results to the Kim and Hwang (1992) study. They found that isolates differed in virulence on
pepper lines, with one isolate causing disease on the commercially marketed, P. capsici-resistant
cultivar „Paladin‟ (Foster and Hausbeck, 2010). The same four isolates were used in QuesadaOcampo and Hausbeck‟s (2010) greenhouse study, in which 42 tomato cultivars and wild
relatives were screened for susceptibility to P. capsici. They found that virulence was dependant
upon the isolate-tomato genotype interaction (Quesada-Ocampo and Hausbeck, 2010).
Genetic recombination during hybridization affects pathogenicity. In Polach and
Webster‟s (1972) study, isolates that varied in pathogenicity to Cucurbitaceous and Solanaceous
crops were mated in vitro. The progeny contrasted to that of its parents in the ability to cause
disease on eggplant, pepper, tomato, squash, and watermelon (Polach and Webster, 1972).
Recently, isolates resistant to the commonly used fungicide, mefenoxam, an oomycete-specific
fungicide, prove to be just as fit and virulent as strains that are sensitive to it, not exhibiting any
reduction in pathogenicity on pepper or squash (Café-Filho and Ristaino, 2008).

12

DISEASE MANAGEMENT

Management in the field. Water plays an important role in the P. capsici life and disease
cycles. Sowing seeds or transplanting seedlings into raised, crowned beds to avoid excess soil
moisture around plant crowns and utilizing black polyethylene mulch can increase yields by
preventing splashing of infested soil onto aboveground plant tissues (Ristaino and Johnston,
1999; Springer and Johnston, 1982). Integrating soil amendments may be a successful
management method for some crops (Kreutzer and Bryant, 1946). Disease has been lessened by
incorporating straw into tomato planting beds (Kretuzer and Bryant, 1946) and growing pepper
in stubble from a no-till wheat cover crop (Ristaino et al., 1997). Staking plants, such as tomato
or squash, to keep foliage and fruit dry is another effective cultural management technique
(Kreutzer and Bryant, 1946).
Applying supplemental irrigation less frequently through the use of furrow irrigation has
shown to reduce the onslaught of disease symptoms and increase yield (Café-Filho et al., 1995).
Employment and frequency of drip irrigation (Ristaino, 1991) and the location of emitters are
integral in disease decline (Café-Filho and Duniway, 1996; Ristaino, 1991). Gains from
managing disease by increasing the distance from the drip tape to the plant may compensate for a
crop loss due to drought stress (Café-Filho and Duniway, 1996). Irrigating susceptible crops
with non-surface water sources is recommended, as infested surface water has been found to be a
significant source of P. capsici inoculum (Gevens et al., 2007). Phytophthora capsici was
identified in Michigan river systems, a naturally fed pond, a pond fed by a deep well, and ditches
tested by baiting traps for three consecutive years (Gevens et al., 2007). Zoospores are able to

13

survive in water at a wide range of temperatures (9 to 32ºC) and can cause fruit infection for up
to five days after being released from sporangia (Granke and Hausbeck, 2010a).
Decreasing the density of plantings may also lessen rot symptoms by reducing relative
humidity and duration of aboveground tissue wetness within the plant canopy (Ngouajio et al.,
2006). The decreased number of plants in a less dense planting may be offset by an increase in
fruit yield (Ngouajio et al., 2006). Crop rotations with non-susceptible hosts with at least three
to four years in between planting a susceptible host are recommended (Ristaino and Johnston,
1999); however, Lamour and Hausbeck (2002) found that even allowing for more time in
between rotations, disease on squash was incited by P. capsici soil inoculum from five years
prior.
There are limited chemical options available to growers that provide adequate control of
P. capsici even when combined with other management tools (Hausbeck and Lamour, 2004).
Metalaxyl, a member of the phenylamide- (PA) fungicide group (FRAC, 2010), has been
intensively used to manage diseases caused by the Peronosporales since the mid-1980s (Davidse
et al., 1989). It specifically inhibits the incorporation of uridine into RNA (Davidse et al., 1989).
Resistance was reported shortly after its commercial introduction in 1977 in P. infestans (Mont.)
de Bary, with resistance in P. megasperma f. sp. medicaginis caused by the insensitivity of the
target site in the RNA polymerase (Cohen and Coffey, 1986; Davidse et al., 1989).
Resistance of P. capsici isolates to mefenoxam (Ridomil Gold [100% of the active
enantiomer contained in metalaxyl, metalaxyl contains 50% of the active ingredient]) has been
reported in some U. S. states (Davey et al., 2008; Dunn et al., 2010; Mathis et al., 1999; Parra
and Ristaino, 1998; Parra and Ristaino, 2001; Ristaino and Johnston, 1999), including Michigan
(Hausbeck and Lamour, 2004; Lamour and Hausbeck, 2000), and globally (Pennisi et al., 1998;

14

Silvar et al., 2006). Isolates may be considered sensitive (S), intermediately sensitive (IS), or
insensitive (I) based upon culture growth on 100 ppm mefenoxam amended-unclarified V8 juice
(UCV8) agar (Lamour and Hausbeck, 2000). The sensitivity of isolates to mefenoxam (MS) is
inherited by a single, incompletely dominant gene unlinked to CT (Lamour and Hausbeck,
2002). After resistance is established in a population, the future utilization of mefenoxam to
manage P. capsici may not be possible. Lamour and Hausbeck (2002) found that when
mefenoxam applications were discontinued (for a two-year period) in a field site, there was no
increase in pathogen sensitivity.
Alternative chemicals applied either in combination with mefenoxam or singly may
reduce the severity of Phytophthora rot and blight. Dimethomorph (Acrobat), zoxamide +
mancozeb (Gavel), and cymoxanil + famoxadone (Tanos) have been effective in controlling
mycelial growth and limiting zoospore production in laboratory studies (Keinath, 2007). Field
trials with captan (Captan 80WDG), copper hydroxide (Kocide 3000), mandipropamid (Revus
2.08SC), fluopicolide (Presidio 4FL), and zoxamide + mancozeb (Gavel 75DF) have been
effective in alternation with mefenoxam (Ridomil Gold), combined with one other, or applied
alone (Adams et al., 2008; Foster and Hausbeck, 2008a; Foster and Hausbeck, 2008b; Holmes et
al., 2007; Jackson et al., 2010). Seed treatments with mefenoxam (Apron XL) and metalaxyl
(Alligiance FL) are also successful in controlling seedling damping-off (Babadoost and Islam,
2003). Recently, isolates of P. capsici have been found to be resistant to iprovalicarb in vitro
(Lu et al., 2010). Resistant isolates also showed limited sensitivity to dimethomorph, flumorph,
and mandipropamid, other chemistries in the carboxylic acid amide- (CAA) fungicide group
(FRAC, 2010; Lu et al., 2010). This suggests that future rotations of these fungicides with others
in different fungicide groups may be necessary to effectively manage P. capsici in the field. A

15

successful chemical management program should alternate between fungicides to postpone
resistance (Hausbeck and Lamour, 2004).
Pre-plant soil fumigation with methyl bromide, which was once commonly used in disease
prevention for protection of P. capsici and other soilborne pathogens, can no longer be used in
many growing areas (Hausbeck and Lamour, 2004). Methyl bromide is labeled as a Class I
Stratospheric Ozone Depleting Substance (Duniway, 2002; Ristaino and Thomas, 1997), and its
phase out was completed in 2005. Michigan Cucurbitaceous and Solanaceous crop growers have
been allowed Critical Use Exemptions from the Environmental Protection Agency (EPA)
(Hausbeck and Lamour, 2004) through 2011. Other fumigants, such as chloropicrin, 1,3dichloropropene (1,3-D), methyl isothiocyanate (MITC) generators, and metam sodium, are
registered and may be applied singly or as mixtures (Duniway, 2002). Unfortunately, these
materials do not have the broad-spectrum activity that methyl bromide has (Duniway, 2002).
Numerous studies have been done to evaluate the effectiveness of other fumigants and nonchemical alternatives to P. capsici in comparison to methyl bromide, with some alternatives
found to provide sufficient control (Hausbeck et al., 2006; Granke et al., 2009a; Ji and Csinos,
2009; Rosskopf et al., 2005).
Management in the greenhouse. Controlling root rot of vegetable transplants,
floriculture crops, and nursery stock in the greenhouse can be difficult due to the warm, moist
environment present. Many Phytophthora spp. can be problematic in greenhouse environments.
For example, P. cryptogea Peth. and Laff. causes root rot of snapdragon (Antirrhinum majus L.),
begonia (Begonia elatior), and petunia (Petunia x hybrida Hort. Vilm. Andr) (Ampuero et al.,
2008; Erwin and Ribeiro, 1996). Phytophthora drechsleri Tucker is a common pathogen of
poinsettia (Euphorbia pulcherrima Willd. Ex Klotzsch); P. nicotianae Breda de Hann (syn. P.

16

parasitica Dastur) causes disease on 58 families, including many vegetables and ornamental
plants, and is the most commonly encountered Phytophthora spp. in containerized plant
production (Daughtrey et al., 1995).
Management of root rot pathogens can be accomplished, in part, by cultural practices.
Sowing seeds or transplanting liners into pasteurized media prevents initial colonization by
pathogens (Dreistadt, 2001). Likewise, using porous, soilless media and allowing it to dry
between watering helps to limit the microclimate in which Phytophthora spp. thrive. Rouging
diseased plants and removing dead foliage decrease inocula sources (Dreistadt, 2001; Hausbeck,
2007). Disinfesting flats, greenhouse benches, and weed control mats with a bleach solution or
disinfectant chemical also aids in the reduction of disease spread (Hausbeck, 2007).
Recirculated and surface irrigation water are often contaminated with Phytophthora spp.
propagules (Dreistadt, 2001; Hong and Moorman, 2005), and may be dispersed through
specialized greenhouse irrigation equipment, such as recirculating water systems (Oh and Son,
2008; Thinggaard and Andersen, 1995; van der Gaag et al., 2001). Granke and Hausbeck‟s
(2010a) study evaluating eleven commercial algaecides with P. capsici zoospores found that
zoospore motility, subsequent mortality, and ability to infect pickling cucumbers were
significantly influenced by the algaecides tested. Thus, the employment of algaecides, not only
to prevent the buildup of algae in greenhouse irrigation systems, but also to reduce Phytophthora
spp. inocula, may be useful (Granke and Hausbeck, 2010a).
Phytophthora spp. isolates (P. citricola Sawada, P. cryptogea, P. drechsleri, P. nicotianae,
and P. palmivora) collected from greenhouses and nurseries in major U. S. floricultural
production states have displayed insensitivity to metalaxyl (or mefenoxam) (Ferrin and
Kabashima, 1991; Hwang and Benson, 2005; Lamour et al., 2003). Fungicides with different

17

modes of action should be rotated to delay the onset of fungicide resistance (Hwang and Benson,
2005). Cyazofamid, dimethomorph (Stature DM), fenamidone (FenStop), fluopicolide (Adorn
4SC), fluopicolide + azoxystrobin (Adorn 4SC + Heritage 50WDG), and fluopicolide +
etridiazole (Adorn 4SC + Terrazole 35WP) are effective in managing Phytophthora spp. in the
greenhouse (Hausbeck and Glaspie, 2009a; Hausbeck and Glaspie, 2009b; Hausbeck and Harlan,
2006). Mono-and di-potassium salts of phosphorous acid (Alude) and dipotassium phosphonate
(Biophos) have reduced root rot caused by P. drechsleri and P. nicotianae in comparison to
inoculated control plants in greenhouse trials (Hausbeck and Glaspie, 2009a; Hausbeck and
Glaspie, 2009b).
Host resistance. Genetic resistance in conjunction with cultural and chemical
management methods is extremely useful in managing disease. However, significant host
resistance to P. capsici has been reported in few crops. „Paladin,‟ a commercially marketed P.
capsici-resistant pepper cultivar, has shown resistance to some isolates, and „Criollo de Morelos
334,‟ (CM-334) a Mexican pepper type distantly related to bell pepper, with small, pungent fruit,
has displayed a high level of resistance to aggressive strains of P. capsici (Foster and Hausbeck,
2010; Thabuis et al., 2004). Recurrent breeding schemes have been developed to transfer the
resistance genes from CM-334 into bell pepper. Resistance in pepper was thought to be
controlled by two independent, dominant genes (Smith et al., 1967). One independent, dominant
gene was shown to be necessary for root rot resistance, and a different, independent, dominant
gene for foliar blight resistance (Walker and Bosland, 1999). Later, resistance to stem blight in
pepper was also found to be governed by a third single gene mechanism; therefore, each of the
three symptoms that P. capsici causes are actually separate disease syndromes (Sy et al., 2005).

18

No cucurbits have significant resistance to date (Hausbeck and Lamour, 2004). However,
the Korean pumpkin cultivar „Danmatmaetdol‟ has exhibited a high level of resistance to diverse
geographic isolates (Lee et al., 2001). Gevens et al. (2006) screened the fruit from 480 cucumber
cultivars and plant introductions (cultigens) throughout a six-year time span with a single isolate
evaluating lesion and sporulation diameter and sporulation density and found that several
cultivars, „Discover,‟ „Excel,‟ and „Vlaspik,‟ had reduced disease. However, none of the
cultigens consistently performed better when compared to the best cultivars (Gevens et al.,
2006).
For summer squash, 33 plant introduction (PI) lines from eight countries in 2001 and 12
PIs from ten countries in 2002, were screened for tolerance to P. capsici in Southwest Michigan
(Goldy and Francis, 2001; Goldy and Hildebrand, 2002). PI 357955 and PI 615115 plants
exhibited 100% survival in 2002, which was not found, however, to be significantly different
than the cultivar „Tigress,‟ planted as a control with an 85% survival rate (Goldy and Hildebrand,
2002). Camp et al. (2009) evaluated summer and winter squash cultivars and PIs in New York,
and found that the zucchini cultivar, „Leopard‟ exhibited significantly less disease than some
yellow squash cultivars and PIs. Padley et al. (2008) screened 115 C. pepo PIs from 24 countries
using three P. capsici isolates collected from Florida. Several PIs exhibited little or no disease,
indicating that there may be resistance present to Phytophthora crown rot (Padley et al., 2008).
Likewise, Chavez et al. (2011) evaluated 119 C. moschata PIs from 39 geographic locations,
finding that PIs 176531, 458740, 442266, 442262, and 634693 exhibited reduced crown rot
symptoms. Padley et al. (2009) succeeded in introgressing resistance into the C. moschata
breeding line, #394-1-27-12 from the University of Florida. This resistance was derived from
two wild species, C. lundelliana and C. okeechobeenesis subsp. okeechobeenesis, and was

19

introduced through hybridizations, self-pollinations, and single plant selections (Padley et al.,
2009).
To conclude, major research has been conducted to date regarding the identification and
biology of P. capsici. This research has helped to progress the integrated disease management of
host crops. In order to continue to successfully control Phytophthora rot and blight,
understanding the differences in susceptibility and symptom progression of crops are essential
for growers and researchers. Additionally, as P. capsici and P. tropicalis are indistinguishable
based on host preference, determining the host range for potentially susceptible crops is critical.
The research objectives of this thesis were to evaluate cucurbit root, crown, and fruit rot caused
by P. capsici and to evaluate the susceptibility of ornamental plants to root rot caused by P.
capsici or P. tropicalis.

20

CHAPTER I: CUCURBIT FRUIT AND SEEDLING RESPONSE
TO PHYTOPHTHORA CAPSICI
ABSTRACT
Cucumber (Cucumis sativus) and squash (Cucurbita spp.) production in Michigan is
hampered by the oomycete pathogen Phytophthora capsici Leonian. Eight cucurbit fruit types
were evaluated with five isolates of P. capsici. Detached fruits were inoculated with a 5-mm
culture plug of mycelia and sporangia and were incubated in a laboratory or greenhouse. Lesion
and pathogen growth diameters were measured and pathogen growth density was visually
assessed. All P. capsici isolates incited fruit rot with significant differences found among fruit
type and pathogen isolate. Straightneck squash (C. pepo), slicing cucumber, and butternut
squash (C. moschata) exhibited more severe disease symptoms of the summer squash, cucumber,
and winter squash fruit tested, respectively.
Seedlings of summer and winter squash were evaluated in greenhouse experiments, in
which P. capsici-infested millet seeds (ca. 1 g) were placed on soilless potting media. Disease
severity assessments were conducted at 2-day intervals for 14 days post inoculation. Crop type,
pathogen isolate, or the crop type*pathogen isolate interaction term were significant for
symptom appearance and area under the disease progress curve values. Susceptibility
differences between butternut squash cultivars „Butterbush‟ and „Waltham‟ and zucchini
cultivars „Leopard‟ and „Zucchini Elite‟ were found. This study indicates that isolate virulence
differs on cucurbit fruit and seedlings, and thus, diverse isolates should be utilized in cucurbit
germplasm screenings for resistance to P. capsici.

21

INTRODUCTION

Phytophthora capsici Leonian, an oomycete pathogen, is a constant threat to the cucurbit
industry in Michigan. Phytophthora capsici was first found to be the causal agent of chile
pepper (Capsicum annuum L.) foliar blight in New Mexico in 1918 (Leonian, 1922), and now
has been reported to cause disease on over 50 plant genera (Farr and Rossman, 2011). Major
susceptible vegetable crops are within the Cucurbitaceae, Fabaceae, and Solanaceae families,
which include cucumber (Cucumis sativus L.), melon (C. melo L.), pumpkin (Cucurbita pepo
L.), summer and winter squash (Cucurbita spp.), lima and snap bean (Phaseolus spp.), bell
pepper (C. annuum), tomato (Solanum lycopersicon L.), and eggplant (S. melongena L.)
(Crossan et al., 1954; Davidson et al., 2002; Drechsler, 1929; Erwin and Ribeiro, 1996; Gevens
et al., 2008; Kreutzer, 1937; Kreutzer et al., 1940; Tompkins and Tucker, 1941). Michigan is
ranked among the top ten states in the United States (U. S.) for the production of many of these
crops, specifically bell pepper, cucumber, snap bean (P. vulgaris L.), squash, and tomato, with a
value of $137 million in 2010 (USDA, 2011b). Specifically, Michigan was ranked number one
in 2010 for harvested ha of pickling cucumber and second for the combination of processing and
fresh market squash with $61.7 million in value and 15,216 harvested ha (USDA, 2011b).
Cucurbit production is particularly at risk, as cucurbits, together with pepper, are
considered to be the most vulnerable to P. capsici (Babadoost, 2004; Tian and Babadoost, 2004).
Pumpkin seedlings are more resistant to P. capsici as they mature (Lee et al., 2001), and squash
fruit exhibit age-related resistance as they transition from the waxy green to green stage (Ando et
al., 2009). However, plants and fruit are susceptible at any stage, and plants may die relatively
quickly following a prolonged rainfall or irrigation period (Kreutzer et al., 1940). Cucumber

22

roots and foliage are somewhat tolerant to P. capsici, but fruit is extremely susceptible
(Hausbeck and Lamour, 2004). Fruit surfaces may exhibit Phytophthora rot symptoms, sunken,
discolored lesions, and pathogen signs, white, powdery growth (mycelia and sporangia), while in
the field or post harvest. Producers have had entire loads of cucurbit fruit rejected due to
postharvest rot (Hausbeck and Lamour, 2004). Disease is favored by warm, wet weather
(Hausbeck and Lamour, 2004), and inocula are disseminated by irrigation water runoff or hard,
driving rain (Granke et al., 2009b; Ristaino and Johnston, 1999).
Chemical and cultural management of Phytophthora rot and blight is challenging.
Phytophthora capsici overwintering structures (oospores) have been found to survive in the soil
to cause subsequent infection for up to five years, making rotations to non-susceptible crops
difficult (Hausbeck and Lamour, 2004). Resistance to mefenoxam, an oomycete-specific
fungicide, is common among isolates in Michigan (Hausbeck and Lamour, 2004; Lamour and
Hausbeck, 2000), other U. S. states (Davey et al., 2008; Dunn et al., 2010; Mathis et al., 1999;
Parra and Ristaino, 1998; Parra and Ristaino, 2001; Ristaino and Johnston, 1999), and globally
(Pennisi et al., 1998; Silvar et al., 2006) where mefenoxam has been greatly utilized. Surface
water has been identified as a potential source of P. capsici inoculum (Gevens et al., 2007), and
asexual propagules (zoospores) may survive in water for up to five days at a wide range of
temperatures (9 to 32ºC) to cause infection (Granke and Hausbeck, 2010a). Increased plant
spacing to decrease the duration of foliar and fruit wetness and canopy humidity, utilization of
drip or furrow irrigation from non-surface water sources, and employment of black plastic mulch
to lessen tissue contact with infested soil (Café-Filho et al., 1995; Gevens et al., 2007; Ngouajio
et al., 2006; Ristaino, 1991; Ristaino and Johnston, 1999) may be practical in some production
systems.

23

Cultivar resistance, in combination with chemical and cultural management practices, is
integral in lessening disease occurrence and severity. „Danmatmaedol,‟ a Korean pumpkin
cultivar, has exhibited increased tolerance to root and crown rot (Lee et al., 2001). Screenings of
C. pepo and C. moschata P. accessions found several plant introduction (PI) lines with reduced
crown rot symptoms (Chavez et al., 2011; Padley et al., 2008). Conversely, fruit screening of a
large collection of cucumber germplasm accessions did not identify any resistant genotypes
(Gevens et al., 2006). Some studies have indicated that seedling and fruit disease response of
Cucurbitaceous and Solanaceous crops is dependant upon isolate virulence (Foster and
Hausbeck, 2010; Kim and Hwang, 1992; Lee et al., 2001; Polach and Webster, 1972; QuesadaOcampo and Hausbeck, 2010; Tian and Babadoost, 2004). Phytophthora capsici isolate 12889
(isolate designation refers to the notation given by the laboratory of M. K. Hausbeck at Michigan
State University) caused disease on commercially-marketed, P. capsici-resistant pepper cultivar
„Paladin‟ (Syngenta Seeds, Inc., Boise, ID), and has shown a high level of virulence on various
crops (Foster and Hausbeck, 2010; Quesada-Ocampo et al., 2009; Quesada-Ocampo and
Hausbeck, 2010).
There are no commercial cucurbit cultivars available with Phytophthora rot resistance,
and land that is not infested with P. capsici inocula in Michigan is becoming limited (Hausbeck
and Lamour, 2004). Differences in virulence of Michigan P. capsici isolates has not been
evaluated on summer or winter squash cultivars, and Michigan growers would benefit from the
knowledge of cultivar susceptibility and disease progression regarding management decisions.
The objectives of this research included 1) evaluating the susceptibility of cucurbits to
root, crown, and fruit rot caused by P. capsici and 2) evaluating the virulence of P. capsici
isolates on cucurbits.

24

MATERIALS AND METHODS

Isolate selection and maintenance. Five P. capsici isolates were used as inocula in fruit
and seedling experiments. Isolates differed in phenotype (mating type = compatibility type [CT],
sensitivity to mefenoxam [MS]) and host origin. Isolates were characterized by CT and MS as
described by Lamour and Hausbeck (2000). Isolate designation refers to the notation given by
the laboratory of M. K. Hausbeck at Michigan State University (MSU). Isolate SF3 (CT = A1)
is intermediately sensitive to mefenoxam and was isolated from pickling cucumber fruit. Isolates
OP97 (CT = A1) and SP98 (CT = A2) are sensitive to mefenoxam and were isolated from the
fruit of pickling cucumber and pumpkin, respectively. Isolate 12889 (CT = A1) is insensitive to
mefenoxam and was isolated from pepper fruit. Isolates were collected from commercial fields
in Michigan and have been shown to be highly virulent on diverse hosts (Foster and Hausbeck,
2010; Gevens et al., 2006; Quesada-Ocampo et al., 2009; Quesada-Ocampo and Hausbeck,
2010). Isolate 13351 (CT = A1) is sensitive to mefenoxam and was isolated from eggplant by
the laboratory of C. D. Smart at Cornell University.
Agar plugs of isolate cultures were obtained from long-term storage (microcentrifuge
tubes containing 1 ml of sterile water and 1 sterile hemp seed at 20ºC) and were plated on
unclarified V8 juice agar (UCV8, 16 g agar, 3 g calcium chloride [CaCO3], 160 ml V8 juice, and
840 ml of distilled water) amended with 0.50 g benomyl, 2 ml ampicillin, and 2 ml rifampicin.
Mycelia and sporangia agar plugs from five-to seven-day old cultures were used to inoculate
cucumber fruit prior to isolate use in experiments to confirm isolate pathogenicity. Tissue was
isolated from fruit and plated on UCV8 agar (12 g agar, 0.3 g CaCO3, 40 ml V8 juice, and 960
ml distilled water) amended with 0.50 benomyl, 2 ml ampicillin, 2 ml rifampicin, and 0.10 g

25

pentachloronitrobenzene (BARP). Cultures were observed using a compound microscope at
100x magnification to confirm P. capsici morphological characteristics according to Waterhouse
(1963), hyphal tipped, and transferred to new UCV8 agar. Cultures were maintained at room
temperature (21 to 23°C) under continuous fluorescent lighting and were transferred every five
to seven days to new UCV8 agar.
Fruit experiment. Screening of summer squash and cucumber fruit, including
straightneck squash (C. pepo), zucchini (C. pepo), pickling cucumber, and slicing cucumber, for
susceptibility to P. capsici was conducted in July 2009 in the laboratory of M. K. Hausbeck at
MSU (Table 1.1). Screening of winter squash fruit, including acorn squash (C. pepo), butternut
squash (C. moschata), spaghetti squash (C. pepo), and pumpkin, for susceptibility to P. capsici
was conducted in September 2009 in a greenhouse at the MSU Plant Science Research
Greenhouses in East Lansing, Michigan (Table 1.1). The experimental design was a split-plot
arrangement, and the experiment was repeated three times. Fruit were replicated eight times for
each isolate and the control. Fruit were obtained from plants grown in commercial fields or
MSU research farms with no history of Phytophthora rot occurrence, which were hand-harvested
at maturity and were of a commercially marketable and similar size (Table 1.1).
Slicing cucumber and squash plants were treated with chlorothalonil (Bravo Weather
Stik; Syngenta Crop Protection, Inc., Greensboro, NC) at 1.75 liters a.i./ha, thiophanate-methyl
(Topsin 4.5FL; Cerexagri Inc., King of Prussia, PA) at 0.59 liter a.i./ha, and copper hydroxide
(Kocide 2000; DuPont Crop Production, Wilmington, DE [winter squash only]) at 0.11 kg/ha at
10-day intervals; pickling cucumber and pumpkin plants were treated with chlorothalonil (Bravo
Weather Stik) at 2.33 liters a.i./ha, alternated with pyraclostrobin (Pristine; BASF Ag Products,
Research Triangle Park, NC) at 1.29 liters a.i./ha at 7-day intervals for control of powdery and

26

downy mildew. Pumpkin plants were treated with imidacloprid (Admire Pro; Bayer
CropScience, Research Triangle Park, NC) at 0.73 liter a.i./ha at transplanting and esfenvalerate
(Asana XL; DuPont Crop Protection) at 0.70 liter a.i./ha at flowering for control of cucumber
beetles. Plants were irrigated and fertilized according to standard management practices.

Table 1.1. Cucurbit fruit evaluated for susceptibility to fruit rot caused by Phytophthora capsici
Crop
Species
Cucumis sativus
(pickling cucumber)
(slicing cucumber)
Cucurbita moschata
(butternut squash)
Cucurbita pepo
(acorn squash)
(pumpkin)
(spaghetti squash)
(straightneck squash)
(zucchini)

Cultivar

Mean fruit
diameter (cm)

Experiment
location

'Vlaspik'
'Cobra'

2.7 - 3.0
4.0 - 5.0

lab
lab

'Butternut Supreme'

10.5 - 12.0

GH

'Table Ace'
'Sorcerer'
'Vegetable'
'Cougar'
'Fortune'
'Reward'
'Spineless Beauty'

11.0 - 12.0
23.0 - 26.5
11.0 - 14.0
4.0 - 4.5
4.0 - 4.5
4.0 - 4.5
4.0 - 4.5

GH
GH
GH
lab
lab
lab
lab

lab = laboratory.
GH = greenhouse.

Fruit were inoculated according to the method described by Gevens et al. (2006). Fruit
were surface disinfested in a 20% bleach (1.23% sodium hypochlorite [NaClO]) solution for 5
min and rinsed with distilled water prior to inoculation. Fruit were allowed to air dry and then
placed into clear incubation chambers (23 x 10 x 32 cm, Potomac Display, Hampstead, MD),
which were disinfested with 70% ETOH. Moist paper towels were placed in chambers to

27

maintain high relative humidity. One 5-mm diameter plug of UCV8 agar with actively growing
mycelia and sporangia from the edge of a five- to seven-day-old culture was used to inoculate
unwounded fruit. Control fruit were inoculated with uncolonized agar. Plugs were placed on
each fruit near the blossom end. Each plug was covered with a sterile cap removed from a 1.7
ml microcentrifuge tube to prevent desiccation. Caps were affixed to the fruit surface with
petroleum jelly. Chambers were placed on laboratory benches under continuous fluorescent
lighting at room temperature (Fig. 1.1 A). Temperature and relative humidity were recorded
using a Watchdog data logger (450 series; Spectrum Technologies, Inc., East Plainfield, IL)
every 30 min, and recordings were used to calculate mean air temperature and relative humidity.
Mean air temperature and relative humidity were 23.0ºC (±5.3ºC) and 96.1% (±3.8%),
respectively.

Figure 1.1. Cucurbit fruit inoculation techniques used to evaluate fruit rot susceptibility to
inoculation with Phytophthora capsici. Fruit incubating in a clear chamber in the laboratory at
high humidity (A). Large fruit in individual clear bags to maintain humidity in the greenhouse
(B). Slicing cucumber fruit at 5 days post inoculation (dpi) (C). Note lesion (L) and pathogen
growth (PG) diameter. Large fruit inoculation technique displayed on a butternut squash fruit
(D). Butternut squash fruit at 8 dpi (E). For interpretation of the references to color in this and
all other figures, the reader is referred to the electronic version of this thesis.

28

Figure 1.1 (con‟t).

B

A

C
L PG

D

E

29

Winter squash fruit were inoculated as described above, with the exception that fruit were
wounded to a depth of 3- to 5-mm with a sterile probe before inoculation and caps were affixed
to the fruit surface with clear tape (Figure 1.1 D). Inoculated fruit were placed into individual
clear, plastic bags on disinfested greenhouse benches (Figure 1.1 B). Bags contained moist
paper toweling to maintain high relative humidity and were closed shut with a rubber band.
Natural light was supplemented with lights during the night. Temperature and relative humidity
were recorded hourly using a Watchdog data logger, and recordings were used to calculate mean
air temperature and relative humidity. Mean air temperature and relative humidity were 22.1ºC
(±2.7ºC) and 94.7% (±4.2%), respectively.
Disease was evaluated at 3 and 5 dpi for summer squash and cucumber, and at 3, 6, and 8
dpi for winter squash. Lesion and pathogen growth diameters were measured perpendicularly
(Figure 1.1 C and E). Pathogen growth density was assessed on a 0 to 3 scale, where 0 = no
growth; 1 = visible, but minimal growth; 2 = moderate growth; and 3 = dense growth. Upon
experiment completion, four pieces of tissue were excised from the margin of symptomatic
lesions from 50% of fruit inoculated with each P. capsici isolate. Asymptomatic control fruit
were also sampled. Tissues were plated onto BARP-amended UCV8 agar. Cultures were
observed three days after plating using a compound microscope at 100x magnification and
confirmed as P. capsici based on morphological characteristics according to Waterhouse (1963).
Isolates were characterized by CT and MS as previously described (Lamour and Hausbeck,
2000). Phenotype of the recovered isolates was compared with the original inoculum.
Statistical analyses. Mean diameter was calculated for lesion and pathogen growth by
averaging two diameter measurements (the second measurement perpendicular to the first) for
the two response variables. Mean lesion and pathogen growth diameters were analyzed by

30

analysis of variance (ANOVA) using the PROC MIXED procedure of SAS version 9.2 (SAS
Institute Inc., Cary, NC). Pathogen isolate was considered the whole plot factor and fruit type
was considered the subplot factor. If variances were heterogeneous for fruit type or pathogen
isolate, the GROUP option of the REPEATED statement (with the lowest AIC value) was used
with degrees of freedom according to Kenward-Roger. If ANOVA was significant for main
effects or interaction terms, treatment means were compared using Fisher‟s protected least
significant difference (LSD) at P = 0.05. Interactions were examined by slicing. Pathogen
growth density was analyzed by proportional odds (or partial proportional odds if the Chi-Square
Score Test for proportional odds assumption was not valid for the model) using the PROC
LOGISTIC procedure of SAS version 9.2 (or the PROC GENMOD procedure of SAS version
9.2 if the partial proportional odds model was applied). The proportional odds model is a logistic
regression analysis procedure appropriate for ordinal response data (McCullagh, 1980). Disease
was not observed on, nor was P. capsici recovered from control fruit, and fruit were removed
from the lesion and pathogen growth diameter and pathogen growth density data sets prior to
analyses.
Seedling experiments. In experiment 1, acorn squash, pumpkin, semi-crookneck squash
(C. pepo), and zucchini were grown to evaluate seedling susceptibility among crops (Table 1.2).
In experiment 2, two cultivars of butternut squash, straightneck squash, and zucchini were grown
to evaluate seedling susceptibility between cultivars of the same crop (Table 1.2). „Butterbush‟
butternut squash was included as a highly susceptible cultivar (Padley et al., 2009). Experiments
were conducted in August 2009 (experiment 1) and December 2009 (experiment 2) at the MSU
Plant Science Research Greenhouses in East Lansing, Michigan. The experimental design for
both experiments was a split-plot arrangement of a randomized complete design. Both

31

experiments were repeated three times. Seedlings were replicated eight times for each isolate
and the control.
In experiment 1 (run 1), seeds were directly sown into square 1.5-liter pots in the
greenhouse using soilless potting media (Baccto Professional Planting Mix, Michigan Peat
Company, Houston, TX). In experiments 1 (runs 2 and 3) and 2, seeds were sown in 128-cell
plug trays filled with soil, and seedling replicates were transplanted into individual 1.5-liter pots
7 days prior to inoculation. Seedlings were grown under a 14-h photoperiod in the greenhouse.
Seedlings were watered as necessary and fertilized three times weekly with 200 ppm of Peter‟s
20-20-20 soluble liquid fertilizer (The Scott‟s Company, Marysville, OH) and were acid treated
once weekly with 10% phosphoric acid. Air temperature and relative humidity were monitored
using a Watchdog data logger every 30 min, and recordings were used to calculate mean air
temperature and relative humidity. Mean air temperature and relative humidity for experiment 1
were 23.1ºC (±6.8ºC) and 40.3% (±34.5%), respectively; mean air temperature and relative
humidity for experiment 2 were 22.5ºC (±7.3) and 21.1% (±2.2%), respectively.

32

Table 1.2. Cucurbit seedlings evaluated in greenhouse experiments for susceptibility to root rot
caused by Phytophthora capsici
Crop
Species
Cucurbita moschata
(butternut squash)

Cultivar

Seed source

'Butterbush'

Burpee Seeds, PA
Johnny's Selected Seeds,
MA

2

Seedway, LLC, MA
Seedway, LLC, MA
Rogers Brand Vegetable
Seed, ID
Harris Moran Seed Co., CA
Harris Moran Seed Co., CA
Harris Seeds, NY
Harris Moran Seed Co., CA

1
1

'Waltham'
Cucurbita pepo
(acorn squash)
(pumpkin)
(semi-crookneck
squash)
(straightneck squash)
(zucchini)

Seedling
experiment

'Table Ace'
'Trojan'
'Gold Star'
'Cougar'
'Lioness'
'Leopard'
'Zucchini Elite'

2

1
2
2
1, 2
2

Seedlings were inoculated at the 2- to 3-true leaf stage (Lee et al., 2001; Padley et al.,
2008) with infested millet seeds. Millet seed inoculum was prepared according to the methods of
Quesada-Ocampo et al. (2009). Approximately 1 g of infested millet seeds were added to the top
of soil around seedling stems. Sterile millet seeds were used to inoculate control plants.
Seedlings were watered after inoculation to prevent immediate inoculum desiccation and to
evenly distribute seeds on the soil surface. Seedlings inoculated with different isolates were
separated 0.3 m apart.
Seedlings were inspected for aboveground root rot symptoms. Disease severity was
evaluated every two days from 0 to 14 dpi on a rating scale adapted from Padley et al. (2008),
where 0 = no visible symptoms; 1 = leaves slightly wilted; 2 = foliar wilting with crown lesion
present at soil line; 3 = plant partially collapsed; 4 = plant completely collapsed; and 5 = death

33

(Figure 1.2). Upon experiment completion, approximately 12.5% of control and seedlings
inoculated with each P. capsici isolate were gently rinsed with water to remove soil. Foliage and
roots were removed, crowns were surface disinfested with 70% ETOH for 1 min, and air-dried.
Four sections of tissue were excised from the crown and plated on BARP-amended UCV8 agar.
Cultures were observed three days after plating using a compound microscope at 100x
magnification and confirmed as P. capsici based on morphological characteristics according to
Waterhouse (1963). Isolates were characterized by CT and MS as previously described (Lamour
and Hausbeck, 2000). Phenotype of the recovered isolates was compared with the original
inoculum.

34

A

B

C

D

E

F

Figure 1.2. Disease severity rating scale used to evaluate seedling susceptibility to root rot
caused by Phytophthora capsici, where A, 0 = no visible symptoms; B, 1 = leaves slightly
wilted; C, 2 = foliar wilting with crown lesion present at soil line; D, 3 = plant partially
collapsed; E, 4 = plant completely collapsed; and F, 5 = death.

Statistical analyses. Disease severity ratings were placed into the area under the disease
progress curve (AUDPC) mathematical formula according to Shaner and Finney (1977).
AUDPC values and symptom appearance dpi data were analyzed by ANOVA using the PROC
MIXED procedure of SAS version 9.2. Symptom appearance dpi data were subjected to the
PROC NLIN procedure prior to using PROC MIXED to satisfy normality assumptions. For
AUDPC values and symptom appearance dpi, pathogen isolate was considered the whole plot

35

factor and crop type was considered the subplot factor. If variances were heterogeneous for crop
type or pathogen isolate, the GROUP option of the REPEATED statement (with the lowest AIC
value) was used with degrees of freedom according to Kenward-Roger. If ANOVA was
significant for main effects or interaction terms, treatment means were compared using Fisher‟s
protected LSD at P = 0.05. Interactions were examined by slicing. Disease was not observed
on, nor was P. capsici recovered from control seedlings in experiments 1 and 2, and seedlings
were removed from the data sets prior to analyses.

36

RESULTS

Fruit experiment. All five P. capsici isolates incited disease and differed significantly in
the three response variables evaluated. Tissue isolations were confirmed to have the same
phenotype as the original inoculum (data not shown). Control fruit did not exhibit disease
symptoms, nor was P. capsici recovered from tissue isolations. Fruits of the same type (summer
squash [straightneck squash and zucchini], cucumber [pickling and slicing cucumber], and
winter squash [acorn, butternut, and spaghetti squash, and pumpkin]) were grouped together for
analyses.
Fruit lesions and pathogen growth were visible on summer squash at 3 dpi. Lesion size
did not significantly differ between fruit types at 3 dpi (Figure 1. 3 A). Fruit type was significant
(P = 0.0273) for pathogen growth diameter. Straightneck squash had more pathogen growth (1.4
cm) in comparison to zucchini (0.7 cm) (Figure 1.3 A). Fruit type was significant for pathogen
growth density (P < 0.0001). Straightneck squash had higher odds (odds = 3.6) of having more
dense pathogen growth compared to zucchini (data not shown). Pathogen isolate was significant
for lesion (P = 0.0253) and pathogen growth (P = 0.0319) diameter. On fruit, isolate SF3 caused
significantly smaller lesions compared to other isolates (1.4 cm) and had no pathogen growth
(0.0 cm) (Figure 1.3 B). Pathogen isolate was not significant (P = 0.5335) for pathogen growth
density after isolate SF3 was removed from the data set. All isolates had similar odds of being
the same density on fruit (data not shown).
Water-soaked lesions and pathogen growth were substantial at 5 dpi for summer squash
(Figure 1.4 A). Fruit type was significant for lesion and pathogen growth diameter (P < 0.0001).
Straightneck squash had significantly larger lesions and more pathogen growth (lesion and

37

pathogen growth diameters = 8.0 and 7.4 cm, respectively) than zucchini (lesion and pathogen
growth diameters = 6.1 and 5.1, respectively) (Figure 1.3 A). Fruit type was significant for
pathogen growth density (P = 0.0088). Straightneck squash had higher odds (odds = 2.5) of
having more dense pathogen growth compared to zucchini (data not shown). Pathogen isolate
was significant at 5 dpi for lesion and pathogen growth diameter (P < 0.0001). Isolates 12889
and 13351 caused significantly larger lesions (8.5 and 8.4 cm, respectively) on fruit compared to
other isolates and had significantly more pathogen growth (7.7 and 7.9 cm, respectively) (Figures
1.3 B and 1.4 A). Isolate SF3 caused significantly smaller lesions (3.9 cm) and had reduced
pathogen growth (2.2 cm) compared to other isolates (Figures 1.3 B and 1.4 A). Isolates OP97,
SP98, 12889, and 13351 had similar odds of being the same density of pathogen growth on both
fruit types, whereas isolate SF3 had higher odds (odds = 12.1, P < 0.0001) of being more dense
on straightneck squash compared to zucchini (data not shown).

Figure 1.3. Lesion and pathogen growth diameters (A) present on summer squash fruit following
inoculation with Phytophthora capsici isolates SF3, OP97, SP98, 12889, and 13351 (B). Disease
was evaluated at 3 and 5 days post inoculation (dpi) in fruit experiment. Bars with a letter in
common are not significantly different for each response variable within each dpi (Fisher‟s
protected LSD, P = 0.05). Disease was not observed on, nor was P. capsici was recovered from
control fruit, and fruit were not included in data analyses.

38

Figure 1.3 (cont‟d).

A

Lesion

Pathogen growth

B

Lesion

Pathogen growth

39

A

B

Control

Control

SF3

SF3

OP97

OP97

SP98
SP98
12889
12889
13351

13351
slicing cucumber

straightneck squash

C
Control

12889

SF3
SP98

OP97
13351
acorn squash

Figure 1.4. Straightneck squash (A) and slicing cucumber fruit (B) at 5 days post inoculation
(dpi) and acorn squash fruit (C) at 8 dpi following inoculation with Phytophthora capsici
isolates SF3, OP97, SP98, 12889, and 13351. Note differences in isolate virulence within
fruit types.
40

Disease was present on cucumber at 3 dpi. Fruit type was significant (P = 0.0235) for
pathogen growth diameter. Slicing cucumber had more pathogen growth (0.5 cm) than pickling
cucumber (0.05 cm) (Figure 1.5 A). Fruit type was significant for pathogen growth density (P =
.0002). Slicing cucumber had higher odds (odds = 7.3) of having any pathogen growth
compared to pickling cucumber (data not shown). The fruit type*pathogen isolate interaction
term for lesion diameter was significant (P < 0.0001) and was examined by slicing by isolate.
All slices were significant. Isolate SF3 caused significantly smaller lesions on both fruit types
compared to other isolates (pickling cucumber = 1.4 cm and slicing cucumber = 0.4 cm) (Figure
1.5 B). Isolate SP98 caused significantly larger lesions (3.2 cm) on slicing cucumber compared
to isolates OP97 and SF3 (Figure 1.5 B). Isolates caused significantly larger lesions on slicing
cucumber compared to pickling cucumber, with the exception of isolate SF3 (Figure 1.5 B).
Isolate SP98 had higher odds of having any growth on fruit compared to isolates SF3 (odds =
16.2), OP97 (odds = 7.5), 12889 (odds = 10.4), and 13351 (odds = 3.2) (data not shown).
At 5 dpi, fruit type was significant (P < 0.0001) for lesion diameter. Slicing cucumber
had significantly larger lesions (7.3 cm) compared to pickling cucumber (6.3 cm) (Figure 1.5 A).
Pathogen growth diameter was similar between fruit types (Figure 1.5 A). Slicing and pickling
cucumber had similar odds of having the same pathogen growth density (P = 0.4270) (data not
shown). Pathogen isolate was significant for lesion (P < 0.0001) and pathogen growth (P =
0.0084) diameter. On fruit, isolate SF3 caused significantly smaller lesions (4.6 cm) and had
reduced pathogen growth (1.9 cm) compared to other isolates (Figure 1.5 C). Isolates SF3,
OP97, SP98, and 12889 had similar odds of having the same density of pathogen growth on both
fruit types, whereas isolate 13351 had higher odds (odds = 16.7, P = 0.0108) of being more
dense on slicing cucumber compared to pickling cucumber (data not shown).

41

A

a

Lesion

Pathogen growth

Figure 1.5. Lesion and pathogen growth (A, B) diameters present on cucumber fruit following
inoculation with Phytophthora capsici isolates SF3, OP97, SP98, 12889, and 13351 (C).
Disease was evaluated at 3 and 5 days post inoculation (dpi) in fruit experiment. Bars with a
letter in common are not significantly different for each response variable within each dpi (A, C)
(Fisher‟s protected LSD, P = 0.05). Bars with a (lower case) letter in common are not
significantly different within each fruit type and (upper case) among isolates (B) (Fisher‟s
protected LSD, P = 0.05). Disease was not observed on, nor was P. capsici recovered from
control fruit, and fruit were not included in data analyses.

42

Figure 1.5 (cont‟d).

B

C

Lesion

Pathogen growth

43

Disease was infrequently observed on winter squash at 3 dpi (data not shown). However,
at 6 dpi, lesions and pathogen growth were observed on all fruit types. Fruit type was significant
for lesion (P = 0.0009) and pathogen growth (P = 0.0007) diameter. Butternut squash had
significantly larger lesions (5.3 cm) compared to pumpkin and spaghetti squash (Figure 1.6 A).
Acorn squash, butternut squash, and pumpkin had similar pathogen growth, significantly greater
than spaghetti squash (1.2 cm) (Figure 1.6 A). Fruit type was significant for pathogen growth
density (P < 0.0001). Acorn squash had similar odds of having any pathogen growth compared
to butternut squash (odds = 1.2) and higher odds than pumpkin and spaghetti squash (odds = 3.1
and 7.4, respectively) (data not shown). Acorn squash had similar odds of having none or
minimal pathogen growth density compared to butternut squash (odds = 1.4), higher odds than
spaghetti squash (odds = 1.9), and lower odds than pumpkin (odds = 0.7) (data not shown).
Acorn squash had similar odds of having moderate to dense pathogen growth compared to
butternut and spaghetti squash (odds = 1.1 and 1.2, respectively) and lower odds than pumpkin
(odds = 0.2) (data not shown). Pathogen isolate was significant for lesion and pathogen growth
diameter (P < 0.0001). Isolate 13351 caused significantly larger lesions (5.6 cm) and had greater
pathogen growth (3.9 cm) in comparison to isolates SF3, OP97, and 12889 (Figure 1.6 B).
Isolate SF3 caused significantly smaller lesions (1.8 cm) and had reduced pathogen growth (0.6
cm) in comparison to other isolates (Figure 1.6 B). Pathogen isolate was significant for pathogen
growth density (P < 0.0001). Isolate 13351 had similar odds of having the same density of
pathogen growth on fruit compared to isolates OP97 and SP98 (odds = 1.4) and higher odds of
being more dense than isolates SF3 and 12889 (odds = 11.3 and 2.5, respectively) (data not
shown).

44

At 8 dpi, fruit type was significant for lesion (P = 0.0002) and pathogen growth (P <
0.0001) diameter. Butternut squash had the largest lesions (9.3 cm) and the most pathogen
growth (7.3 cm), which was statistically similar to acorn squash (Figure 1.6 A). Pumpkin and
spaghetti squash had significantly smaller lesions and less pathogen growth (Figure 1.6 A). Fruit
type was significant for pathogen growth density (P < 0.0001). Acorn squash had similar odds
of having any pathogen growth compared to butternut squash (odds = 1.4) and higher odds than
pumpkin and spaghetti squash (odds = 5.4 and 6.8, respectively) (data not shown). Acorn squash
had lower odds of having none or minimal pathogen growth density compared to butternut
squash (odds = 0.5), higher odds than spaghetti squash (odds = 2.9), and similar odds to pumpkin
(odds = 1.1) (data not shown). Acorn squash had lower odds of having moderate to dense
pathogen growth compared to butternut squash and pumpkin (odds = 0.6 and 0.1, respectively)
and similar odds to spaghetti squash (odds = 1.1) (data not shown). Pathogen isolate was
significant for lesion and pathogen growth diameter (P < 0.0001). Similar to 6 dpi, isolate 13351
caused the largest lesions (10.2 cm) and had the most pathogen growth (8.3 cm) at 8 dpi,
significantly more than other isolates (Figures 1.4 C and 1.6 B). Isolate SF3 caused the smallest
lesions (3.4 cm) and had reduced pathogen growth (1.6 cm) (Figures 1.4 C and 1.6 B). Pathogen
isolate was significant for pathogen growth density (P < 0.0001). Isolate 13351 had higher odds
of being more dense than all isolates, specifically isolates SF3 (odds = 8.5), OP97 (odds = 1.8),
SP98 (odds = 1.8), and 12889 (odds = 2.4) (data not shown).

45

Figure 1.6. Lesion and pathogen growth diameters (A) present on winter squash fruit following
inoculation with Phytophthora capsici isolates SF3, OP97, SP98, 12889, and 13351 (B). Disease
was evaluated at 6 and 8 days post inoculation (dpi) in fruit experiment. Bars with a letter in
common are not significantly different for each response variable within each dpi (Fisher‟s
protected LSD, P = 0.05). Disease was not observed on, nor was P. capsici was recovered from
control fruit, and fruit were not included in data analyses.

46

Figure 1.6 (cont‟d).

A

Lesion

Pathogen growth

B

Lesion

Pathogen growth

47

Seedling experiments. All isolates incited disease on squash seedlings. Tissue isolations
were confirmed to have the same phenotype as the original inoculum (data not shown). Control
seedlings did not exhibit disease symptoms, nor was P. capsici recovered from tissue isolations.
Disease symptom appearance was significant for the crop type*pathogen isolate
interaction term (P = 0.0169) in experiment 1. The interaction was examined by slicing by
isolate. All slices were significant, except for the crop type*isolate SF3 slice. Semi-crookneck
squash inoculated with isolates OP97, SP98, 12889, and 13351 began exhibiting disease first, on
average, at approximately 8 dpi (7.7, 7.8, 8.0 and 8.1 dpi, respectively) (Figure 1.7). Pumpkin
inoculated with isolate 13351 developed symptoms between 8 to 9 dpi (8.5 dpi) earlier than
acorn squash and zucchini (Figure 1.7). Acorn squash inoculated with isolates OP97, SP98, and
12889 showed disease significantly later than other crops inoculated with these isolates (11.4,
10.7, and 11.1 dpi, respectively). There were no significant differences regarding symptom onset
of isolate SF3 with any cucurbit (Figure 1.7).
Crop type (P < 0.0001) and pathogen isolate (P = 0.0003) were significant for AUDPC
values. Semi-crookneck squash had the greatest AUDPC value (AUDPC value = 29),
significantly larger than acorn squash, pumpkin, and zucchini (AUDPC values = 15, 18, and 15,
respectively) (Table 1.3). Cucurbits inoculated with isolate SF3 had the lowest AUDPC value
(AUDPC value = 8) (Table 1.4). Cucurbits inoculated with isolates OP97, SP98, 12889, and
13351 had statistically similar AUDPC values (Table 1.4).

48

Figure 1.7. Symptom appearance days post inoculation (dpi) of cucurbit seedlings evaluated in
experiment 1 for susceptibility to root rot caused by Phytophthora capsici isolates SF3, OP97,
SP98, 12889, and 13351. Bars with a (lower case) letter in common are not significantly
different within each crop and (upper case) among isolates (Fisher‟s protected LSD, P = 0.05).
Disease was not observed on, nor was P. capsici recovered from control seedlings, and seedlings
were not included in data analyses.

49

Table 1.3. Area under the disease progress curve (AUDPC) values for experiment 1 evaluating
cucurbit seedling susceptibility to root rot caused by Phytophthora capsici
Crop

x

y

AUDPC

z

15 ab
Acorn squash
Pumpkin
18 b
Semi-crookneck squash
29 c
Zucchini
15 a
x
Disease was not observed on, nor was P. capsici recovered from control seedlings, and
seedlings were not included in data analysis.
y
AUDPC values were calculated by using disease severity ratings every two days, where 0 = no
visible symptoms; 1 = leaves slightly wilted; 2 = foliar wilting with crown lesion present at soil
line; 3 = plant partially collapsed; 4 = plant completely collapsed; and 5 = death.
z
Means within columns followed by the same letter are not significantly different (Fisher‟s
protected LSD, P = 0.05).

Table 1.4. Area under the disease progress curve (AUDPC) values for experiment 1 evaluating
acorn squash, pumpkin, semi-crookneck squash, and zucchini seedling susceptibility to root rot
caused by Phytophthora capsici isolates SF3, OP97, SP98, 12889, and 13351
x

Isolate
SF3
OP97
SP98
12889
13351

AUDPC

y

z

8a
20 b
23 b
22 b
25 b

x

Disease was not observed on, nor was P. capsici recovered from control seedlings, and
seedlings were not included in data analysis.
y
AUDPC values were calculated by using disease severity ratings every two days, where 0 = no
visible symptoms; 1 = leaves slightly wilted; 2 = foliar wilting with crown lesion present at soil
line; 3 = plant partially collapsed; 4 = plant completely collapsed; and 5 = death.
z
Means within columns followed by the same letter are not significantly different (Fisher‟s
protected LSD, P = 0.05).

50

In experiment 2, symptom appearance was significant for crop type (P = 0.0117).
„Cougar‟ and „Lioness‟ straightneck squash and „Waltham‟ butternut squash developed
symptoms between 9 and 11 dpi (9.6, 9.7, and 10.5 dpi, respectively) before both cultivars of
zucchini („Leopard‟ = 11.2 and „Zucchini Elite‟ = 11.0) and „Butterbush‟ butternut squash (11.1
dpi). There was no significant difference between cultivars of the same crop regarding disease
onset (data not shown).
The crop type*pathogen isolate interaction term was significant for AUDPC values (P <
0.0001). The interaction was examined by slicing by isolate. All slices were significant, except
for the crop type*isolate SF3 slice. The greatest AUDPC value attained was „Cougar‟ inoculated
with isolate SP98 (AUDPC value = 30) (Table 1.5), which was statistically similar to isolates
12889 and 13351 paired with „Cougar.‟ The AUDPC values of „Lioness‟ inoculated with any
isolate were not significantly different from „Cougar‟ (Table 1.5). However, the AUDPC value
of „Waltham‟ inoculated with isolate 13351 (AUDPC value = 11) was significantly less than
„Butterbush‟ (AUDPC value = 20) (Table 1.5). Likewise, the AUDPC value of „Zucchini Elite‟
inoculated with isolate 13351 (AUPDC = 15) was significantly less than „Leopard‟ (AUDPC
value = 20) (Table 1.5). AUPDC values of butternut squash and zucchini cultivars inoculated
with isolates SF3, OP97, SP98, and 12889 did not significantly differ from each other (Table
1.5).

51

Table 1.5. Area under the disease progress curve (AUDPC) values for experiment 2 evaluating cucurbit cultivar seedling
susceptibility to root rot caused by Phytophthora capsici isolates SF3, OP97, SP98, 12889, and 13351
x

AUDPC

y

OP97

Isolate
SP98

12889

13351

Crop, cultivar
Butternut squash

SF3

'Butterbush'
'Waltham'
Straightneck squash
'Cougar'
'Lioness'
Zucchini
'Leopard'
'Zucchini Elite'

0 aA
1 aA

7 bAB
3 aA

19 cA
15 bA

14 bcAB
10 bA

20 cB
11 bA

3 aA
3 aA

17 bD
17 bCD

30 cB
27 cB

25 cC
26 cC

25 cC
23 bcBC

1 aA
3 aA

11 bBC
11 bB

18 bcA
17 bA

15 bcAB
16 bB

20 cB
15 bA

z

x

AUDPC values were calculated by using disease severity ratings every two days, where 0 = no visible symptoms; 1 = leaves slightly
wilted; 2 = foliar wilting with crown lesion present at soil line; 3 = plant partially collapsed; 4 = plant completely collapsed; and 5 =
death.
y
Disease was not observed on, nor was P. capsici recovered from control seedlings, and seedlings were not included in data
analysis.
z
Means within rows followed by the same (lower case) letter are not significantly different within each cultivar, and means within
columns followed by the same (upper case) letter are not significantly different within each isolate (Fisher‟s protected LSD, P = 0.05).

52

DISCUSSION

The results of this study indicate that the susceptibility of cucurbits to P. capsici varies.
Susceptibility differences were confirmed by significant differences in lesion and pathogen
growth size and pathogen density observed on fruit and significant differences in symptom
appearance and AUDPC values of seedlings.
Similar to what Ando et al. (2009) noted, summer squash fruit, including straightneck
squash and zucchini, were very susceptible to P. capsici. However, their study did not indicate
significant differences in fruit susceptibility (evaluated as the stage of disease progression on
fruit surfaces following inoculation) (Ando et al., 2009). Lesions and pathogen growth were
observed on both straightneck squash and zucchini fruit in this study at 3 dpi. However, by
measuring lesion and pathogen growth size, significant differences in fruit susceptibility were
observed at 5 dpi, with straightneck squash exhibiting greater susceptibility. Grouping similar
crop types (summer squash, cucumber, and winter squash) together for analyses lessened the
variability among fruit. In this study, slicing cucumber fruit were more susceptible than pickling
cucumber with greater disease (larger lesions) observed at 5 dpi. But, pathogen growth size and
density did not differ among fruit at 5 dpi.
The inoculation technique developed by Gevens et al. (2006), replicated by Ando et al.
(2009), and modified for use in this study for winter squash, was effective in evaluating fruit
susceptibility to Phytophthora rot. However, affixing the cap to the surface by petroleum jelly
was not successful in preventing moisture loss from the P. capsici plug in the greenhouse (tested
prior to fruit experiments, data not shown). Covering the cap with clear tape was effective in
keeping the plug moist until the initial infection occurred. Also similar to what Ando et al.

53

(2009) observed, winter squash fruit, exhibited symptoms later than summer squash and
cucumber in this study. Acorn and butternut squash were the most susceptible of the winter
squash having larger lesions and more pathogen growth compared to pumpkin and spaghetti
squash.
Differences of isolate virulence were observed among P. capsici isolates in the fruit
experiment. Isolates 12889 and 13351 were the most virulent on summer squash, and isolate
13351, followed by SP98, were the most virulent on winter squash. Isolate SF3 was the least
virulent. However, isolate SF3 (from pickling cucumber) caused larger lesions on pickling
cucumber in comparison to slicing cucumber at 3 dpi. There was no significant difference in
lesion size between cucumber fruit inoculated with isolate SF3 at 5 dpi.
The virulence of P. capsici isolates differed in seedling experiments. Overall, symptoms
on semi-crookneck squash and pumpkin were observed first with isolate 13351, followed by
isolates OP97, SP98, and 12889. Additionally, the AUDPC values of semi-crookneck squash
and all seedlings inoculated with isolate 13351 were greater than for other cucurbits and isolates,
respectively. In experiment 2, there were no differences observed in symptom onset between
cultivars of the same crop. Similar to the results of Camp et al. (2009), there were no significant
differences in disease severity between cultivars of the same crop with regards to straightneck
squash. In this study, significant differences in AUDPC values between butternut squash and
zucchini cultivars were observed following inoculations with isolate 13351. Overall, greater
AUDPC values were attained by seedlings in experiment 1 versus experiment 2. Air temperature
was similar for both experiments; however relative humidity was comparably higher for
experiment 1, which was conducted in summer, than for experiment 2, which was conducted in
winter. The survival of mycelia and sporangia in plant tissue has been shown to decrease with a

54

lower relative humidity (Schlub, 1983). Granke and Hausbeck (2010b) found that isolate SP98
produced significantly more sporangia on fruit tissue at 60% relative humidity in comparison to a
lower relative humidity (~35%). Hence, the lower relative humidity during experiment 2 may
have impacted disease severity; nonetheless, relative humidity was still suitable for pathogen
growth. Additionally, different cultivars were tested in experiments 1 and 2, which also may
have influenced AUDPC values.
Comparable results regarding differences in P. capsici isolate virulence have been
observed in other studies. Experiments evaluating pepper fruit and plant (Foster and Hausbeck,
2010; Kim and Hwang, 1992; Polach and Webster, 1972; Ristaino, 1990), tomato plant
(Quesada-Ocampo and Hausbeck, 2010), and cucurbit fruit and plant (Lee et al., 2001; QuesadaOcampo et al., 2010; Tian and Babadoost, 2004) cultivars and accessions in greenhouse and field
studies found virulence differences of isolates collected from various hosts. However, Gevens et
al. (2006) did not find any differences in isolate (OP97, SF3, SFF3 [CT = A2, insensitive to
mefenoxam], and SP98) virulence in initial cucumber fruit rot cultivar and cultigen (PI) studies,
and accordingly, used one P. capsici isolate (OP97) for subsequent screenings. Similar results to
Gevens et al. (2006) were obtained by Quesada-Ocampo et al. (2009); they found no difference
in P. capsici isolate (OP97, SFF3, SP98, and 12889) virulence on Fraser fir (Abies fraseri Pursh
Poir.) saplings.
In studies by Foster and Hausbeck (2010) and Quesada-Ocampo and Hausbeck (2010),
isolate 12889 was the most virulent. In this study, isolates 12889 and 13351 collected from the
Solanaceous crops, pepper and eggplant, respectively, were the most virulent on summer squash
fruit. Likewise, isolate 13351 was the most virulent on winter squash fruit. In seedling
experiment 1, isolate 13351 was the most virulent, and in experiment 2, straightneck squash

55

cultivars inoculated with isolate SP98 (collected from pumpkin) had the greatest AUDPC values,
although statistically similar to the AUDPC values attained with isolates 12889 and 13351.
Isolates SF3 and OP97, both collected from the Cucurbitaceous host, pickling cucumber, were
less virulent on cucurbit fruit and seedlings than isolates collected from Solanaceous hosts.
Although, isolates collected from Solanaceous hosts were highly virulent on squash fruit
and seedlings in this study and in the studies conducted by Foster and Hausbeck (2010) and
Quesada-Ocampo and Hausbeck (2010), does not definitively conclude that isolates recovered
from Solanaceous hosts are more virulent than isolates recovered from other hosts. For instance,
Lee et al. (2001) observed the converse in their study; isolates collected from pumpkin caused
significantly greater disease on pumpkin than isolates collected from pepper. Isolate SP98 was
consistently virulent on fruit and seedlings in this study.
The results obtained in this study support that crop rotations of eggplant, pepper, or tomato
in between cucurbit and non-susceptible host rotations are unadvisable, as the most virulent
isolates on cucurbits were those collected from Solanaceous hosts. Crop rotations are typically
used as a component of an integrated pest management (IPM) program to control disease caused
by P. capsici (Lamour and Hausbeck, 2004). A three- to four-year rotation to a non-susceptible
crop is recommended between susceptible crop plantings (Ristaino and Johnston, 1999).
However, even the recommended rotational duration may not provide significant control, as P.
capsici oospores may remain in the soil for as long as five or more years to cause infection
(Hausbeck and Lamour, 2004).
Furthermore, as the cucurbit response was dependent on isolate virulence, and isolate
origin varied geographically, indicates that a crop response to the specific P. capsici population
in a field may differ among regions. Although, no differences in susceptibility to root and crown

56

rot were found between the straightneck squash cultivars evaluated in this study, „Waltham‟ and
„Zucchini Elite‟ seedlings exhibited reduced disease in comparison to „Butterbush‟ and
„Leopard,‟ respectively. While, the amount of disease observed on „Waltham‟ or „Zucchini
Elite‟ would likely not be acceptable according to grower standards when paired with isolates
SP98, 12889, or 13351, but if a similar crop type*pathogen isolate interaction was observed in
the field as was in our study with isolates SF3 or OP97, and combined with IPM practices,
disease may potentially not be as problematic and plants may survive to yield fruit, decreasing
the likelihood of a total crop loss. As only two cultivars of butternut squash and zucchini were
evaluated in this study, other experiments are necessary to ascertain if there are less susceptible,
currently available cultivars that growers would easily be able to incorporate into their
production systems.
Germplasm screening by Chavez et al. (2011) and Padley et al. (2008) identified summer
and winter squash lines that exhibit reduced crown rot, which may be useful in breeding work to
develop future cultivars that have significant resistance. Resistance to crown rot has been
introgressed into the C. moschata breeding line #394-1-27-12 from the University of Florida and
was derived from two wild species, C. lundelliana and C. okeechobeenesis subsp.
okeechobeenesis (Padley et al., 2009). Crown rot resistance was found to be controlled by three
dominant genes, and it is unknown whether the foliar blight resistance that #394-1-27-12 also
exhibits is conferred by the same mechanism (Padley et al., 2009). Varying levels of pepper
resistance to the disease syndromes of root rot, stem blight, and foliar blight exist, and three
different genetic mechanisms were found to be responsible for these separate disease syndromes
(Barksdale et al., 1984; Sy et al., 2005). Differences in this study were observed regarding
pumpkin and other winter squash fruit rot in comparison to seedling root and crown rot. Future

57

germplasm studies evaluating cucurbit cultivars and accessions to all diseases (root, crown, and
fruit rot and foliar blight) would be worthwhile, not only to establish whether the same genetic
mechanisms are involved for all disease symptoms and to discern whether they are separate
syndromes, but also for growers, so they can be aware of which cultivars would work best in
their production systems. Additionally, in accordance with studies by Quesada-Ocampo et al.
(2010, 2011), results from this study indicate that diverse isolates should be utilized in cucurbit
germplasm screenings. Isolate 13351, which was especially virulent in this study, was collected
from eggplant in New York. Other cucurbit fruit and seedling P. capsici screenings have used
same-state isolates collected from Cucurbitaceous hosts (Chavez et al., 2011; Gevens et al.,
2006; Padley et al., 2008).
In the short term, cucurbit growers do not have options regarding resistant cultivars. With
this limitation, Michigan growers may benefit from planting several desirable cultivars in a field,
as one cultivar may be more tolerant to the local P. capsici isolates in comparison to other
cultivars. An additional viable option may include growing a crop with reduced fruit
susceptibility, such as zucchini in comparison to straightneck squash or spaghetti squash in
comparison to butternut squash. Also observed in this study, was that symptom onset differed
among the cucurbit fruit and seedlings tested. These results can transition into field situations, as
growers can expect diseased yellow squash seedlings and summer squash fruit to exhibit
symptoms before winter squash.
In this study, disease development did not occur uniformly without surface wounding of
winter squash fruit (tested prior to experiments); disease occurred only on two pumpkins without
surface wounding prior to inoculation (tested prior to fruit experiments, data not shown). Some
cucurbit fruit have increased tolerance to P. capsici as they mature (Ando et al., 2009); however,

58

age-related resistance is negated in cucumbers by surface wounding (Granke and Hausbeck,
2010b). The negation of age-related resistance by wounding in other cucurbit fruit is not known
at this time, and hence, care should be taken to avoid superficial wounds when harvesting and
storing fruit.

59

ACKNOWLEDGMENTS

This research is based upon work supported by the USDA CSREES Special Research Grant
Program under Award No.: 2009-34572-19990 and the Pickle and Pepper Research Committee
of MSU, Pickle Packers International, Inc. I would like to thank Bartley Farms, MI, T. Superak
of Harris Moran Seed Co., CA, and the Southwest Michigan Research and Extension Center, MI
for fruit and seed donations; A. Bloemers, A. Cortright, B. Cortright, A. Cook, S. Glaspie,
T. VanderMaas, and B. Webster for technical assistance; M. Meyer for document review;
and K. Ananyeva and A. Worth for help with statistical analyses.

60

CHAPTER II: ORNAMENTAL PLANT SUSCEPTIBILITY
TO PHYTOPHTHORA CAPSICI OR P. TROPICALIS
ABSTRACT

Phytophthora capsici and P. tropicalis Aragaki & J. Y. Uchida, previously conspecific
with P. capsici, share many hosts. Phytophthora tropicalis was placed in a separate taxon based
on morphological characteristics, absence of or minimal growth at 35 C, and having weak or no
pathogenicity on pepper (Capsicum annuum). Two Fabaceous (Lupinus and Lathyrus) and five
Solanaceous (Calibrachoa, Browallia, Nicotiana, Nierembergia, and Petunia) ornamental plant
genera and seven million bells (C. x hybrida) cultivars were evaluated with four P. capsici
isolates and one P. tropicalis isolate in greenhouse studies. Plants were inoculated with infested
millet seeds (ca. 1.5 g) placed in soilless potting media and were inspected for aboveground root
rot disease symptoms for 42 days post inoculation. Plant height and canopy width were
measured (cm) at 7-day intervals and were used to calculate a mean volume approximation
3

(cm ). Million bells and flowering tobacco (N. x sanderae) exhibited reduced mean volumes,
and P. capsici and P. tropicalis were recovered from tissue. Disease was observed on and
pathogen recovery was feasible from squash (Cucurbita pepo, included as a P. capsicisusceptible control), sweet pea (L. latifolius), and lupine (L. polyphyllus). The potential host
range of P. capsici has expanded to flowering tobacco, lupine, million bells, and sweet pea. The
potential host range of P. tropicalis has expanded to flowering tobacco, squash, and sweet pea.
Although, all million bells cultivars were susceptible to P. capsici and P. tropicalis in
experiments, „Celebration Purple Star‟ displayed reduced disease following inoculations with
both pathogens. Phytophthora capsici-inoculated „Callie Gold with Red Eye,‟ „Million Bells‟
Cherry Pink,‟ and „Superbells White‟ displayed reduced disease compared to other cultivars.

61

INTRODUCTION

Michigan is ranked third in sales among the top five floricultural states in the United States
(U. S.) for the total production of annuals, perennials, cut flowers, foliage plants, and propagative
materials (USDA, 2011a). In 2010, floricultural sales in Michigan were approximately $389
million, about 9.8% of the U. S. total (USDA, 2011a). Flowering ornamentals, which are grown
for their aesthetic value, require diligent disease management (Baker and Linderman, 1979). An
estimated 20% of the production costs of greenhouse-grown plants are attributed to disease
prevention and control (Baker and Linderman, 1979). Generally, the entire plant is marketed,
and the sale is directly related to how attractive the foliage and blossoms are (Daughtrey and
Benson, 2005). Plants exhibiting aboveground root rot symptoms, such as crown lesions,
stunting, and wilting, are not salable and result in financial losses to the wholesale grower. A
grower‟s reputation may be risked if diseased materials are supplied to retail outlets or sold
directly to consumers (Baker and Linderman, 1979). High humidity, warm temperatures, and
moist conditions present in greenhouse and nursery environments are conducive to the growth,
reproduction, and colonization of root rot pathogens.
Phytophthora spp., which are pathogens of many floriculture crops, incite root, crown, and
stem rot of flowering annuals and perennials (Daughtrey et al., 1995). Propagules may initially
be introduced to the greenhouse via shipments of infected seed, cuttings, or plants (Daughtrey
and Benson, 2005). Phytophthora spp. may be dispersed through greenhouse irrigation
equipment, such as recirculating water systems (Oh and Son, 2008; Thinggaard and Andersen,
1995; van der Gaag et al., 2001) or by reusing infested flats and placing susceptible plants on
infested benches or weed control mats (Hausbeck, 2007).

62

Phytophthora cryptogea Peth. and Laff. causes root rot of snapdragon (Antirrhinum majus
L.), begonia (Begonia elatior), and petunia (Petunia x hybrida Hort. Vilm. Andr) (Ampuero et
al., 2008; Erwin and Ribeiro, 1996). Phytophthora drechsleri Tucker is a common pathogen of
poinsettia (Euphorbia pulcherrima Willd. Ex Klotzsch), and P. nicotianae Breda de Hann (syn.
P. parasitica Dastur) has a wide host range causing disease on host plants in 58 families
(Daughtrey et al., 1995). Phytophthora tropicalis Aragaki & J. Y. Uchida causes disease on
numerous herbaceous ornamentals, including stem and foliar blight of annual vinca
(Catharanthus roseus [L.] G. Don) (Hao et al., 2010) and corm and bulb rot of cyclamen
(Cyclamen persicum Mill.) (Gerlach and Schubert, 2001). Greenhouse studies found additional
P. tropicalis-susceptible floriculture crops, including begonia (Begonia sp.), gerbera daisy
(Gerbera jamesonii), lupine (Lupinus albus L.), and dusty miller (Senecio bicolor subsp.
cineraria) (Hong et al., 2008).
Phytophthora tropicalis was previously conspecific with P. capsici Leonian (formerly in
the „P. palmivora Butler MF4‟ subgroup, reclassified to P. capsici [Alizadeh and Tsao, 1985;
Brasier and Griffin, 1979; Uchida and Aragaki, 1980]), but was given a separate species taxon in
2001 based on morphological characteristics, absence of or minimal growth at 35 C, and having
weak or no pathogenicity on pepper (Capsicum annuum L.) (Aragaki and Uchida, 2001).
However, there is still no precise distinction between P. capsici and P. tropicalis regarding
morphology and isolate origin, and there is considerable host overlap between the species
(Lamour, 2009). Both cause disease on the tropical crops macadamia (Macadamia spp.) and
cacao (Theobroma cacao L.), as well as eggplant (Solanum melongena L.) (Erwin and Ribeiro,
1996). Phytophthora capsici and P. tropicalis cause disease on tropical ornamental plants grown
in temperate regions of the U. S. as flowering annuals or foliage plants, such as flamingo flower

63

(Anthurium spp.), carnation (Dianthus spp.), pothos (Epipremnum spp.) (Orlikowsi et al., 2006;
Wick and Dicklow, 2001), and pincushion flower (Leucospermum spp.) (Aragaki and Uchida,
2001; Donahoo and Lamour, 2008; Oudemans and Coffey, 1991). More recently in 2007,
million bells (Calibrachoa x hybrida Cerv.) plants with crown and stem rot in a wholesale New
York greenhouse were reported as being infected with P. capsici (Daughtrey et al., 2007). PCR
and sequencing of internal transcribed spacer (ITS) region and restriction fragment length
polymorphism (RFLP) of mitochondrial DNA (Cox 1 and Cox 2) identified this isolate as P.
capsici (Daughtrey, 2010). PCR sequencing and analysis of ten genes (four mitochondrial and
six nuclear) later identified this isolate as P. tropicalis (see Quesada-Ocampo et al., 2011).
Although, P. capsici and P. tropicalis share common morphological characteristics and hosts,
having two distinct species is supported based on genetic and phylogenetic analyses (Donahoo
and Lamour, 2008; Quesada-Ocampo et al., 2011; Zhang et al., 2004).
Phytophthora capsici is a limiting factor in the production of susceptible crops in Michigan
(Hausbeck and Lamour, 2004), as well as throughout the U. S. (Babadoost, 2000; Isakeit, 2007;
Mathis et al., 1999; Ploetz and Haynes, 2000; Ristaino and Johnston, 1999), and globally
(Hwang and Kim, 1995; Silvar et al., 2006). Many P. capsici-susceptible vegetables include
members of the Fabaceae and Solanaceae families (Davidson et al., 2002; Erwin and Ribeiro,
1996; Gevens et al., 2008; Hausbeck and Lamour, 2004). The susceptibility of floriculture crops
in these families to P. capsici has not been well investigated. Million bells and lupine are
reported Solanaceous (Stehmann et al., 2009) and Fabaceous hosts of P. tropicalis, respectively
(Daughtrey et al., 2007; Hong et al., 2008; see Quesada-Ocampo et al., 2011). Additionally, few
studies have investigated million bells response to plant pathogens (Omer et al., 2011).
Calibrachoa was previously included in the Petunia Jussieu genus, but was given its own taxon

64

in 1985 (by Wijsman and de Jon and based on differing chromosome numbers [Petunia 2n = 14
and Calibrachoa 2n = 18]) (Facciuto et al., 2006; Stehmann et al., 2009), and did not achieve
popularity in the floricultural industry until the first hybrids were introduced in the early-1990s
(Schultz-Nelson, 2008).
As P. tropicalis and P. capsici are closely related taxa (Donahoo and Lamour, 2008),
screening ornamental Fabaceous and Solanaceous plants to both pathogens is important
regarding floricultural crop disease management in Michigan and elsewhere. This is especially
true if floriculture plants are produced in the same greenhouse or nursery as P. capsicisusceptible vegetable transplants.
The objectives of this research included 1) evaluating the susceptibility of Fabaceous and
Solanaceous ornamental plants to P. capsici and P. tropicalis and 2) evaluating the susceptibility
of million bells cultivars to P. capsici and P. tropicalis.

65

MATERIALS AND METHODS

Isolate selection, P. tropicalis isolate identification, and isolate maintenance. Four P.
capsici isolates and one P. tropicalis isolate were used as inocula in experiments. Isolates
differed in phenotype (mating type = compatibility type [CT], sensitivity to mefenoxam [MS], an
oomycete-specific fungicide) and host origin and were characterized by CT and MS as described
by Lamour and Hausbeck (2000). Isolate designation refers to the notation given by the
laboratory of M. K. Hausbeck at Michigan State University (MSU). Isolates OP97 (CT = A1)
and SP98 (CT = A2) are sensitive to mefenoxam and were isolated from fruit of pickling
cucumber (Cucumis sativus L.) and pumpkin (Cucurbita pepo L.), respectively. Isolate 12889
(CT = A1) is insensitive to mefenoxam and was isolated from pepper fruit. Isolates were
collected from commercial fields in Michigan and have been shown to be highly virulent on
various hosts (Foster and Hausbeck, 2010; Quesada-Ocampo et al., 2009; Quesada-Ocampo and
Hausbeck, 2010). Isolate 13351 (CT = A1) is sensitive to mefenoxam and was isolated from
eggplant by the laboratory of C. D. Smart at Cornell University.
Phytophthora tropicalis isolate 13715 (CT = A1), obtained from M. L. Daughtrey at
Cornell University, is sensitive to mefenoxam and was isolated from million bells. Isolate 13715
was previously reported as P. capsici (Daughtrey et al., 2007), but was identified as P. tropicalis
following PCR and sequencing of ten genes (see Quesada-Ocampo et al., 2011). The gene
sequences of isolate 13715 were compared to 20 P. capsici isolates, including two type cultures
(ATCC 64531 and ATCC 15399), nine P. tropicalis isolates, including the type culture (ATCC
76651), and 10 isolates that displayed an intermediate genotype between P. capsici and P.
tropicalis from the study by Quesada-Ocampo et al. (2011) using nucleotide BLAST2. Results

66

from BLAST2 indicated that isolate 13715 did not correspond to P. capsici, and e-values and
identity percentages were the same for both P. tropicalis and intermediate isolates. Hence, CLC
DNA workbench (CLC bio LLC, Cambridge, MA) was used to align the sequences of each gene
and to manually determine nucleotide changes in specific positions which were different to P.
tropicalis and intermediate isolates. Taking into account these specific changes, isolate 13715
was identified as P. tropicalis.
Agar plugs of isolate cultures were obtained from long-term storage (microcentrifuge
tubes containing 1 ml of sterile water and 1 sterile hemp seed at 20ºC) and plated on unclarified
V8 juice agar (UCV8, 16 g agar, 3 g calcium chloride [CaCO3], 160 ml V8 juice, and 840 ml of
distilled water) amended with 0.50 g benomyl, 2 ml ampicillin, and 2 ml rifampicin. Five-mm
diameter mycelia and sporangia agar plugs from five- to seven-day-old cultures were used to
inoculate straightneck squash (C. pepo) fruit prior to use in experiments to confirm isolate
pathogenicity. Five days post inoculation (dpi), symptomatic tissue was excised and plated on
UCV8 agar (12 g agar, 0.3 g CaCO3, 40 ml V8 juice, and 960 ml distilled water) amended with
0.50 g benomyl, 2 ml ampicillin, 2 ml rifampicin, and 0.10 g pentachloronitrobenzene (BARP).
Cultures were observed three days after plating using a compound microscope at 100x
magnification to confirm P. capsici morphological characteristics according to Waterhouse
(1963). Phytophthora tropicalis was confirmed according to Aragaki and Uchida (2001).
Isolates were hyphal tipped and transferred to new UCV8 agar. Cultures were maintained at
room temperature (21 to 23°C) under continuous fluorescent lighting and were transferred every
five to seven days to new UCV8 agar.
Pathogenicity on Fabaceous and Solanaceous plants (experiment 1). Seven
ornamental plant types from two families were grown in experiment 1 to evaluate susceptibility

67

to root rot caused by P. capsici and P. tropicalis (Table 2.1). Straightneck squash was included
as a P. capsici-susceptible control (Table 2.1). Experiment 1 was conducted in March 2010 in a
greenhouse at the MSU Horticulture Teaching and Research Center in Holt, Michigan and in
October 2010 in a greenhouse at the MSU Plant Science Research Greenhouses in East Lansing,
Michigan. The experimental design was a split-plot arrangement of a randomized complete
design, and the experiment was repeated three times. Number of replicates differed among runs
and plant types (Table 2.1). Run 3 did not include million bells due to the inability to acquire the
same cultivars from commercial producers that were used in runs 1 and 2 for run 3.
Pathogenicity on million bells (experiment 2). Six cultivars of million bells were grown
in experiment 2 to evaluate susceptibility to root rot caused by P. capsici and P. tropicalis (Table
2.2). Experiment 2 was conducted in February 2011 in two separate greenhouse ranges at the
MSU Plant Science Research Greenhouses in East Lansing, Michigan. The experimental design
was a split-plot arrangement of a randomized complete design, and the experiment was repeated
twice. There were twelve replicates of each cultivar. Isolates 13715 and SP98 were used as
inoculum.

68

Table 2.1. Ornamental plants evaluated in experiment 1 for susceptibility to root rot caused by Phytophthora capsici and P. tropicalis
Host
Family
Cucurbitaceae
Fabaceae

Solanaceae

Species
Cucurbita pepo
(straightneck squash)
Lathyrus latifolius
(sweet pea)
Lupinus polyphyllus
(lupine)
Browallia speciosa
(bush violet)
Calibrachoa x hybrida
(million bells)

Cultivar

Number of
replicates

Plug or seed source

'Cougar'

12

Rispens Seeds, CA

1, 2, 3

'Latifolia Mixed'

12

C. Raker & Sons, MI

1, 2, 3

'Gallery Pink'

10

C. Raker & Sons, MI

1, 2, 3

'Bells Silver'

12

C. Raker & Sons, MI

1, 2, 3

'Callie Gold with Red Eye'
'Starlette Sunset'

8
12

C. Raker & Sons, MI
Sunbelt Greenhouses, GA

1, 2
1, 2

z

x

Run

y

Nicotiana x sanderae
'Perfume Deep Purple'
12
C. Raker & Sons, MI
(flowering tobacco)
1, 2, 3
Nierembergia scoparia
'Purple Robe'
12
(cup flower)
C. Raker & Sons, MI
1, 2, 3
Petunia x hybrida
'Prism Sunshine'
12
(petunia)
C. Raker & Sons, MI
1, 2, 3
x
Plants evaluated were received as rooted cuttings or seedlings from commercial producers. Straightneck squash was grown from
seed.
y
Run 3 did not include million bells due to the inability to acquire the same cultivars from commercial producers that were used in
runs 1 and 2 for run 3.
z
Ten replicates were evaluated in runs 1 and 2, and twelve replicates were evaluated in run 3.

69

Table 2.2. Million bells (Calibrachoa x hybrida) cultivars evaluated in experiment 2 for
susceptibility to root rot caused by Phytophthora capsici and P. tropicalis
Cultivar
Source*
Habit
Sunbelt Greenhouses, GA
'Cabaret Yellow'
trailing
Sunbelt Greenhouses, GA
'Callie Gold with Red Eye'
trailing
Sunbelt Greenhouses, GA
'Can-Can Apricot'
mounded, trailing
Mast Young Plants, MI
'Celebration Purple Star'
semi-trailing
'Million Bells' Cherry Pink' Sunbelt Greenhouses, GA
upright, trailing
Four Star Greenhouse, MI
'Superbells White'
mounded, trailing
*Cultivars evaluated were received as rooted cuttings from commercial producers.

Inoculation and evaluation. Rooted cuttings or seedlings were received from
commercial producers (Tables 2.1 and 2.2). Plant replicates were transplanted into individual
3

386.7 cm pots containing soilless potting media (Baccto Professional Planting Mix, Michigan
Peat Company, Houston, TX) 14 days prior to inoculation. „Cougar‟ straightneck squash seeds
(Rispens Seeds, Inc., Beecher, IL) were sown in a 128-cell plug tray filled with soil 14 days prior
to inoculation. Straightneck squash seedling replicates were transplanted into individual 386.7
cm3 pots containing soil 7 days prior to inoculation. Plants in experiments 1 (run 3) and 2 were
grown under a 14-h photoperiod to mimic light conditions from experiment 1 (runs 1 and 2).
Plants were watered as necessary and fertilized three times weekly at 200 ppm with Peter‟s 2020-20 soluble liquid fertilizer (The Scott‟s Company, Marysville, OH). Plants were acid treated
once or three times weekly with 10% phosphoric acid. In experiment 2, plugs were drenched
with thiophanate-methyl (OHP 6672 4.5L; OHP, Inc., Mainland, PA) two days prior to
transplanting, and plants were drenched twelve days after transplanting at the rate of 124.9 ml
formulated product per liter for prevention of black root rot (Thielaviopsis basicola).
Temperature and relative humidity in greenhouses were recorded hourly with a Watchdog data

70

logger (450 series; Spectrum Technologies, Inc., East Plainfield, IL), and recordings were used
to calculate mean air temperature and relative humidity. Mean air temperature and relative
humidity for experiment 1 were 21.1ºC (±17.3ºC) and 55.5% (±39.7%), respectively; mean air
temperature and relative humidity for experiment 2 were 23.4ºC (±12.5) and 30.5% (±26.6%),
respectively.
Infested millet seed was used to inoculate seedlings. Millet seed inoculum was prepared
according to the methods of Quesada-Ocampo et al. (2009). Infested millet seeds (ca. 1 g) were
placed on the surface of soil near plant stems in experiment 1 (runs 1 and 2). At 14 dpi,
approximately 0.5 g of infested millet seeds were added to the soil because root rot development
was slow. In experiments 1 (run 3) and 2, millet seeds were buried to a depth of 2 to 3 cm and
about 1 cm away from the crown to prevent inoculum from desiccating before infection could
take place. Sterile millet seeds were used to inoculate control plants. Plants were watered after
inoculation to prevent immediate inoculum desiccation.
Plants were inspected for aboveground root rot disease symptoms every two days from 0
to 42 dpi. Plant height and canopy width of each replicate was measured (cm) at 7-day intervals
from 14 to 42 dpi in experiment 1 and from 0 to 42 dpi in experiment 2. Crown and root tissue
of 50% of inoculated plants were excised upon death or experiment completion. Asymptomatic
control plants were also sampled. Excess roots and foliage were removed, and plants were
gently rinsed with water to eliminate remaining potting soil. Plants were surface disinfested in a
5% bleach solution (0.31% NaClO) for 3 min, submerged in distilled water for 1 min, and airdried. Straightneck squash plants were surface disinfested with 70% ETOH for 1 min and airdried. Two sections of tissue were excised from the crown and roots and plated on BARPamended UCV8 agar. Cultures were observed three days after plating using a compound

71

microscope at 100x magnification and confirmed as P. capsici based on morphological
characteristics according to Waterhouse (1963). P. tropicalis was confirmed according to
Aragaki and Uchida (2001). Isolates were characterized for CT and MS as previously described
(Lamour and Hausbeck, 2000). Phenotype of the recovered isolates was compared with the
original inoculum.
Statistical analyses. Plant height and mean canopy width (calculated by averaging two
width measurements [the second measurement perpendicular to the first]) were used to calculate
2

plant volume (volume of a cylinder [V = πr h]) to standardize height and canopy width
3

measurements among plants. Mean plant volume (cm ) of each plant type was determined by
averaging the volume of the plant type-pathogen isolate pairing at every interval. Mean plant
volume data was square root transformed prior to analyses to satisfy normality assumptions and
were analyzed by analysis of variance (ANOVA) using the PROC MIXED procedure of SAS
version 9.2 (SAS Institute Inc., Cary, NC). Pathogen isolate was considered the whole plot
factor and plant type was considered the subplot factor. Auto-regressive of order 1 with
heterogeneous variance- (experiment 1, runs 1 and 2 and experiment 2) or compound symmetry
with heterogeneous variance- (experiment 1, all runs combined) covariance structures (with the
lowest AIC and BIC values) were used to make conclusions on mean plant volume. Plant
replicate volume calculations of each plant type-pathogen isolate pairing for each 7-day interval
were placed into the area under the disease progress curve (AUDPC) mathematical formula
according to Shaner and Finney to assess stunting or death over time (1977) and is referred to
hereafter as area under the plant growth curve (AUPGC). Larger AUPGC values signify greater
plant volume. AUPGC values were square root transformed prior to analyses to satisfy
normality assumptions and were analyzed by ANOVA using the PROC MIXED procedure of

72

SAS version 9.2. Pathogen isolate was considered the whole plot factor and plant type was
considered the subplot factor. If variances were heterogeneous for plant type or pathogen isolate,
the GROUP option of the REPEATED statement (with the lowest AIC value) was used with
degrees of freedom calculated according to Kenward-Roger. If ANOVA was significant for
mean plant volume or AUPGC value main effects or interaction terms, treatment means were
compared using Fisher‟s protected least significant difference (LSD) at P = 0.05. Interactions
were examined by slicing. Estimates and ± estimate standard errors were back transformed to
report least square means and standard errors.

73

RESULTS

Pathogenicity, disease symptoms, and pathogen recovery. In experiment 1, disease
onset of P. capsici and P. tropicalis-inoculated flowering tobacco and lupine began at
approximately 17 and 19 dpi, respectively. Disease incidence varied among plant type-pathogen
isolate pairings; some plants remained healthy in appearance, and others were severely diseased
(Table 2.3). Symptomatic plants were stunted in comparison to control plants, had chlorotic
foliage that became necrotic, followed by foliar and stem wilting, and occasional death (Figure
2.1 A-C). Roots of symptomatic plants ranged from brown and decayed to only slightly rotted.
Pathogen recovery from flowering tobacco and lupine varied among isolates (Table 2.4).
In experiments 1 and 2, P. tropicalis-inoculated million bells exhibited necrotic crown
lesions, progressing to the stems at approximately 12 dpi. Asymmetrical foliar wilting generally
followed, in which stems on one side of plants wilted before the others, with older foliage closer
to the base wilting first, followed by leaves further up stems (Figure 2.1 D). All aboveground
tissues lost turgidity, and plants died (Figure 2.1 E). Roots were brown and decayed. Rapid
death typically ensued after initial symptom onset for most cultivars. Disease incidence of P.
capsici-inoculated million bells differed among cultivar-pathogen isolate pairings (Tables 2.3
and 2.5). Symptomatic plants resembled those of P. tropicalis-inoculated plants (stunting in
comparison to the control plants, crown lesions, asymmetrical foliar wilting, and death).
Pathogen recovery varied among cultivars and isolates (Tables 2.4 and 2.6).
Phytophthora capsici-inoculated straightneck squash first began exhibiting aboveground
root rot symptoms (wilting and crown lesions) at approximately 5 dpi. As dpi increased,
seedlings collapsed, becoming completely shriveled. Straightneck squash inoculated with P.

74

tropicalis developed water-soaked crown lesions (Table 2.3) at approximately 7 dpi. Lesions
continued to progress to the stems, but did not extend beyond cotyledons. As dpi increased,
lesions became hardened and ceased to enlarge (Figure 2.1 F, G). First- and second-true leaves
exhibited a loss of turgidity; however, seedlings did not collapse or die. Phytophthora tropicalis
was frequently recovered from root and crown tissue (Table 2.4).
Chlorosis of sweet pea foliage was observed (Table 2.3 and Figure 2.1 H) beginning at 26
dpi, with necrosis following. Pathogen recovery varied among isolates (Table 2.4). No disease
symptoms were observed on petunia (Table 2.3), but P. capsici recovery from asymptomatic
plants was possible (Table 2.4). Phytophthora capsici- and P. tropicalis-inoculated bush violet
and cup flower did not become diseased (Table 2.3), nor were pathogens recovered from root or
crown tissue (Table 2.4). The tissue isolations from which P. capsici or P. tropicalis were
recovered from were confirmed to have the same phenotype as the original inoculum (data not
shown).

Figure 2.1. Aboveground root rot symptoms caused by Phytophthora capsici or P. tropicalis in
experiments 1 or 2. Stunting (A) and death of flowering tobacco (Nicotiana x sanderae) (B),
foliar and stem wilting of lupine (Lupinus polyphyllus) (C), uneven foliar wilting (D) and death
(E) of million bells (Calibrachoa x hybrida), crown lesions on straightneck squash (Cucurbita
pepo) (F, G), and chlorotic and necrotic foliage of sweet pea (Lathyrus latifolius) (H).

75

Figure 2.1 (con‟t).

C

B

A

E

D

F

G

H

76

Table 2.3. Disease incidence of ornamental plants evaluated in experiment 1 for susceptibility to Phytophthora capsici isolates OP97,
SP98, 12889, and 13351 and P. tropicalis isolate 13715
Disease incidence (%)
Isolate
SP98
12889
0.0
0.0
0.0
0.0
11.1
38.9
43.7
65.6

*

uninoculated
OP97
13351
13715
Plant
Bush violet
0.0
0.0
0.0
0.0
Cup flower
0.0
0.0
0.0
0.0
Flowering tobacco
0.0
16.7
22.2
41.7
Lupine
0.0
78.1
71.9
81.3
**
Million bells
'Callie Gold with
Red Eye'
0.0
6.3
25.0
6.3
6.3
100.0
'Starlette Sunset'
0.0
25.0
12.5
20.8
16.7
100.0
Petunia
0.0
0.0
0.0
0.0
0.0
0.0
Straightneck squash
0.0
100.0
100.0
100.0
100.0
86.0
Sweet pea
0.0
5.6
8.3
5.6
8.3
16.7
*
Disease incidence was calculated by dividing the number of plants that exhibited aboveground root rot symptoms by the total number
of plants inoculated with each isolate.
**
Million bells were evaluated in runs 1 and 2 only.

77

Table 2.4. Recovery of Phytophthora capsici isolates OP97, SP98, 12889, and 13351 and P. tropicalis isolate 13715 from ornamental
plants evaluated in experiment 1 for susceptibility to root rot
Pathogen recovery (%)
Isolate
SP98
12889
0.0
0.0
0.0
0.0
22.2
16.7
62.5
50.0

*

13715
uninoculated
OP97
13351
Plant
0.0
Bush violet
0.0
0.0
0.0
Cup flower
0.0
0.0
0.0
0.0
Flowering tobacco
0.0
38.9
5.6
44.4
Lupine
0.0
43.8
75.0
57.8
**
Million bells
'Callie Gold with Red Eye'
0.0
12.5
25.0
0.0
37.5
100.0
'Starlette Sunset'
0.0
50.0
25.0
0.0
33.3
100.0
Petunia
0.0
5.6
5.6
5.6
11.1
0.0
Straightneck squash
0.0
100.0
100.0
100.0
100.0
75.0
Sweet pea
0.0
5.6
0.0
0.0
5.6
22.2
*
Pathogen recovery was calculated by the percentage of positively identified P. capsici or P. tropicalis isolates recovered from root or
crown tissue of 50% of each plant type inoculated with each isolate and control plants.
**
Million bells were evaluated in runs 1 and 2 only.

78

Table 2.5. Disease incidence of million bells (Calibrachoa x hybrida) cultivars evaluated in
experiment 2 for susceptibility to Phytophthora capsici and P. tropicalis
*

Disease incidence (%)
Isolate
uninoculated
SP98
13715
Cultivar
0.0
'Cabaret Yellow'
75.0
100.0
0.0
'Callie Gold with Red Eye'
62.5
100.0
0.0
'Can-Can Apricot'
92.0
100.0
0.0
'Celebration Purple Star'
75.0
100.0
0.0
'Million Bells' Cherry Pink'
79.2
100.0
0.0
'Superbells White'
79.2
100.0
*
Disease incidence was calculated by dividing the number of plants that exhibited aboveground
root rot symptoms by the total number of plants inoculated with each isolate.

Table 2.6. Recovery of Phytophthora capsici and P. tropicalis from million bells (Calibrachoa x
hybrida) cultivars evaluated in experiment 2 for susceptibility to root rot
*

Pathogen recovery (%)
Isolate
uninoculated
SP98
13715
Cultivar
'Cabaret Yellow'
0.0
91.7
100.0
'Callie Gold with Red Eye'
0.0
100.0
100.0
'Can-Can Apricot'
0.0
91.7
100.0
'Celebration Purple Star'
0.0
25.0
100.0
'Million Bells' Cherry Pink'
0.0
91.7
100.0
'Superbells White'
0.0
16.7
100.0
*
Pathogen recovery was calculated by the percentage of positively identified P. capsici or P.
tropicalis isolates recovered from root or crown tissue of 50% of each cultivar inoculated with
each isolate and control plants.

79

Pathogenicity on Fabaceous and Solanaceous plants (experiment 1). Runs conducted in
March 2010 (runs 1 and 2) were grouped together for analyses, and separate analyses were
conducted for all runs combined (runs were not found to be significantly different when million
bells were removed from runs 1 and 2 data sets, data not shown).
The plant type*pathogen isolate*dpi interaction term was significant for mean plant
volume for runs 1 and 2 (Table 2.7). All pathogen isolate*dpi slices were significant (P <
0.0001) when the three-way interaction term was sliced. All plant type*pathogen isolate slices
were significant, except for the straightneck squash*isolates SP98 (P = 0.4051), 12889 (P =
0.1435), and 13351 (P = 0.4121) slices. All „Callie Gold with Red Eye‟ and „Starlette Sunset‟
million bells*dpi slices were significant, except for the „Callie Gold with Red Eye‟*14 dpi slice
(P = 0.4032) and „Starlette Sunset‟*21 dpi slice (P = 0.1375) when the three-way interaction
term was sliced by plant type*dpi. The flowering tobacco*35 dpi was significant (P = 0.0062)
when the three-way interaction term was sliced.
For all runs combined, the plant type*pathogen isolate and plant type*dpi interaction
terms were significant (Table 2.8). The straightneck squash slice was significant (P < 0.0001)
when the plant type*pathogen isolate interaction term was sliced by pathogen isolate. All isolate
slices were significant (P < 0.0001), except for the isolate 13715 (P = 0.3797) and untreated (P =
0.1459) slices when the interaction term was sliced by plant type. All plant type and dpi slices
were significant (P < 0.0001) when plant type*dpi interaction term was sliced, except for the
straightneck squash slice (P = 0.3429).
In runs 1 and 2 at 35 dpi, flowering tobacco inoculated with isolates 12889, 13351, and
13715 were significantly smaller than control plants (data not shown). At 42 dpi, inoculatedflowering tobacco and lupine were smaller than control plants, although not significantly smaller

80

(Figure 2.2). „Callie Gold with Red Eye‟ million bells inoculated with isolate 13715 were
significantly smaller than P. capsici-inoculated and control plants beginning at 21 dpi (data not
shown) and continuing until 42 dpi (Figure 2.2). At 14 dpi, „Starlette Sunset‟ million bells
inoculated with isolates OP97 and 12889 were significantly smaller compared to control plants
(data not shown). „Starlette Sunset‟ million bells inoculated with isolate 13715 were
significantly smaller than control and P. capsici-inoculated plants beginning at 28 dpi (data not
shown) and continuing until 42 dpi (Figure 2.2). Straightneck squash inoculated with isolate
13715 were smaller, although not significantly, than control plants beginning at 21 dpi (data not
shown) and continuing until 42 dpi (Figure 2.2).
For all runs combined, control and inoculated plants were similar in volume (volume =
mean over all 7-day intervals), except for straightneck squash (Table 2.9). Inoculated-flowering
tobacco and lupine were smaller than control plants, although not significantly smaller (Table
2.9). Significant differences were found regarding volume (volume = mean over all inoculated
and control plants) at 7-day intervals for all plant types, except for straightneck squash (Figure
2.3).

81

Table 2.7. ANOVA result for mean plant volume for experiment 1 (runs 1 and 2) evaluating
ornamental plant susceptibility to root rot caused by Phytophthora capsici isolates OP97, SP98,
12889, and 13351 and P. tropicalis isolate 13715
Effect
P-value
Plant type
<0.0001*
Pathogen isolate
<0.0001*
Dpi
<0.0001*
Plant type x pathogen isolate
<0.0001*
Plant type x dpi
<0.0001*
Pathogen isolate x dpi
<0.0001*
Plant type x pathogen isolate x dpi <0.0001*
* = significant (Fisher‟s protected LSD, P = 0.05).
dpi = days post inoculation.

Table 2.8. ANOVA result for mean plant volume for experiment 1 (all runs combined)
evaluating ornamental plant susceptibility to root rot caused by Phytophthora capsici isolates
OP97, SP98, 12889, and 13351 and P. tropicalis isolate 13715
Effect
P-value
Plant type
<0.0001*
Pathogen isolate
0.0699 ns
Dpi
<0.0001*
Plant type x pathogen isolate
0.0035*
Plant type x dpi
<0.0001*
Pathogen isolate x dpi
0.1248 ns
Plant type x pathogen isolate x dpi
0.1106 ns
* = significant (Fisher‟s protected LSD, P = 0.05).
ns = not significant (Fisher‟s protected LSD, P = 0.05).
dpi = days post inoculation.

82

3

Figure 2.2. Mean plant volume (cm ) of ornamental plants at 42 days post inoculation evaluated
in experiment 1 (runs 1 and 2) for susceptibility to root rot caused by Phytophthora capsici
isolates OP97, SP98, 12889, and 13351 and P. tropicalis isolate 13715. Bars represent ±
standard error. MB = million bells.

83

3

Table 2.9. Mean plant volume (cm ) of ornamental plants in experiment 1 (all runs combined) evaluating susceptibility to root rot
caused by Phytophthora capsici isolates OP97, SP98, 12889, and 13351 and P. tropicalis isolate 13715
*

Plant

uninoculated

OP97

Mean plant volume
Isolate
SP98
12889

13351

13715

**

3288.4 a
Bush violet
3060.5 a
3341.6 a
3275.4 a
2898.0 a
3263.2 a
Cup flower
7920.2 a
6925.7 a
9362.2 a
7729.1 a
7531.2 a
7532.9 a
Flowering tobacco
10884.7 a
9892.5 a
10749.5 a
8630.4 a
10720.5 a
8347.6 a
Lupine
8187.9 a
6797.7 a
7557.4 a
6799.9 a
6643.8 a
6348.3 a
Petunia
6689.5 a
6075.8 a
6745.4 a
7149.9 a
6953.8 a
6385.6 a
Straightneck squash
11086.0 b
28.1 a
4.7 a
9.6 a
5.1 a
9322.2 b
Sweet pea
10561.7 a
10930.7 a
11489.7 a
9141.1 a
11151.4 a
9395.7 a
*
Plant height and canopy width measurements (cm) taken at 7-day intervals from 14 to 42 days post inoculation were used to calculate
2

a volume approximation (volume of a cylinder [V = πr h]) to standardize height and canopy width measurements among plants.
Mean volume was obtained by the replicate volume mean of each plant type over all intervals.
**
Means within rows followed by the same letter are not significantly different within each plant type (Fisher‟s protected LSD, P =
0.05).

84

3

Figure 2.3. Mean plant volume (cm ) of ornamental plants in evaluated in experiment 1 (all runs
combined) for susceptibility to root rot caused by Phytophthora capsici isolates OP97, SP98,
12889, and 13351 and P. tropicalis isolate 13715 at 7-day intervals from 14 to 42 days post
inoculation (dpi). Bars represent ± standard error.

85

The plant type*pathogen isolate interaction term was significant for AUPGC values for
runs 1 and 2 (Table 2.10). The „Callie Gold with Red Eye‟ and „Starlette Sunset‟ million bells,
straightneck squash, and all isolate slices were significant when the interaction term was sliced
by plant type and pathogen isolate (Table 2.10). For all runs combined, the plant type*pathogen
isolate interaction term was significant, and the straightneck squash and P. capsici isolate slices
were significant when the interaction term was sliced by plant type and pathogen isolate (Table
2.11).
„Callie Gold with Red Eye‟ and „Starlette Sunset‟ million bells inoculated with isolate
13715 had significantly lower AUPGC values than P. capsici-inoculated and control plants
(AUPGC values = 26997.8 and 80309.9, respectively) in runs 1 and 2 (Table 2.12). Inoculatedflowering tobacco and lupine had lower AUPGC values than control plants in runs 1 and 2 and
all runs combined, although not significantly lower (Tables 2.12 and 2.13). Straightneck squash
inoculated with P. capsici isolates had significantly lower AUPGC values than isolate 13715inoculated and control plants in runs 1 and 2 and all runs combined (Tables 2.12 and 2.13).
AUPGC values for straightneck squash inoculated with isolate 13715 were lower, although not
significantly lower, than control plants in runs 1 and 2 and all runs combined (Tables 2.12 and
2.13).

86

Table 2.10. ANOVA result for area under the plant growth curve (AUPGC) values and plant
type*pathogen isolate slices for experiment 1 (runs 1 and 2) evaluating ornamental plant
susceptibility to root rot caused by Phytophthora capsici isolates OP97, SP98, 12889, and 13351
and P. tropicalis isolate 13715
Effect
Plant type
Pathogen isolate
Plant type x
pathogen isolate

Slice

Plant

Isolate

P-value
<0.0001*
<0.0001*
<0.0001*

Plant type x
pathogen
isolate

Bush violet
Cup flower
Flowering tobacco
Lupine
MB 'Callie Gold with
Red Eye'
MB 'Starlette Sunset'
Petunia
Straightneck squash
Sweet pea

0.9352 ns
0.7901 ns
0.1062 ns
0.4796 ns

uninoculated
OP97
SP98
12889
13351
13715
* = significant (Fisher‟s protected LSD, P = 0.05).
ns = not significant (Fisher‟s protected LSD, P = 0.05).
MB = million bells.

87

<0.0001*
<0.0001*
0.8083 ns
<0.0001*
0.3188 ns
<0.0001*
<0.0001*
<0.0001*
<0.0001*
<0.0001*
<0.0001*

Table 2.11. ANOVA result for area under the plant growth curve (AUPGC) values and plant
type*pathogen isolate slices for experiment 1 (all runs combined) evaluating ornamental plant
susceptibility to root rot caused by Phytophthora capsici isolates OP97, SP98, 12889, and 13351
and P. tropicalis isolate 13715
Effect
Plant type
Pathogen isolate
Plant type x
pathogen isolate

Slice

Plant

Isolate

P-value
<0.0001*
0.0401*
<0.0001*

Plant type x
pathogen
isolate

Bush violet
Cup flower
Flowering tobacco
Lupine
Petunia
Straightneck squash
Sweet pea
uninoculated
OP97
SP98
12889
13351
13715

* = significant (Fisher‟s protected LSD, P = 0.05).
ns = not significant (Fisher‟s protected LSD, P = 0.05).

88

0.9999 ns
0.9813 ns
0.8274 ns
0.9875 ns
0.9994 ns
<0.0001*
0.9741 ns
0.1114 ns
<0.0001*
<0.0001*
<0.0001*
<0.0001*
0.4283 ns

Table 2.12. Area under the plant growth curve (AUPGC) values of ornamental plants evaluated in experiment 1 (runs 1 and 2) for
susceptibility to root rot caused by Phytophthora capsici isolates OP97, SP98, 12889, and 13351 and P. tropicalis isolate 13715
*

uninoculated

Plant

OP97

AUPGC
Isolate
SP98
12889

13351

13715

**

87320.3 a
Bush violet
73218.9 a
72527.9 a
80174.0 a
82529.8 a
84262.5 a
Cup flower
215360.0 a
186399.4 a 223956.1 a
195328.6 a
223606.0 a
207380.1 a
Flowering tobacco
487958.1 a
407810.0 a 398893.3 a
343255.4 a
406980.2 a
335125.2 a
Lupine
270545.6 a
219801.6 a 251923.7 a
222878.1 a
237130.0 a
210561.7 a
Million bells
„Callie Gold with
Red Eye‟
204665.8 b
177047.4 b 184427.3 b
165600.2 b
176265.6 b
26997.8 a
„Starlette Sunset‟
300797.4 b
292810.9 b 316305.0 b
286685.3 b
330809.0 b
80309.9 a
Petunia
214554.2 a
206661.2 a 210203.9 a
233008.9 a
244688.5 a
214026.5 a
Straightneck squash
431780.4 b
1306.1 a
253.4 a
547.0 a
261.5 a
368266.9 b
Sweet pea
223880.4 a
166643.6 a 194560.4 a
157133.0 a
172499.0 a
172283.1 a
*
Plant height and canopy width measurements (cm) taken at 7-day intervals from 14 to 42 days post inoculation were used to calculate
2

a volume approximation (volume of a cylinder [V = πr h]) to standardize height and canopy width measurements among plants.
3

Volume measurements (cm ) were placed into the area under the disease progress curve (AUDPC) formula to assess stunting or death
over time. Larger AUPGC values signify greater plant volume.
**
Means within rows followed by the same letter are not significantly different within each plant type (Fisher‟s protected LSD, P =
0.05).

89

Table 2.13. Area under the plant growth curve (AUPGC) values of ornamental plants evaluated in experiment 1 (all runs combined)
for susceptibility to root rot caused by Phytophthora capsici isolates OP97, SP98, 12889, and 13351 and P. tropicalis isolate 13715
*

uninoculated

Plant

OP97

AUPGC
Isolate
SP98
12889

13351

13715

**

102329.6 a
Bush violet
97937.7 a 106641.4 a
102925.5 a
91397.4 a
101576.1 a
Cup flower
230803.4 a
204114.2 a 276518.2 a
224287.5 a
218070.3 a
219548.5 a
Flowering tobacco
323590.3 a
294990.2 a 316665.1 a
243789.1 a
310405.0 a
212843.8 a
Lupine
234633.7 a
196417.4 a 220383.3 a
193828.9 a
181663.5 a
184926.8 a
Petunia
201053.6 a
187705.6 a 205952.6 a
216355.2 a
213536.4 a
196408.5 a
Straightneck squash
321194.2 b
627.2 a
112.6 a
259.7 a
288.0 a
273602.2 b
Sweet pea
304097.1 a
310728.2 a 315742.8 a
256886.8 a
321874.7 a
259335.6 a
*
Plant height and canopy width measurements (cm) taken at 7-day intervals from 14 to 42 days post inoculation were used to calculate
2

a volume approximation (volume of a cylinder [V = πr h]) to standardize height and canopy width measurements among plants.
3

Volume measurements (cm ) were placed into the area under the disease progress curve (AUDPC) formula to assess stunting or death
over time. Larger AUPGC values signify greater plant volume.
**
Means within rows followed by the same letter are not significantly different within each plant type (Fisher‟s protected LSD, P =
0.05).

90

Pathogenicity on million bells (experiment 2). The cultivar*pathogen isolate*dpi
interaction term was significant for mean plant volume (Table 2.14). All pathogen isolate*dpi
slices were significant, except for isolate SP98*21 (P = 0.0489) and 42 (P = 0.3410) dpi slices
when the three-way interaction term was sliced. All cultivar*pathogen isolate slices were
significant (P < 0.0001) when the three-way interaction term was sliced. Significant cultivar*dpi
slices varied among cultivars and dpi (Table 2.14).
Most control and inoculated cultivars were statistically similar in mean plant volume
from 0 to 14 dpi. At 21 dpi, „Cabaret Yellow‟ and „Can-Can Apricot‟ P. capsici-inoculated
plants were significantly larger than P. tropicalis-inoculated plants (data not shown). At 28 dpi,
mean plant volume of „Million Bells‟ Cherry Pink‟ significantly differed among inoculated and
control plants (data not shown). At 42 dpi, mean plant volumes of „Cabaret Yellow‟ and „CanCan Apricot‟ plants differed significantly among inoculated and control plants, with P.
tropicalis-inoculated plants having significantly smaller volumes, followed by P. capsiciinoculated plants, and then control plants (Figure 2.4). „Callie Gold with Red Eye,‟ „Million
Bells‟ Cherry Pink,‟ and „Superbells White‟ control and P. capsici-inoculated plants were
significantly larger than plants inoculated with P. tropicalis (Figure 2.4). „Celebration Purple
Star‟ inoculated and control plants were statistically similar in volume from 0 (data not shown)
to 42 dpi (Figure 2.4).

91

Table 2.14. ANOVA result for mean plant volume and cultivar*days post inoculation (dpi)
slices of the three-way interaction term for experiment 2 evaluating million bells (Calibrachoa x
hybrida) cultivar susceptibility to root rot caused by Phytophthora capsici and P. tropicalis
Effect
Cultivar
Pathogen isolate
Dpi
Cultivar x
pathogen isolate
Cultivar x dpi
Pathogen isolate x
dpi
Cultivar x
pathogen isolate x
dpi

Slice

Cultivar

Dpi

P-value
<0.0001*
<0.0001*
<0.0001*
<0.0001*
<0.0001*
<0.0001*

Cultivar x
pathogen
isolate x
dpi
'Cabaret Yellow'

'Callie Gold with Red Eye'

'Can-Can Apricot'

* = significant (Fisher‟s protected LSD, P = 0.05).
ns = not significant (Fisher‟s protected LSD, P = 0.05).

92

0
7
14
21
28
35
42
0
7
14
21
28
35
42
0
7
14
21
28
35
42

<0.0001*
0.2310 ns
0.9989 ns
0.0548 ns
0.0003*
<0.0001*
<0.0001*
<0.0001*
0.1914 ns
0.4244 ns
0.0545 ns
<0.0001*
<0.0001*
<0.0001*
<0.0001*
0.2211 ns
0.3733 ns
0.0350*
0.0003*
<0.0001*
<0.0001*
<0.0001*

Table 2.14 (cont‟d).
Effect
Cultivar x
pathogen isolate x
dpi

Slice
Cultivar x
pathogen
isolate x
dpi

Cultivar

Dpi

P-value

„Celebration Purple Star‟

0
7
14
21
28
35
42

0.0584 ns
0.5766 ns
0.6642 ns
0.6569 ns
0.7379 ns
0.1268 ns
0.1497 ns

0
7
14
21
28
35
42
0
7
14
21
28
35
42

0.0013*
0.0604 ns
0.6528 ns
0.0002*
<0.0001*
<0.0001*
<0.0001*
<0.0001*
0.3258 ns
0.7397 ns
0.4320 ns
0.0205*
<0.0001*
<0.0001*

'Million Bells' Cherry
Pink'

'Superbells White'

* = significant (Fisher‟s protected LSD, P = 0.05).
ns = not significant (Fisher‟s protected LSD, P = 0.05).

93

3

Figure 2.4. Mean plant volume (cm ) of million bells (Calibrachoa x hybrida) cultivars at 42
days post inoculation evaluated in experiment 2 for susceptibility to root rot caused by
Phytophthora capsici and P. tropicalis. Bars with a letter in common are not significantly
different within each cultivar (Fisher‟s protected LSD, P = 0.05).

94

The cultivar*pathogen interaction term was significant for AUPGC values (Table 2.15).
All cultivar and isolate slices were significant when the interaction term was sliced, except for
the „Celebration Purple Star‟ slice (Table 2.15).
„Million Bells‟ Cherry Pink‟ control plants had the greatest AUPGC value (AUPGC
value = 2242.3) of all control plants, and the greatest value (AUPGC value = 1830.2) of P.
capisci-inoculated plants (Figure 2.5). Control „Callie Gold with Red Eye,‟ „Celebration Purple
Star,‟ „Million Bells‟ Cherry Pink,‟ and „Superbells White‟ and P. capsici-inoculated plants were
statistically similar in AUPGC values (Figure 2.5). „Cabaret Yellow‟ had the lowest AUPGC
value (AUPGC = 1177.5) following inoculation with P. capsici (Figure 2.5). „Celebration
Purple Star‟ had the greatest AUPGC value (AUPGC value = 1266.1) of P. tropicalis-inoculated
plants (Figure 2.5). AUPGC values of inoculated and control „Celebration Purple Star‟ plants
were statistically similar (Figure 2.5).

95

Table 2.15. ANOVA result for area under the plant growth curve (AUPGC) values and
cultivar*pathogen isolate slices for experiment 2 evaluating million bells (Calibrachoa x
hybrida) cultivars for susceptibility to root rot caused by Phytophthora capsici and P. tropicalis
Effect
Cultivar
Pathogen isolate
Cultivar x
pathogen isolate

Slice

Cultivar

Isolate

P-value
<0.0001*
0.0026*
0.0004*

Cultivar x
pathogen
isolate

'Cabaret Yellow'
'Callie Gold with Red Eye'
'Can-Can Apricot'
'Celebration Purple Star'
'Million Bells' Cherry Pink'
'Superbells White'
uninoculated
P. capsici
P. tropicalis

* = significant (Fisher‟s protected LSD, P = 0.05).
ns = not significant (Fisher‟s protected LSD, P = 0.05).

96

<0.0001*
<0.0001*
<0.0001*
0.7713 ns
<0.0001*
<0.0001*
0.0002*
0.0056*
<0.0001*

Figure 2.5. Area under the plant growth curve (AUPGC) values of million bells (Calibrachoa x
hybrida) cultivars evaluated in experiment 2 for susceptibility to root rot caused by Phytophthora
capsici and P. tropicalis. Plant volume measurements were placed into the area under the
disease progress curve (AUDPC) formula to assess stunting or death over time. Larger AUPGC
values signify greater plant volume. Bars with a letter in common are not significantly different
within each cultivar (Fisher‟s protected LSD, P = 0.05).

97

DISCUSSION

The results of this study indicate that the potential host range of P. capsici has expanded
to flowering tobacco, lupine, million bells, and sweet pea, and the potential host range of P.
tropicalis has expanded to flowering tobacco, straightneck squash, and sweet pea. The
susceptibility to root rot caused by P. capsici or P. tropicalis was confirmed by conducting
Koch‟s postulates and observed volume differences of inoculated versus uninoculated control
plants. To our knowledge, these plant species have not been previously documented as hosts of
P. capsici or P. tropicalis. Additionally, this and other studies and recent reports, validate that P.
capsici can no longer be completely regarded as a pathogen of annual Cucurbitaceous and
Solanaceous vegetable crops, and P. tropicalis is not confined to woody perennials (Aragaki and
Uchida, 2001; Donahoo and Lamour, 2008; Erwin and Ribeiro, 1996; Gerlach and Schubert,
2001; Hao et al., 2010; Hong et al., 2008; Orlikowski et al., 2006; Oudemans and Coffey, 1991).
Many other Phytophthora spp. cause disease on the P. capsici and P. tropicalissusceptible plants found in this study. Lupinus and Nicotiana spp. are hosts of a number of
Phytophthora spp. (Erwin and Ribeiro, 1996; Farr and Rossman, 2011). Specifically, L. albus L.
and N. glutinosa L. (tobacco) are reported hosts of P. tropicalis and P. capsici, respectively (Farr
and Rossman, 2011; Hong et al., 2008). Thus, it is not unforeseen that the plants evaluated in
this study were susceptible to root rot caused by P. capsici or P. tropicalis. Significant
differences in mean plant volume were found for flowering tobacco inoculated with P. capsici
isolates 12889 and 13351 and P. tropicalis isolate 13715. Phytophthora capsici and P.
tropicalis-inoculated lupine had lower mean plant volume and AUPGC values, although not
significantly lower, than control plants.

98

All million bells cultivars were susceptible to P. capsici and P. tropicalis in this study.
However, their susceptibility differed. Death of all cultivars following inoculations of P.
tropicalis occurred by 42 dpi, with the exception of „Celebration Purple Star,‟ which showed
reduced disease symptoms. Differences in cultivar response to P. capsici were observed; „Callie
Gold with Red Eye,‟ „Celebration Purple Star,‟ „Million Bells‟ Cherry Pink,‟ and „Superbells
White‟ P. capsici-inoculated and control plants were statistically similar in mean plant volume
and AUPGC values. Phytophthora capsici-inoculated „Cabaret Yellow‟ and „Can-Can Apricot‟
were significantly smaller in volume and had significantly lower AUPGC values compared to
control plants. Cultivars evaluated in this experiment differed in habit, which may account for
the differences observed. Future experiments evaluating additional million bells series and
cultivars within those respected series with P. capsici and P. tropicalis are necessary to provide a
more comprehensive cultivar tolerance designation. Pathogen recovery from million bells, as
well as other plant types evaluated in this study, was common; although, not at 100% recovery.
Phytophthora spp. recovery from woody or ornamental host tissue is difficult (Hong et al., 2008;
Quesada-Ocampo et al., 2009), and by experiment completion, the crowns of several of the plant
types in this study were tough and woody.
Straightneck squash seedlings, included as a P. capsici-susceptible control, became
diseased following inoculations with P. tropicalis. However, mean plant volume (from 21 to 42
dpi) and AUPGC values did not significantly differ between P. tropicalis-inoculated and control
straightneck squash. Hong et al. (2008) observed damping-off of cucumber seedlings following
inoculation of P. tropicalis; hence, disease symptoms observed on straightneck squash in this
study is not unexpected.
No significant differences in mean plant volume or AUPGC values were found for sweet

99

pea in this study. Nonetheless, foliar chlorosis and necrosis were observed on inoculated plants,
and P. capsici and P. tropicalis recovery from tissue was feasible. Recovery of P. capsici was
possible from petunia, but no disease symptoms were noted. Several weed species have been
reported to be alternate P. capsici hosts (French-Monar et al., 2006; Ploetz and Haynes, 2000;
Ploetz et al., 2002). Although, some species remain asymptomatic, pathogen recovery from
tissue is possible, suggesting that these weeds may contribute as an inoculum source. As this
study focused on the susceptibility of ornamental plants to P. capsici or P. tropicalis, not if
dissemination of inocula from an infected plant to a susceptible plant is probable, future
experiments are necessary to discern if plant types which remain relatively asymptomatic, such
as petunia or sweet pea, may contribute as a significant source of P. capsici inoculum in the
greenhouse. Disease was not observed on, nor were pathogens recovered from bush violet or
cup flower in this study. Bush violet and cup flower are not reported hosts of any Phytophthora
spp. (Farr and Rossman, 2011).
Differences in virulence of P. capsici isolates were observed on some susceptible plants in
this study. Isolates 12889 and 13351, collected from pepper and eggplant, respectively, caused a
higher level of disease on flowering tobacco in comparison to isolates OP97 and SP98, collected
from pickling cucumber and pumpkin, respectively. However, a greater sampling of isolates is
necessary to elucidate if Solanaceous isolates are more virulent than those collected from other
host families on flowering tobacco. Studies by Foster and Hausbeck (2010) and QuesadaOcampo and Hausbeck (2010) found that isolate 12889 was significantly more virulent on the
Solanaceous crops pepper and tomato (S. lycopersicon L.), than isolates collected from cucurbits.
Disease incidence and plant volume (mean plant volume and AUPGC values) varied
between runs in experiment 1 (data not shown). Runs 1 and 2 were conducted in summer, and

100

environmental conditions (air temperature) were more extreme than in run 3, which was
conducted in fall. Despite a diligent watering regime, plants in runs 1 and 2 exhibited wilt
symptoms on occasion due to low soil moisture. By submitting „Caroline‟ rhododendron
(Rhododendron sp.), a highly tolerant cultivar to P. cinnamomi, to drought or flooding stress
prior to inoculation with P. cinnamomi, severe root and crown rot symptoms were produced
(Blaker and MacDonald, 1981). Similar results were obtained regarding tomato and P.
nicotianae (Ristaino and Duniway, 1989). Phytophthora nicotianae-inoculated tomato plants
that were water stressed pre or post inoculation, showed significantly more disease than those
that were not subjected to water stress (Ristaino and Duniway, 1989). Although, the same plant
types were susceptible to root rot caused by P. capsici or P. tropicalis in all runs, differences
observed regarding disease incidence and plant volume may be explained by water stress due to
high air temperature. Nonetheless, air temperatures were suitable for plant and pathogen growth,
and no statistical differences were found among runs.
The results of this study confirm the necessity for continued integrated pest management
(IPM) practices, especially exclusion and eradiation, in greenhouse and nursery production. As
many ornamental plants are supplied as vegetative propagules, produced in locations other than
where they will be grown on at, and sent to wholesalers and retailers nationwide (Daughtrey and
Benson, 2005; Lamour et al., 2003), pathogen introduction can readily occur. For example, P.
ramourum-infected camellia (Camellia spp.) and rhododendron nursery stock were shipped from
west coast production nurseries throughout the U. S. (Rizzo et al., 2005). Phytophthora
nicotianae isolates collected from several eastern U. S. production greenhouses were similar in
phenotype and amplified fragment length polymorphism (AFLP) profile, suggesting that same
clonal linage was present at these various facilities (Lamour et al., 2003). Furthermore, infected

101

plants may appear healthy and not show disease until stress is instigated by suboptimal
environmental conditions (Lamour et al., 2003), and in which case, inoculum spread may have
already occurred.
On a local scale, water used for irrigation, such as surface water sources, is frequently
contaminated with Phytophthora spp. propagules (Dreistadt, 2001; Hong and Moorman, 2005).
Naturally occurring P. tropicalis infections have been reported in multiple greenhouses and
nurseries in the U. S. (Hao et al., 2010; Hong et al., 2006) and Europe (Cacciola et al., 2006;
Gerlach and Schubert, 2001; Orlikowski et al., 2006). Specifically, P. tropicalis was detected in
irrigation water at a Virginian nursery during two consecutive growing seasons and was isolated
from diseased garden center Japanese andromeda (Pieris japonica [Thunb.] D. Don ex G. Don)
and rhododendron (R. catawbiense Michx.) foliage (Hong et al., 2006). Additionally in
Michigan, land free of P. capsici infestation is becoming limited (Hausbeck and Lamour, 2004),
and surface water (runoff fed into water bodies from infested fields) has been recognized as a
source of P. capsici inoculum (Gevens et al., 2007). Laboratory studies found that P. capsici
asexual propagules, zoospores, may survive in water for up to five days at a wide range of
temperatures (9 to 32ºC) to cause infection (Granke and Hausbeck, 2010a).
After the initial infection occurs, asexual propagules may be further disseminated
throughout a facility (Lamour et al., 2003) via infested potting soil that may be leached into
irrigation water or nutrient solutions, which is reused and dispersed to other plants (Oh and Son,
2008). Phytophthora cryptogea-infested nutrient solution administered through ebb-and-flow
(EBB) systems have been documented to cause root rot of gerbera daisy (Thinggaard and
Andersen, 1995; van der Gaag et al., 2001), and nutrient solution containing P. nicotianae
zoospores has been found to cause root rot of kalanchoe (Kalanchoe blossflediana) when used in

102

an EBB system (Oh and Son, 2008). Depending on the production facility, management
strategies may be exercised to eradicate pathogens or reduce their dispersal in irrigation water,
such as filtration, heat or chlorine treatments, or the addition of organic or inorganic compounds
(Hong and Moorman, 2005). In recirculating irrigation or nutrient solution systems, disease may
be lessened by employing capillary mats or nutrient-flow wick culture (Oh and Son, 2008; van
der Gaag et al., 2001). Laboratory studies evaluating eleven algaecides with P. capsici
zoospores found that the tested algaecides influenced zoospore motility, mortality, and
subsequent ability to cause disease (Granke and Hausbeck, 2010a). Additionally, many
Phytophthora spp., P. capsici, P. citricola Sawada, P. cryptogea, P. drechsleri, P. nicotianae,
and P. palmivora, isolates collected from production fields, greenhouses, and nurseries have
displayed insensitivity to mefenoxam, a phenylamide- (PA) fungicide, or metalaxyl (50% of the
active enantiomer, mefenoxam contains 100%) adding to the difficulty of pathogen control
(Ferrin and Kabashima, 1991; Hausbeck and Lamour, 2004; Hwang and Benson, 2005; Lamour
and Hausbeck, 2000; Lamour et al., 2003).
Successful crossing of P. capsici and P. tropicalis has occurred in vitro, with resulting
progeny possessing the ability to hybridize with P. capsici (Donahoo and Lamour, 2008).
However, no natural hybridization of the two species has been reported (Donahoo and Lamour,
2008). Nonetheless, the host overlap of P. capsici and P. tropicalis is increasing and pathogen
dispersal among and within production facilities is possible. In Michigan, P. capsici is chiefly a
pathogen of field-grown vegetable crops during the growing season (Hausbeck and Lamour,
2004). However, once introduced into a heated greenhouse, the successful colonization of and
infection by P. capsici on vegetable transplants and ornamental plants may likely not be
restricted to the field season. Future experiments evaluating the dissemination of P. capsici and

103

P. tropicalis inocula from infested sources (irrigation water, nutrient solution, soil, greenhouse
benches, and weed control mats) to susceptible plants are necessary. Meanwhile, to mitigate
disease, an IPM program involving careful scouting and thorough sanitation, fungicide rotations,
and in accordance with Hong et al. (2008), avoidance of rotations of P. tropicalis- and P.
capsici-susceptible hosts on the same greenhouse benches or weed control mats, is suggested.

104

ACKNOWLEDGMENTS

This research is based upon work supported by the Specific Cooperative Agreement #59-1907-0096 between the USDA-ARS Biological Integrated Pest Management Unit, Ithaca, NY, and the
Michigan State University Department of Plant Pathology, East Lansing, MI as part of the
Floriculture and Nursery Research Initiative, and the American Floral Endowment. I would like
to thank M. Daughtrey for providing the Phytophthora tropicalis isolate used in this study;
T. Gallagher, L. Granke, B. Harlan, L. Quesada-Ocampo, L. Rodriguez-Salamanca, J. Schmidt,
and A. Vargas-Berdugo for technical assistance; M. Meyer and R. Naegele for document review;
and K. Ananyeva, L. Ruan, and W. Yang for help with statistical analyses.

105

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