IMPACT OF IRRIGATION REGIME AND HOST CULTIVAR ON DOLLAR SPOT OF
CREEPING BENTGRASS
By
Nancy May Dykema

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
Plant Pathology – Master of Science
2014

ABSTRACT
IMPACT OF IRRIGATION REGIME AND HOST CULTIVAR ON DOLLAR SPOT OF
CREEPING BENTGRASS
By
Nancy May Dykema
Dollar spot (DS), caused by Sclerotinia homoeocarpa, is an important disease of
turfgrass. Irrigation practices and host resistance can impact disease incidence and
aesthetics on fairway turfgrass. This study was conducted to determine the impact of
irrigation regime and creeping bentgrass cultivar on DS incidence as well as vegetative
compatibility group (VCG) of the pathogen. Irrigation was applied at either 2200 h daily
or twice weekly, or 0500 h daily to three creeping bentgrass cultivars, ‘Declaration’,
‘SRP-1WM’, and ‘L-93’. ‘Declaration’ and ‘SRP-1WM’ were considered resistant to DS
and ‘L-93’ was considered susceptible. Plots watered at 2200 h daily exhibited
significantly less disease than those irrigated at 2200 h twice weekly, regardless of
creeping bentgrass cultivar. ‘SRP-1WM’ developed the least amount of DS each year
among all cultivars. In 2011 and 2013, the ‘SRP-1WM’ plots receiving daily irrigation
(AM or PM) did not significantly differ and exhibited less DS than those irrigated at 2200
h twice weekly for the same cultivar.
Recovered isolates of S. homoeocarpa were scored for VCG based on barrage
zone formation to determine if irrigation regime or creeping bentgrass cultivar
corresponded uniquely to VCG. Results indicated that VCGs B and F, averaging 45 and
33% overall, respectively, were the most predominant pairing regardless of irrigation
regime or host cultivar. These results indicate that specific irrigation regimes or host
cultivars investigated in this study similarly influenced VCG.

To my husband,
Mark
For your endless love and support

iii

ACKNOWLEDGEMENTS

There are many people who have offered me words of encouragement and
support while I became a non-traditional graduate student and to whom I would like to
express tremendous gratitude. First and foremost, I am forever grateful to my husband,
Mark, and my children, Kristen and Scott, for their love and patience while I “took time
for me.” Mark, you are amazing; I would not have succeeded without your steadfast
support. I appreciate my sisters, Connie, Kathleen, and Pat, and my dad, Oliver, who
were always ready with kind words of encouragement any time I needed them.
I extend my thanks to Dr. Joe Vargas for his open mindedness and for providing
me the opportunity to pursue my degree, even if my timing was unexpected. Thank you
to Dr. Willie Kirk whose technical guidance and expertise was invaluable and from
whom I learned so much, as well as to Dr. Kevin Frank, for his thoughtful insight and
ever-patient demeanor to keep me grounded.
I would also like to thank Paul Giordano for his encouragement and friendship
and Eric Galbraith for his immense support. My gratitude is also extended to the crew at
the Hancock Center for their assistance and companionship.
I doubt that I could have achieved my goal without my friends. I am indebted to
Jerri Gillett, for her unwavering camaraderie and for always being ready with a shoulder
to lean on and advice that never lead me astray. I also deeply appreciate the support of
my friends, Angie Miller, Gayle McGhee, and Linda Colon.

iv

TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................vi
LIST OF FIGURES ......................................................................................................... vii
CHAPTER I: IMPACT OF IRRIGATION REGIME AND CREEPING BENTGRASS
CULTIVAR ON DOLLAR SPOT ON FAIRWAY TURF IN MICHIGAN............................. 1
Abstract ................................................................................................................ 2
Introduction ........................................................................................................... 3
Materials and Methods ....................................................................................... 11
Results ............................................................................................................... 16
Discussion .......................................................................................................... 26
APPENDIX...........................................................................................................30
LITERATURE CITED...........................................................................................37
CHAPTER II: CHARACTERIZATION OF VEGETATIVE COMPATIBILITY GROUP OF
SCLEROTINIA HOMOEOCARPA ISOLATES FROM THREE CULTIVARS OF
AGROSTIS STOLONIFERA AND THREE IRRIGATION REGIMES…………………….44
Abstract .............................................................................................................. 45
Introduction ......................................................................................................... 46
Materials and Methods ....................................................................................... 50
Results ............................................................................................................... 55
Discussion .......................................................................................................... 60
LITERATURE CITED ......................................................................................... 63
CONCLUSIONS ............................................................................................................ 67

v

LIST OF TABLES

Table 1. The effect of irrigation regime and creeping bentgrass cultivar on dollar spot in
2011.…………………………………………………………………..………….………….…19
Table 2. The effect of irrigation regime and creeping bentgrass cultivar on dollar spot in
2013.……………………………………………………………………….…………………...24
Table 3. 2011 Daily average maximum/minimum air temperatures………..…………....31
Table 4. 2012 Daily average maximum/minimum air temperatures……………………...32
Table 5. 2013 Daily average maximum/minimum air temperatures……………………...33
Table 6. 2011 Daily total precipitation, average relative humidity, and estimated
potential evapotranspiration (PET)………………………………………………….……….34
Table 7. 2012 Daily total precipitation, average relative humidity, and estimated
potential evapotranspiration (PET)…………………………………………………………..35
Table 8. 2013 Daily total precipitation, average relative humidity, and estimated
potential evapotranspiration (PET)…………………………………………………….…….36
Table 9. Percentage of total number of S. homoeocarpa isolates recovered for each
VCG from irrigation x cultivar experiment………………………………….…………..……56
Table 10. Frequency of Vegetative Compatibility Group (VCG) of Sclerotinia
homoeocarpa isolated from creeping bentgrass under three irrigation regimes from the
Hancock Turfgrass Research Center (East Lansing, MI) in 2011………………………..58
Table 11. Vegetative compatibility group (VCG) of Sclerotinia homoeocarpa isolated
from creeping bentgrass using three irrigation programs at the Hancock Turfgrass
Research Center (East Lansing, MI)………………………………………………………...59

vi

LIST OF FIGURES

Figure 1. 2011 Average dollar spot incidence among irrigation regimes……………......16
Figure 2. 2011 Average dollar spot incidence among creeping bentgrass cultivars…...17
Figure 3. 2011 Relative Area Under the Disease Progress Curve (RAUDPC) for
irrigation program and creeping bentgrass cultivar………………………………………...20
Figure 4. 2013 Average dollar spot incidence among irrigation regimes..………….…...22
Figure 5. 2013 Average dollar spot incidence among creeping bentgrass cultivars…...23
Figure 6. 2013 Relative Area Under the Disease Progress Curve (RAUDPC) for
irrigation program and creeping bentgrass cultivar………………………………………...25
Figure 7. Aerial photograph of the field trial site.............................................................52
Figure 8. Vegetative compatibility assay among isolates of Sclerotinia homoeocarpa on
potato dextrose agar amended with red food coloring after 7 days……………………...56

vii

CHAPTER I:
IMPACT OF IRRIGATION REGIME AND CREEPING BENTGRASS CULTIVAR ON
DOLLAR SPOT ON FAIRWAY TURF IN MICHIGAN

1

Abstract
Irrigation regime and cultivar selection can impact disease incidence on fairway
turfgrass. Improvements in dollar spot (Sclerotinia homoeocarpa FT Bennett) resistance
of various cultivars of creeping bentgrass (Agrostis stolonifera L.) have been achieved
based on breeding efforts. An integrated approach to dollar spot management has been
attained by combining irrigation regime and host cultivar. Three irrigation regimes in
combination with three creeping bentgrass cultivars were studied for interactive impacts
on dollar spot development over three years. Irrigation was applied in one of three
programs: 0500 h daily, 2200 h daily, or 2200 h twice weekly. Total seasonal irrigation
applications were within 1 cm for all irrigation treatments and were calculated and
calibrated to deliver approximately equivalent amounts of water weekly. The cultivars of
creeping bentgrass tested were ‘Declaration’ and ‘SRP-1WM’, considered resistant
cultivars, and ‘L-93’, a susceptible cultivar. Relative area under the disease progress
curve (RAUDPC) was calculated from dollar spot incidence data each year. In 2012,
dollar spot pressure was extremely low and no conclusions were made using this data.
However, in 2011 and 2013, plots irrigated at 2200 h daily exhibited significantly less
dollar spot than those irrigated at 2200 h twice weekly, regardless of cultivar of creeping
bentgrass. The least amount of dollar spot developed on ‘SRP-1WM’ in each year of the
trial. In 2011 and 2013, the ‘SRP-1WM’ plots receiving daily irrigation (0500 or 2200 h)
were not significantly different from each other at p = 0.05 but exhibited less dollar spot
than those receiving the 2200 h twice weekly program. The 2200 h daily irrigation
program resulted in less dollar spot than the 2200 h twice weekly irrigation program
when approximately equivalent amounts of water were applied weekly.

2

Introduction
Water is vital to life and consumption is continually scrutinized and even
regulated through legislation in some parts of the USA (Cisar et al., 2004). Restrictions
for recreational use of water, such as for turf management, have resulted from these
regulations
Texas:

(Southwest

Florida:

www.swfwmd.state.fl.us/conservation/restrictions/;

www.tceq.texas.gov/drinkingwater/trot/droughtw.html;

Sanderson,

2013).

Approximately 80% of maintained turfgrass on golf courses in the USA is irrigated,
where nearly 100% of greens, tees, and fairways are irrigated (Throssell et al., 2009).
Various conservation efforts are often voluntarily implemented by turfgrass managers,
including improvements to irrigation systems (Lyman et al., 2007). Water management
is just one facet of the complex of strategies used to maintain amenity turf. Other daily
practices include mowing. Pest management is also of great concern for turf managers,
often consuming large portions of maintenance budgets (Vargas, 2005).
Dollar spot is a plant disease found worldwide on most turfgrass species (Smiley
et al., 2005) and is caused by the fungus Sclerotinia homoeocarpa FT Bennett, although
its taxonomy is under contention (Kohn, 1979; Whetzel, 1945; Baldwin and Newell,
1992). Dollar spot is considered to be one of the most economically important diseases
on amenity turf due to costs associated with control (Vargas, 2005).
Symptoms of dollar spot in stands of turfgrass include blighted, circular spots
ranging in size up to 5 cm in diameter on fairways and putting greens, or 15 cm on
athletic fields or home lawns. The disease was named “dollar spot” due to the
resemblance of the spots on putting greens and fairways to the size of a silver dollar
(Bennett, 1937). Spots are often sunken and can coalesce, developing into larger

3

irregular-shaped areas (Smiley et al., 2005). White, cottony mycelia may be visible
under environmental conditions conducive to development of dollar spot and especially
in the presence of dew. On leaf blades, dollar spot lesions become chlorotic, watersoaked and turn white to tan-colored, with a reddish-brown perimeter in species other
than Poa annua (Vargas, 2005). Lesions can occur transversely across a leaf or may
spread longitudinally from the apex of the leaf.
Dollar spot is prevalent from late spring through the end of fall in the northern
USA (Vargas, 2005). The pathogen spreads by direct contact with neighboring plants
and by dissemination of infected clippings via equipment such as mowers, and by
humans and animals (Allen et al., 2005). The aerial mycelia infect the leaf on contact if
the host is susceptible and environmental conditions are conducive. The fungus
survives periods of unfavorable conditions as mycelia in infected plant tissue and debris
or as stromata. Infection occurs during periods of high humidity in the turf canopy with
temperatures typically between 15-30°C. Dollar spot is often worse under conditions of
low nitrogen fertility and soil moisture levels below field capacity (Smiley et al., 2005;
Vargas, 2005; Walsh et al., 1999).
Management of dollar spot is accomplished through the use of chemical, cultural,
biological, and genetic means. Although chemical management is among the most
common practices to reduce dollar spot, integrated management practices are
frequently employed (Vargas, 2005; Watkins et al., 2001). Successful dollar spot
management occurs with a combination of cultural practices and fungicide usage.
Biological control of dollar spot has been attained through the use of commercially
available products containing antimicrobial agents (biofungicides), however, biological

4

control products tend to be most successful under conditions of low to moderate
disease severity (Smiley et al., 2005; Vargas, 2005; Walsh et al., 1999). Commercially
available cultivars of creeping bentgrass (Agrostis stolonifera L.) have been developed
that are less susceptible to dollar spot as compared to older cultivars (NTEP, 2010;
Belanger et al., 2004; Bonos, 2006; Bonos et al., 2006). Because infection of plants by
S. homoeocarpa occurs under conditions where relative humidity exceeds 85% in the
canopy or in the presence of dew, dew removal is a cultural practice implemented to
manage dollar spot (Beard and Batten, 1982; Delvalle et al., 2011; Ellram et al., 2007;
Koch, 2012). Other cultural methods include rolling using a standard lightweight
turfgrass rolling machine (Giordano et al., 2012), fertility (Couch and Bloom, 1960;
Golembiewski and Danneberger, 1998; Landschoot and McNitt, 1997; Markland et al.,
1969; Watkins et al., 2001; Williams, et al., 1996), and removal of clippings (Williams et
al., 1996). Irrigation programming has been investigated for its impact on dollar spot
(Jiang et al., 1998; McDonald et al., 2006). The effect of leaf wetness on dollar spot has
been studied (Walsh et al., 1999; Williams et al., 1996).
The effect of irrigation timing, frequency, application volume, and leaf wetness on
various turfgrass diseases has been studied extensively (Couch and Bloom, 1960;
Fidanza and Dernoeden, 1996; Gross et al., 1998; McDonald et al., 2006; Roberts et
al., 2011; Watkins et al., 2001; Williams et al., 2001; Williams et al., 1996). Delvalle et
al. (2011) demonstrated that daily dew removal reduced the severity of dollar spot
compared with not removing dew and that dew removal extended the length of dollar
spot control using fungicides. Williams et al. (1996) investigated the effects of dew
removal in the morning (AM) at 0800 h and in the afternoon (PM) at 1300 h in

5

combination with fertility level and removal or return of clippings and found that AM dew
removal resulted in significant reduction in dollar spot as compared to PM removal.
Many recommendations from university extension websites offer conflicting
suggestions regarding the optimal time of day to irrigate turfgrass. Some of these
publications suggest nighttime irrigation while others encourage avoidance of nighttime
irrigation, citing an increased risk for disease potential. For the most part, according to
these recommendations, the best time to water lawns falls into one of two categories:
either water at nighttime or water in the morning. Some suggest the use of morning
irrigation based on the idea that it will dilute guttation water which formed on plants
overnight and can serve as a nutrient source for pathogens, and that nighttime irrigation
will encourage disease development (Dernoeden, 2003; U of NH Cooperative
Extension, 1994; Polomski and Shaughnessy, 1999; Fech, 2013; Voigt; Fresenburg,
2010; Pound and Street, 2013). Other extension articles suggest avoidance of morning
irrigation, especially if the leaf wetness (LW) period is extended, and instead to irrigate
at night because less evaporation occurs and winds are lower so water is presumed to
be applied more efficiently (Smith, 2008; Toski and Skinner, 2012; Swift, 1996; Peacock
and Bruneau, 2006). However, whether morning or nighttime irrigation was
recommended, most of these publications typically state that the watering regime is
most efficient when applied during less windy and cooler times of day so less
evaporation takes place. Interestingly, there seems to be a general consensus that an
important issue to consider when choosing when to water is that the LW period not be
extended, which is said to encourage disease development.

6

Two of the most common, and often disputed, approaches are deep and
infrequent or light and frequent irrigation scheduling. Deep and infrequent irrigation
typically refers to watering which generally occurs two times weekly or less and usually
involves applying enough water to penetrate into the soil approximately 3 cm beyond
the root zone. Daily irrigation has been shown to reduce turfgrass quality (Jordan et al.,
2003; Richie et al., 2002) and result in shallow rooting (Fu and Dernoeden, 2009;
Jordan et al., 2003; Qian et al., 1997). However, benefits of daily irrigation have been
reported which include improved turf quality (Fry and Huang, 2004, Fu and Dernoeden,
2009) and disease reduction (Jiang et al., 1998, McDonald et al., 2006, Melvin and
Vargas, 1994).
Frequency of irrigation affects plants in several ways. Qian and Fry (1996)
demonstrated that deep and infrequent irrigation resulted in more extensive rooting in
Zoysiagrass (Zoysia japonica Steud.) as compared with light daily applications.
Similarly, Fu and Dernoeden (2009) showed that creeping bentgrass under deep and
infrequent irrigation led to the production of roots with a larger surface area, greater
length, and were more numerous than turf irrigated on a light daily basis. Benefits of
daily irrigation have been reported as well. JinMin and Dernoeden (2009) investigated
watering frequency and found that watering creeping bentgrass on a light frequent basis
resulted in very good color and quality throughout most of the trial period, while the
deep infrequent program exhibited acceptable quality for one year only. In comparing
light daily irrigation to infrequent irrigation, Melvin and Vargas (1994) observed less
necrotic ring spot with daily irrigation as compared with infrequent irrigation programs.

7

McDonald et al. (2006) found deep and infrequent irrigation at 0600 h resulted in
higher incidence of dollar spot than light and daily irrigation at 2100 h. Conversely, Miller
(2012) demonstrated that deep and infrequently irrigated plots receiving 76 cm of
irrigation annually resulted in lower dollar spot incidence compared to plots receiving
60-70 cm annually. In a greenhouse study, Couch and Bloom (1960) found that turf
under higher levels of drought stress developed greater dollar spot incidence than turf
watered to levels approaching field capacity. In a comparison of two irrigation regimes
for their effect on dollar spot, Watkins et al. (2001) found that irrigation had no impact on
dollar spot. Applying irrigation at 0500 h, Jiang et al. (1998) showed that plots receiving
daily irrigation had twice the number of dollar spots compared with those irrigated to
80% of water lost to evapotranspiration (ET), which received 200% less water than the
daily irrigated plots, while brown patch was suppressed with daily irrigation. Fidanza and
Dernoeden (1996) discovered that morning irrigation (0500 h) consistently reduced
brown patch severity compared with evening irrigation (2200 h).
Researchers have investigated the effect ET has on irrigation of turfgrass and its
impact on disease and quality. Evapotranspiration is the sum of amount of water lost
from soil by evaporation and from plants by transpiration. Potential ET (PET) is the
amount of water that would be lost by evaporation and transpiration if water was not
limited in the system. PET can be used a predictor of irrigation requirements for crops.
Numerous researchers have demonstrated that irrigation to 80% ET has resulted in
acceptable turfgrass quality (DaCosta and Huang, 2006; Jiang et al., 1998; Richie et al.,
2002; Roberts et al., 2011). Roberts et al. (2011) reported that plots irrigated to 80% ET
typically exhibited the least amount of anthracnose and best turfgrass quality. Dacosta

8

and Huang (2006) showed that turfgrass quality was maintained at an acceptable level
throughout the summer in New Jersey when turf was irrigated to 80% ET while in the
fall of the year, 40% ET replacement maintained acceptable quality. Jiang et al. (1998)
showed that plots receiving daily irrigation suppressed brown patch (Rhizoctonia solani)
and had twice the number of dollar spot infection centers compared with those irrigated
to 80% ET.
Creeping bentgrass is the most common cool season turfgrass used on golf
courses (Lyman et al., 2007). Cultivars differ in susceptibility and may influence disease
development. Turfgrass breeding efforts have led to improvements in disease
resistance among many species of turfgrass (Belanger et al., 2004; Bonos, 2006; Bonos
et al., 2006). Cultivars of creeping bentgrass have varying levels of susceptibility to
diseases such as dollar spot (Casler et al., 2007; Cole et al., 1969; Settle et al., 2001)
as well as differences in speed of recovery from diseases (Settle, et al., 2001; Vincelli et
al., 1997). Settle et al. (2001) and Abernathy et al. (2001) identified ‘L-93’ as having
higher dollar spot resistance than other cultivars in trials. Cole et al. (1969) revealed
differences in dollar spot susceptibility and infection center diameter among creeping
bentgrass cultivars.
The use of disease-resistant cultivars of creeping bentgrass may lead to a
reduction in fungicide application rates or total number of fungicide applications made
(Settle et al., 2001). Making curative rather than preventive fungicide applications for the
control of dollar spot resulted in fewer fungicide applications made in more resistant
creeping bentgrass cultivars by Settle et al. (2001). Vincelli et al. (1997) found that
numerous cultivars of creeping bentgrass showed improved recovery from dollar spot

9

while many exhibited poor recovery. This indicates tremendous potential for reduced
fungicide inputs when the use of less susceptible cultivars or those with faster
recuperative potential are selected.
Many disease-resistant cultivars of creeping bentgrass with differing levels of
resistance are commercially available; researchers have investigated the use of
blended plantings. Abernathy et al. (2001) demonstrated that the use of resistant
cultivars of creeping bentgrass in a blend suppressed dollar spot while the use of
susceptible cultivars increased dollar spot. More recently, development of interspecific
hybrids of dollar spot resistant creeping bentgrass and colonial bentgrass (Agrostis
capillaris L.) created by Belanger et al. (2004) yielded hybrids with excellent dollar spot
resistance in field tests. Based on 2010 National Turfgrass Evaluation Program (NTEP)
progress report data, creeping bentgrass cultivars ‘Declaration’ and ‘SRP-1WM’ were
considered to have relatively high resistance to dollar spot, while ‘L-93’ was
comparatively more susceptible (NTEP, 2010). ‘Crenshaw’ is highly susceptible to dollar
spot (NTEP, 2002).
Both irrigation program and cultivar selection can impact disease development;
these factors were investigated in an integrated approach aimed at reducing dollar spot
in creeping bentgrass. The objective of this research was to study the impact of three
irrigation programs, specifically irrigating at 0500 h daily (daily morning), at 2200 h daily
(daily nighttime), or at 2200 h twice weekly (infrequent nighttime), in combination with
three creeping bentgrass cultivars (‘Declaration’, ‘SRP-1WM’, ‘L-93’) on DS incidence
on fairway turf in East Lansing, Michigan.

10

Materials and Methods
Research was conducted at the Hancock Turfgrass Research Center on the
campus of Michigan State University in East Lansing, MI from Jun 2011 through Sep
2013. Field plots were established in Oct 2010 on a renovated Colwood-Brookstone
loam soil site. Nine 11 x 11 m irrigation plots were set up, each of which possessed a
Toro TR50 sprinkler head (The Toro Company, Riverside, CA) located in plot corners.
Within each irrigation plot, twelve plots measuring 2.7 x 3.7 m were established; four
plots were seeded to one of three creeping bentgrass (Agrostis stolonifera L.) cultivars
on 8 Oct 2010. Buffer strips surrounding irrigation plots were seeded to the dollar spotsusceptible cultivar ‘Crenshaw’. Plots were mowed three times weekly at approximately
13 mm with clippings returned.
-1

During the trial period in 2011, the study received a total of 146 kg N ha

from

either 18-9-18 or 24-2-12 granular fertilizer monthly. The herbicide Mec Amine-D (2,4-D,
-1

Mecoprop-p and Dicamba) (Loveland Products, Greeley, CO) was applied at 30 L ha

on 21 Jun 2011 for the control of broadleaf weeds. Insects were controlled with carbaryl
(Sevin; Bayer Environmental Science, Research Triangle Park, NC) on 21 Jul 2011,
-1

applied at 50 L ha

for the control of black cutworms (Agrotis ipsilon).

Using a modified version of the Goodman and Burpee (1991) technique, the
-1

research site was inoculated with 210 kg ha

of sand/cornmeal topdressing inoculum

mixture on 17 Jun 2011 in order to encourage uniform disease development. A mixture
of silica sand and cornmeal (Quaker Oats Co., Chicago, IL) (2:1, v/v) and 2.4% potato
dextrose broth (Becton, Dickinson and Co, Sparks, MD) (5% v/v) was placed in 15 x 25
11

cm aluminum baking pans, covered with two layers of aluminum foil and placed into a
second aluminum baking pan. The sand-cornmeal mixture was autoclaved at 121 C for
-1

45 minutes. S. homoeocarpa on potato dextrose agar (PDA: 39 g PDA L

water;

Becton, Dickinson and Co, Sparks, MD ) in 9 cm petri dishes was cut into 30-100 pieces
and placed on the sterile sand-cornmeal. After incubation at 22 ± 2 C for 2 weeks, the
media was chopped and kneaded by hand to break down clumps. The inoculum used
was a mixture of equal proportions comprised of an isolate of S. homoeocarpa FT
Bennett representing each of six different vegetative compatibility groups (VCGs) of the
fungus which had previously been isolated from MI turfgrass affected by DS by Powell
(1998). The blended sand-cornmeal inoculum was applied to field plots at a rate of 2.1
kg 100 m

-2

using a Gandy lawn spreader (Model 36H13, Gandy Co, Owatonna, MN).

These VCGs were designated VCG A through VCG F.
The study was fertilized monthly in 2012 using 18-3-18 granular fertilizer totaling
-1

122 kg N ha

for the study duration. The herbicide amicarbazone (Xonerate; Arysta
-1

LifeScience North America LLC, Cary, NC) was applied to the study site at 360 ml ha

in an effort to reduce Poa annua infiltration. To alleviate thatch build up in the turf, on 17
Aug 2012, plots were sand topdressed and the sand was worked into the turf using a
greens mower (John Deere and Co., Moline, IL) fitted with Vibe V vibratory greens
rollers (Turfline, Inc., Moscow Mills, MO). This was done to reduce scalping in the plots
caused by mowing.
In 2012, no inoculations were planned. Instead, the fungal population sampled in
2011 was to be compared to that from 2013 to study impacts of treatment application on

12

the fungal population during that time period. However, due to lack of disease in the
study site by early Sep 2012, the plots were re-inoculated on 13 Sep 2012 by spreading
-1

turfgrass clippings collected from an infested fairway at 40 kg ha

using an Andersons

SR2000 rotary fertilizer spreader (The Andersons, Inc., Maumee, OH). The clipping
method was used in 2012 because it took considerably less time to prepare than the
Goodman method employed in 2011.
-1

The study received 120 kg N ha

annually in 2013 using 18-3-18 granular

fertilizer applied monthly during the trial period. On 29 Aug 2013, the plant growth
regulator Trinexapac-ethyl (Primo Maxx; Syngenta Crop Protection, Inc, Greensboro,
-1

NC) was applied at 440 ml ha .
On 6 Jun 2013, the study site was inoculated using sand/cornmeal topdressing
-1

inoculum at 160 kg ha

as described for 2011. The topdressing mixture was infested

with S. homoeocarpa VCG B only due to high virulence exhibited in laboratory and
greenhouse screening by VCG B. Additionally, Deng et al (2002), Viji et al (2004), and
DeVries (2006) found VCG B to be the most predominant in populations, while Powell
and Vargas (2001) found VCG B to be among the most common VCG recovered from
field isolations in Michigan.
Three irrigation programs were tested. Irrigation treatments were applied at 0500
h daily (daily morning), 2200 h daily (daily nighttime) or 2100 h twice weekly (infrequent
nighttime) to three replicate plots. Toro TR50 sprinklers were located in plot corners
and delivered 0.318 cm per 10 minutes to irrigation plots. Treatments applied on a daily
basis were designed to replace approximately 80% PET. This rate was chosen based
13

on the results of researchers who demonstrated disease reduction based on 80% ET
replacement (Jiang et al., 1998; Roberts et al., 2011). PET data were obtained from the
Enviro-weather

Automated

Weather

(http://www.agweather.geo.msu.edu/mawn),

Station
formerly

known

Network
as

the

website
Michigan

Automated Weather Network (MAWN). The PET recorded from the website was used
as a reference PET and was calculated based on the FAO Penman-Monteith equation
(Allen et al., 1998.) Daily irrigation applications to replace 80% ET were adjusted twice
weekly, similar to the technique used by Watkins et al. (2001). Average daily irrigation
applications totaled 0.3, 0.4, and 0.2 cm in 2011, 2012, and 2013, respectively.
Infrequent nighttime applications were made twice each week from Jun through midSep, except as described below. Seasonal irrigation applications for daily irrigated plots
were 31.2, 22.7, and 20.4 cm, and for infrequent irrigation, season totals were 29.9,
22.0, and 20.5 cm in 2011, 2012, and 2013, respectively. Approximately equivalent
amounts of water were applied to all irrigation treatments weekly. Plots were not
irrigated on 25-26 Jun and 27-28 Aug 2011, 10-15 Aug 2012, or 23-24 Aug 2013 due to
heavy precipitation. Seasonal precipitation amounts during the trial period were 18.2,
11.4, and 16.7 cm for 13 Jun – 23 Sep 2011, 8 Jul – 11 Sep 2012, and 13 June – 7 Sep
2013, respectively.
The study site was renovated and plots were seeded in Oct 2010. Creeping
bentgrass cultivars selected for this trial included both dollar spot resistant (‘Declaration’
and ‘SRP-1WM’) and DS susceptible (‘L-93’) representatives. These cultivars were
chosen based on 2010 NTEP results (NTEP, 2010). Areas surrounding the irrigation

14

plots were seeded to ‘Crenshaw’, a highly susceptible cultivar, in order to promote the
establishment and spread of dollar spot around the study site.
Plots were rated for dollar spot 10, 3, and 8 times in 2011, 2012, and 2013,
respectively. Plots were visually rated for dollar spot incidence on a 0 to 100% scale,
where 0 = no disease observed and 100 = entire plot area blighted. Area under the
disease progress curve (AUDPC) values were calculated using the formula AUDPC =

∑

where

percent plot area blighted), and

= 1, 2, 3…

-1,

is the amount of disease (ie.

is the th rating (time in days) (Campbell and Madden,

1990). AUDPC is a measure of the average percent plot area blighted times the number
of days between the ratings.

Relative AUDPC (RAUDPC) was calculated for each

season by dividing the AUDPC by the maximum potential AUDPC which is 100 times
the trial duration in days (ie. 100*total trial days), and then multiplying by 100 to expand
the scale. (Olanya and Campbell, 1990). This allows for simple comparisons across
experiments since all data are converted to a 0-100 scale.
The trial consisted of a 3 x 3 factorial arrangement of treatments (three levels of
irrigation regime and three levels of creeping bentgrass cultivar) in a randomized
complete block design with split plots. Irrigation program was the whole plot treatment
with three replications, and creeping bentgrass cultivar was the sub plot treatment with
four replications within each whole plot. Disease data for 2011 and 2013 were square
root transformed, but actual means for RAUDPC values are provided in the data tables.
Data were subjected to analysis of variance (ANOVA) (least squares method) using
JMP 10.0.0 (SAS Institute Inc., Cary, NC). Means were separated using Tukey’s
Honestly Significant Difference test (p=0.05).
15

Results
In 2011, dollar spot developed slowly through Jul and early Aug, and peaked in
early Sep. Average dollar spot incidence data by irrigation program and by cultivar are
provided in Figures 1 and 2. The main effects of irrigation regime, cultivar, and the
interaction between the two were significant (Table 1).

Figure 1. 2011 Average dollar spot incidence among irrigation regimes. Values
represent mean percent of plot area blighted by dollar spot averaged over all cultivars
(n=36). Error bars represent the standard error of the mean (SEM).

50
40
30
20
10

9/
8
9/
17
9/
23

9/
2

8/
8
8/
18
8/
26

8/
1

0
7/
13
7/
20

% plot area blighted

60

Rating date
Infrequent nighttime
Daily morning
Daily nighttime

16

Figure 2. 2011 Average dollar spot incidence among creeping bentgrass cultivars.
Values represent mean percent of plot area blighted by dollar spot averaged over all
irrigation treatments (n=36). Error bars represent the standard error of the mean (SEM).

60
40
20
0

7/
13
7/
20
8/
1
8/
8
8/
18
8/
26
9/
2
9/
8
9/
17
9/
23

% plot area blighted

80

Rating date
'L-93'
'Declaration'
'SRP-1WM'

17

The interaction of cultivar ‘L-93’ with daily nighttime irrigation resulted in
significantly less dollar spot than with infrequent nighttime irrigation even though
approximately equivalent volumes of water were applied weekly (Table 1). Likewise, the
interaction of ‘Declaration’ with daily nighttime irrigation resulted in significantly less
dollar spot than ‘Declaration’ with infrequent nighttime irrigation. ‘SRP-1WM’ with daily
nighttime irrigation resulted in significantly less dollar spot than ‘SRP-1WM’ with
infrequent nighttime irrigation (Figure 3). Daily nighttime irrigation programs exhibited
significantly less dollar spot in ‘L-93’ and ‘Declaration’ than the daily morning irrigation
programs for ‘L-93’ and ‘Declaration’, but those same irrigation treatments for ‘SRP1WM’ were not statistically different (Table 1.) Plots receiving daily morning irrigation
were not significantly different than those irrigated infrequently during the night for all
cultivars (Table 1).
The RAUDPC was calculated for the period from 13 Jul through 23 Sep 2011, a
total of 71 days, and ranged from 1.02 to 40.63. RAUDPC values indicated that daily
nighttime irrigation programs had significantly lower incidence of dollar spot than daily
morning irrigation or infrequent nighttime irrigation programs. Over the duration of the
season, ‘SRP-1WM’ exhibited the lowest RAUDPC dollar spot value among all cultivars,
while ‘Declaration’ developed significantly more dollar spot than ‘SRP-1WM’ but less
than ‘L-93’. ‘L-93’ was the most susceptible cultivar, resulting in significantly higher
RAUDPC dollar spot levels among all cultivars over all irrigation programs.

18

Table 1. The effect of irrigation regime and creeping bentgrass cultivar on dollar
spot in 2011.
RAUDPC ((AUDPC/Maximum Potential AUDPC) x 100)
'L-93'
'Declaration'
'SRP-1WM'
b

Infrequent nighttime
Daily morning
Daily nighttime

40.63 a
37.07 a
28.43 b

Irrigation Program
Infrequent nighttime
Daily morning
Daily nighttime

23.26 a
19.24 b
13.11 c

Cultivar
'L-93'
'Declaration'
'SRP-1WM'

35.38 a
16.27 b
3.96 c

Analysis of Variance
Irrigation (I)
Cultivar (C)
IxC
a

21.13 c
17.81 c
9.87 d

a

8.01 de
2.85 ef
1.02 f

<.0001
<.0001
0.0428

Data were analyzed using a square root transformation, but actual means are
shown.
b
For main effects of Irrigation Program and Cultivar, means in the same column
followed by the same letter are not significantly different while interactions are
compared among all treatments at p=0.05 according to Tukey’s HSD.

19

Figure 3. 2011 Relative Area Under the Disease Progress Curve (RAUDPC) for
irrigation program and creeping bentgrass cultivar. The effect of irrigation
scheduling program on dollar spot in three creeping bentgrass cultivars measured as
RAUDPC from 13 Jul through 23 Sep 2011, a total of 71 days. RAUDPC values were
the calculated means (n=12) of each irrigation x cultivar interaction and were calculated
as described in the text. Bars sharing common letters are not significantly different at
p=0.05 for all interactions (Tukey’s HSD).

50
a

RAUDPC

40

a
b

30

c

20

c
d

10

de
ef

0

'L-93'

'Declaration'

'SRP-1WM'

Creeping Bentgrass Cultivar
Infrequent nighttime
Daily morning
Daily nighttime

20

f

Dollar spot pressure was extremely low for the 2012 season. On average, less
than 1% plot area was blighted with dollar spot (based only on three rating dates, data
not shown). As a result, RAUDPC was extremely low (0.00 to 0.81).

Although

statistically significant, dollar spot epidemics in 2012 did not reach levels to which
informative or conclusive inferences could be made, and thus will not be discussed or
presented.
Disease pressure developed steadily through Jul and early Aug in 2013 and
continued to build into early Sep (Figures 4 and 5). No significant interaction between
irrigation program and cultivar was found. When applying approximately the same
weekly volume of water, daily irrigated plots, whether morning or nighttime, developed
significantly less dollar spot than those irrigated infrequently during the night for all
cultivars in 2013 (Table 2). Dollar spot incidence was lower under a daily nighttime
irrigation regime as compared with daily morning watering (Table 2). Similar to results
from 2011, ‘L-93’ was the most susceptible cultivar, exhibiting the highest level of dollar
spot. ‘SRP-1WM’ was consistently the most resistant cultivar in the study (Table 2).

21

Figure 4. 2013 Average dollar spot incidence among irrigation regimes. Values
represent mean percent of plot area blighted by dollar spot averaged over all cultivars
(n=36). Error bars represent the standard error of the mean (SEM).

50
40
30
20
10

Rating date
Infrequent nighttime
Daily morning
Daily nighttime

22

9/
18

9/
5

8/
18

8/
10

8/
1

7/
24

7/
7

0

7/
1

% plot area blighted

60

Figure 5. 2013 Average dollar spot incidence among creeping bentgrass cultivars.
Values represent mean percent of plot area blighted by dollar spot averaged over all
irrigation treatments (n=36). Error bars represent the standard error of the mean (SEM).

60
40
20

Rating date
'L-93'
'Declaration'
'SRP-1WM'

23

9/
5
9/
18

8/
1
8/
10
8/
18

7/
7
7/
24

0
7/
1

% plot area blighted

80

Table 2. The effect of irrigation regime and creeping bentgrass cultivar on dollar
spot in 2013.
RAUDPC ((AUDPC/Maximum
a
Potential AUDPC) x 100)
Irrigation
b
Infrequent nighttime
29.09 a
Daily morning
21.65 b
Daily nighttime
18.04 c
Cultivar
'L-93'
38.58 a
'Declaration'
16.66 b
'SRP-1WM'
13.53 c
ANOVA
Irrigation (I)
<.0001
Cultivar (C)
<.0001
IxC
>0.05
a

Data were analyzed using a square root transformation, but actual means are
shown.
b
For main effects of Irrigation Program and Cultivar, means in the same column
followed by the same letter are not significantly different while interactions are
compared among all treatments at p=0.05 according to Tukey’s HSD.

The RAUDPC values were calculated for the season from 1 Jul through 18 Sep
2013 and ranged from 13.53 to 29.02 (Table 2). When comparing the effect of irrigation
regime on each cultivar independently, both of the daily irrigation programs resulted in
significantly less dollar spot incidence than those irrigated infrequently at night for ‘L-93’
and ‘SRP-1WM’ (Figure 6). For ‘Declaration’, the daily nighttime watering program had
significantly less dollar spot than all other irrigation treatments, although plots irrigated
daily in the morning had significantly less dollar spot than those on the infrequent
nighttime program (Figure 6). Within each cultivar, infrequent nighttime irrigation
resulted in significantly higher dollar spot levels than either of the daily irrigation
programs (Figure 6).

24

Figure 6. 2013 Relative Area Under the Disease Progress Curve (RAUDPC) for
irrigation program and creeping bentgrass cultivar. The effect of irrigation
scheduling program on dollar spot in three creeping bentgrass cultivars measured as
RAUDPC from 1 Jul through 18 Sep 2013, a total of 79 days. RAUDPC values were the
calculated means (n=12) of each irrigation x cultivar combination and were calculated
as described in the text. Since there was no significant interaction between irrigation
regime and cultivar, comparisons among irrigation programs were made within each
cultivar. Bars sharing common letters are not significantly different at p=0.05 for all
interactions (Tukey’s HSD).

60

RAUDPC

50
40

a
b

b

30
a

20

b

a
c

b

10
0

'L-93'

'Declaration'

'SRP-1WM'

Creeping Bentgrass Cultivar
Infrequent nighttime
Daily morning
Daily nighttime

25

b

Discussion
Application of equivalent weekly volumes of irrigation to a fairway in MI on a daily
basis, as compared to an infrequent basis, resulted in a significant reduction in dollar
spot incidence. This conclusion supports work conducted by McDonald et al. (2006) that
showed that plots receiving daily irrigation had lower incidence of dollar spot when
compared to infrequent irrigation. Although the frequency of application was either daily
or infrequent (twice weekly) in each of the studies, the time of day for the applications
and the total weekly amount of water applied differed in that study. McDonald et al.
(2006) compared light and frequent (daily) irrigation applications made at 2100 h with
infrequent applications made at 0600 h; there was no comparison made between
different irrigation frequencies made at the same time of day. Additionally, efforts to
apply the same amount of water to frequent and infrequent irrigation plots was
attempted for the first year of the study, after which irrigation amounts applied to each
treatment varied due to high amounts of natural precipitation which affected application
parameters for the study (McDonald et al., 2006).
In evaluating the effects of irrigation frequency on dollar spot severity in the field,
the findings of Jiang et al. (1998) and Watkins et al (2001) were not supported. Jiang et
al. (1998) reported that severity of dollar spot was enhanced by daily irrigation
compared with irrigation 3 times weekly at a level to return 80% evapotranspiration
(ET); however, irrigation volumes were 200% higher in the plots that were irrigated daily
compared to those irrigated 3 times weekly. This large difference in water application
may have influenced the outcome of the study. Efforts were made in the current study to
maintain equivalent water applications to all plots on a weekly basis and the outcomes

26

suggested that incidence of dollar spot was impacted by the time of day or frequency of
irrigation applications rather than volume of water applied.
The results of this study demonstrated that nighttime irrigation on a daily basis
resulted in a reduction in dollar spot. Benefits reported for nighttime irrigation include a
reduction in loss of water from evaporation than daytime irrigation, providing more
efficient use of irrigation water (Smith, 2008). Nighttime irrigation may also dilute and
displace dew on turf plants. The extension of the duration of the leaf wetness period in
turf has been shown to increase disease incidence (Gross et al., 1998; Uddin et al.,
2003; Vargas, et al., 1993). However, if dew is already present as was the case in this
study, the duration of the leaf wetness period would not be extended by watering at
2200 h as would potentially occur by watering at 1700 h, for example. The duration of
the leaf wetness period would be extended in the case of late day or evening irrigation
which could lead to increased disease potential. In Kentucky, Williams et al. (1998)
showed that leaf wetness duration did not appear to be reduced by any significant factor
when dew was removed prior to 0400 h. In that trial, dew was already present on the
turf at 2200 h.
The development of improved cultivars of turfgrass with greater levels of disease
resistance has impacted disease epidemics. Research focusing on development of
dollar spot on resistant creeping bentgrass cultivars has increased the number made
commercially available. Results indicate that since ‘L-93’ is more susceptible than either
‘Declaration’ or ‘SRP-1WM’, choosing a dollar spot resistant cultivar may allow turfgrass
managers to reduce fungicide inputs for dollar spot management. By combining this
potential with careful cultural management practices, it is anticipated that even greater

27

reductions in pesticide use could be attained. The importance of cultivar selection
should not be underestimated when designing integrated disease management
programs.
By utilizing less susceptible creeping bentgrass cultivars and combining this
effort with an irrigation program designed to reduce disease incidence, less dollar spot
was achieved in this study. The direct comparison of daily versus infrequent irrigation,
with time of day and weekly volume applied held equivalent, indicated that daily
irrigation resulted in less dollar spot than infrequent irrigation. Although infrequent
nighttime and daily morning watering programs were similar in 2011 and significantly
different in 2013, with daily morning irrigation resulting in less dollar spot, this
comparison is not entirely justified since time of day of application was different. A better
comparison would have been to include an infrequent morning program to compare with
daily morning programs. However, due to space limitations, this was not possible in the
current trial but would be interesting to investigate in future studies. Irrigating daily as
compared with infrequently at 2200 h using the same weekly volume of water resulted
in less dollar spot on fairway turf in MI. Results were not conclusive when comparing
morning and nighttime daily irrigation regimes since differences occurred for some
cultivars but not others between the 2011 and 2013 seasons. This could be due to
variation in evapotranspiration rates among cultivars.
It is possible that watering at 2200 h diluted guttation fluids and delayed fungal
growth as compared with irrigating at 0500 h. Daily irrigation may prevent drought stress
in turfgrass while plants under higher drought stress are more susceptible to disease.
Additionally, plots receiving daily irrigation may experience an elevation in bacterial

28

populations compared with those receiving infrequent irrigation. Those plots being
watered daily may avoid a dry down period experienced in plots receiving less frequent
irrigation programming. Moisture is highly influential with regards to soil bacterial
populations, some of which can be inhibitory to pathogens.
Due to reduced dollar spot incidence observed in this study, daily nighttime
watering would be the preferred method of irrigation where dollar spot is the major
disease. Additionally, the amount of dollar spot may be reduced with the use of resistant
cultivars. Combining host resistance with daily nighttime irrigation could possibly reduce
the use of fungicides for dollar spot by reducing disease incidence, thereby having a
positive impact on the environment and budget for crop protection.

29

APPENDIX

30

APPENDIX

Table 3. 2011 Daily average maximum/minimum air temperatures.
June
Air Max Air Min
(°F)*
(°F)*
74.8
61.4
71.0
50.5
75.6
51.6
89.4
60.7
85.4
56.8
84.7
55.6
92.9
73.8
91.4
74.9
82.4
57.4
60.0
52.9
75.2
58.1
66.2
50.3
78.4
52.2
76.5
50.1
67.3
47.5
73.0
57.7
77.8
53.2
82.6
56.0
80.2
56.6
76.0
59.8
86.8
66.8
79.9
63.9
70.1
62.1
64.5
58.3
77.7
57.4
81.1
52.2
78.3
58.1
70.9
61.3
78.7
53.3
83.0
51.4

July
August
September
Air Max Air Min Air Max Air Min Air Max Air Min
Day
(°F)
(°F)
(°F)
(°F)
(°F)
(°F)
1
85.6
58.4
88.8
71.6
89.1
64.1
2
91.6
68.9
85.7
68.3
90.5
71.1
3
86.7
66.1
84.0
70.2
88.7
68.5
4
85.4
62.7
80.8
64.9
73.4
58.9
5
87.6
57.7
84.5
61.9
59.5
48.1
6
86.1
67.6
80.8
69.9
66.6
46.2
7
79.6
56.5
81.1
66.3
67.2
48.9
8
86.1
56.8
81.2
-64.4
55.6
9
89.4
60.2
79.6
63.2
76.7
59.1
10
89.0
66.2
73.9
60.4
78.8
60.1
11
80.1
66.8
77.8
53.3
77.6
57.5
12
86.1
66.2
78.7
55.2
81.8
58.9
13
78.1
59.4
79.5
60.6
76.4
54.2
14
80.0
51.5
72.9
58.4
65.4
43.8
15
84.5
61.8
79.2
56.5
59.8
38.9
16
90.9
57.4
83.3
53.5
58.9
36.3
17
91.2
64.8
80.6
55.4
64.8
45.9
18
90.4
74.6
82.8
62.7
69.3
43.8
19
93.0
72.6
83.4
53.3
66.7
55.9
20
92.9
70.3
80.7
59.8
71.0
46.5
21
93.9
72.0
76.7
56.5
74.7
51.9
22
82.0
65.7
76.2
49.3
67.4
49.8
23
86.8
67.7
80.3
53.2
63.9
50.9
24
86.3
67.2
87.8
65.5
64.8
43.9
25
86.9
71.4
76.9
58.9
70.1
49.5
26
82.5
65.0
79.5
51.5
69.3
49.1
27
77.4
56.5
81.9
55.9
65.7
43.8
28
86.1
69.2
75.1
54.8
61.7
52.0
29
87.2
70.4
76.4
47.9
62.4
49.3
30
88.2
62.4
77.2
53.7
52.3
39.9
88.0
64.2
79.9
61.2
*Air temperature taken at 1.5 m from ground.
Source: Enviro-weather (http://www.agweather.geo.msu.edu/mawn/station.asp?id=htc)

31

Table 4. 2012 Daily average maximum/minimum air temperatures.

Day
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30

June
Air Max Air Min
(°F)*
(°F)*
53.0
46.1
70.2
46.1
75.7
56.8
71.2
52.2
68.5
49.1
77.5
43.8
78.7
51.1
80.5
49.3
84.5
62.7
88.2
58.7
80.3
64.8
74.5
57.7
72.7
47.5
79.0
47.8
85.7
53.3
86.9
62.6
80.0
59.6
79.6
57.6
91.5
73.4
90.1
71.3
84.8
62.9
80.5
58.2
82.7
52.8
84.2
64.7
73.8
54.7
80.7
46.6
86.7
52.7
94.5
66.2
87.0
64.5
87.5
65.0

July
Air Max Air Min
(°F)
(°F)
90.8
61.0
94.7
61.0
92.4
70.1
96.9
72.4
95.7
68.4
100.9
68.9
92.3
73.2
83.2
63.9
86.2
57.5
82.6
61.1
85.2
54.7
88.0
53.9
90.4
59.4
90.3
63.8
88.4
68.0
92.6
65.9
95.9
75.2
84.7
68.0
74.8
65.9
80.5
60.0
85.1
53.9
89.2
65.1
93.4
71.7
83.2
65.6
88.4
55.8
84.5
69.3
79.3
65.7
80.9
59.9
85.5
53.8
87.6
59.6
85.4
62.7

August
Air Max Air Min
(°F)
(°F)
83.8
60.6
89.0
61.7
94.2
66.0
89.4
68.1
81.8
66.1
83.1
50.7
86.4
54.5
83.7
65.1
71.4
58.6
62.5
57.7
72.2
57.2
76.5
55.9
70.9
60.0
78.0
60.6
82.6
56.9
75.9
60.9
73.4
57.1
75.1
44.9
77.6
51.0
75.6
49.2
78.1
49.2
80.3
49.1
86.9
50.4
88.2
58.6
89.7
58.6
88.7
63.8
84.4
67.9
78.9
58.6
81.2
50.7
84.2
51.6
90.9
68.1

September
Air Max
Air Min
(°F)
(°F)
80.8
61.2
83.2
64.2
89.9
60.2
83.9
66.1
79.0
63.9
83.7
63.7
76.4
57.8
68.0
52.9
71.2
47.9
74.0
41.8
79.1
51.0
82.4
55.4
78.4
55.7
70.4
51.5
73.6
41.6
76.3
47.9
75.7
50.7
64.4
43.5
65.0
38.8
68.3
47.7
67.1
43.4
58.7
44.2
55.8
40.6
61.9
37.5
67.7
49.5
71.3
49.6
65.0
47.2
69.2
45.7
71.7
40.3
64.4
44.8

*Air temperature taken at 1.5 m from ground.
Source: Enviro-weather (http://www.agweather.geo.msu.edu/mawn/station.asp?id=htc)

32

Table 5. 2013 Daily average maximum/minimum air temperatures.
June
Air Max Air Min
(°F)*
(°F)*
76.3
63.3
67.4
50.1
67.6
40.0
71.1
48.1
68.7
50.4
68.0
54.7
67.2
49.8
73.4
46.2
76.3
57.1
73.4
59.9
79.4
62.1
73.6
59.1
78.4
60.3
76.9
55.2
75.9
54.6
78.6
62.6
83.8
58.0
70.6
52.9
75.3
45.9
81.2
51.6
84.8
59.6
86.4
67.2
87.0
68.7
85.5
66.2
81.1
65.6
81.2
62.5
84.3
65.1
78.3
62.9
73.1
62.6
77.4
60.2

July
August
September
Air Max Air Min Air Max Air Min Air Max Air Min
Day
(°F)
(°F)
(°F)
(°F)
(°F)
(°F)
1
72.4
55.8
75.7
60.7
81.9
65.3
2
72.0
55.3
70.7
54.3
71.5
58.4
3
80.9
59.5
76.7
53.0
73.6
56.6
4
78.0
59.5
73.6
56.2
78.2
51.5
5
84.2
62.9
69.8
50.2
70.4
50.8
6
83.3
65.7
78.1
61.3
75.1
46.0
7
81.4
67.3
81.4
63.0
79.0
57.7
8
80.2
67.4
77.2
59.0
72.6
57.1
9
86.3
65.9
78.7
53.0
81.9
54.4
10
84.4
66.8
79.5
58.8
90.2
70.2
11
79.6
57.8
76.9
57.5
86.3
69.3
12
80.2
53.1
71.4
61.8
77.0
55.7
13
82.5
56.5
67.8
53.2
59.8
40.8
14
88.2
61.6
70.3
48.6
66.9
38.1
15
92.1
67.7
72.1
44.9
59.3
46.0
16
89.8
72.2
78.8
48.1
61.9
44.9
17
90.8
72.2
78.8
48.6
67.1
33.4
18
90.6
71.5
81.4
51.6
75.2
44.3
19
91.1
74.2
80.6
53.1
77.8
57.5
20
84.4
69.2
83.4
58.7
73.2
65.5
21
82.6
61.7
84.7
63.5
66.1
53.8
22
80.3
65.6
78.9
64.2
54.6
39.3
23
79.5
58.4
80.6
58.9
62.1
35.0
24
72.7
51.6
81.4
52.7
67.4
35.8
25
77.2
49.3
83.8
56.6
73.6
39.3
26
77.4
57.5
83.5
68.0
76.0
38.3
27
74.1
55.1
86.4
69.6
76.5
40.7
28
64.4
49.7
82.2
66.7
76.6
45.4
29
68.1
55.7
88.4
61.4
68.4
47.7
30
79.3
46.9
85.1
66.5
69.8
44.9
67.2
63.1
78.3
65.9
*Air temperature taken at 1.5 m from ground.
Source: Enviro-weather (http://www.agweather.geo.msu.edu/mawn/station.asp?id=htc)

33

Table 6. 2011 Daily total precipitation, average relative humidity, and estimated potential
evapotranspiration (PET).
June

Day

Tot.
Ppt.a

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30

--------0.01
0.44
----0.05
0.51
----0.24
0.26
0.02
0.05
-------

Avg.
Rel.
Hum.b
44.7
49.7
61.5
60.2
54.7
57.7
58.5
52.1
69.0
83.8
80.6
68.2
47.7
53.9
77.6
75.3
74.4
67.3
62.2
77.3
78.9
78.1
80.4
82.7
62.3
60.3
72.7
67.0
60.5
65.7

July

August

September

Est.
PETc

Tot.
Ppt.

Avg.
Rel.
Hum.

Est.
PET

Tot.
Ppt.

Avg.
Rel.
Hum.

Est.
PET

Tot.
Ppt.

0.27
0.20
0.19
0.22
0.22
0.21
0.24
0.27
0.13
0.05
0.11
0.11
0.22
0.22
0.10
0.14
0.12
0.21
0.20
0.11
0.16
0.14
0.08
0.07
0.21
0.21
0.12
0.17
0.22
0.18

----------0.34
------0.20
---0.03
0.02
---1.28
1.62
1.61
--

64.8
67.9
60.2
49.5
58.2
53.4
59.3
57.3
55.3
57.6
78.4
63.2
57.9
59.6
61.0
57.1
60.7
71.6
64.9
68.3
59.5
75.7
76.2
74.9
62.9
53.2
79.6
80.4
72.4
60.7

0.19
0.23
0.22
0.24
0.24
0.23
0.17
0.22
0.21
0.21
0.09
0.23
0.21
0.19
0.18
0.22
0.23
0.16
0.20
0.20
0.26
0.09
0.14
0.15
0.22
0.23
0.09
0.14
0.19
0.22

-0.06
0.77
--0.09
-0.02
0.28
---0.31
0.17
---0.01
-0.49
0.02
0.45
0.41
-------

64.9
72.1
77.0
79.5
72.3
83.3
79.4
75.4
74.6
63.1
60.8
68.9
79.4
87.7
69.1
66.6
70.9
67.5
65.4
80.8
68.5
63.9
70.3
73.9
68.5
73.9
69.3
63.1
66.6
68.9

0.21
0.11
0.13
0.09
0.17
0.07
0.13
0.12
0.2
0.2
0.2
0.2
0.1
0.1
0.2
0.2
0.2
0.2
0.2
0.1
0.2
0.2
0.2
0.2
0.1
0.2
0.2
0.2
0.2
0.1

-0.01
-0.11
--0.06
-0.33
0.04
---0.16
----0.73
0.01
----0.02
0.61
0.09
-0.35
0.13

31
-65.3 0.23
-75.5
0.1
Total precipitation (inches).
b
Average relative humidity (percent).
c
Estimated PET (inches per day).
Source: Enviro-weather (http://www.agweather.geo.msu.edu/mawn/station.asp?id=htc)
a

34

Avg.
Rel.
Hum.
69.9
65.9
73.1
74.5
70.5
70.0
79.5
84.1
81.4
79.3
79.2
73.4
65.2
78.5
62.5
71.4
66.7
72.3
87.9
78.2
74.3
73.7
81.1
78.6
84.6
82.0
79.1
86.3
88.5
81.7

Est.
PET
0.17
0.19
0.16
0.11
0.08
0.11
0.08
0.04
0.08
0.11
0.11
0.15
0.13
0.06
0.11
0.07
0.10
0.12
0.04
0.11
0.11
0.09
0.06
0.09
0.05
0.06
0.06
0.04
0.04
0.05
0.11

Table 7. 2012 Daily total precipitation, average relative humidity, and estimated potential
evapotranspiration (PET).

Day

Tot.
Ppt.a

June
Avg.
Rel.
Hum.b

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30

0.58
----0.01
----0.02
0.02
-----0.37
--0.05
----------

90.3
63.8
53.3
60.3
66.5
59.6
56.4
51.0
44.7
51.6
66.5
57.1
52.5
50.6
52.3
60.4
63.3
72.7
52.9
51.5
64.2
63.1
57.8
56.7
53.9
52.1
50.5
50.2
54.6
56.1

Est.
PETc
0.03
0.17
0.23
0.17
0.15
0.21
0.21
0.22
0.25
0.24
0.15
0.23
0.19
0.22
0.25
0.21
0.15
0.14
0.26
0.26
0.18
0.19
0.20
0.19
0.21
0.21
0.24
0.25
0.20
0.23

Tot.
Ppt.

July
Avg.
Rel.
Hum.

----0.03
--------0.20
---0.40
0.26
0.01
--0.04
-0.02
0.19
-----

56.9
56.0
60.5
59.9
67.1
61.2
63.2
54.3
57.1
50.8
49.7
49.9
56.9
60.8
66.4
58.6
50.2
74.1
84.7
64.9
62.1
62.5
57.6
55.7
59.9
76.2
80.0
64.4
58.4
56.7

Est.
PET

August
Avg.
Tot.
Rel.
Est.
Ppt. Hum. PET

September
Avg.
Tot.
Rel.
Est.
Ppt. Hum. PET

0.21
0.20
0.21
0.23
0.18
0.23
0.23
0.22
0.18
0.21
0.21
0.22
0.21
0.22
0.22
0.23
0.29
0.14
0.08
0.19
0.20
0.19
0.25
0.22
0.21
0.11
0.12
0.20
0.19
0.21

---0.11
----0.43
0.70
0.38
-0.03
0.05
-0.06
---------0.12
0.20
----

--0.05
0.64
--0.28
0.06
----0.20
0.36
--0.01
0.19
0.01
0.06
0.16
0.06
0.06
0.01
0.02
0.01
-----

59.9
62.5
58.7
66.6
60.4
54.0
57.0
52.0
83.9
90.4
82.0
66.7
80.3
73.3
68.7
76.0
67.9
61.4
65.5
63.4
60.4
61.4
60.2
54.4
56.2
61.4
65.9
60.8
63.1
61.6

0.20
0.16
0.21
0.19
0.23
0.21
0.20
0.19
0.03
0.04
0.07
0.16
0.05
0.15
0.15
0.10
0.15
0.16
0.16
0.15
0.17
0.17
0.17
0.18
0.19
0.18
0.14
0.16
0.15
0.17

31
0.31
66.5 0.19
-48.6 0.21
Total precipitation (inches).
b
Average relative humidity (percent).
c
Estimated PET (inches per day).
Source: Enviro-weather (http://www.agweather.geo.msu.edu/mawn/station.asp?id=htc)
a

35

59.5
62.9
65.3
73.2
79.1
63.4
68.3
69.0
61.8
63.1
57.5
55.2
67.5
61.8
59.9
56.0
59.9
63.6
60.0
57.6
60.5
69.4
67.0
60.1
65.3
63.7
63.2
61.2
59.0
67.2

0.12
0.13
0.15
0.09
0.10
0.14
0.07
0.12
0.13
0.13
0.16
0.16
0.10
0.13
0.13
0.13
0.11
0.09
0.13
0.13
0.09
0.07
0.08
0.12
0.10
0.09
0.09
0.10
0.11
0.09

Table 8. 2013 Daily total precipitation, average relative humidity, and estimated potential
evapotranspiration (PET).

Day

Tot.
Ppt.a

June
Avg.
Rel.
Hum.b

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30

0.26
--------0.86
-0.89
1.09
0.01
-0.45
0.13
------0.18
0.40
-0.04
0.20
---

76.3
74.2
57.5
53.5
46.6
64.7
72.9
66.6
65.0
84.2
74.9
82.9
72.3
64.6
72.2
71.0
59.9
63.3
55.5
56.3
60.4
63.7
65.0
72.8
81.1
79.1
78.7
82.5
76.6
61.3

Est.
PETc
0.12
0.11
0.18
0.19
0.16
0.14
0.09
0.16
0.18
0.08
0.17
0.07
0.17
0.20
0.12
0.18
0.24
0.19
0.20
0.21
0.20
0.21
0.21
0.18
0.10
0.13
0.16
0.12
0.11
0.16

Tot.
Ppt.

July
Avg.
Rel.
Hum.

-0.06
0.06
--0.04
0.07
0.16
0.10
-----0.44
0.02
--0.03
--0.24
0.41
--0.05
0.02
0.05
0.02
--

58.7
72.3
76.4
72.2
63.9
75.1
79.2
79.5
80.5
69.1
58.2
68.5
63.0
69.3
74.5
69.6
69.5
69.2
62.9
60.4
63.4
75.1
72.1
60.1
62.3
67.5
76.7
77.2
75.0
69.9

Est.
PET

August
Avg.
Tot.
Rel.
Est.
Ppt. Hum. PET

September
Avg.
Tot.
Rel.
Est.
Ppt. Hum. PET

0.14
0.10
0.14
0.16
0.19
0.15
0.12
0.11
0.16
0.21
0.19
0.19
0.22
0.20
0.18
0.22
0.20
0.22
0.25
0.23
0.18
0.15
0.15
0.17
0.19
0.14
0.13
0.10
0.10
0.17

-----0.01
0.19
----0.65
---------0.23
----1.71
1.40
-0.13

------0.09
-0.32
--0.06
--0.06
0.01
--0.01
0.03
--------0.12
--

71.0
78.9
63.6
64.2
72.1
77.9
78.5
66.2
64.6
63.7
66.1
86.8
64.8
62.2
71.8
66.1
67.4
69.6
66.3
71.4
68.8
78.1
59.8
65.1
67.4
68.5
82.2
83.7
76.3
75.1

0.17
0.09
0.19
0.15
0.09
0.10
0.14
0.18
0.19
0.16
0.15
0.04
0.14
0.14
0.12
0.16
0.18
0.15
0.18
0.15
0.17
0.11
0.18
0.17
0.18
0.14
0.09
0.07
0.16
0.16

84.5
72.5
63.8
72.4
62.3
68.8
70.6
72.2
81.5
62.0
67.2
70.6
67.7
67.3
81.5
76.7
74.5
68.5
81.1
84.3
69.7
65.9
77.0
74.9
72.7
74.8
72.3
72.9
79.8
79.8

31
0.04 0.42
88.5 0.04
-79.5 0.09
Total precipitation (inches).
b
Average relative humidity (percent).
c
Estimated PET (inches per day).
Source: Enviro-weather (http://www.agweather.geo.msu.edu/mawn/station.asp?id=htc)
a

36

0.07
0.13
0.16
0.16
0.14
0.14
0.08
0.13
0.10
0.19
0.15
0.13
0.09
0.12
0.03
0.07
0.11
0.13
0.08
0.05
0.11
0.06
0.09
0.11
0.11
0.11
0.13
0.13
0.05
0.09

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37

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41

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removal. Crop Science 36: 1304-1309.

43

CHAPTER II:
CHARACTERIZATION OF VEGETATIVE COMPATIBILITY GROUP OF SCLEROTINIA
HOMOEOCARPA ISOLATES FROM THREE CULTIVARS OF AGROSTIS
STOLONIFERA AND THREE IRRIGATION REGIMES

44

Abstract
Many vegetative compatibility groups (VCG) of Sclerotinia homoeocarpa FT
Bennett have been identified, with researchers reporting anywhere from 4 to 54 distinct
VCGs. In this study, on each of two dates, a total of 180 isolates of S. homoeocarpa
were recovered from a creeping bentgrass (Agrostis stolonifera L.) fairway at the
Hancock Turfgrass Research Center in East Lansing, Michigan which was subjected to
three different irrigation program treatments (infrequent nighttime, daily nighttime, or
daily morning) and seeded to three different creeping bentgrass cultivars (‘Declaration’,
L-93’, or ‘SRP-1WM’). Recovered isolates were scored for VCG based on barrage zone
formation in an effort to determine if irrigation regime or creeping bentgrass cultivar
corresponded to VCG prevalence. Results indicated that VCG B and F, averaging 45
and 33% overall, respectively, were the most predominant while no isolates of VCG A
were recovered, and only 6% of isolates from one collection date were VCG D. VCG B
was the most commonly recovered VCG for all irrigation regimes and all creeping
bentgrass cultivars, followed by VCG F, except for the infrequent nighttime irrigation
program on the second sampling date when VCG F isolates were more predominant
than VCG B. Influence of the irrigation regimes or creeping bentgrass cultivars tested in
this fairway trial in Michigan did not vary regarding selection of VCG in Agrostis
stolonifera.

45

Introduction
Dollar spot (Sclerotinia homoeocarpa FT Bennett) is a major disease of warmand cool-season turfgrass (Smiley et al., 2005). Although numerous methods for control
of dollar spot are available, the use of fungicides remains among the most reliable
management strategy, causing dollar spot to be one of the most economically important
diseases of turf (Vargas, 2005). Characteristic symptoms of dollar spot on golf course
putting greens, tees, and fairways include blighted, tan-colored, sunken spots up to 5
cm in diameter, reaching up to 15 cm diameter in taller turf stands such as in golf
course roughs, athletic fields and home lawns. Spots may coalesce, resulting in larger
blighted areas. In the mornings when sufficient dew is present on the turfgrass, white,
fluffy fungal mycelia may be observed. Leaf symptoms may begin with chlorotic, watersoaked lesions which become white to tan-colored, typically with reddish-brown
margins. Typically, dollar spot is favored under conditions of high humidity and
temperatures that range from 15 to 30°C. Dollar spot may occur for a majority of the
growing season in the northern USA, usually from late spring through fall (Smiley et al.,
2005; Vargas, 2005). The disease is thought to spread mainly by mycelial growth from
soil and infected foliage to nearby plants and by direct transport of infected clippings on
golf equipment or machinery. The taxonomy of the fungus is inconclusive due to the
lack of a fertile teleomorph, which may be an indicator that populations are clonal and
vegetative compatibility group (VCG) variability is a result of vegetative change during
mitosis.
Various management strategies including chemical, cultural, biological, and host
resistance, are employed to combat dollar spot, with chemical fungicide usage the most

46

common, particularly under high disease pressure conditions. Cultural management
strategies for control of dollar spot have been studied. The impact of removal of dew on
dollar spot by mowing with either a walking or triplex mower or pulling a floor squeegee
over plots (Beard and Batten, 1982; Delvalle et al., 2011; Ellram et al., 2007) and
lightweight rolling with a turfgrass rolling machine (Giordano et al., 2012) has been
investigated. Additionally, the effect of fertility using various rates and types of fertilizers
(Bloom and Couch, 1960; Landschoot and McNitt, 1997; Markland et al., 1969),
avoidance of drought stress by applying various amounts of water (Couch and Bloom,
1960), and management of irrigation programs with both time of day and frequency of
application of irrigation (Jiang et al., 1998; McDonald et al., 2006) have also been
studied.
Creeping bentgrass (Agrostis stolonifera L.) is the most common cool-season
turfgrass used on golf courses (Lyman et al., 2007). Cultivars of creeping bentgrass
differ in susceptibility to S. homoeocarpa (Bonos et al., 2006; Casler et al., 2007; Cole
et al., 1969; NTEP, 2010). Based on results from the National Turfgrass Evaluation
Program (NTEP) in 2010, ‘Declaration’ and ‘SRP-1WM’ were among the least
susceptible cultivars of creeping bentgrass available, while ‘L-93’ was more susceptible
to dollar spot (NTEP, 2010). Although host resistance to dollar spot impacts disease
development, the mechanism behind this resistance is not well-understood.
Isolates of Sclerotinia homoeocarpa can be categorized by vegetative
compatibility group (VCG). Fungal strains are said to be vegetatively compatible if
hyphal fusion occurs when the isolates are grown together and form a stable
heterokaryon, in which case they are members of the same VCG. For this to occur, the

47

strains must possess identical alleles at particular sets of loci (Leslie, 1993). Hyphae
that do not anastomose are considered incompatible. In an incompatible reaction, a
barrage zone develops at the hyphal junction (Sonoda, 1989). Formation of a barrage
zone can be characterized by the development of dark zones of pigment where noncompatible hyphae converge (Leslie, 1993). The central portion of the barrage zone
contains dead or collapsing and degenerating cells as described by Newhouse et al.
(1991), while either edge of the zone may contain dense, aerial mycelia (Leslie, 1993).
Deng et al. (2002) described three incompatible reactions as gap, line-gap, and
barrage. In a gap reaction, a 3-10 mm gap with two dark lines at the edges formed and
was visible on the back of potato dextrose agar plate. A line-gap reaction was described
as having a narrow gap 1 mm across with one or two dark lines on the underside of the
media plate.
Sonoda et al. (1989) identified 54 distinct VCGs in accessions of S.
homoeocarpa from Florida using barrage formation as an indicator of incompatibility,
while Jo et al. (2008) regrouped 11 VCGs using barrage zone characterization to 5
VCGs using nitrate-nonutilizing (nit) mutants. Other researchers have reported differing
numbers of VCGs in S. homoeocarpa (Chakraborty et al., 2006; Viji et al., 2004). In a
study examining the response of bentgrass cultivars to isolates of S. homoeocarpa from
ten different VCGs, Chakraborty et al. (2006) determined that bentgrass cultivars
exhibited significant differences in dollar spot severity and that fungal isolates differed
significantly in virulence. In Michigan, Powell and Vargas (2001) demonstrated that
representatives of six S. homoeocarpa VCGs were present throughout the season from
spring into fall, with two VCGs, denoted as VCG A and VCG B, identified as the most

48

predominant. Isolates of S. homoeocarpa VCG F were considered seasonal and were
identified only in late summer epidemics for the first two years of the study; however, in
the third year, isolates of S. homoeocarpa VCG F were only recovered in the early
summer epidemics (Powell and Vargas, 2001). It is unclear whether this seasonal
variation was due to environmental conditions or some other factor. Viji et al. (2004)
identified a correlation between virulence and VCG in S. homoeocarpa, while
Chakraborty et al. (2006) demonstrated that fungal isolates differed significantly in
aggressiveness regardless of host cultivar or species of grass. This supports the idea
that by using a few highly virulent fungal isolates, turfgrass breeders could select for
resistance to a wide range of isolates.
The objective of this study was to determine whether a) different irrigation
regimes and b) different creeping bentgrass cultivars, alone or in combination,
influenced the population VCG characteristics of isolates of S. homoeocarpa recovered
from field plots.

49

Materials and Methods
A field study was conducted at the Hancock Turfgrass Research Center on the
campus of Michigan State University in East Lansing, MI. Overall, the effect of different
irrigation programs and cultivars of creeping bentgrass (Agrostis stolonifera L.) on the
appearance of vegetative compatibility group (VCG) of isolates of Sclerotinia
homoeocarpa recovered from the experimental plots in Jul and Aug 2011 was
examined. Three irrigation programs were situated in positions determined by
statistically generated random allocations in 11 x 11 m plots and replicated three times.
Irrigation programs investigated were daily irrigation at 0500 h (daily morning), daily
irrigation at 2200 h (daily nighttime) or irrigation at 2200 h twice weekly (infrequent
nighttime). Within the irrigation plots, three creeping bentgrass cultivars, ‘Declaration’,
‘L-93’, and ‘SRP 1-WM’, were seeded to 2.7 x 3.7 m split plots and replicated four times.
Buffer strips surrounding irrigation plots were seeded to the DS-susceptible cultivar
‘Crenshaw’ (Figure 7.)
-1

The study site received a total of 146 kg N ha

from either 18-9-18 or 24-2-12

granular fertilizer monthly. The herbicide Mec Amine-D (2, 4-D, Mecoprop-p and
-1

Dicamba) (Loveland Products, Greeley, CO) was applied at 30 L ha

on 21 Jun 2011

for the control of broadleaf weeds. Insects were controlled with carbaryl (Sevin; Bayer
Environmental Science, Research Triangle Park, NC) on 21 Jul 2011, applied at 50 L
-1

ha

for the control of black cutworms (Agrotis ipsilon).
Virulence of isolates of S. homoeocarpa was determined by center-point

inoculation of ‘A4’ cultivar of creeping bentgrass (Agrostis stolonifera cv ‘A4’). S.

50

-1

homoeocarpa was grown on potato dextrose agar (PDA: 39 g PDA L

water; Becton,

Dickinson and Co, Sparks, MD). One 5 mm agar plug containing mycelia of S.
homoeocarpa was placed in the center of each of two creeping bentgrass pots which
were placed in quart-sized plastic bags containing 150 ml water. Bags were sealed and
incubated at 22 ± 2 C for 10 days. All fungal isolates tested successfully infected ‘A4’
creeping bentgrass. Representatives of VCGs A-F, which were confirmed to be virulent,
were used for the duration of the trial.
Plots were inoculated with S. homoeocarpa-infested sand-cornmeal topdressing
mixture. The inoculum was prepared using a modified version of the method of
Goodman and Burpee (1991). A mixture of silica sand and cornmeal (Quaker Oats Co.,
Chicago, IL) (2:1, v/v) and 2.4% potato dextrose broth (Becton, Dickinson and Co,
Sparks, MD) (5% v/v) was placed in 15 x 25 cm aluminum baking pans, covered with
two layers of aluminum foil and placed into a second aluminum baking pan. The sandcornmeal mixture was autoclaved at 121 C for 45 minutes. S. homoeocarpa on potato
-1

dextrose agar (PDA: 39 g PDA L

water; Becton, Dickinson and Co, Sparks, MD ) in 9

cm petri dishes was cut into 30-100 pieces and placed on the sterile sand-cornmeal.
After incubation at 22 ± 2 C for 2 weeks, the media was chopped and kneaded by hand
to break down clumps. This procedure was followed for each of six different VCG
isolates (A, B, C, D, E, and F) of S. homoeocarpa which were previously collected from
turfgrass in MI by Powell (1998). A mixture containing equivalent amounts of each VCG
was blended together by hand for field application. On 17 Jun 2011, the blended sandcornmeal inoculum was applied to field plots at a rate of 2.1 kg 100 m
lawn spreader (Model 36H13, Gandy Co, Owatonna, MN).
51

-2

using a Gandy

Dollar spot-infected leaf tissue was collected on 15 Jul and 25 Aug 2011. Twenty
samples of symptomatic leaf tissue from each creeping bentgrass cultivar were
randomly collected from every irrigation plot. Samples were stored in 5.7 x 8.9 cm coin
envelopes (Quality Park Products, St. Paul, MN) at 1 - 4 C.

Figure 7. Aerial photograph of the field trial site. Irrigation plots are labeled with
irrigation regime. Sub-plots within irrigation plots were established to either
‘Declaration’, ‘L-93’, or ‘SRP-1WM’ cultivars of creeping bentgrass.

52

To recover fungal isolates from collected leaf tissue, Individual blades with
-1

advancing disease margins were placed onto acidified water agar (24 g agar L

water

with 10 ml lactic acid) and incubated at 22 ± 2 C for 3 days. After development of
mycelia characteristic of S. homoeocarpa from the blades of grass, a 3 mm diameter
-1

agar plug from each fungal isolate was transferred to PDA (39 g PDA L

water).

Pairing to VCG for each isolate was performed on PDA plates amended with red
-1

food coloring (10 drops L

water with 39 g PDA; McCormick and Co., Inc., Hunt Valley,

MD) to help delineate barrage zone formation (Kohn et al., 1990). For each recovered
isolate, a 2 mm mycelial plug was placed at the center of a food color amended PDA
plate. For one sample plate, a plug of each of tester strains belonging to VCGs A-E
were plated 3.5 cm apart surrounding the central sample plug. A second sample plate
contained VCG F and the recovered isolate. Each plate also contained a pairing with
the recovered isolate to ensure self-fusion and to act as a negative control indicated by
the absence of barrage formation. Each recovered isolate and combination of tester
isolates was plated three times. Compatibility of the test isolates with known VCG
isolates was determined after 7 days of incubation at 22 C. Incompatible reactions were
scored as the presence of dark barrage zones at the contact margins of cultures
(Newhouse and MacDonald, 1991). Reactions were scored as compatible when hyphal
growth was confluent. For reactions that were not readily resolved, the incubation period
was increased to 3 weeks. For slow growing recovered isolates, in order to encourage
adequate growth for VCG pairing and to avoid overgrowth of the recovered isolate with
faster growing known VCG isolates, slow growing recovered isolates were re-plated 3 to

53

4 days prior to the addition of the tester isolates. This prevented faster growing isolates
from overrunning slower growing isolates.

54

Results
Isolates were readily recovered from the acidified water agar plates after
incubation. Mycelia were thin and sparse, but once transferred to PDA, cultures
generally grew rapidly and filled the plate after incubation for 4 days. On PDA, mycelial
growth appeared typical for S. homoeocarpa, and ranged from dense, fluffy and white to
darker brown mycelia. Some recovered isolates grew very slowly with sparse mycelial
growth, typically brownish in color. When being scored for vegetative compatibility,
these isolates were incubated for 3-4 days prior to the addition of tester strains onto the
scored agar plate in order to allow sufficient growth and avoid being overrun by faster
growing tester isolates.
After incubation, vegetative compatibility of recovered specimens was generally
easy to determine. All self-fusion reactions were confluent and served as an example of
a compatible reaction on every plate. Incompatible reactions were identified based on
the formation of dark barrage zones or the presence of abundant, dense, aerial mycelia
at the junction of isolates (Figure 8). For each collection date, the overall percentage of
the total number of specimens recovered is listed by VCG (A-F) in Table 9. The number
of specimens classified within each VCG followed the same trend on both collection
dates except for VCG D, which was not recovered on 15 Jul. Vegetative compatibility
group B was the most prevalent, recovered at 47 and 42% on 15 Jul and 25 Aug,
respectively, followed by VCGs F, E and C. VCGs B and F accounted for 78% of the
VCGs of the collected fungal specimens. No isolates were scored as compatible with
VCG A. On 15 Jul, none of the recovered specimens exhibited compatible reactions
with VCG D, while on 25 Aug, only about 5% were identified.

55

Figure 8. Vegetative compatibility assay among isolates of Sclerotinia
homoeocarpa on potato dextrose agar amended with red food coloring after 7
days. The center isolate in each plate is the recovered sample and is surrounded by
VCGs A-E (clockwise from top). Recovered samples were tested for compatibility with
VCG F on separate plates. Incompatible reactions are identified by the formation of a
dark or dense barrage zone. “S” denotes the position of the recovered sample used to
confirm self-fusion and as a negative control on each plate. “C” indicates a compatible
reaction. Plate A was scored as compatible with VCG E, Plate B with VCG B, and Plate
C was not compatible with VCGs A-E tested here.

S

S

C

S

C

Plate A

Plate B

Plate C

Table 9. Percentage of total number of S. homoeocarpa isolates recovered for
each VCG from irrigation x cultivar experiment.
Sampling
Date
15-Jul-11
25-Aug-11
Average

Frequency of recovery of VCG groups of
S. homoeocarpa (%)
A*
B
C
D
E
F
0
47
10
0
13
30
0
42
7
6
9
36
0
44.8
8.5
2.8
10.3
33.2

* VCGs A, B, C, D, E, and F are named as per Powell and Vargas (2001).

56

VCG B isolates were predominant on all cultivars at both dates followed by VCG
F, except for ‘L-93’ when, on 25 Aug, VCGs B and F were recovered in equivalent
frequencies (Table 10). On 15 Jul, less than 18% of the isolates were identified as either
VCG C or E. On 25 Aug, VCGs C, D, and E were identified between 3 and 13% of the
time. The distribution of VCG pairing of recovered isolates was similar among cultivars.
Percentages of isolates in each VCG by irrigation regime are listed in Table 11.
Data from both daily irrigation treatments (morning and nighttime) follow the same
trends as for the effect of cultivar. VCGs B and F were recovered more frequently from
each irrigation regime treatment than any other VCG. However, samples recovered
from the plots that were irrigated infrequently at night resulted in 58 and 31% VCG B
and F, respectively on 15 Jul while they were 33 and 47% VCG B and F, respectively on
25 Aug. This was the only time VCG B was not the most prevalent isolate recovered.

57

Table 10. Frequency of Vegetative Compatibility Group (VCG) of Sclerotinia
homoeocarpa isolated from creeping bentgrass under three irrigation regimes
from the Hancock Turfgrass Research Center (East Lansing, MI) in 2011.
Frequency of Vegetative Compatibility Groups (VCG) of
Sclerotinia homoeocarpa (%)
Creeping Bentgrass Cultivar

a

b

VCG
A
B
C
D
E
F
SEM

c

d

15-Jul-11
‘Declaration’
0
49
14
0
6
31
8.0

25-Aug-11

‘SRP-1WM’
0
46
8
0
14
32

‘L-93’
0
48
7
0
18
28

7.6

7.7

‘Declaration’
0
39
11
8
13
29
5.9

‘SRP-1WM’
0
46
7
3
8
37
8.0

a

‘L-93’
0
42
4
6
6
42
8.1

Creeping bentgrass cultivars were irrigated at 0500 h daily, 2200 h daily, or 2200 h
twice weekly.
b
Date of sample.
c

d

VCGs A, B, C, D, E, and F are named as per Powell and Vargas (2001).
SEM = Standard Error of the Mean per cultivar (N=9).

58

Table 11. Vegetative compatibility group (VCG) of Sclerotinia homoeocarpa
isolated from creeping bentgrass using three irrigation programs at the Hancock
Turfgrass Research Center (East Lansing, MI).
Frequency of Vegetative Compatibility Groups (VCG) of
Sclerotinia homoeocarpa (%)
b

VCG
A
B
C
D
E
F

c

SEM

d

0500 h
Daily
0
41
13
0
16
30
6.7

Irrigation Regime

15-Jul-11
2200 h 2200 h Twice
Daily
Weekly
0
0
43
58
9
7
0
0
18
4
30
31
7.0

9.5

a

0500 h
Daily
0
40
9
10
8
33
6.5

25-Aug-11
2200 h 2200 h Twice
Daily
Weekly
0
0
53
33
6
8
4
7
9
6
28
47
8.3

a

7.6

Samples collected from ‘Declaration’, ‘L-93’, and ‘SRP-1WM’ cultivars of creeping
bentgrass were combined for each irrigation program (0500 h daily, 2200 h daily, and
2200 h twice weekly).
b
Date of sample.
c

d

VCG A, B, C, D, E, and F are named as per Powell and Vargas (2001).
SEM = Standard Error of Mean per irrigation regime (N=9).

59

Discussion
The results of this study indicate similarity in the association among the irrigation
regimes or creeping bentgrass cultivars tested to a particular VCG of Sclerotinia
homoeocarpa. The majority (78% of total) of recovered isolates were classified as either
VCG B or F, regardless of irrigation program or creeping bentgrass cultivar. On the 15
Jul collection date, nearly 47% of all isolates collected paired with VCG B, followed by
30% with VCG F. Similarly, on the 25 Aug collection date, isolates expressing
compatible reactions with VCG B numbered 42% of the total collected while 36% were
compatible with VCG F. Powell and Vargas (2001) found VCG A to be the most
predominant of the S. homoeocarpa isolates recovered in MI, followed by VCG B.
Although the current study did not recover specimens belonging to VCG A, the findings
support those of Deng et al. (2002), Viji et al. (2004), and DeVries (2006) who
demonstrated that VCG B was predominant in the populations that were investigated.
Although VCGs A-F were introduced as initial inoculum in equivalent amounts as
field inoculum for the trial, not all of the VCGs were recovered at either evaluation date.
It is possible that some VCGs, such as VCGs B and F, were more virulent than others.
Viji et al. (2004) classified S. homoeocarpa isolates into four virulence groups based on
DS severity: highly, moderately, and weakly virulent and avirulent. Certain virulence
groups corresponded to specific VCGs; however, this was not confirmed upon AFLP
analysis Viji et al. (2004). VCG B was comprised of isolates in all four virulence group
classifications. Contrary to findings of the current study where no VCG A isolates were
recovered, VCG A isolates were highly virulent in the Viji et al. (2004) study. All of the S.
homoeocarpa VCG tester isolates used for field inoculation in this study were found to

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be virulent prior to field inoculations, but the level of virulence as described by Viji et al.
(2004) was not determined.
In examining isolate recovery by creeping bentgrass cultivar, VCG B was
predominant, followed by VCG F, except at the 25 Aug sampling date from the cultivar
‘L-93’ from which VCGs B and F were recovered in equivalent amounts. VCGs B and F
represented 75 to 80% of the isolates recovered on 15 Jul and 68 to 84% from 25 Aug.
These results indicated that the most predominant VCGs were recovered from all of the
cultivars and no VCG corresponded to a specific cultivar. Similarly, when evaluating
isolate recovery by irrigation regime, VCG B followed by VCG F were most prevalent,
except in the infrequent nighttime irrigation program plots on 25 Aug when VCG F was
more prevalent than VCG B isolates. This was the only occurrence when VCG B was
not the most frequently recovered isolate. Even though the trend from the first to second
collection reversed for the infrequent nighttime irrigation program, 89 and 80% of
recovered isolates were represented by a combination of VCGs B and F for each date,
respectively.

If further collections had been made, perhaps on an annual basis

throughout the study, a better understanding of the potential impact of infrequent
irrigation on VCG of S. homoeocarpa could have been elucidated.
It may be possible that S. homoeocarpa VCGs could be favored by certain
environmental conditions, such as soil moisture level or the timing of irrigation
applications to turf, which may be affected by irrigation programming since no VCG A
isolates were recovered in this study. If irrigation patterns which favor less virulent
isolates of S. homoeocarpa could be identified, this could lead to another means of
reducing DS pressure under field conditions. Irrigation programming could then be

61

patterned to favor less virulent isolates of S. homoeocarpa. When examining VCGs of
recovered specimens of S. homoeocarpa from fairway plots subjected to one of three
different irrigation regimes, each containing creeping bentgrass cultivars ‘Declaration’,
‘L-93’, and ‘SRP-1WM’, VCG recovery was similar for each irrigation regime or cultivar.

62

LITERATURE CITED

63

LITERATURE CITED

Beard, J.B. and S.M. Batten. 1982. Turf management for golf courses. MacMillan
Publishing Co., New York, N.Y., USA. 642 pp.
Bloom, J.R. and H.B. Couch. 1960. Influence of environment on diseases of turfgrasses. I. Effect of nutrition, pH, and soil moisture on Rhizoctonia brown patch.
Phytopathology 50: 532-535.
Bonos, S.A., B.B. Clarke and W.A. Meyer. 2006. Breeding for disease resistance in the
major cool-season turfgrasses. Annual Review of Phytopathology 44: 213-234.
Casler, M., G. Jung, S. Bughrara, A. Hamblin, C. Williamson and T. Voigt. 2007.
Development of creeping bentgrass with multiple pest resistance. USGA Green Section
Record 45: 15-19.
Chakraborty, N., T. Chang, M.D. Casler and G. Jung. 2006. Response of bentgrass
cultivars to Sclerotinia homoeocarpa isolates representing 10 vegetative compatibility
groups. Crop Science 46: 1237-1244.
Cole, H., J. Duich, L. Massie and W. Barber. 1969. Influence of fungus isolate and grass
variety on Sclerotinia dollar spot development. Crop Science 9: 567-570.
Couch, H.B. and J.R. Bloom. 1960. Influence of environment on diseases of turfgrasses. II. Influence of nutrition, pH and soil moisture on Sclerotinia dollar spot.
Phytopathology 50: 761-763.
Delvalle, T.C., P.J. Landschoot and J.E. Kaminski. 2011. Effects of dew removal and
mowing frequency on fungicide performance for dollar spot control. Plant Disease 95:
1427-1432.
Deng, F., M. Melzer and G. Boland. 2002. Vegetative compatibility and transmission of
hypovirulence-associated dsRNA in Sclerotinia homoeocarpa. Canadian Journal of
Plant Pathology 24: 481-488.
DeVries, R.E. 2006. Genetic diversity of Sclerotinia homoeocarpa delineated by AFLP
and vegetative compatibility. Master’s Thesis. University of Tennessee.
Ellram, A., B. Horgan and B. Hulke. 2007. Mowing strategies and dew removal to
minimize dollar spot on creeping bentgrass. Crop Science 47: 2129-2137.

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Giordano, P.R., T.A. Nikolai, R. Hammerschmidt and J.M. Vargas. 2012. Timing and
frequency effects of lightweight rolling on dollar spot disease in creeping bentgrass
putting greens. Crop Science 52: 1371-1378.
Goodman, D.M., and L.L. Burpee. 1991. Biological control of dollar spot disease of
creeping bentgrass. Phytopathology 81:1438-1446.
Jiang, H., J. Fry and N. Tisserat. 1998. Assessing irrigation management for its effects
on disease and weed levels in perennial ryegrass. Crop Science 38: 440-445.
Jo, Y.-K., S. Chang, J. Rees and G. Jung. 2008. Reassessment of vegetative
compatibility of Sclerotinia homoeocarpa using nitrate-nonutilizing mutants.
Phytopathology 98: 108-114.
Kohn, L.M., I. Carbone and J.B. Anderson. 1990. Mycelial interactions in Sclerotinia
sclerotiorum. Experimental Mycology 14: 255-267.
Landschoot, P. and A. McNitt. 1997. Effect of nitrogen fertilizers on suppression of
dollar spot disease of Agrostis stolonifera L. International Turfgrass Society Research
Journal 8: 905-911.
Leslie, J.F. 1993. Fungal vegetative compatibility. Annual Review of Phytopathology 31:
127-150.
Lyman, G.T., C.S. Throssell, M.E. Johnson, G.A. Stacey and C.D. Brown. 2007. Golf
course profile describes turfgrass, landscape, and environmental stewardship features.
Appl. Turf. Sci. http://www.plantmanagementnetwork.org/pub/ats/research/2007/profile/
(verified 24-Nov. 2013).
Markland, F., E. Roberts and L. Frederick. 1969. Influence of nitrogen fertilizers on
Washington creeping bentgrass, Agrostis palustris Huds. II. Incidence of dollar spot,
Sclerotinia homoeocarpa infection. Agronomy Journal 61: 701-705.
McDonald, S.J., P.H. Dernoeden and C.A. Bigelow. 2006. Dollar spot and gray leaf spot
severity as influenced by irrigation, chlorothalonil, paclobutrazol, and a wetting agent.
Crop Science 46: 2675-2684.
Newhouse, J.R. and W.L. MacDonald. 1991. The ultrastructure of hyphal anastomoses
between vegetatively compatible and incompatible virulent and hypovirulent strains of
Cryphonectria parasitica. Canadian Journal of Botany 69: 602-614.
NTEP. 2010. ‘2008 National Bentgrass (Fairway/Tee) Test. 2010 Data Progress Report
NTEP No. 11-2. Available at http://www.ntep.org/reports/bt08f/bt08f_11-2/bt08f_112.htm (verified 24-Nov 2013). National Turfgrass Evaluation Program Beltsville
Agricultural Research Center West. Beltsville, MD.

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Powell, J. and J. Vargas Jr. 2001. Vegetative compatibility and seasonal variation
among isolates of Sclerotinia homoeocarpa. Plant Disease 85: 377-381.
Powell, J.F. 1998. Seasonal variation and taxonomic clarification of the dollar spot
pathogen, Sclerotinia homoeocarpa. PhD Dissertation. Michigan State University. East
Lansing, MI.
Smiley, R.W., P.H. Dernoeden and B.B. Clarke. 2005. Compendium of turfgrass
diseases. 3rd ed. The American Phytopathological Society. St. Paul, MN.
Sonoda, R.M. 1989. Vegetative compatibility groups among Sclerotinia homeocarpa
from leaves of Paspalum notatum. Proceedings-Soil and Crop Science Society of
Florida. 48:35-36.
Vargas, J.M., Jr. 2005. Management of turfgrass diseases. 3rd ed. John Wiley & Sons,
Inc. Hoboken, NJ.
Viji, G., W. Uddin, N. O'Neill, S. Mischke and J. Saunders. 2004. Genetic diversity of
Sclerotinia homoeocarpa isolates from turfgrasses from various regions in North
America. Plant Disease 88: 1269-1276.

66

CONCLUSIONS

67

Chapter 1: Irrigation regime and cultivar field study

Results of this research indicated that irrigation regime and host resistance
impact dollar spot (DS) of creeping bentgrass on fairway turf in MI. In 2011, ‘SRP-1WM’
watered at 2200 h twice weekly (infrequently), 0500 h daily, or 2200 h daily exhibited
significantly less DS than ‘Declaration’ watered in the same manner. ‘L-93’ developed
significantly more DS compared to ‘Declaration' when each cultivar was irrigated
infrequently or daily. These data demonstrate the importance of host resistance with
regard to DS incidence when irrigation programming is comparable. When irrigated at
2200 h twice weekly, all cultivars developed significantly more DS than when irrigated at
2200 h daily with similar total weekly amounts of applied water ha-1. Irrigating with
approximately the same weekly volume of water at 2200 h on a daily basis yielded
significantly less DS than on an infrequent basis regardless of host cultivar. Watering at
2200 h daily compared to 0500 h daily yielded significantly less DS for ‘L-93’ and
‘Declaration’, however, for ‘SRP-1WM’, daily irrigation treatments did not significantly
affect DS incidence.
Data from 2013 supported the findings from 2011. For the main effect of
irrigation, 2200 h daily irrigation averaged over all cultivars for the season exhibited
significantly less DS than either 0500 h daily or 2200 h twice weekly irrigation
programming. Both daily irrigation regimes developed significantly less DS than the
infrequent irrigation program on a seasonal basis when averaged for all cultivars. There
are several explanations which may account for this difference in impact on DS resulting
from irrigation programming. It is possible that under conditions of daily irrigation, soil

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moisture content was maintained at a consistently higher level when compared with
infrequent irrigation, even though the same weekly volumes were applied. The
infrequent applications may have wet the soil to a deeper level within the soil profile at
the time of application, but then may have resulted in the top 1 - 2 cm drying down
between irrigation events.
Another consideration is that by watering daily, the leaf wetness duration may
have been affected. Dew formation and its duration on plant leaves may have been
altered (ie diluted or displaced) daily as compared with infrequent programming which
would only affect the dew twice weekly. It is unknown whether the interruption of dew
formation, as would occur at the 2200 h irrigation event, or its displacement at 0500 h,
would influence pathogen growth. By slowing or interfering with fungal growth, reduced
disease incidence could result.
Yet another consideration is that by supplying irrigation to the plants daily rather
than allowing the soil to dry between irrigation events, plant health could be affected. In
reducing potential plant stress which could result from drying, maintaining more
consistent soil moisture might allow plants to focus energy on disease resistance rather
than physiological processes associated with drought stress.
By combining the use of host resistance and irrigation management, a reduction
in DS of creeping bentgrass may be achieved. This combination of cultural practices
may be of importance in areas where DS is problematic. Further evaluation of the
mechanism regarding soil moisture, disease development and impact on microbial
populations is needed. Additionally, further research examining the role of guttation

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water, canopy humidity, and leaf wetness duration may enhance current knowledge
regarding the S. homoeocarpa infection process, leading to a better understanding of
parameters that may enhance or inhibit infection of turfgrass.

Chapter 2: Vegetative compatibility group study

Vegetative compatibility group (VCG) pairings indicate similar correspondence of
VCG with host resistance or irrigation regime. Isolates recovered from samples
collected were predominantly paired with VCG B followed by VCG F for every cultivar
and every irrigation regime. The exceptions were from the 25 Aug sampling when equal
numbers of VCGs B and F were recovered in the ‘L-93’ plots. On that same date,
samples from infrequent nighttime irrigation programming yielded more VCG F than B.
This was the only occurrence when VCG B was not the most prevalent compatibility
type isolated. Attempts by other researchers have been made to correlate VCG among
S. homoeocarpa accessions to fungicide resistance, virulence, geographic distribution,
host, and seasonal epidemics. Although there have been a few reported successes in
correlating VCGs to studied factors, the association of VCG in S. homoeocarpa with
clearly defined factors has largely eluded researchers.

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