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thesis entitled

EVALUATION OF SPRING-PLANTED BRASSICA COVER
CROPS FOR USE IN MUSKMELON (Cucumis melo L.) AND
EGGPLANT (Solanum melongena L.) PRODUCTION
SYSTEMS

presented by

Victoria Joy Ackroyd

has been accepted towards fulfillment
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MS. degree in Horticulture

 

 

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EVALUATION OF SPRING-PLANTED BRASSICA COVER CROPS FOR USE IN
MUSKMELON (Cucumis melo L.) AND EGGPLANT (Solanum melongena L.)
PRODUCTION SYSTEMS

By
Victoria Joy Ackroyd

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Horticulture

2010

ABSTRACT
EVALUATION OF SPRING-PLANTED BRASSICA COVER CROPS FOR USE IN
MUSKMELON (Cucumis melo L) AND EGGPLANT (Solanum melongena L.)
PRODUCTION SYSTEMS
By
Victoria Joy Ackroyd

Members of the Brassica family produce glucosinolates which upon hydrolysis are
reported to impact soilbome pathogen populations. Oilseed radish (Raphanus
sativus (L) var. oleiferus Metzg (Stokes)), Oriental mustard (Brassicajuncea (L.)
Czern.), and yellow mustard (Sinapis aIba L.) are Brassicas that are often used as
cover crops. This research evaluated suitability of these species for use as spring-
planted cover crops. In one field experiment, cover crops were spring-planted
preceding muskmelon (Cucumis melo L. Group Reticulatus) and eggplant (Solanum
melongena L). In another field experiment, oilseed radish and Oriental mustard I
were spring-planted, then muskmelon, honeydew (Cucumis meio L. Group
lnodorous), and cucumber (Cucumis sativus L.) were planted at five day intervals
after cover crop incorporation to study cover crop phytotoxicity. In the laboratory,
lyophilized and non-lyophilized oilseed radish root and shoot extracts were tested
on muskmelon, honeydew, and cucumber germination and radicle growth. Results
indicate that while these cover crops do aid in nutrient cycling, they do not confer
significant protection against soilbome disease caused by Verticillium dahliae Kleb.
under our field conditions. They should be used with caution due to their ability to

inhibit cucurbit seed germination and cash crop growth. Laboratory results further

suggest phytotoxic compounds are likely primarily volatile in nature.

Dedicated to Grandma Vicki, 1945 - 2009. Wish you could have seen this.

ACKNOWLEDGEMENTS

My thanks to Dr. Mathieu Ngouajio, my major advisor, for his guidance,
patience, and insight. Thank you also to my committee, Dr. Mary Hausbeck and Dr.
Ron Goldy, for their advice and aid. I also thank Dr. Gregory Lang for his
perspective on graduate student life and development. My thanks to the USDA
Methyl Bromide Transitions Program (Grant # 2006-51102-03565) for funding
this work.

No one works in a vacuum, and so I thank my laboratory group: Buck, Drey,
Ajay, Zachary, and Erin. My thanks also to Dr. Nair and Dr. Yunbao Liu, for their
help with lab procedures and equipment. This work would not have been possible
without the aid of our undergraduate assistants: Djoko, Pam, Adam, Rebekah, and
Sam. My deepest thanks to Dave Francis, the crew at the South West Michigan
Research and Extension Center, and Bill Chase for their help accomplishing the
field work. I am grateful to grower Ron Eding for his donation of oilseed radish
biomass. My thanks to the staff in the Department of Horticulture office ladies for
their help with all things paperwork-related: Sherry, Lorri, joyce, and Rita.

I also thank my fellow graduate students, particularly Elizabeth and
Sheridan Kelley, for their camaraderie and advice. I owe the deepest debt of
gratitude to my chosen kin — Grandma Vicki Phillips, Aunt ‘Buttons' Watts, and
grandparents Don and joy Ackroyd. It's been a long and rough road, but somehow
we endure. Finally, my thanks to Ronald ‘Trouble' Nussbaum — fellow traveler and

dearest love. Love you most!

TABLE OF CONTENTS

LIST OF TABLES .............................................................................................................. vi
LIST OF FIGURES .............................................................................................................. x
CHAPTER 1: Introduction ................................................................................................ 1
CHAPTER 2: Literature Review ...................................................................................... 5

CHAPTER 3: Field Evaluation of Spring-Planted Brassica Cover Crops for
Performance in Muskmelon (Cucumis melo L.) and Eggplant (Solarium melongena

L.) Cropping Systems
Abstract .............................................................................................................. 34
Introduction ....................................................................................................... 34
Materials and Methods ...................................................................................... 36
Results ................................................................................................................ 41
Discussion .......................................................................................................... 48

CHAPTER 4: Impact of Brassica Cover Crops on Cucurbit Germination and Yield

Abstract .............................................................................................................. 79

_ Introduction ....................................................................................................... 80
Materials and Methods ...................................................................................... 84
Results ................................................................................................................ 89
Discussion .......................................................................................................... 96
CHAPTER 5: Conclusions and Future Work .............................................................. 131
LITERATURE CITED .................................................................................................... 139

LIST OF TABLES
Table 2.1. Glucosinolates found in Brassica cover crops (pg. 24-27) ........................ 24

Table 2.2. Soilborne pathogens reported to be susceptible to green manures.
Susceptibility alone does not imply a reduction in disease in the field (pg. 28-
30) .................................................................................................................................. 28

Table 3.1. Pesticide application schedule for eggplant and muskmelon plots in
2008 and 2009 (pg. 56-57) ........................................................................................... 56

Table 3.2. Mean monthly and long term (8-year) temperature and precipitation
during cover crop growth and melon and eggplant growth in 2008 and 2009 at
SWMREC Michigan.* ...................................................................................................... 58

Table 3.3. Monthly and long term (8-year) growth degree days (GDD) during cover
crop growth and melon and eggplant growth in 2008 and 2009 at SWMREC
Michigan. Base temperature = 10°.* ............................................................................. 59

Table 3.4. Brassica cover crop stand count and dry weight biomass prior to soil
incorporation ................................................................................................................. 60

Table 3.5. Average soil nitrate levels (ppm) as affected by biofumigation with
Brassica cover crops and soil treatments. Data for 2008 and 2009 were

combined ....................................................................................................................... 61

Table 3.6. Average soil ammonium levels (ppm) in 2009 as affected by
biofumigation with Brassica cover crops and soil treatments ................. _ ................ 62

Table 3.7. Melon ‘Athena' and eggplant ‘Classic’ stand as affected by biofumigation
with Brassica cover crops and soil treatments.* ......................................................... 63

Table 3.8. Eggplant biomass as affected by the previous Brassica cover crops and
soil treatments.* ............................................................................................................. 64

Table 3.9. Effect of cover crops and soil treatments on eggplant ‘Classic’ height
(cm) in 2008.* ................................................................................................................... 65

Table 3.10. Effect of cover crops and soil treatments on eggplant ‘Classic’ height
(cm) in 2009.* ................................................................................................................ 66

vi

Table 3.11. Effect of cover crops and soil treatments on eggplant ‘Classic’
chlorophyll Ievels.* ......................................................................................................... 67

Table 3.12. Visual estimates of direct-seeded melon ‘Athena’ plot plant vigor as
affected by biofumigation with Brassica cover crops and soil treatments in
2009 ................................................................................................................................. 68

Table 3.13. Visual estimates of transplanted melon ‘Athena' plot plant vigor as
affected by biofumigation with Brassica cover crops and soil treatments in
2009 ................................................................................................................................ 69

Table 3.14. Visual estimates of eggplant plot plant health and vigor as affected by
biofumigation with Brassica cover crops and soil treatments in 2008 ..................... 70

Table 3.15. Visual estimates of eggplant plot plant health and vigor as affected by
biofumigation with Brassica cover crops and soil treatments in 2009 ..................... 71

Table 3.16. Percentage of ‘Classic’ eggplant with verticillium wilt syrhptoms as
affected by biofumigation with Brassica cover crops and soil treatments in
2008* .............................................................................................................................. 72

Table 3.17. Percentage of ‘Classic’ eggplant with verticillium wilt symptoms as
affected by biofumigation with Brassica cover crops and soil treatments in
2009.* .............................................................................................................................. 73

Table 3.18. Direct-seeded muskmelon ‘Athena’ marketable and total yield as
affected by biofumigation with Brassica cover crops and soil treatments in
2009.* .............................................................................................................................. 74

Table 3.19. Transplanted muskmelon ‘Athena’ total yield as affected by
biofumigation with Brassica cover crops and soil treatments in 2009.* ................. 75

Table 3.20. Eggplant ‘Classic’ yield in kg/plant, fruit no./plot, and harvest index
(fruit weight divided by total plant weight) as affected by biofumigation with
Brassica cover crops and soil treatments in 2008* ................................................... 76

Table 3.21. Eggplant ‘Classic' yield in kg/plant, fruit no./plot, and harvest index

(fruit weight divided by total plant weight) as affected by biofumigation with
Brassica cover crops and soil treatments in 2009.* ................................................... 77

vii

Table 4.1. Mean monthly and long term (8 year) average temperatures and
precipitation during cover crop and cucurbit growth in 2009 at HRTC Michigan.*“
....................................................................................................................................... 107

Table 4.2. Monthly and long term (8 year) average growth degree days (GDD)
during cover crop and cucurbit grth in 2009 at HTRC Michigan.* Base
temperature = 10°C.“ ................................................................................................. 108

Table 4.3. Cover crop biomass production (dry weight) for field experiments
conducted in 2009 at the Horticulture Teaching and Research Center (HRTC), East
Lansing, MI.* ............................................................................................................... 109

Table 4.4. Emergence percentages of three cucurbit crops planted after oilseed
radish, Oriental mustard, or no cover crop. Each percentage is generally the
average of four replications, each planted with 24 seeds of each crop. OSR= Oilseed
radish, OM=Oriental mustard, CK=control. There was no plant date by cover crop
significance at the P=0.05 level with the exception of D0 cucumber.* .................. 110

Table 4.5. Emergence percentages of three cucurbit crops planted after oilseed
radish, Oriental mustard, or no cover crop. Each percentage is the average of seven
planting dates, each consisting of four replications wherein each replication was
planted with 24 seeds of each crop. OSR= Oilseed radish, OM=Oriental mustard,
CK=contr01111

Table 4.6. Number of remaining vines left at harvest (out of 12) of cucumber
planted after oilseed radish, Oriental mustard, or no cover crop. Each value is the
average of four replications. OSR= oilseed radish, OM=Oriental mustard,
CK=controI ................................................................................................................... 112

Table 4.7. Oilseed radish harvest and non-Iyophilized aqueous extract data and
corresponding field equivalent rates. The ratio of DI water to biomass was 1:1

(L:kg) ............................................................................................................................ 113

Table 4.8. Oilseed radish harvest and Iyophilized extract data and corresponding
field equivalent rates ................................................................................................... 114

Table 4.9. Oilseed radish non-Iyophilized and lyophilized aqueous extract
treatments. Root and shoot concentrations were the same.* ................................. 115

viii

Table 4.10. Properties of oilseed radish non-Iyophilized and Iyophilized root and
shoot aqueous extracts and deionized water. Values in parentheses are the
standard deviations; values are the average of 8 readings. Values are for the full
strength concentration (100% and 8 g/ L) of extracts ............................................. 116

Table 4.11. Effect of an acetic acid solution on muskmelon seed germination and
root length. Numbers in parentheses are standard deviations ............................... 117

Table 4.12. Germination rates of cucurbits exposed to seven concentrations of
oilseed radish Iyophilized shoot aqueous extract. Each value is the average of three
experimental runs, each consisting of three replications of ten seeds each .......... 118

Table 4.13. Germination rates of cucurbits exposed to seven concentrations of
oilseed radish Iyophilized root aqueous extract. Each value is the average of three
experimental runs, each consisting of three replications of ten seeds each. Data
could not be combined due to trial by treatment interaction ................................... 119

Table 4.14. Radicle lengths (mm) of cucurbits exposed to seven concentrations of
oilseed radish Iyophilized shoot aqueous extract. Each value is the average of three
experimental runs, each consisting of three replications of ten seeds each.
Averages were calculated using the number of seeds germinated, not the number
of seeds tested ............................................................................................................. 120

Table 4.15. Radicle lengths (mm) of cucurbits exposed to seven concentrations of
oilseed radish Iyophilized root aqueous extract. Each value is the average of three
experimental runs, each consisting of three replications of ten seeds each.
Averages were calculated using the number of seeds germinated, not the number
of seeds tested. Data could not be combined due to trial by treatment
interaction ................................................................................................................... 121

Table A1 Number of cucumber, muskmelon, and honeydew vines left 30 d after
planting and dry vine weight (g). The maximum number of possible vines left for
muskmelon and honeydew was 24; for cucumber it was 12. A weight of 0 means
the vine mass was too small for the scale to detect (pg. 135-138) ......................... 135

LIST OF FIGURES
Images in this thesis are presented in color.
Figure 2.1. Eggplant ‘Classic’ leaf displaying symptoms of verticillium wilt ............ 3 1
Figure 2.2. Eggplant ‘Classic’ plants displaying symptoms of verticillium wilt........32

Figure 4.1. Individual fresh vine weights (g) at harvest of cucumber planted after
oilseed radish, Oriental mustard, or no cover crop. Each value is the average of four
replications. OSR= oilseed radish, OM=Oriental mustard, CK=control. NS (not
significant) at the p-level of 0.05 for differences among cover crop treatments...122

Figure 4.2. Average cucumber fruit produced per plant from seeds planted after
oilseed radish, Oriental mustard, or no cover crop. Values are the averages of all
seven planting dates. OSR= Oilseed radish, OM=Oriental mustard, CK=control. NS
at the P=0.05 level for differences between cover crop treatments with the
exception of the cull fruit. Different letters indicate significant difference; Means
followed by the same letter do not differ significantly at the P=0.05 level. NS is not
significant; LSDo.os = 0.15 ............................................................................................ 123

Figure 4.3. Cucumber marketable yield (g) per plant from plants planted after
oilseed radish, Oriental mustard, or no cover crop. OSR= Oilseed radish,
OM=Oriental mustard, CK=control. NS (not significant) at the P=0.05 level for
differences between cover crop treatments ............................................................ 124

Figure 4.4. Cucumber total yield (g) per plant from plants planted after oilseed
radish, Oriental mustard, or no cover crop. OSR= Oilseed radish, OM=Oriental
mustard, CK=control. NS (not significant) at the P=0.05 level for differences
between cover crop treatments ................................................................................. 125

Figure 4.5. Yield (g) per cucumber vine in each treatment averaged over the seven
plant dates. Bars with different letters are significantly different at the P=0.05
level. LSD = 43.35. Standard error bars are 5% of each value .............................. 126

Figure 4.6. Germination percentages of three cucurbit crops exposed to six extract
concentrations of non-Iyophilized oilseed radish root aqueous extract. Each
percentage is the average of three experimental runs, each consisting of three
replications of ten seeds each .................................................................................... 127

Figure 4.7. Germination percentages of three cucurbit crops exposed to six extract
concentrations of oilseed radish non-Iyophilized shoot aqueous extract. Each
percentage is the average of three experimental runs, each consisting of three
replications of ten seeds each .................................................................................... 128

Figure 4.8. Radicle lengths (mm) of cucurbits exposed to six concentrations of
oilseed radish non-Iyophilized shoot aqueous extract. Each value is the average of
three experimental runs, each consisting of three replications of ten seeds each.
Averages were calculated using the number of seeds germinated, not the number
of seeds tested ............................................................................................................. 129

Figure 4.9. Radicle lengths (mm) of cucurbits exposed to six concentrations of
oilseed radish non-Iyophilized root aqueous extract. Each value is the average of
three experimental runs, each consisting of three replications of ten seeds each.
Averages were calculated using the number of seeds germinated, not the number
of seeds tested ............................................................................................................. 130

xi

CHAPTER 1: Introduction

CHAPTER 1: Introduction

Sustainable agricultural methods have become increasingly popular due to
consumer demand, increased input costs, and more stringent regulation of
pesticides. “Sustainable" is an amorphous term as there is no set definition, but
most definitions account for economics, environment, and social issues (Pannell
and Schilizzi 1999). Sustainable practices are viewed as being economically
feasible for producers, as environmentally low-impact as possible, and fair to the
involved human parties (Pannell and Schilizzi 1999). Different producers may
place emphasis on one or more aspects of the definition of sustainability, or
consider all equally (Pannell and Schilizzi 1999). In some cases, a single practice
can incorporate all three elements: reduced pesticide use saves money while
exposing the environment and farm workers to smaller amounts of potentially
harmful chemicals. In some cases, a given production method may be sustainable
in one way but less so in another (Pannell and Schilizzi 1999). For example,
decreased herbicide use, while beneficial to the environment, requires use of more
expensive and physically demanding weed management methods such as manual

hoeing and/ or use of fossil fuels for cultivation.

Producers employ a variety of methods that can be classified as sustainable
(Reganold et al. 1990). These practices often result in decreased pesticide use,
preservation of the farmland quality, and increased profit margins (Reganold et al.
1990). Depending on the crop and a producer’s specific needs/concerns, one or
more methods may be used (Reganold et al. 1990). Crop rotation can reduce pest
and disease pressure (Reganold et al. 1990). Use of row covers excludes insects

2

from crops and allows for earlier plant dates. Plastic mulch decreases need for
herbicides and maximizes water use. Reduced and no tillage systems prevent

erosion and preserve soil structure.

Cover crops provide a variety of benefits; interest in them and use of them
has increased (Mutch and Snapp 2003). Programs such as Sustainable Agriculture
and Food Systems at Michigan State University actively promote the use of cover
crops (SAFS 2009), since they can preserve soil quality, suppress weeds, aid in

nutrient cycling, and serve as animal feed (Mutch and Snapp 2003).

Brassica cover crops are of particular interest because they produce
glucosinolates. When plants are flailed and tilled into the soil, glucosinolates
degrade to biocidal isothiocyanates (lTCs). ITCs impact soilborne pathogens
including Phytopthora spp. (Dunne et al. 2003) and Fusarium gramineanim
Schwabe, Rhizoctonia solam' ].G. Kuhn, and Gaeumannomyces graminis var. tritici J.
Walker (Kirkegaard et al. 1996, Sarwar et al. 1998). 2-phenylethyl ITC and
propenyl ITC (two common by-products of glucosinolate break down in Brassicas)
are more efficient at suppressing fungal pathogens than methyl ITC (a synthetic
fumigant) (Sarwar et al. 1998). Studies have also shown ITCs to suppress weed
seed germination, a tool which must be used with caution (Norsworthy et al. 2006,
Norsworthy and Meehan 2005a, Norsworthy and Meehan 2005b). Haramoto and
Gallandt (2005a) found Brassica cover crops decreased cash crop stand count by
23-34% and delayed emergence by roughly 2 days. However, this impact was no

different than other short-season cover crops (such as red clover).

Brassica cover crops provide many of the same benefits as other cover
crops by improving soil structure and aeration, and aiding in nutrient cycling
(potentially reducing fertilizer rates while optimizing yield). Wang et al. (2008)
found celery and onion production systems in Michigan benefit from the nutrient
cycling properties of Brassica cover crops. Collins et al. (2007) determined 29% of

N uptake by a mustard crop was later recycled by the potato cash crop.

There is minimal research on use of Brassica cover crops in Michigan,
particularly as spring cover crops. Given interest in cover crops, problems
confronting Michigan growers, need for scientific research in sustainable
agriculture, and Brassica cover crop potential, it is time this deficiency is
addressed. We hypothesize that due to their production of glucosinolates and their
nutrient cycling capabilities, Brassica cover crops will reduce impact of soilbome
diseases, promote cash crop growth after establishment, improve crop yield, and

affect crop germination and establishment in vitro as well as in vivo.

Objectives of this work are to: a) determine effects of Brassica cover crops
on verticillium wilt incidence in eggplant and b) determine effects of Brassica

cover crops on cucurbit and eggplant establishment, growth, and yield.

CHAPTER 2: Literature Review

CHAPTER 2: Literature Review

Cash Crops

Michigan ranks second to California in terms of agricultural diversity,
contributing 71.3 billion dollars to Michigan’s yearly economy (based on estimates
using data from 2007) (Holton 2009). Two crops contributing to this diversity are
eggplant (Soianum melongena L.) and muskmelon (Cucumis melo L. Group
Reticulatus). In 2005 about 223 ha (550 A) of muskmelon worth $2.4 million were
harvested; in the same year, 85 ha (210 A) of eggplant worth $1.11 million were

harvested (M DA 2006).

Eggplant and muskmelon are warm season crops requiring a long season to
produce fruit (days to maturity vary by cultivar and climate conditions and range
from 80-120). Transplants are set in the field as soon as temperatures reliably stay
above 13° C (55° F) (about the first week of june). Both crops favor well drained,
sandy to loamy soils with a pH of 6.0-6.8 (Kemble 1996, Kemble et al. 1998, OVG
2010). Eggplant are heavy feeders; general recommendations are to apply 128
kg/ha (114 lb/A) N and 128 kg/ha (114 lb/A) each of P205 and K20 before
installing plastic mulch (OVG 2010). General recommendations are to apply 56
kg/ha (50 lb/A) N, 56-112 kg/ha (50-100 lb/A) P205, and 112-168 kg/ha (100-150
lb/A) K20 before laying plastic mulch, if using fertigation (OVG 2010). Soil tests to
determine the amount of P and K needed are advisable (OVG 2010). Under
favorable weather conditions, eggplant are picked twice a week starting early

August and ending late September; muskmelons are harvested three to four times

starting mid july and ending mid September though in hot weather they may

require picking every other day (OVG 2010).

Both crops are prone to disease and insect infestation. Aphids, Colorado
beetles (eggplant) and cucumber beetles (muskmelon), flea beetles, and mites may
infest eggplant and muskmelon (Kemble 1998 et al., Kemble 1996, OVG 2010).
According to Kemble (1996), aphids are best controlled through weed eradication
along field edges and reflective mulches while the Ohio Vegetable Production
Guide (OVG) (2010) states cucumber beetles may be controlled via insecticides.
Mites can be controlled by scouting and spot-applications of miticides; flea beetles
are best controlled through use of row covers (in small and/or organic operations)
(Kemble 1996) and insecticides (OVG 2010). Muskmelon is prone to bacterial wilt
(Erwim'a tracheiphilia Smith), powdery mildew (Ersiphe cichoreacearum D.C.),
downy mildew (Pseudoperonospora cubensis Berk. 8: MA Curtis), anthracnose
(Colletotrichum Iagenarium (Pass) Ellis 8!. Halst), fusarium wilt (Fusarium
oxysporum f. sp. melonis W.C. Snyder 8!. HM Hansen), altemaria leaf spot
(Alternaria cucumerina (Ellis & Everh.) j.A. Elliott), and damping-off (Pythium spp.)
(Kemble 1996, OVG 2010). These diseases are controlled through a combination of
plant resistance, crop rotation, fungicides, weed control, and careful fertilization
(Kemble 1996, OVG 2010). F usarium wilt is the soilborne disease of most concern
in Michigan muskmelon production (Hausbeck, personal communication).
Common eggplant diseases include altemaria leaf spot (Alternaria solam' Sorauer),
anthracnose (CoIIeotn'chum lagenarium (Pass) Ellis 8: Halst), and verticillium wilt
(Verticillium dahliae Kleb.) (OVG 2010). These diseases are controlled through

7

crop rotation, fungicides, and choosing well-drained sites (OVG 2010). Verticillium
wilt is of particular concern in eggplant production in Michigan due to the
difficulty of controlling it without methyl bromide (Ngouajio, personal

communication).

Verticillium dahliae is a fungal pathogen with a simple life cycle. It thrives
under temperatures of 25-28° C (Agrios 2005). It produces short-lived conidia as
well microsclerotia; the microsclerotia are its resting structure (Agrios 2005).
Verticillium dahliae overwinters via microsclerotia in the soil and mycelia in
infected plant debris (Agrios 2005). Symptoms develop slowly and first appear on
lower/outer parts of the plant; leaves develop chlorotic lesions that turn necrotic
and then drop off, while upper leaves may wilt (Agrios 2005) (Figures 2.1 and 2.2).
Verticillium wilt gradually becomes more severe and virulent as inoculum
accumulates over the years (Agrios 2005). It is controlled through crop rotation,
use of resistant cultivars and disease-free plants, soil fumigation, and soil

solarization (Agrios 2005).

Methyl Bromide

Methyl bromide (MB) is a preplant soil fumigant and a post-harvest
commodities fumigant (Carpenter et al. 2000; Ristaino and Thomas 1997). It has
been used to control weeds, soilborne pests such as nematodes, and plant
pathogens including fungi and bacteria (Carpenter et al. 2000; EPA 2009c). Methyl
bromide is of particular value in controlling verticillium wilt (Wilhelm 1980).

Methyl bromide historically has been used primarily on strawberries, peppers,

ornamentals, tobacco, grapes, and melons (Ristaino and Thomas 1997). California
and Florida growers relied most heavily on it (Carpenter et al. 2000). Methyl
bromide is reported to be harmful to the ozone layer; for this reason, provisions
were made in the 1992 Montreal Protocol on Substances that Deplete the Ozone
Layer and the 1991 US. Clean Air Act to phase out MB (Carpenter et al. 2000; EPA
2009b). Today it can only be used with a special exemption; Michigan, among
other states, has an exemption to control soilbome diseases in muskmelon and
eggplant production (EPA 2009a). Methyl bromide is becoming increasingly
expensive and its use is highly restricted, making the search for alternatives of

paramount importance (Ngouajio, personal communication).

Researchers have been searching for years for viable alternatives to MB; as
yet their success has been varied. Hausbeck (2007) has stated finding alternatives
is vital for growers of crops in the Solanaceae (eggplant family) and Cucurbitaceae
(squash and melon family). The most viable alternatives to MB will likely be
combinations of several methods (including development/use of disease resistant
cultivars and use of cover crops) which decrease soilbome pathogen levels while
encouraging plant grth (Carpenter et al. 2000; Martin 2003; Ristaino and
Thomas 1997). Martin (2003) also notes past reliance on MB has indirectly led to a
dearth of information on specific pathogens and how they interact with their hosts
(as well as pathogen control methods other than MB), and remedying this situation
will likely lead to new production practices. Suitable alternatives to MB will
probably vary by crop and region (Carpenter et al. 2000; Martin 2003). The

current regulatory climate is such that chemical based fumigants will likely be

9

 

highly restricted, and restrictions (e.g. buffer zones) will make use difficult E
(Duniway 2002).

According to Duniway (2002), there are no alternatives to MB that are as
effective, multi-spectrum (against pathogens, weeds, and nematodes), and easy to
apply. Currently registered MB alternatives include chloropicrin (CP), telone (1,3-
D), methyl isothiocyanate (MITC) generators like metam sodium and dazomet,
methyl iodide, and propylene oxide; potential MB fumigant alternatives which are
undergoing EPA registration include sodium azide, propargyl bromide, and
dimethyl disulfide (EPA 2009c). Methyl bromide alternatives are not likely to be
used separately, but rather would be mixed together or applied sequentially
(Duniway 2002). Such MB alternatives are not themselves without hazards. All
synthetic fumigants create concerns in regards to groundwater contamination,
worker exposure, and chronic exposure (Duniway 2002). Only chloropicrin, 1,3-D,
and metam sodium are broad spectrum enough to be considered likely
replacement candidates (Duniway 2002). Methyl bromide is currently applied in
conjunction with CP (in addition to its pest suppressive capacities, CP is an easily
detectable chemical that serves as an indicator of MB presence /exposure; MB is
odorless and lethal). However, CP alone is not likely to be a complete alternative as
it is not effective against nematodes and some weeds (Ristaino and Thomas 1997).
Metam sodium is viewed as unreliable due to application difficulty; it does not
move easily through soil (Duniway 2002). Duniway (2002) states, however, that
its efficacy could be optimized. Metam sodium does not mix well with other

fumigants but could be applied after other fumigants (sequentially) to increase

10

 

weed and pathogen control (Duniway 2002). Dazomet can also be difficult to L
apply; care must be taken to avoid phytotoxicity (Duniway 2002). Methyl iodide
when combined with CP (50:50 ratio) works as well as MB:CP to destroy V. dahliae
Kleb inoculum (Duniway 2002). The type and application method of MB
alternative used will depend on the crop and system (Duniway 2002).

Cover Crops

One avenue of inquiry in the search for MB alternatives involves use of
cover crops, which provide a number of benefits. Cover crops help prevent erosion,
increase nutrient cycling, and add soil organic matter. According to Snapp et al.
(2005), routinely incorporating cover crops into a production system leads to an
increase in soil organic matter content, which improves soil physical aspects such
as water and nutrient holding capacities and aeration; these benefits often lead to
increased cash crop yields. Collins et al. (2007) found 29% of the N uptake by a
mustard crop was later recycled by the potato cash crop, meaning the cover crop
can reduce amount of N fertilizer that needs to be used on even a heavy feeding
crop like potatoes. Oilseed radish (Raphanus sativus (L.) var. oleiferus Metzg
(Stokes)) is an efficient scavenger of N; the Daikon cultivar has been shown to
recycle more than 22 kg/ha (20 lb/A) N in two months on a muck soil (Ngouajio

and Mutch 2004).

A variety of cover crops, including Brassicas, are grown in Michigan (Mutch
and Snapp 2003). Brassica cover crops are cool-season annuals characterized by
deep taproots, broad leaves, and small seeds; they originated in the Mediterranean
(Snapp et al. 2006). They can germinate in soils as cool as 4°C (40° F), grow to

11

 

heights of 76-102 cm (30-45 in.) with roots 31-91 cm (1-3 ft) long, and mature in L
4—6 weeks (Snapp et al. 2006). Two broad categories of Brassicas are typically
grown as cover crops in Michigan: mustards (Brassicajuncea (L.) Czern., B. nigra
(L.) W.D.j. Koch, B. napus L., and B. hirta Moench) (Snapp et al. 2006) and oilseed
radish (N gouajio and Mutch 2004). Mustards prefer well-drained, neutral to
slightly acidic or basic soil and large amounts of moisture (Snapp et al. 2006);
oilseed radish tolerates moderate drought (Ngouajio and Mutch 2004). Mustards
are generally sown at 10-17 kg/ha (9 - 15 lb/A) (Snapp et al. 2006) while oilseed
radish is sown at 11-22 kg/ha (10 - 20 lb/A) (Ngouajio and Mutch 2004). If soil
tests reveal the need for fertilizer, N can be applied at 112 kg/ha (100 lb/A); it is
also advisable to add sulfur (6:1 N:S ratio), given the crucial role this element plays
in production of biocidal compounds (Snapp et al. 2006). When grown as a cover
crop, Brassicas do not suffer from diseases or pests severely enough to warrant
control methods, other than growing them in rotation with non-Brassica crops
(Ngouajio and Mutch 2004, Snapp et al. 2006).

Brassicas are quantitative long day flowerers (long days hasten flowering),
though carbohydrate supply plays a role (plants can be induced to flower under
short day conditions with addition of sucrose to the growth medium) (Friend et al.
1984). Some mustard species are highly sensitive to long days, making them
difficult as a spring crop due to flowering occurring before peak biomass
production has occurred (Snapp et al. 2006). Brassica juncea (L) Czern. ‘Pacific
Gold’ can produce 2,240 kg/ha (2,000 lb/A) of biomass as a spring crop, and 3,360-

5,600kg/ha (3,000—5,000 lb/A) of biomass as a fall crop (Snapp et al. 2006).

12

Oilseed radish can produce 8,960-11,200 kg/ha (8,000-10,0001b/A) of dry
biomass (Ngouajio and Mutch 2004). While they are a cool season crop, mustards
and oilseed radish do not survive hard freezes (air temperatures below -4°C (25°
F)), especially once they are past the seedling stage (Ngouajio and Mutch 2004,
Snapp et al. 2006). Ideally Brassicas are flailed and tilled in to the soil at full
flowering; irrigation should be applied before and after the process to maximize
biofumigation (Snapp et al. 2006). Plants should not be allowed to go to seed as
they can become weedy (Snapp et al. 2006).

Aside from the general benefits of cover crops, Brassica species are
potentially valuable tools because they are allelopathic. The term ‘allelopathy‘ was
first used by Molisch (1937) to describe the phenomenon wherein plants influence
each other via chemical means. Originally the term encompassed both positive and
negative effects, but currently it typically means the ‘inhibitory effect of a
compound added to the environment’ (Choesin and Boerner 1991).
Allelochemicals are common in the plant world and include organic acids,
alkaloids, alcohols, aldehydes, glycosides, tannins, and terpenes (Szczepanski 1977).
A variety of factors dictate levels at which allelochemicals are present in the
environment. Such factors include plant species, quantity and type of
allelochemical produced, and the environment itself (e.g. soil composition). Crop
and weed species vary in susceptibility to allelochemicals (Oleszek 1987).

The allelopathic compounds of interest in Brassicas such as yellow mustard,
oilseed radish, and Oriental mustard include glucosinolates. Glucosinolates are

sulfur based compounds comprised of a thioglucose group, an R-group (carbon

13

 

".

.b 0’ -‘a ar’ _

side chain) which gives each compound its name, and a sulphonated oxime ..
(Mayton et al. 1996). When in the presence of myrosinase enzyme and water they

break down to form isothiocyanates (ITCs), organic cyanides, oxazolidinethiones,

and ionic thiocyanate (Kirkegaard and Sarwar 1998). The glucosinolates isolated

from Brassica species are either aliphatic, aromatic, or indolyl in structure (Brown

and Morra 1997; Kirkegaard and Sarwar 1998). Some are highly volatile while

others are water soluble (Brown and Morra 1997). Major glucosinolates found in

Brassica cover crops include sinigrin (Oriental mustard) and glucosinalbin (yellow

mustard) (Kirkegaard and Sarwar 1998), among others (Table 2.1).

These compounds have been shown to exhibit biocidal properties that
impact a variety of pathogens in the soil (Brown and Morra 1997; Sarwar et al.
1998) (Table 2.2). Soil characteristics such as water content influence breakdown
of glucosinolates into ITCs and help determine how much of an effect addition of
cover crops to the soil will have (Brown and Morra 1997, Morra and Kirkegaard
2002). Warm temperatures and microbial activity decrease phytotoxicity while
low temperatures lead to higher phytotoxicity (Mason-Sedun and lessop 1986).
Mason-Sedun and jessop (1986) determined rate of phytotoxicity decrease is
greatest at 24° C and least at 0° C. Some glucosinolates are more potent than
others. According to Bialy et al. (1990), allyl and 2-phenethyl ITCs are highly active
compounds. ITC quantities matter, as well; the more residue present, the greater
the phytotoxic effects (Mason-Sedun and jessop 1986). Turk and Tawaha (2002,

2003) demonstrated as concentrations of B. nigra aqueous extracts increased,

14

 

germination rates of lentil (Lens culinaris Medik) and wild oat (Avenafatua L.) I:

decreased.
Cover Crops and Soilborne Pathogens

Brassica cover crops are of interest because of their potential to decrease
soilborne pathogen populations via volatile and water soluble compounds. Volatile
compounds disperse completely through the soil, making them effective potential
anti-pathogen compounds (Mayton et al. 1996).

Brassica cover crops can impact soilbome pathogen populations (Table 2.2)
and decrease disease incidence/ severity. Broccoli residues were found to decrease
V. dahliae microsclerotia, as well as decrease incidence and severity of verticillium
wilt in cauliflower by 50% (Xiao et al. 1998). According to Mayton (1996), B.
juncea L and B. nigra L. inhibited Fusarium sambucinum Fuckel. Dunne et al.
(2003) determined B. juncea suppressed fungi more effectively than B. napus, and
suppression was generally due to the fungicidal properties of compounds
contained in cover crop residues. Sarwar et al. (1998) demonstrated ITCs were
more effective than synthetic chemicals on some fungal species.

The literature does not conclusively support the notion that use of Brassica
cover crops results solely in a decrease in pathogen populations or disease levels.
Bensen et al. (2009) discovered that while B. juncea and S. aIba cover crops
decreased disease in lettuce in the short term, with long term use there was no
significant decrease in disease. Brassica napus was found to encourage pathogen

growth: in an experiment by Mayton et al. (1996), B. napus increased F.

15

 

sambucinum growth. Mazzola et al. (2001) determined high-glucosinolate B. napus '-
seed meal did not consistently suppress Pythium spp. in apple orchard soil. Njoroge
et al. (2008) discovered Pythium spp. populations increased in some plots in which
Brassica cover crops were used as a green manure compared to an untreated
control (though plots planted with Brassicas had higher levels of Pythium spp.
compared to unplanted plots to begin with). In another experiment, B. napus, B.
juncea, and S. alba did not affect V. dahiiae or Fusarium spp. population levels, nor
did they suppress disease in the following processing tomato crop (Hartz et al.
2005). Wiggins and Kinkel (2005) found B. napus failed to decrease verticillium
wilt and potato scab disease (when disease pressure was medium-high). Brassica
napus is also ineffective against Pythium spp., R. solani, and nematodes (johnson et
al. 1992). Brassicajuncea and S. alba failed to decrease Sclerotim'a minor jagger
soil sclerotia levels (Bensen et al. 2009). Sometimes the results from Brassica
cover crop use are mixed. While a broccoli (Brassica oleracea var. italica Plenck)
green manure failed to decrease V. dahliae infection of potato, a decrease in
inoculum levels and disease severity was observed (Ochiai et al. 2007). Pinkerton
et al. (2000) discovered B. napus decreased Phytophthora cinnamomi Rands and V.
dahliae populations somewhat, but did not decrease incidence of verticillium wilt.
A number of factors determine how effective Brassica cover crops are at
decreasing pathogen populations and may help explain contradictory findings.
Levels and composition of glucosinolates play a role: higher levels of biofumigants
tend to be more effective (Blok et al. 2000; Dunne et al. 2003). Mayton et al. (1996)

attributed failure of B. napus to decrease F. sambucinum growth to its lack of allyl

16

 

”‘1‘

[TC production. Furthermore, Brassica cover crops produce less active compounds
than would be used to treat a field if they were synthetic fumigants (Hartz et al.
2005). Glucosinolate degradation is highly inefficient under field conditions,
meaning it is possible insufficient levels of lTCs result (Hartz et al. 2005, Morra and
Kirkegaard 2002). Different cover crops produce different levels of key
compounds, as do the same cultivars at different sites (Hartz et al. 2005): Njoroge
et al. (2008) found B. juncea produced more glucosinolates per in2 than B. napus.
Sarwar et al. (1998) showed different ITCs have different degrees of toxicity; in
general, the shorter the ITC chain the greater the toxicity. Sarwar et al. (1998) also
demonstrated ITC toxicity is partially dependent on application method (in vitro,
ITCs applied to the headspace of the container resulted in different toxicity levels
than those mixed with the plating medium). Innate susceptibility of each pathogen
is a factor. Dunne et al. (2003) determined Phytophthora spp. had differing levels of
susceptibility, and a pathogen may be susceptible to one Brassica cover crop but
tolerant of others. Another consideration is pathogen structure being tested, since
‘fungal biotypes’ differ in sensitivity to lTCs (Sarwar et al. 1998). Some pathogens,
such as Sclerotim'a sclerotiorum (Lib.) de Bary, can adapt to the presence of ITCs,
diminishing ITC efficacy (Rahmanpour et al. 2009). Soil type may play a role, as
soils with high cation exchange capacities (CECs) may be prone to binding to lTCs
and thus decreasing ITC activity (Goldy, personal communication). Finally, Blok et
al. (2000) postulate that in cases where the cover crop is tilled under and then

covered with air-tight plastic, glucosinolates play a secondary role while it is

17

primarily fermentation and creation of anaerobic conditions that decreases

pathogen populations.

The literature is divided over the notion that glucosinolates alone are
responsible for Brassica cover crop impacts on pathogens (Larkin and Griffin
2006; Mazzola et al. 2001). Cohen and Mazzola (2006) have demonstrated low-
glucosinolate Brassica seed meal can change composition of soil microbe
communities, leading to increases in Pythium spp. population. Total bacterial and
actinomycete populations (including Pythium spp.) increased in soil treated with B.
napus seed meal (Mazzola et al. 2001). Hoagland et al. (2008) found low-
glucosinolate B. napus and S. alba seed meal amendments also lead to an increase
in Pythium spp. populations. The ways in which Brassica biomass can impact
microbial communities are diverse and intricate; one such way is by serving as a
carbohydrate source for sufficiently opportunistic organisms (Cohen and Mazzola
2006). Furthermore, Brassica species are themselves vulnerable to pathogens: one
such pathogen is Altemaria spp. (Ishimoto et al. 2000). Lu et al. (2010) determined
that when a susceptible Brassica cover crop cultivar was grown, F. oxysporum
(varying races) populations increased; when a resistant Brassica cover crop was
grown, populations decreased (Lu et al. 2010).

Cover Crops and Seed Germination

Studies have tested impact of the active chemical components in Brassica
cover crops on weed seed germination and weed density and biomass production.

Norsworthy and Meehan (2005a) found low ITC levels stimulate weed emergence

18

 

-n. 1.:-
w

 

while at the highest concentrations they suppress weed seed emergence by at least is
37%. According to Norsworthy et al. (2006), susceptibility varies across weed
species: they found purple nutsedge is more easily suppressed by lTCs than yellow
nutsedge. In some cases, the effect can be thorough: ITCs were found to decrease
emergence of Texas panicum by 98%, while emergence of large crabgrass was
reduced 98%-100% (Norsworthy and Meehan 2005a). Different forms of ITCs
have differing levels of impact; in general, the two most effective ITCs are the
phenyl and 3-methylthiopropyl forms (Norsworthy and Meehan 2005b).
Norsworthy and Meehan (20053) also determined application techniques help
determine efficacy: loss of volatilized compounds needs to be minimized. Even
though weed emergence may be reduced, without competition weed biomass may
not be reduced (Norsworthy and Meehan 2005a). Wang et al. (2008) reported
Brassica cover crops reduce weed density (compared to the cover crop-less
control) and affect composition of weed communities that do become established,
but also indicated additional methods were still needed to achieve adequate weed

control.

Brassica cover crops can impact cash crop germination and growth.
Haramoto and Gallandt (2005a) found these cover crops decreased average stand
count of bioassay species by 23-34% and delayed emergence by roughly 2 days
under field conditions. They also found, however, that Brassica cover crops did not
significantly differ from other short season cover crops (such as red clover) in

impact on cash crop establishment.

19

Multiple laboratory experiments have shown that compounds produced by
Brassicas can impact germination and growth. Turk and Tawaha (2002) found B.
nigra aqueous extracts decreased lentil germination and inhibited lentil seedling
growth. They determined plant radicles are more sensitive to extracts than
hypocotyls (2002, 2003). Turk and Tawaha (2003) found B. nigra aqueous extracts
negatively impacted wild oat germination and seedling growth. According to
Brown and Morra (1996), the types of glucosinolates present (not just
glucosinolate concentration) determine level of seed germination inhibition. Bialy
et al. (1990) found concentrations of 500 ppm of allyl, benzyl, and 2-phenethyl
ITCs would lead to 30, 10, and 100% wheat (Triticum aestivum Songle) seed
germination inhibition. At 300 ppm 2-phenethyl ITC inhibited wheat seed
germination by 40% (Bialy et al. 1990). Bialy et al. (1990) found 300 ppm of 2
phenethyl ITC retarded wheat root growth by 97% and wheat coleoptile growth by
96%. According to Oleszek (1987), B. nigra and B. juncea volatiles caused the
most damage of several Brassica species tested on wheat, barnyard grass
(Echinochloa crus-galli (L.) P. Beauv.), and lettuce (Lactuca sativa L.). Oleszek
(1987) determined Cruciferous volatile compounds severely inhibit lettuce
germination. Water soluble compounds from B. napus also inhibit lettuce
germination (Brown and Morra 1996). The plant part the compounds are derived
from can impact germination and growth. Brown and Morra (1996) determined
volatile compounds from B. napus roots inhibited lettuce germination more than
those from stems or leaves. Turk and Tawaha found B. m’gra leaf extracts tend to

be more toxic than extracts fi‘om other plant parts such as roots and stems (2002,

20

 

2003). Brown and Morra (1996) also determined water soluble compounds from i
B. napus roots delayed lettuce germination, whereas those from leaves and stems
completely inhibited it. Turk and Tawaha (2003) postulate B. nigra’s allelopathic
effects are short term and mainly impact germination.

Not all studies support the conclusion that glucosinolate byproducts are
phytotoxic in the field. Choesin and Boerner (1991) found B. napus appears not to
be allelopathic - allyl ITCs applied at the levels typically found in soil had no
impact on ‘target plants' (Medicago sativa L). Further, different B. napus genotypes
producing differing levels of allyl ITC had no difference in their impact on M. sativa
(Choesin and Boerner 1991). Green bean (Phaseolus vulgaris L) growth and yield
were not affected by Brassica cover crop incorporation (Haramoto and Gallandt
2005b). Brassica impacts on germination and growth are dependent on both
Brassica species and weed/ crop species (Oleszek 1987), perhaps explaining why
M. sativa and green bean showed no response. Some cash crops may benefit from
allelochemicals: evidence indicates that at low levels, Brassica residues can

stimulate plant grth (Mason-Sedun and jessop 1986).

A variety of factors impact effect of Brassica cover crops on seed
germination besides compound type and concentration. Soil type is one such
factor. Mason-Sedun and lessop (1986) found Brassica residues incorporated in to
a sandy soil have a greater impact (delayed emergence, reduced growth/yield) on

wheat than those incorporated in to a clay soil.

21

a. {ll-ill

Allelopathy alone may not explain impact of Brassica cover crops on other 5—
plants; cover crop interactions with microbes also likely play a role. Cohen and
Mazzola (2006) found Pythium infections were greatest when seedlings were
planted immediately after Brassica seed meal incorporation; a delay of 4 weeks
greatly decreased infection rates. This differs from a study by Mazzola et al. (2001)
in which B. napus L seed meal was found to be phytotoxic to apple seedlings, even
12 weeks after soil incorporation. Mazzola et al. (2001) state glucosinolate by-
products alone are unlikely to be the cause. Hoagland et al. (2008) postulated the
observed increase in Pythium populations was at least partly the cause of observed
decreases in weed and wheat seed germination and increases in seedling
mortality. Treatments in the Hoagland experiments (2008) with B. juncea seed
meal suppressed Pythium (likely due to the nature of its ITCs), resulting in less
severe decreases in germination and increases in seedling mortality. Damage

observed by the authors was attributed to the ITCs.

Need for this Work and Objectives

Scientists have been searching for alternatives to MB since the early 19905
(Carpenter et al. 2000). Many of them are investigating chemical fumigants; this is
understandable, given chemicals generally provide rapid, obvious, consistent
results. However, some authors argue chemicals are only a stop gap solution
(Ngouajio, personal communication). If effective, biological alternatives may offer
benefits above and beyond control of one specific problem, and may be one of the
long term solutions growers need. Brassica cover crops are potentially one such
biological alternative and therefore warrant further investigation, especially in

22

relation to how they impact cash crop seed germination and growth. Compounds '-
from plants in the Brassica family impact cash crop germination and growth in the
laboratory (Bialy et al. 1990; Brown and Morra 1996; Mason-Sedun and lessop
1988; Oleszek 1987; Turk and Tawaha 2002); less work has been done in the field.
A fair amount of research has been done in vitro and in the greenhouse; there is a
need to verify these results are valid under field conditions. Different geographical
and climatic regions may require different solutions. Results obtained in other
states may not apply in Michigan. Brassica cover crops need to be evaluated in our
climate and geographic location. They also need to be evaluated in terms of the
system in which they are used. Little research has been done with respect to
Brassica cover crops grown immediately prior to the cash crop in the same
growing season (Ngouajio, personal communication). Brassica cover crops also
need to be evaluated in terms of which (if any) of them is most effective for
addressing specific problems (e.g. soilborne diseases, weed infestations, or

unfavorable soil chemical /biological / physical properties).

23

 

 

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Figure 2.1 Eggplant 'Classic' leaf displaying symptoms of verticillium wilt;

31

 

Figure 2.2. Eggplant ‘Classic' plants displaying symptoms of verticillium wilt.

32

CHAPTER 3: Field Evaluation of Spring-Planted Brassica Cover Crops for
Performance in Muskmelon (Cucumis melo L.) and Eggplant (Solanum

melongena L.) Cropping Systems

33

CHAPTER 3: Field Evaluation of Spring-Planted Brassica Cover Crops for
Performance in Muskmelon (Cucumis melo L.) and Eggplant (Solanum

melongena L.) Cropping Systems

ABSTRACT

A two year field study was conducted in Benton Harbor, M1, to examine
impact of oilseed radish (Raphanus sativus (L) var. oleiferus Metzg (Stokes)),
Oriental mustard (Brassica juncea (L) Czern.), and yellow mustard (Sinapis alba L)
on eggplant (Solanum melongena L) and muskmelon (Cucumis melo L.) growth and
yield. Additional treatments included a microbial amendment (Terra Clean® plus
SoilBuilder'"), fallow (- control) and methyl bromide (+ control). The experiment
had a randomized complete block design with three replications. Cash crops were
rated for vigor, disease presence (eggplant), height (eggplant), yield, and fresh
biomass production (eggplant). All three cover crops reduced stand in direct-
seeded muskmelon and transplant survival; eggplant transplant survival was not
affected. Yellow mustard reduced eggplant growth and plant vigor. Cover crops
did not affect verticillium wilt incidence or eggplant yield. The cover crops tested
should be used with care, and sufficient time should be allowed between cover

crop incorporation and seed planting/cucurbit transplantation.
INTRODUCTION

Only California exceeds Michigan in terms of agricultural diversity;
agriculture contributes $71.3 billion annually to Michigan's economy (Holton

2009). Two crops that contribute to Michigan’s diversity are eggplant (Solanum

34

melongena L) and muskmelon (Cucumis melo L. Group Reticulatus). Growers
harvested about 223 ha (550 A) of muskmelon worth $2.4 million in 2005; in the
same year, growers harvested 85 ha (210 A) of eggplant worth $1.11 million (MDA
2006). Both eggplant and muskmelon are warm season crops. Both crops are
susceptible to a variety of soilbome diseases. Eggplant is particularly susceptible
to verticillium wilt caused by Verticillium dahliae Kleb. This is of particular
concern given the phase-out of methyl bromide, which for years was used to
control soilbome diseases such as verticillium wilt (Wilhelm 1980). While
Michigan producers still have exemptions to use methyl bromide for Solanaceous
and Cucurbitaceous crops (EPA 2009a), these exemptions will likely also be
phased out. Hausbeck (2007) has stated finding alternatives to methyl bromide is

of paramount importance for production of these crops.

Martin (2003) determined there is unlikely to be one general panacea to
replace methyl bromide; rather, a variety of methods will likely be used in
combination. One potential method is use of Brassica cover crops. In addition to
aiding in nutrient cycling, preventing erosion, and improving soil structure (Mutch
and Snapp 2003), Brassica cover crops produce compounds which are toxic to
some soilbome pathogens (Kirkegaard and Sarwar 1998). As an added benefit,
these compounds have also been shown to inhibit weed seed germination

(Norsworthy et al. 2006, Norsworthy and Meehan 2005a and 2005b).

Given their useful properties, short growth cycle (SS-65 days from seed to

flowering), and tolerance of cool temperatures, Brassica cover crops such as

35

yellow mustard (Sinapis alba L), oilseed radish (Raphanus sativus (L) var.
oleiferus Metzg (Stokes)), and Oriental mustard (Brassica juncea (L) Czem.)
should be investigated in an eggplant/muskmelon short rotation system. Since
these cash crops are not transplanted into the field until early June, there is time to
grow Brassica cover crops if they are planted in early April. Since eggplant and
muskmelon require significant amounts of fertilizer, they would likely benefit from
nutrient cycling properties of the cover crops. Given the cash crops’ susceptibility
to soilborne diseases, any reduction in soil pathogen levels by the tilled-under

cover crop would also be beneficial.

There has been minimal research in to use of Brassica cover crops in most
vegetable systems; there has been no research in to use of Brassica cover crops as
an early spring cover crop before muskmelon and eggplant. The objectives of this
study were to determine effects of spring-planted Brassica cover crops on a)
verticillium wilt incidence in eggplant production; b) eggplant growth and yield;

and c) melon stand establishment, plant growth, and yield.
MATERIALS AND METHODS

Experimental Site and Procedures

Yellow mustard 'Tilney', oilseed radish ‘Defender’, and Oriental mustard
‘Forge’ were planted at the Michigan State University South West Michigan
Research and Extension Center (SWMREC) in Berrien County, Michigan, on April 4,
2008 and April 1, 2009. Planting rates in 2008 were: oilseed radish, 22.4 kg/ha

(20.01b/A); Oriental mustard, 8.5 kg/ha (7.6 lb /A) ; and yellow mustard, 10.4

36

kg/ha (9.3 lb/A). In 2009 planting rates were: oilseed radish, 11.2 kg/ha (10.0
lb/A); Oriental mustard, 6.7 kg/ha (6.01b/A); and yellow mustard, 9.0 kg/ha (8.0
lb/A). Cover crops had been planted in these plots in 2007; treatments remained
the same throughout the three years. The soil was an Oakville series fine sand
transitioning to loamy sand. This experiment had a randomized complete block
design with three replications. Individual plots were 135 m2. Treatments were
Oriental mustard, yellow mustard, oilseed radish, bare soil (- control), methyl
bromide (+ control), and microbial amendment. Cover crops were sown using a
John Deere 450 drill. Methyl bromide was applied May 19, 2008 and May 21, 2009
(448 kg/ha (400 lb/A), 50:50 mix of methyl bromide and chloropicrin); plastic
mulch and drip tape were installed at the same time. In the microbial amendment
treatment, Terra-Clean® (BioSafe Systems, 36 Commerce St, Glastonbury CT) (19
L/ha) (2 gal/A) was applied first to disinfest the soil followed two hours later by
SoilBuilder’" (Advanced Microbial Solutions, LLC., SouthPilot Point, TX)
application at the rate of 19 L/ha (2 gal /A) on June 9, 2008 and June 9, 2009. The
active ingredient in Terra-Clean® is the oxidizing agent hydrogen dioxide, which
hypothetically controls Phytophthora spp., Pythium spp., Fusarium spp., Rhizoctonia
spp., Verticillium spp., and Thielaviopsis spp. in the soil (BioSafe Systems, LLC.
2010). The company also claims it can ‘stimulate plant growth, root development,
and nutrient uptake’ (BioSafe Systems, LLC. 2010). Soilbuilder‘“ contains one
million colony-forming units/mL of microbes including Bacillus spp.,
actinomycetes, cyanobacteria, algae, protozoa, and their fermentation by-products

(Advantage Microbial, LLC. 2010). The company claims it can restore soil

37

microbial populations, increase plant nutrient uptake, reduce soil compaction, and
improve water retention (Advantage Microbial, LLC 2010). Cover crops were
flailed and incorporated into the soil at the flowering stage with a rotovator on
June 3, 2008 and June 4, 2009. Care was taken to avoid cross contamination. Beds
were then shaped and covered with plastic mulch; irrigation drip tape was also
installed. The entire plot was split into two with the eggplant (Solanum melongena
L. ‘Classic’) and muskmelon (Cucumis melo L. ‘Athena’) in the north and south side,
respectively in 2008. In 2009 the two crops were rotated.

On June 10, 2008 and June 12, 2009, all crops were planted. Eggplant
transplants were planted in four rows with two middle ones serving as data rows.
In 2008 melon was direct-seeded in four rows as indicated above. In 2009 melon
was both direct-seeded (two rows) and transplanted (two rows) to test the effects
of treatments on melon transplants. The muskmelons were re-seeded June 25,
2008 and June 25, 2009. In addition to guard rows, there were guard plants at the
beginning and end of each row. These were honeydew (Cucumis melo L Group
lnodorus ‘Earlibrew’) in the muskmelon rows and eggplant ‘Ghostbuster’ in the
eggplant rows. In both years eggplants were spaced 0.5 m apart (14 plants per
bed; 13,047 plants/ha (5,280 plants/AD while muskmelons were spaced 1 m apart
(7 plants per bed; 6,523 plants/ ha (2,640 plants/AD; both years two muskmelon
seeds were seeded per hole, were thinned to 7 plants per bed.

Two weeks after planting, eggplants were staked. Three weeks after planting,
fertigation commenced. Plants received 1.1 kg of N / ha (1.01b/A) per week in the

form of a 4-0-8-2 (Ca) fertilizer through the drip irrigation system; irrigation ran

38

three times a week, and fertigation occurred once per week No accounting in the
irrigation schedule was made for rain because beds were covered with plastic and
no excessively heavy rain events occurred during the growing season. Pesticides
were applied as per standard grower practices (Table 3.1). Rows were hoed and
beds hand-weeded as necessary.

Data Collection

Prior to cover crop incorporation plant samples were taken from each cover
crop treatment, in two areas of 50 cm x 50 cm each. The number of plants in each
sample was counted, and samples dried at 60° C for two weeks. Excess soil was
shaken off the samples, and they were weighed. Samples consisted of entire plants
(roots, stems, leaves, and flowers).

Data collection on eggplants and muskmelons began two weeks after
planting, with stand counts. Another stand count was done on the muskmelons
two weeks after re-seeding. Eggplant height was taken weekly. Chlorophyll content
of the eggplant leaves was measured twice in 2008 and once in 2009 using a SPAD
chlorophyll meter. Subjective scores were taken of eggplant plots, also weekly. The
subjective score used a scale of 1 to 10, with 1 being ‘all plants are dead' and 10
being ‘all plants look vigorous, healthy, and productive'. A count of plants showing
symptoms of verticillium wilt was taken in each eggplant plot each week in the
2008 and 2009. On july 28, 2008, 2 eggplant plants from the guard rows were
harvested and sent to the Michigan State University (MSU) Plant Diagnostics Lab to
confirm the presence of verticillium wilt; it was confirmed by isolating Verticillium

dahliae from symptomatic tissue and culturing it on Verticillium-selective medium

39

(MSU Plant Diagnostics Lab, personal communication). Soil samples were taken
every three weeks, on average, using a soil probe and about 20 soil cores per
eggplant plot. Soil was stored in plastic bags in a 4° C cooler. Nitrate extraction
was performed using the KCl method, and then samples were analyzed for nitrate
and ammonia levels by Michigan State University Soil Testing Laboratory.

The first eggplant harvest in 2008 was August 11 ; in 2009 it was August 6.
Harvests continued every two to three weeks until September 28, 2008 and
September 24, 2009. Eggplants were separated in to Grade 1 (US No.1), Grade 2
(US No. 2), or cull (Unclassified) (USDA-AMS 1953). They were then counted and
weighed. Bird damage was not counted against the fruit. On the last harvest date,
all fruits were harvested. Plants were then cut at ground level, and fresh above
ground biomass was weighed.

Muskmelon harvests began August 26, 2008 and August 21, 2009. It was not
until the September 8, 2008 harvest that a re-seeding mistake was realized. The
last harvest date in 2009 was September 24; in that time period melons were
harvested almost weekly. They were sorted as either marketable or cull, counted
and weighed. Marketable fruits met the definitions for US Grade Fancy - Grade 2;
culled fruit did not (USDA-AMS 2008). As with eggplants, bird damage was not
counted against muskmelons.

Statistical Analysis

SAS (version 9.2) was used to perform analysis of variance on the data, then

means were separated using Fisher’ 5 least significant difference at the P = 0.05

level when significant differences in means were detected. Data from 2008 and

40

 

2009 were combined when there was no year by treatment interaction, except
when data varied considerably by year.

RESULTS

Weather

Weather influences crop growth and yield and disease development, so it is
an important consideration in any field experiment. Low temperatures at SWMREC
during April - September were comparable in 2008 and 2009, but 2009
experienced cooler daily maximum temperatures (Table 3.2). April and july of
2009 experienced particularly decreased highs - the average high in April 2009
was 13.80 C vs. 16.290 C in April 2008 while july 2009 high temperatures averaged
24.80 C compared to the 2008 average of 27.50 C. july 2008’s high temperature was
on par with the 8 year average July temperature of 27.70 C.

Growing Degree Days (GDD) in 2009 reflect the slightly lower 2009 average
temperatures (Table 3.3). There were 1,335.7 GDD in 2009 compared to 1421.4
GDD in 2008 and the 8 year average of 1,509.3 GDD. April, lune, july, August, and
September 2009 all had fewer GDD than their counterparts in 2008. july 2009
(peak of the growing season) saw 297.1 GDD compared to 368.1 GDD in july 2008.
May was the only exception to this trend - there were 166.6 GDD in 2009
compared to 113.1 GDD in 2008.

Both 2008 and 2009 were wetter than average. In 2008 SWMREC received a
total of 583.7 mm of rain; in 2009 it was 511.1 mm (Table 3.2). The average is
459.5 mm. While it appears 2008 was rainier than 2009, close to 300 mm of that

total came during a heavy rain event in September 2008. Up until September

41

(during the majority of the growing season), 2009 was wetter than 2008 with a
total of 487.2 mm of rain compared to 2009’s 290.7 mm of rain. April and lune of
2009 received significantly more rain than their 2008 counterparts. Cool weather
slows crop growth, while wet weather encourages disease spread.

Cover Crops

Each of the cover crops produced similar amounts of biomass in 2008 and
2009 (Table 3.4). Oilseed radish produced 6,086 kg/ ha in 2008 and 4,173 kg/ ha in
2009 dry biomass. Oriental mustard produced 3,487 kg/ha dry biomass in 2008
and 2,843 kg/ha biomass in 2009. Yellow mustard values fell in between those of
the other two cover crops both years.

There were significant differences among cover crop biomass production in
2008: oilseed radish produced 6,086 kg/ ha compared to Oriental mustard and
yellow mustard, which produced 3,487 and 3,641 kg/ ha, respectively. There were
no significant differences among cover crop dry biomass production and stand
count in 2009. In 2009, Oriental mustard had a stand of 244 plants /m2, which was
significantly different than the 100.7 and 119.3 plants/m2 of oilseed radish and
yellow mustard.

Soil Nitrogen Levels

The data for 2008 and 2009 were combined as there was no treatment by
year interaction. The nitrate levels in oilseed radish plots were significantly higher
than those in all other plots (Table 3.5) in june of both years. In july the nitrate
levels were significantly higher in Oriental mustard (7.6 ppm) and oilseed radish

(7.3 ppm) plots than in other treatment plots, which had nitrate levels ranging

42

from 4.5 to 5.5 ppm. In September there were no significant differences in nitrate
levels, which ranged from 2.3 to 4.1 ppm. There was no significant difference in
ammonium levels in 2009 (Table 3.6).
Cash Crop Stand Count and Transplant Survival

There were significant differences in muskmelon stand counts in 2008 and
2009. In 2008, plots in methyl bromide and control treatments had 100% stand
count, while those in microbial amendment treatments had 78.6% stand count
(Table 3.7). These values differed significantly from those in yellow mustard
treatment (40.5%), which in turn differed significantly from those in Oriental
mustard (11.9%) and oilseed radish (0%) treatments. In 2009 the pattern was
similar. Control and methyl bromide treatments had stand counts of 88.1% and
85.7%; microbial amendment treatment had a stand count of 69.0%, which
differed significantly from the control but not from methyl bromide. The oilseed
radish, Oriental mustard, and yellow mustard treatments had stand counts of 0%,
1.2%, and 2.4%; these values differed significantly from those in other treatments.

In 2009 plots were also planted with muskmelon transplants. The survival
rate in methyl bromide, control, and microbial amendment treatments ranged
from 85.7% to 100.0%; this differed significantly from the survival rate in cover
crop treatments, which ranged from 45.2% to 50.0% (Table 3.7).

Eggplant transplants were grown and planted each year. There was no
significant difference in transplant survival among treatments (Table 3.7).

Eggplant Growth

43

In 2008 there were significant differences in eggplant fresh biomass
production at the end of the season (Table 3.8). Plants in microbial amendment
treatment produced the most biomass on average (16.1 kg/plot), while those in
yellow mustard treatment produced the least (11.7 kg/plot). There was no
significant difference in fresh biomass production in 2009; values ranged from 5.4
to 8.1 kg/plot, less than in 2008.

There were differences in eggplant height during the 2008 growing season.
Values were not significantly different for the first date data was taken (June 23,
one week after transplanting) but were significantly different for the three
following weeks (Table 3.9). Plants in methyl bromide and control treatments
were generally taller than those in cover crop treatments, ranging from 18.5 to
19.6 cm on June 30, compared to 15.4 to 16.4 cm for the cover crop treatment
plants. Values were not significantly different for the rest of the July data

There were also differences in eggplant height during the 2009 growing
season (Table 3.10), but the pattern differed from that in 2008 (Table 3.9). Not
until July 23 and July 30 were there significant differences in eggplant height:
plants in the methyl bromide treatment were significantly taller than those in the
other treatments.

There was no significant difference in leaf chlorophyll levels for the second
data collection dates in 2008 (July 28) and 2009 (August 6). There was a
significant difference for the first data collection date in 2008 (July 21): plants in
methyl bromide treatment had significantly lower chlorophyll levels than other

treatments except Oriental and yellow mustards (Table 3.11). Values ranged from

44

an average of 42.3 for methyl bromide treatment to 49.2 for control and microbial
amendment treatments. Lower chlorophyll levels indicate plants were less
stressed/actively growing (Goldy, personal communication).

Cash Crop Health and Vigor

With the exception of stand count, data for muskmelon for the 2008 field
season were unusable. Muskmelon subjective scores showed no clear patterns in
2009. In general, direct-seeded muskmelon plots in oilseed radish and Oriental
mustard treatments ranked lowest (worst), while those in methyl bromide,
control, and microbial amendment treatments ranked highest (best) (Table 3.12).
The transplanted muskmelon plots in control and methyl bromide treatments
generally ranked highest, especially early in the growing season; those in the
yellow mustard treatment ranked lowest (Table 3.13).

Eggplant visual scores also followed no clear patterns. In 2008, plots in
methyl bromide treatment had higher scores than those in yellow mustard
treatment (on July 7 the methyl bromide plot rating average was 8.3 out of 10
compared to the yellow mustard plot rating average of 4.5; on July 14 the methyl
bromide value was 8.3 out of 10 compared to the yellow mustard value of 6.3)
(Table 3.14). On September 28, yellow mustard plots had a rating of 6.8 out of 10,
significantly less than the rest of the treatments’ ratings. For the dates not
mentioned, there was no significant difference. In 2009 visual ratings were
significantly different at the start of the growing season, but were not at the end

(Table 3.15). From mid-July to early August, plots in methyl bromide treatment

45

scored highest (ranging from 8.2 — 10 out of 10) while those in microbial
amendment treatment scored lowest (6.0 - 7.8 out of 10).

There were no clear patterns in verticillium wilt infection in eggplant. In
2008, plots in methyl bromide treatment had higher percentages of symptomatic
plants than those in yellow mustard treatment (44.0% compared to 14.3% on July
21, 57.1% compared to 26.2% for July 28) (Table 3.16). In 2008, plants in methyl
bromide treatment often had similar percentages of symptomatic plants to those
in control treatment. Most of the dates had no significant differences in
percentages of symptomatic plants among treatment plots. In 2009, verticillium
wilt symptoms did not appear until relatively late — mid-July. Once verticillium wilt
symptoms appeared, however, the majority of plants showed symptoms of the
disease (Table 3.17). Plants in the methyl bromide treatment in 2009 had the
lowest percentages of symptomatic plants (for example, on July 30 100.0% of
plants in all treatments except methyl bromide were symptomatic; a significantly
different average of 14.3% of plants were symptomatic in methyl bromide
treatment). From August - September 2009 there were no significant differences
in verticillium wilt symptom percentages among treatments.

Yield

In direct-seeded muskmelon plots in 2009 there were no significant
differences in average marketable or total fruit weight per plot (Table 3.18). There
were differences in total fruit number produced per plot, though not in marketable
fruit number per plot. Plants in methyl bromide and control treatments produced

22.5 and 21.8 fruits on average (with a fruit weight of 15.7 and 14.3 kg/ plot),

46

significantly more than those in other treatments (except microbial amendment
treatment) which produced 8.7 -— 10.3 fruits per plot (and a fruit weight of 18.1 —
22 kg/ plot).

In transplanted muskmelon plots there were significant differences in
marketable number, total number, and total weight of fruit produced (Table 3.19).
Plants in methyl bromide and oilseed radish plots produced 13.5 and 14.0
marketable fruits per plot on average, while those in yellow mustard and microbial
amendment treatments both produced significantly less (9.7 fruits per plot on
average). Plants in methyl bromide, oilseed radish, and control plots produced
19.7 to 22.0 fruits total while those in the rest produced 13.7 to 14.7 fruits. There
was no significant difference in marketable yields. Plants in oilseed radish
treatment produced 40.6 kg total fruit/ plot, significantly more than those in other
treatments (ranging from 27.1 to 31.4 kg/plot).

In 2008 plants in methyl bromide treatment produced a significantly higher
number of (marketable eggplant (86.3) than those in other treatments, as well the
lowest number of culls (50.2) (Table 3.19). Plants in methyl bromide and control
treatments produced a significantly larger mass of eggplant compared to those in
yellow mustard treatment (22.8 kg and 23.0 kg compared to 15.9 kg, respectively).
Plants in methyl bromide and control treatments produced a significantly lower
number of cull fruits (55.7 and 50.2, respectively) compared to those in microbial
amendment treatment (70.7). Plants in yellow mustard treatment produced the
least number of marketable fruit (58.3) and the least number of total fruit (115.5).

There were no significant differences in the weight of fruit produced. Harvest

47

index I was calculated by dividing the marketable fruit mass produced by the total
plant mass (fruit weight plus fresh above ground plant weight); harvest index [I
was calculated by dividing the total fruit mass produced by the total plant mass.
Harvest index [I was were significantly different: plants in methyl bromide
treatment had a significantly higher harvest index I (0.57) and harvest index II
(0.66) than those in microbial amendment (0.47 and 0.58) and oilseed radish
(0.50, 0.60) treatments. Harvest indices for plants in methyl bromide and control
treatments were similar.

In 2009 plants in oilseed radish treatment produced the highest number of
marketable fruit per plot (65.2) while those in the methyl bromide treatment
produced the lowest number (40.7) (Table 3.20). Oilseed radish plots produced
the most total fruit in terms of number (118.8) compared to methyl bromide plots
(79.3). There was no significant difference in amount of fruit weight produced by
each plot, except for weight of cull fruit produced. Plants in methyl bromide
treatment produced less cull fruit (kg) than those in oilseed radish and microbial
. amendment treatments. Harvest indices were not significantly different.
DISCUSSION

The cover crops produced roughly equivalent amounts of biomass in 2008
and 2009. Oilseed radish produced more dry biomass than yellow and Oriental
mustards in 2008. In general, 2009 was cooler and wetter than 2008, creating sub-
optimal grth conditions. These results suggest that none of these three cover
crops is consistently superior to the others in terms of biomass production, and

thus other criteria should be used to determine which cover crop to plant.

48

The literature on cover crops suggested plots in oilseed radish and Oriental
mustard treatments would have higher levels of nitrates during the growing
season, and they did. Dry weights for Oriental mustard and yellow mustard were
not significantly different in 2008, suggesting Oriental mustard is a more efficient
scavenger of nitrogen than yellow mustard. If nitrogen leaching is of primary
concern, Oriental mustard would be the more efficient of the two crops to plant.
Collins et al. (2007) showed a mustard (Brassica hirta Moench) cover crop can take
up and then release a notable amount of nitrogen which the following cash crop
can then use. Given the comparatively large amount of biomass produced both
years by oilseed radish and the deep taproot of oilseed radish, it follows that plots
planted in oilseed radish would have the highest nitrogen levels (as there was
more biomass to hold N and then decay). Ngouajio and Mutch (2004) have stated
that oilseed radish efficiently recycles nitrogen. That there were no Isignificant
differences in plot nitrate levels by September suggests cover crops had
thoroughly biodegraded and released their stored N, which was either used by the
cash crops or leached away.

In addition to their nutrient-recycling capabilities, cover crops proved
problematic in that they reduced muskmelon emergence in both 2008 and 2009.
This finding concurs with the literature. Brassica cover crops are allelopathic in
laboratory tests (Bialy et al. 1990; Brown and Morra 1996; Mason-Sedun and
Jessop 1988; Oleszek 1987; Turk and Tawaha 2002; Turk and Tawaha 2003). They
also decrease cash crop and weed stand counts in the field (Haramoto and Gallandt

2005a)

49

Brassica cover crops can alter soil microbial population levels and structures
and encouraging the growth of Pythium spp. populations (Mazzola et a1. 2001).
Pythium spp. are a common cause of damping-off, which would also explain the
decreased stand counts in the cover crop treatments. Mazzola et al. (2001)
postulated the effect on cash crops varies by Brassica species and damage may be
attributed to allelopathy, microbial population changes, or a combination of the
two depending on the cover crop. Allelopathy and increased Pythium spp.
populations could also explain decreased survival of transplanted muskmelon;
given that eggplant transplants show no such decrease in survival, the cause is
more likely allelopathy than Pythium spp. population changes. While Pythium spp.
impacts many crops, plants have been shown to vary in their vulnerability to
Brassica cover crops (Norsworthy et al. 2006; Oleszek 1987).

Among the cover crops, yellow mustard seems to have the most deleterious
effect on eggplant height and biomass production. In general, plants in methyl
bromide and control did better than those in cover crop treatments, suggesting
cover crops were negatively impacting cash crop growth. This observation
supports laboratory research that has shown cover crops to have a negative
impact on cash crop growth (Bialy et al. 1990; Oleszek 1987; Turk and Tawaha
2002). It contradicts field research, however, that has shown Brassica cover crops
to have no such impact on a green bean (Phaseolus vulgaris L) crop planted soon
after cover crop incorporation in to the soil (Haramoto and Gallandt 2005b).

Brassica cover crops have been shown previously to clearly decrease

soilborne pathogen populations in the laboratory (Angus et al. 1994; Dunne et al.

50

2003; Kirkegaard et al. 1996; Lewis and Papavizas 1971; Lewis and Papavizas
1974; Mazzola et al. 2001; Nastruzzi et al. 1996; Sarwar et al. 1998). Greenhouse
and field studies have shown Brassica residues to be capable of decreasing
soilborne disease incidence (Blok et al. 2000; Larkin and Griffin 2006; Snapp et al.
2007). Most relevantly to this study, broccoli (Brassica oleracea var. italica Plenck)
residues decreased verticillium wilt incidence in cauliflower (Subbarao et al.
1999). There was no such clear connection between cover crop use and
verticillium wilt incidence in this study, which coincides with research suggesting
Brassica cover crops do not control disease in the field (Njoroge et al. 2008).
Brassica cover crops did not decrease verticillium wilt incidence in a following
cash crop (Hartz et al. 2005; Pinkerton et al. 2000; Wiggins and Kinkel 2005). In
one year (2009), yellow mustard decreased the verticillium wilt rating for some
data points. In 2008, verticillium wilt likely was not a severe problem in eggplant
(methyl brbmide and control treatment plots did not significantly differ in disease
rating values). In 2009 verticillium wilt did not develop until fairly late in the
growing season (mid-July), but it was severe (100% of plants in most plots were
symptomatic). Given yellow mustard plots had lower disease ratings in a severe-
disease year, it is possible this cover crop can impact verticillium wilt in the field.
However, yellow mustard also has one of the clearest impacts on cash crop growth
and yield and thus the benefits must be weighed against the costs or losses.
Additional studies need to be conducted in controlled environments to clearly test

the effect of yellow mustard on verticillium wilt as well as various cash crops.

51

Furthermore, in terms of muskmelon vigor, the harm the cover crops did to
muskmelon stand count directly impacted subjective ratings. The visual
impression of plots where plants had to be re-seeded was of decreased vigor.

Brassica cash crops had varying impacts on muskmelon yield parameters. In
general, plants in control and methyl bromide treatments produced larger
numbers (and heavier weights) of fruit than those in cover crop treatments. Of the
three cover crops, oilseed radish was the least harmful: transplanted plants in this
treatment produced more marketable fruit than those in non-control and non-
methyl bromide treatments and also the largest total fruit weight of any of the
treatments. Yellow mustard was arguably the most harmful: plants in this
treatment routinely produced the least amount of fruit, both in number and total
mass. There was no significant difference in the number or mass of marketable
fruit and total fruit mass produced by direct-seeded muskmelon plants. Those in
yellow mustard, oilseed radish, and Oriental mustard treatments did produce
significantly less total fruit (in terms of number) than those in methyl bromide and
control treatments. Taken together, these results suggest that a). spring-planted
Brassica cover crops confer no yield advantage on these two cash crops, b). these
cover crops can decrease yield, and c). the decrease in yield is likely primarily a
result of the cover crops’ negative impact on muskmelon stand. Effects on
muskmelon yield were likely due to a confluence of two factors. First, the cover
crops’ impact on cash crop growth meant fewer flower buds, less fruit, and less
biomass to sustain the fruit. Second, muskmelons in the cover crop treatments had

to be re-seeded or the transplants replaced. In Michigan, the growing season is so

52

short that some plants may not have had the chance to produce fruit because their
plant date was 1 to 2 weeks later than that of most plants in other treatments.

Results were less straightforward with eggplant data. Many harvest
parameters with significant differences in 2008 showed no such differences in
2009. As with muskmelon, plants in yellow mustard treatment tended to produce
lower numbers and smaller masses of fruit. In 2008, plants in yellow mustard
treatment produced less marketable fruit (kg) than those in all other treatments,
except oilseed radish and Oriental mustard. Plants in yellow mustard treatment
also produced fewer marketable fruit and fewer fruit total than those in control
and methyl bromide treatments, while producing larger numbers of cull fruit. In
2009 there was an interesting reversal: plants in methyl bromide treatment
produced the smallest number of marketable and total fruit; however, there was
no significant difference among treatments in terms of the total mass of fruit
produced or the mass of marketable fruit produced, suggesting plants in this
treatment produced fewer, larger fruit.

In 2008 there were significant differences in the harvest indices. Plots in the
methyl bromide had a higher harvest index I than those in the yellow mustard,
oilseed radish, and microbial amendment treatments. Plots in the methyl bromide
treatment also had a higher harvest index ll than those in the microbial
amendment and oilseed radish treatments. For both indices, methyl bromide and
control values were similar. In 2009 there were no significant differences among

harvest indices.

53

The general lack of consistent impact on muskmelon and eggplant yield
coincides with literature on spring planted Brassica cover crops. Haramoto and
Gallandt (2005b) also found no effect on the yield of a green bean crop planted
immediately after a Brassica cover crop. This result does, however, contradict the
finding of Hartz et al. (2005) that Brassica cover crops increased head lettuce
(Lactuca sativa L.) yield. It also contradicts the discovery that onion (AIIium cepa
L.) and celery (Apium graveolens L.) crops benefit from a preceding Brassica cover
crop (Wang et al. 2008, Wang et al. 2010). It should be noted that in the two
studies listed above, cover crops were planted in the fall preceding cash crops, and
not immediately preceding them. Differing results in the studies noted could thus
be due to dissipation of allelochemicals or return of microbial populations to their
‘normal' state before the planting of the cash crop. It should also be noted that in

the two studies listed above, the crops are considerably different from

muskmelon/ eggplant.

CONCLUSIONS

Brassica cover crops planted in the spring and incorporated in to soil about
two weeks prior to cash crop planting provided some benefits and posed some
challenges. Benefits include increased soil nitrate levels during the growing season
and some (likely not practically significant) protection against verticillium wilt (in
the case of yellow mustard). At the same time, Brassica cover crops can have a
severe impact on cash crop stand count when the crops are direct-seeded less than

two weeks after cover crop incorporation. An unacceptable stand reduction was

54

also recorded in muskmelon transplants. Yellow mustard can have a strong
negative impact on eggplant growth and subjective vigor scores. Cover crops
overall did not increase amount and weight of fruit produced by eggplant or
muskmelon, but they did decrease the number of fruit produced (in direct-seeded
muskmelon).

More research needs to be done to determine the nature of the impact of
Brassica cover crops on muskmelon germination and transplant survival, and to
determine if this impact extends to other cucurbits which are commonly direct-
seeded in Michigan. lf negative effects observed are due to allelochemicals, then a
safe plant back period after cover crop incorporation needs to be identified. Such a
plant back period would be significantly longer than the 10 to 14 days used in this
study. In that case, spring planting of cover crops would not be a viable option for
eggplant and cucumber production in Michigan due to the short growing season.
Late summer or fall planting of the cover crops could be alternative windows.

However, those scenarios need to be tested.

55

Table 3.1. Pesticide application schedule for eggplant and muskmelon plots in

 

 

2008 and 2009.

Application Date Pesticide(s) Rate

June 19, 2008 Champ 2F 1 1/3 pt/A
Maneb 7SDF 1 lb/A

July 1, 2008 Champ 2F 1 1/3 pt/A
Maneb 75DF 1 lb/A

July 10, 2008 Champ 2F 1 1/3 pt/A
Dithane DF 1 1h lb/A
Asana XL 6 oz/A

July 18, 2008 Champ 2F 1 1/3 pt/A
Dithane DF 2 lb/A
Thiodan 50 1 lb/A

July 24, 2008 Champ 2F 1 1/3 pt/A
Dithane DF 2 lb/A
Thiodan 50 1 lb/A

August 5, 2008 Champ 2F 1 1/ 3 pt/A
Dithane DF 2 lb/A
Thiodan 50 1 lb/A

August 15, 2008 Agrimek 10 oz/A
Asana XL 8 oz/A
Champ 2F 1.5 pt/A
Echo 2 pt/A

August 29, 2008 Agrimek 10 oz/A
Asana XL 8 oz/A
Champ 2F 1.5 pt/A
Echo 2 pt/A

September 10, 2008 Champ 2F 1.5 pt/A

- Echo 2 pt/A

June 26, 2009 Champ 1 1/ 3 pt/A
Pencozeb 1.5 lb/A
Thiodan 15. lb/A

July 8, 2009 Champ 1 1/ 3 pt/A
Pencozeb 1.5 lb/A
Asana XL 6 oz/A

July 17, 2009 Champ 1 1/3 pt/A
Pencozeb 1.5 lb/A
Asana XL 6 oz/A

 

56

Table 3.1. cont.

 

 

Application Date Pesticide(s) Rate
July 27, 2009 Asana XL 6 oz/A
Champ 1 1/ 3 pt/A
Equus 2 pts/A
August 4, 2009 Thiodan 1.5 lb/A
Prevecure 1.2 pts/A
Champ 1 1/3 pt/A
Equus 2 pts/A
August 10, 2009 Thiodan 1.51b/A
Prevecure 1.2 pts/A
Champ 1 1/ 3 pt/A
Equus 2 pts/A
August 19, 2009 Champ 1 1/ 3 pt/A
Equus 2 pts/A
Thiodan 1.5 lb/A
Ranman 2.75 oz/A
August 31, 2009 Champ 1 1 / 3 pt/A
Equus 2 pts/A
Thiodan 1.5 lb/A
Previcure 1.2 pts/A
September 11, 2009 Champ 1 1/3 pt/A
Equus 2 pts/A

 

57

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Table 3.3. Monthly and long term (8-year) growing degree days (GDD) during
cover crop grth and melon and eggplant growth in 2008 and 2009 at SWMREC
Michigan. Base temperature = 10°C.*

 

 

 

Month Monthly average GDD
2008 2009 8 year average

April 86.5 61.9 81.1

May 1 13.1 166.6 162.5

June 293.7 280.9 299.4

July 368.1 297.1 374.7
August 328.8 309.8 355.4
September 237.2 219.4 236.3
Total 142 1.4 1335.7 1509.3

 

‘ Cover crops were sown on April 4, 2008 and April 1, 2009. Melons and eggplants
were planted June 10, 2008 and June 12, 2009.

59

Table 3.4. Brassica cover crop stand count and dry weight biomass prior to soil
incorporation.

 

 

 

 

 

Stand (plants m2) Dry weight kg/ha
2008 2009 I 2008 2009
Treatment
Oilseed radish 190.7* 100.7 b 6086.5 a 4173.3
Oriental mustard 182.7 244.0 a 3487.5 b 2843.0
Yellow mustard 194.0 119.3 b 3641.5 b 3000.0
LSD0.os NS 91.1 321.7 NS

 

*Values with different letters are significantly different at the P=0.05 level. N5 is
not significant at the P=0.05 level.

Table 3.5. Average soil nitrate levels (ppm) as affected by biofumigation with
Brassica cover crops and soil treatments. Data for 2008 and 2009 were combined.

 

 

 

 

Soil nitrate levels (ppm)
Treatment June July September
Methyl bromide 5.7 c* 4.5 b 2.3
Oilseed radish 8.2 a 7.6 a 4.3
Oriental mustard 7.4 ab 7.3 a 4.1
Yellow mustard 6.1 c 4.9 b 2.9
Microbial amendment 6.7 be 5.5 b 3.0
Control 6.4 be 5.4 b 3.4
LSDo.os = 1.2 1.2 NS

 

* Means within a column followed by the same letter are not significantly different
at the P=0.05 level. NS is not significant at the P=0.05 level.

61

Table 3.6. Average soil ammonium levels (ppm) in 2009 as affected by
biofumigation with Brassica cover crops and soil treatments.

 

2009 soil ammonium levels (ppm)

 

 

 

Treatment June July September
Methyl bromide 08* 1.0 0.7
Oilseed radish 0.8 0.8 0.9
Oriental mustard 0.8 0.9 0.9
Yellow mustard 0.9 0.9 0.8
Microbial amendment 0.8 0.9 0.6
Control 0.7 0.9 0.7
LSDo,os = NS N S NS

 

* N8 is not significant at the P=0.05 level.

62

Table 3.7. Melon ‘Athena’ and eggplant ‘Classic’ stand as affected by biofumigation
with Brassica cover crops and soil treatments.*

 

 

 

 

 

Muskmelon (%) Eggplant (%)

Direct-seed Transplant Transplanted
Treatment 2008 2009 2009 2008-2009
Methyl bromide 100.0 a** 85.7 ab 97.6 a 100.0
Oilseed radish 0.0 c 0.0 c 50.0 b 100.0
Oriental mustard 11.9 c 1.2 c 45.2 b 99.3
Yellow mustard 40.5 b 2.4 c 45.2 b 100.0
Microbial amendment 78.6 a 69.0 b 85.7 a 100.0
Control 100.0 a 88.1 a 100.0 a 100.0
LSDo.os = 23.2 17.7 26.6 NS

 

*Muskmelons were direct-seeded on June 10, 2008 and June 12, 2009 or
transplanted on June 12, 2009. Eggplant was transplanted on June 10, 2008 and
June 12, 2009. Stand count was taken on June 23, 2008 and June 25, 2009.
”Means within a column followed by the same letter are not significantly different
at the P=0.05 level. NS is not significant at the P=0.05 level.

63

Table 3.8. Eggplant fresh shoot biomass as affected by the previous Brassica cover
crops and soil treatments.*

 

 

 

Treatment 2008 (kg/plot)" 2009 (kg/plot)
Methyl bromide 13.0 bc 6.9
Oilseed radish 14.7 ab 8.1
Oriental mustard 12.7 bc 6.7
Yellow mustard 11.7 c 5.4
Microbial amendment 16.1 a 7.3
Control 14.2 ab 7.6
LSDo,os = 2.2 NS

 

*Biomass was collected on September 28, 2008 and September 24, 2009 at the end
of the harvest season.

* *Means within a column followed by the same letter are not significantly
different at the P=0.05 level. NS is not significant at the P=0.05 level.

Table 3.9. Effect of cover crops and soil treatments on eggplant ‘Classic’ height
(cm) in 2008*

 

 

 

Treatment Jun. 23 Jun. 30 Jul. 7 Jul. 14 Jul. 21 Jul. 28
Methyl bromide 17.6 19.6 a 22.5 a 30.2 a 36.1 44.4
Oilseed radish 14.3 15.4 c 18.8 c 25.7 b 35.8 * 43.0
Oriental mustard 15.4 16.2 c 19.8 be 25.5 bc 34.5 45.1
Yellow mustard 15.0 16.4 be 18.0 c 22.6 c 32.3 40.5
Microbial amendment 17.0 18.5 ab 22.8 a 28.4 ab 36.0 44.5
Control 16.2 18.5 ab 21.5 ab 29.5 a 37.1 43.8
LSDo.os = NS** 2.3 2.1 3.1 NS NS

 

*Eggplants were transplanted on June 10, 2008. Plant spacing was 1.5 m (5.5’)
between beds and 0.5 m (1.5’) inside the rows (13,047 plants/ha) (5,280
plants/A).

** Means within a column followed by the same letter are not significantly
different at the P=0.05 level.

65

Table 3.10. Effect of cover crops and soil treatments on eggplant ‘Classic’ height

 

 

 

(cm) in 2009.*

Treatment Jun. 25 Jul. 2 Jul. 9 Jul. 16 Jul. 23 Jul. 30
Methyl bromide 18.9* 23.0 28.6 37.9 47.0 a 53.0 a
Oilseed radish 18.5 22.2 26.4 37.8 46.0 ab 49.5 ab
Oriental mustard 18.4 2 1.2 26.0 36.0 42.0 bcd 46.5 bc
Yellow mustard 17.3 20.8 25.8 34.9 40.5 cd 42.7 c
Microbial amendment 19.0 22.5 26.9 34.5 38.3 d 44.0 c
Control 19.4 22.7 28.7 37.7 43.1 abc 47.1 bc
LSDo.os = NS NS NS NS 4.7 5.4

 

*Eggplants were transplanted on June 12, 2009. Plant spacing was 1.5 m (5.5’)

between beds and 0.5 m (1.5') inside the rows (13,047 plants/ha) (5,280

plants/A).

** Means within a column followed by the same letter are not significantly

different at the P=0.05 level.

66

Table 3.11. Effect of cover crops and soil treatments on eggplant ‘Classic’
chlorophyll levels.*

 

 

 

 

Average SPAD readingM

Treatment July 21, 2008 July 28, 2008 August 6, 2009
Methyl bromide 42.3 b*** 44.5 51.8
Oilseed radish 49.0 a 46.3 52.0
Oriental mustard 47.7 ab 45.8 53.7
Yellow mustard 47.1 ab 46.4 53.2
Microbial amendment 49.2 a 43.3 52.2
Control 49.2 a 47.5 53.0
LSDo.os = 3.9 NS NS

 

*Eggplants were transplanted on June 12, 2009. Plant spacing was 1.5 m (5.5’)
between beds and 0.5 m (1.5') inside the rows (13,047 plants/ha) (5,280

plants /A).

“Values for 2008 are the average of five readings; values for 2009 are the average
of seven readings.

***Means within a column followed by the same letter are not significantly
different at the P=0.05 level.

67

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Table 3.18. Direct-seeded muskmelon ‘Athena’ marketable and total yield as
affected by biofumigation with Brassica cover crops and soil treatments in 2009.*

 

 

 

Marketable Total Marketable Total

fruit fruit yield yield
Treatment (no. /plot)** (no. [plot] (kg/plot) (kg/plot)
Methyl bromide 6.5*** 15.7 a 12.2 22.5
Oilseed radish 7.2 10.3 be 17.4 22.0
Oriental mustard 6.5 10.3 be 16.4 21.0
Yellow mustard 6.3 8.7 c 14.3 18.1
Microbial amendment 8.3 13.0 ab 17.5 23.0
Control 8.7 14.3 a 17.9 21.8
LSDo.os = NS 3.0 NS N S

 

*Plant spacing was 1.5 m (5.5') between beds and 1.0 m (3.0') inside rows (6,523
plants/ha) (2,640 plants /A). Plants were direct-seeded on June 12, 2 009.
“Fruits were sorted in to marketable fruit and fruit which failed to meet the
standards (culls) (USDA-AMS 2008).

***Means within a column followed by the same letter are not significantly
different at the P=0.05 level. N5 is not significant at the P=0.05 level.

74

Table 3.19. Transplanted muskmelon ‘Athena' total yield as affected by
biofumigation with Brassica cover crops and soil treatments in 2009.*

 

 

 

Treatment Marketable Total fruit Marketable Total yield
fruit (no/plot] yield (kg/ plot)
(no. /plot)** (kg/plot)
Methyl bromide 13.5 ab*** 19.7 a 24.7 31.4 b
Oilseed radish 14.0 a 22.0 a 30.8 40.6 3
Oriental mustard 10.3 bc 13.7 b 23.6 27.4 b
Yellow mustard 9.7 c 14.7 b 20.3 27.4 b
Microbial amendment 9.7 c 14.7 b 20.5 27.1 b
Control 12.2 abc 19.3 a 23.2 30.2 b
LSDo.os = 3.2 4.4 NS 7.3

 

*Plant spacing was 1.5 m (5.5') between beds and 1.0 m (3.0') inside rows (6,523
plants/ha) (2,640 plants/A). Plants were transplanted on June 12, 2009.

. “Fruits were sorted in to marketable fruit and culls fruit which failed to meet the
standards (culls) (USDA-AMS 2008).

*** Means within a column followed by the same letter are not significantly
different at the P=0.05 level. NS is not significant at the P=0.05 level.

75

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77

CHAPTER 4: Impact of Brassica Cover Crops on Cucurbit Germination and
' Yield

78

CHAPTER 4: Impact of Brassica Cover Crops on Cucurbit Germination and
Yield

ABSTRACT

There is evidence to suggest Brassica cover crops can be phytotoxic to cash
crops, especially when the cash crop is planted too soon after cover crop
incorporation. A field experiment was completed in which honeydew (Cucumis
melo L. Group lnodorus), muskmelon (Cucumis melo L Group Reticulatus), and
cucumber (Cucumis sativus L.) seeds were planted at 5 d intervals after the
incorporation of oilseed radish (Raphanus sativus (L) var. oleiferus Metzg
(Stokes)), Oriental mustard (Brassicajuncea (L) Czem.), and no cover crops.
Cucumber emergence was reduced in oilseed radish treatment for the Day 0
planting. There were no other significant differences in emergence for individual
planting dates and cash crops among treatments. At cucumber harvest (SO-60 d
after planting) there were no significant differences in individual vine weight or
marketable fruit mass per vine, though plants in oilseed radish and Oriental
mustard treatments produced significantly more total fruit mass per vine than
those in the control treatment. There were no significant differences in marketable
fruit number per vine, though plants in the oilseed radish treatment produced
more culled fruit per vine in the control. Bioassays using both non-Iyophilized and
lyophilized root and shoot aqueous extracts of oilseed radish were performed in
the laboratory on muskmelon, cucumber, and honeydew. Germination rates and
radicle elongation measurements showed both extracts impacted all three crops to
varying degrees. Germination ranged from 64.4-98.9% at the 0% concentrations to

0—2.2% at the 100% (1kg fresh weight cover crop: 1 L water) concentrations for

79

non-Iyophilized extracts. Muskmelon germination was least sensitive to the
extracts, followed by cucumber and then honeydew. Cucumber and muskmelon
root grth was equally inhibited by non-Iyophilized shoot extract, while
honeydew grth was mildly stimulated at S and 12.5% concentrations.
Honeydew root growth was least inhibited by non-Iyophilized root extract,
followed by cucumber and then muskmelon. Overall non-Iyophilized root extract
was more potent than non-Iyophilized shoot extract, while the reverse was true of
Iyophilized extracts. In addition, non-Iyophilized extracts were far more inhibitory
than Iyophilized extracts.
INTRODUCTION

Current economic and regulatory conditions present growers with
increasing challenges. Consumers demand environmentally-friendly production
practices as local, state, and federal government regulations make pesticide use
more difficult Fertilizer prices increase yearly. Production challenges include
maintaining soil quality (high soil fertility and low pest pressure) and using
pesticides and fertilizers more efficiently. One tool that may be of use in meeting
these challenges is cover crops. Cover crops provide a variety of services in a
production system. They decrease erosion, aid nutrient cycling, preserve soil
quality, and suppress weeds (Mutch 2009). Cover crops in the Brassica family such
as oilseed radish [Raphanus sativus (L) var. oleiferus Metzg (Stokes)), Oriental
mustard (Brassicajuncea (L) Czem.), and yellow mustard (Sinapis alba L.) have
also been shown to impact plant pathogen populations in the soil (Sarwar et al.

1998). Given the phase-out of methyl bromide, any practice that is

80

environmentally-friendly, economically feasible, and which decreases soilborne
disease incidence is likely to be well received by growers, consumers, and the
government. Use of Brassica cover crops in field rotations could be one such
practice.

Our previous field study involving muskmelon (Cucumis melo L. Group
Reticulatus) and honeydew (Cucumis melo L. Group lnodorus) suggested oilseed
radish, Oriental mustard, and yellow mustard may be phytotoxic, especially to seed
germination and seedling growth. Haramoto and Gallandt (2005a) also determined
Brassica cover crops decreased cash crop emergence by 23-34% and delayed
germination 2 d (though their impact was similar to that of other short-season
cover crops such as red clover). Laboratory experiments have proven compounds
produced by Brassicas can inhibit weed seed germination (Norsworthy et al. 2006;
Norsworthy and Meehan 2005a and 2005b). Studies with Brassica m'gra Moench
aqueous extracts have shown the extracts to inhibit germination and growth of
lentils (Lens culinaris Medik.) and wild oats (Avenafatua L) (Turk and Tawaha
2002 and 2003). Wheat (Triticum aestivum Songle) germination is likewise
inhibited by compounds produced by Brassica cover crops (Bialy et al. 1990).
Phytotoxicity would typically be a problem in production systems where the cover
crop is tilled under and another crop is immediately planted, as in a
muskmelon/ eggplant (Solanum melongena L.) short rotation wherein the cover
crop is planted in April and tilled under in late May.

There are several potential avenues through which Brassica cover crops

could be impacting cucurbit germination. Molisch (1937) first coined the word

81

allelopathy to describe the situation wherein plants interact with each other via
chemical compounds. These interactions can be positive or negative, though today
the word allelopathy implies negative interactions (Choesin and Boerner 1991).
One well-known example would be that of black walnut ([ugIans nigra L.), which
exudes chemicals from its roots that prevent many plants from growing beneath
its canopy. Allelochemicals are common throughout the plant world and include
(but are not limited to) organic acids, alkaloids, alcohols, aldehydes, glycosides,
tannins, and terpenes (Szczepanski 1977). While allelochemicals are common, a
variety of factors dictate levels at which they are present in the environment Such
factors include the plant species present, the quantity and type produced, and the
environment itself (e.g. soil moisture levels and soil composition). Furthermore,
crop and weed species vary in their susceptibility to allelochemicals (Oleszek
1987)

Brassica cover crops produce multiple classes of compounds that are
implicated as being allelopathic. The primary class of interest is glucosinolates,
which when degraded by hydrolysis produce biologically-active compounds called
isothiocyanates (ITCs) (Kirkegaard and Sarwar 1998). Some of these glucosinolate
by-products are water-soluble, while some are highly volatile (Brown and Morra
1996). Other compounds from glucosinolate hydrolysis that may be allelopathic
are organic cyanides and oxazolidinethione (Brown and Morra 1996).

Allelopathy is not the only mechanism by which Brassica cover crops could
impact germination. These cover crops do not solely act by killing pathogens/weed

seeds. Cohen and Mazzola (2006) have demonstrated low-glucosinolate Brassica

82

seed meal can change the composition of soil microbe communities, leading to
increases in the Pythium spp. population. Hoagland et al. (2008) found low-
glucosinolate Brassica napus and Sinapis alba seed meal amendments also lead to
an increase in Wthium spp. populations. Njoroge et al. (2008) discovered Pythium
spp. populations increased in some plots in which Brassica cover crops were used
as a green manure compared to an untreated control (though plots planted with
Brassicas had higher levels of Pythium spp. compared to unplanted plots to begin
with). Pythium spp. is‘one of the pathogens that can cause seedling damping-off.
Cohen and Mazzola (2006) found Pythium infections were greatest when seedlings
were planted immediately alter seed meal incorporation; a delay of 4 weeks
greatly decreased infection rates. The ways in which Brassica biomass can impact
microbial communities are diverse and intricate; one such way is by serving as a
carbohydrate source for sufficiently opportunistic organisms (Cohen and Mazzola
2006). Hoagland et al. (2008) postulated the observed increase in Rythium
populations was at least partly the cause of the observed decreases in weed and
wheat seed germination and increases in seedling mortality. Treatments in the
Hoagland experiments (2008) with Brassica juncea (L.) Czem. seed meal
suppressed Pythium (likely due to the nature of its ITCs), resulting in less severe
decreases in germination and increases in seedling mortality. What damage the
authors did observe they attributed to ITCs.

One last way in which cover crops could impact seed germination and
seedling establishment is by changing soil structure (Cortland, personal

communication). Cover crops that have been incorporated but have not yet

83

decomposed sufficiently could create a situation wherein seeds are planted but are
not in good contact with soil /moisture, in cavities created by the decomposed
crops.

Brassica cover crops have the potential to be a valuable tool. More
information is needed on their impact on cash crops, especially on those which are
direct-seeded. Brassica cover crops may be impacting germination directly
through allelopathic mechanisms, or indirectly through altering soil microbial
community structure or soil structure. Producers need to know how long to wait
between cover crop incorporation and cash crop seeding. The objectives of this
study are to a) verify that Brassica cover crops impact cucurbit germination in the
field; b). determine a safe plant-back period; and c). determine if the cover crops
are allelopathic.

MATERIALS AND METHODS
Field Experiment - Site and Procedures

Oilseed radish ‘Defender‘ and Oriental mustard ‘Forge' were planted at the
Michigan State University Horticulture Teaching and Research Center in Holt,
Michigan on May 8, 2009. Monthly temperature and rainfall for the site are
presented in Table 4.1 and growing degree days in Table 4.2. The plot previously
was fallow for three years. The soil was Thetford loamy sand (sandy, mixed, mesic,
Psammaquentic Hapludalfs). The experiment was a split plot design with four
replications and two factors (cover crop was the main plot factor while crop
planting date was the subplot factor). Cover crop factor included oilseed radish

(11 kg/Ha) (10.0 lb/A), Oriental mustard (7 kg/ Ha) (6.01b/A), and bare ground

84

control. Cover crops were planted on May 8, 2009 using a Marliss drill. Cover crop
plots were 116 m2 each. Ammonium nitrate fertilizer (22 kg N / Ha) (20 lb N /A)
was applied May 27, 2009. Cover crops were sampled for biomass production
(Table 4.3) and flail mowed and tilled under June 24, 2009 when most plants were
at flowering stage. Black plastic and drip tape were installed immediately
afterwards on raised beds.

‘ Each main plot (cover crop) treatment was divided in to seven subplots,
each of which was randomly assigned a planting date. The cucurbits were planted
at five day intervals after cover crop incorporation (CCI) (D0, D5, D10, D15, D20,
D25, and D30). Furthermore, each subplot bed had three rows of holes.
Muskmelon ‘Athena' was planted in the center row of each bed, while pickling
cucumber ‘loumey and honeydew ‘Earlibrew’ were planted in the outer rows of
holes. Two seeds per hole were sown by hand. Rows were 1 m apart, center to
center. Holes within each row were 0.5 m apart. Irrigation was applied for 2 - 3
hours after each cucurbit planting, except when soil was damp to a depth of five
cm due to previous rainfall. No fertilizer and no other irrigation were applied to
the cucurbits due to the amount of rain received over the summer. Plots were hoed
once.

Laboratory Experiments - Materials and Methods

Oilseed radish was planted by grower Ron Eding on September 14, 2009 in
Hamilton, Ml (42° 40' 39” N, 86° 0' 22" W). The soil was a Houghton muck soil with
80% organic matter. A total of 528 plants were harvested from 15 randomly

chosen 0.5 m2 quadrats on November 30, 2009. Plants were 0.3 - 0.6 m tall and

85

had not yet initiated flower buds. The plants were rinsed in tap water and then in
deionized water to remove dirt, air-dried for 1 d, and weighed. Roots were
separated from shoots; the two plant parts were processed separately. The
biomasses were processed in a commercial grade blender (CB-10; Waring
Commercial®, Torrington, CT) with deionized water (1 L to 1 kg biomass) for 90 -
120 sec. The resulting extracts were strained through cheesecloth. Both extracts
were then filtered through Whatman #4 filter paper. These extracts will
henceforth be referred to as the non-Iyophilized extracts. Some of the strained
liquids were further centrifuged at 10,000 rpm for 10 minutes (RCSC; Sorvall®
Instruments, DuPont, Wilmington, Del.), and then the supernatants were freeze-
dried using a lyophilizer. The resulting powders were mixed to allow for
uniformity and stored at 4° C until use. Solutions made with these powders will

henceforth be referred to as Iyophilized extracts.

Experiments testing both extracts were designed as a completely
randomized design with three replications. Each experiment was repeated three
times. Non-Iyophilized extract was diluted to make solutions of 5, 12.5, 25, 50, and
100% strength. Deionized water was' the control. For assays using Iyophilized
extracts, extracts were dissolved in deionized water to create concentrations of 0,
0.25, 0.5, 1, 2, 4, and 8 g/ L by serial dilution of a stock solution containing 8 g/L.
Crops tested were cucumber ‘journey‘, muskmelon ‘Athena', and honeydew
‘Earlibrew’. Ten seeds of each crop were placed in 10 cm Petri dishes on Whatman
#4 filter paper, then 3.0 ml of each extract dilution was placed into each Petri dish.

Dishes were sealed with parafilm. Seeds were incubated in a Conviron grth

86

chamber (Controlled Environments, LTD, Winnipeg, Manitoba, Canada) at 21° C in

the dark for 6 d.

The EC and pH of the 100% concentration treatments for non-Iyophilized
extract, the 8 g/ L Iyophilized extract and deionized water were measured with a
Horiba D-24 pH /conductivity meter (Spectrum Technologies, Inc). An experiment
was run using deionized water acidified with acetic acid (vinegar) to a pH of 5.48
to determine if the extracts' acidity was a confounding variable. Three milliliters of
vinegar solution were placed in each of four Petri dishes containing ten seeds each;
deionized water was the control. As with the bioassays, seeds were incubated at
21°C for 6 d. As the ECs of water and extracts were similar, no experiment was run

to test the possibility of EC being a confounding variable.
Field Equivalent Concentration Estimates

Field equivalent rates (FER) (extract concentrations that would typically
occur under field conditions) were estimated for both types of extracts. For the
non-Iyophilized sample, FER was calculated based on area harvested, total fresh
biomass, and amount of extract obtained (Table 4.7). For Iyophilized extracts, FER
estimates were based on amount of extract produced and harvested area (Table
4.8). Both estimates rely on the assumptions that 15 cm of cover crop are
incorporated, extracted materials are released simultaneously and immediately
into the environment, and 3 ml of aqueous extract were added to each Petri dish
(equal to 46.88 L per cubic meter of soil). The FER estimation equation for

Iyophilized extract is as follows (after Hill 2006):

87

FER = E(g)* P(cm3)/A(cm2)* D{cm)* W(L)

Where FER is estimated field equivalent rate, E is total extract dry weight, P is Petri
dish volume (64 cm”), A is area harvested, D is assumed depth of cover crop
incorporation (15 cm), and W is extract volume added to each Petri dish during
bioassays (3 ml). Extracts are likely released over time in the field, not
simultaneously as assumed. These estimated field rates are thus likely higher than

would be found under field conditions.

Data Collection

In the field experiment, two 50 by 50 cm cover crop biomass samples were
taken from each plot prior to cover crop incorporation. Samples were dried at 60°
C for 14 d. Cucurbit seed emergence was recorded every 5 d. At 30 d after planting,
muskmelon and honeydew plants were cut at ground level, counted, and placed in
a drier for an average of 21 d at 60° C. One cucumber seedling from each hole
containing two seedlings was likewise cut and dried to determine biomass
production. Cucumbers were harvested at 50 to 60 d after planting, sorted into
USDA Grade 1, 2, 3 or cull fruit, and weighed. At the time of harvest cucumber
plants were cut at ground level, counted, and weighed to determine fresh biomass.
Fresh cucumber vine weights per bed were divided by number of vines remaining
in each bed to eliminate biases resulting from varying germination and survival
rates. Cucumber yields were also divided by number of plants remaining in each

bed for the same reason.

In laboratory experiments, on day 6 Petri dishes were opened and root
length and germination rates were measured. Root length was measured using a
digital caliper (Avenger Products, Boulder City, NV, USA). A seed was considered to

be germinated if the root was at least 2 mm long.

Statistical Analysis
Field experiment data were analyzed using SAS PROC GLM and ANOVA (SAS

v 9.2, SAS Institute, Cary, North Carolina).

Data from the three laboratory experiments using non-lyophilized extracts
were combined because there was no experiment by treatment interaction. Data
from lyophilized shoot extracts treatments were likewise combined. Data from
lyophilized root extracts treatments were not combined due to an experiment by
treatment interaction; data from this part of each experiment were analyzed
individually. Germination and root length were analyzed using SAS PROC GLM and

ANOVA (SAS v 9.2, SAS Institute, Cary, North Carolina). ‘

For all data, means were separated using Fisher’s protected LSD. A P-value

of 0.05 was used to determine significance.

RESULTS
FIELD EXPERIMENT
Weather
Because weather impacts crops, a year with weather extremes may result in
skewed data. Temperature, rainfall, and growth degree day (GDD) data were

obtained from the Michigan Automated Weather Network (MAWN) for 2009 and

89

the preceding eight years (Tables 4.1 and 4.2). Low temperatures from May-
September in 2009 were similar to those of the eight year average, with the July
low being the most different (13.4° C average in 2009 compared to the eight year
average of 15.1° C). The average high temperatures for 2009 were likewise similar
to average, though July was again slightly cooler than average (24.9° C compared to
27.8° C). The slightly lower temperatures (both for lows and highs) resulted in just
under 70 fewer Growing Degree Days (GDD) in 2009 than on average (1263.6 GDD
in 2009 compared to an average of 1341.8 GDD). July in particular saw a notable
decrease in GDD: 290.2 in 2009 vs. 359.5 on average. Overall lower temperatures
(and thus fewer GDD) could slow crop growth, especially those that thrive in the
heat such as cucurbits. During the 5 d immediately after the D5 planting,
temperatures were especially cool, resulting in extremely low germination rates
for all crops and treatments (including the control). To mitigate impact of weather
the first 5 d after planting the germination rates in the results tables were
determined 10 d after planting.

While 2009 was slightly cooler, on average, it was also slightly rainier. In
total HTRC received 424.7 mm of rain from May-September 2009, whereas on
average it receives 393.3 mm of precipitation during those months. lune and
August were notably rainier, with 126.2 and 104.7 mm falling when on average
those months receive 80.8 and 59.4 mm, respectively. Slightly wetter weather may
encourage the disease proliferation, and 2009 was an especially severe year for
downy mildew.

Cover Crop Biomass

90

The oilseed radish cover crop had fewer plants on average per m2 quadrat
than the Oriental mustard (Table 4.3). This is because oilseed radish seeds are
larger than those of Oriental mustard. Oilseed radish produced 4,677 kg/ ha
compared to 5,250 kg/ha for Oriental mustard. Oriental mustard seemed better
adapted to the cool and wet spring than the oilseed radish, which would explain
why the mustard was close to full flower when the radish was barely opening its
buds. Mustards are also more sensitive to day length than oilseed radish. Under
ideal circumstances the two species flower at roughly the same time after planting.
Cash Crop Emergence

There were few significant differences in the cucurbit crop emergence
across treatments for each planting date (Table 4.4). Cucumber was impacted by
treatments when it was planted immediately after cover crop incorporation (D0).
Only 45.8% emerged in oilseed radish treatment compared with 72.9% emergence
in the control. Emergence percentage for cucumbers in Oriental mustard treatment
was statistically similar to the other two treatments. Across the 30 day planting
period, cucumber showed a significant difference in emergence levels: 84.6% of
seeds in control emerged, while 79.4% in Oriental mustard treatment and 77.2%
in oilseed radish treatment emerged (Table 4.5). Control treatment had a
significantly higher overall emergence percentage than oilseed radish treatment,
while Oriental mustard treatment emergence rate was not significantly different
than that of the other two treatments. Cover crop treatments did not affect
muskmelon and honeydew seed emergence.

Cash Crop Growth

91

Cucumbers were thinned to two plants per hole 30 d after planting and
allowed to grow a total of 50-55 d to harvest At harvest, remaining vines were
counted and weighed fresh. There was no significant difference in number of vines
remaining among treatments (Table 4.6). On average 9.9-10.3 out of 12 possible
vines remained per bed.

There was no significant difference in average individual vine weights;
however, all values for control treatment were lowest (Figure 4.1)..

Cucumber Yield

There was no significant difference in production of Grades 1, 2, and 3 fruit
per vine; there was a significant difference in the average number of culled fruit
per vine averaged over the seven plant dates (Figure 4.2). Plants in oilseed radish
treatment produced 0.54 culled fruit per vine, significantly differing from those in
the control which produced 0.36 culled fruit per vine. There was no significant
difference between Oriental mustard treatment and the other two treatments.
There was no significant difference in marketable yields per vine among
treatments (Figure 4.3). There was a significant difference between treatments in
terms of total yield and average yield over all planting dates (Figure 4.4). Averaged
over all the planting dates, oilseed radish and Oriental mustard treatment vines
produced 230.5 and 198.4 g of fruit mass/vine compared to 142.8 g of fruit
mass/vine in the control; the cover crop treatment plants thus produced
significantly more total fruit mass /vine than control plants (Figure 4.5).
LABORATORY EXPERIMENT

Extract Yield and Characteristics

92

Oilseed radish produced roughly three times more shoot tissue than root
tissue (Table 4.7). The harvest netted 4.40 kg of root tissue and 12.78 kg of shoot
tissue. The large proportion of shoot to root tissue was due to the fact the taproot
was not well developed at cover crop sampling. When macerated in a 1:1 ratio of
biomass: deionized water and strained through cheesecloth, the results were 5.90
and 13.85 L of root and shoot tissue aqueous extract, respectively. This extract was
then filtered through filter paper, for a final total of 3.43 L of non-lyophilized root
aqueous extract and 10.07 L of non-lyophilized shoot aqueous extract Lyophilized
root and shoot extracts yielded 110.0 and 181.0 g of dry powder from 2.29 and
5.79 kg of fresh biomass, respectively (Table 4.8). The FER for non-lyophilized
extract was 0.20 and 0.67 ml/Petri dish for root and shoot extracts, respectively
(Table 4.7). Lyophilized extract FER was 4.00 and 8.88 g/L for root and shoot
extracts, respectively (Table 4.8). Specific treatment concentrations for both

lyophilized and non-Iyophilized extracts are listed in Table 4.9.

The pH of the deionized water was 8.16 while that of root and shoot
extracts was 6.02 and 5.80, respectively (Table 4.10). The EC of the deionized
water, root extract, and shoot extract was 28.10, 29.50 and 25.38 mv, respectively.
An experiment was conducted using an acetic acid solution with a pH of 5.4;
germination for muskmelon seeds in this treatment was 97.5% (with a standard
deviation of 5) while that of seeds in the deionized water control was 100% (with
a standard deviation of 0) (Table 4.1 1). Average root length in the acetic acid

treatment was 35.7 mm, while in the control it was 36.3 mm.

93

Germination

Both root and shoot non-lyophilized aqueous extracts significantly reduced
germination of honeydew, muskmelon, and cucumber (Figures 4.6 and 4.7). As
concentration of both extracts increased from 0 to 100%, inhibition became more
pronounced. Whereas 64.4-98.9% of seeds germinated at 0% extract
concentration, at 100% extract concentration germination rates ranged from 0 to

2.2%.

Shoot lyophilized extract had varying impacts on the crops (Table 4.12).
There were no differences in muskmelon germination rates across treatments. As
concentration increased, germination of both cucumber and honeydew seeds
decreased. At the 0 g/ L concentration, 80.0% of honeydew germinated while
20.0% germinated at the 8 g/ L concentration; at the 0 g/ L concentration, 82.2% of
cucumber germinated while at the 8 g/ L concentration, 42.2% germinated. The
effects were not as pronounced with lyophilized root extract (Table 4.13). Data for
the three trials could not be combined due to trial by treatment interaction. For
some trials in which the differences were significant, cucumber germination
ranged from 86.7% at the 0 g/ L concentration to 40.0% at the 8 g/ L concentration.
The same was true of honeydew: germination ranged from 90.0% at 0 g/ L to
60.0% at 8 g/L. Muskmelon germination was not inhibited by lyophilized root

extracts.

Crops also varied in their sensitivity to non-lyophilized extracts.

Muskmelon germination was most severely impacted at the 50% and 100% non-

94

lyophilized root extract concentrations and 100% shoot extract concentration
while cucumber germination inhibition began at 5% root concentration mark and
12.5% shoot concentration mark (Figures 4.6 and 4.7). Honeydew germination
was most sensitive to non-lyophilized extracts and began to be significantly

inhibited around 5% concentrations of root and shoot extract.

Radicle Length

Both root and shoot non-lyophilized aqueous extracts of oilseed radish
negatively inhibited radicle elongation of cucumber, honeydew, and muskmelon
(Figures 4.8 and 4.9). As extract concentration increased, impact on radicle length
became more severe. Crop radicle lengths ranged from 22.8 to 37.0 mm at 0%
extract concentration to a total grth inhibition at 100% extract concentration.
Even at 50% extract concentration radicle lengths ranged only from 0 to 5.7 mm.
The one exception was honeydew; at concentrations of 5% and 12.5% both root

and shoot non-Iyophilized extracts stimulated root growth.

As with germination rates, lyophilized extracts had a less pronounced
impact on radical elongation than non-lyophilized extracts (Figures 4.8 and 4.9,
Tables 4.14 and 4.15). None of the crops were affected by lyophilized shoot
extracts. In lyophilized root extract treatments, however, as concentration
increased root length generally decreased (Table 4.15). In Trial 1, for example,
average muskmelon root length decreased from 43.4 mm in the 0 g/L treatment to
11.4 mm in the 8 g/ L treatment Similarly, average cucumber root length

decreased from 23.1 mm in the 0 g/L treatment to 9.8 mm in the 8 g/L treatment

95

(Trial 2). Unlike cucumber and muskmelon, honeydew showed no clear association

between extract concentration and radical length.

DISCUSSION
FIELD EXPERIMENT

As indicated in Table 4.1, May 2009 was cooler and rainier than average.
This undoubtedly slowed cover crop growth and reduced biomass production.
Kirkegaard and Sarwar (1998) showed biomass levels determine the amount of
glucosinolates (and thus breakdown products like ITCs) present. Decreased
biomass means decreased amounts of glucosinolates released in to the
environment, and thus potentially decreased impacts on weeds /pathogens/cash
crops.

There was no significant Brassica cover crop impact on cucurbit emergence
in this field study, except for cucumber emergence on plant date D0. This may be
due to a combination of cucumber being more sensitive to cool weather and/ or
increased sensitivity to chemicals under adverse weather conditions. The overall
finding of this field study is in contrast with our previous observations (Chap. 3)
and previous research indicates Brassica cover crops and their chemical by-
products can impact seed germination. Bialy et al. (1990) found ITCs can inhibit
wheat (Triticum aestivum Songle) germination, though the effect varies by specific
ITC and concentration. Turk and Tawaha (2002, 2003) have demonstrated
Brassica nigra Moench aqueous extracts can inhibit lentil (Lens culinaris Medik.)
and wild oat (Avenafatua L.) germination. Norsworthy et al. (2006) and

Norsworthy and Meehan (2005a and 2005b) have demonstrated lTCs are active

96

against a wide range of weed seeds. Brown and Morra (1996) found B. napus
extracts inhibit lettuce (Lactuca sativa L) germination. These and other studies
differ from this one in that they were done in the laboratory and/or using pure
chemical extracts and/ or using smaller types of seeds. Carefully controlled
laboratory results do not always translate in to similar field results. A variety of
factors including soil structure and composition, weather, and amount of
chemicals that actually enter the environment in a given time period may mitigate
effects of compounds that otherwise would have an evident impact on
germination. Unlike in the laboratory, seeds in the field were not exposed to the
entire amount of chemical by-products all at once.

The results from this study are similar to those found by Haramoto and
Gallandt (20053) in a field study, in which cucumber germination was not
impacted by a yellow mustard cover crop. The germination of other cash crops in
that study, however, was impacted by the Brassica cover crops. Other studies have
found Brassica cover crops to impact cash crop germination in the field. Brassica
napus decreased lettuce (Lactuca sativa L.) and spinach (Spinacia oleracea L.)
germination (Kruidhof et al. 2009). Timing and treatment of the cover crop matter,
as well. Kruidhof et al. (2009) found that B. napus biomass inhibited lettuce and
spinach establishment in the first 2 - 3 weeks when finely macerated, while cut
biomass inhibited cash crop establishment after the first 2 — 3 weeks.

The results of this field study differ from those in Ch. 3, where oilseed
radish and Oriental mustard in particular inhibited muskmelon seed germination.

There are several possibilities as to why the results differ. Hoagland et al. (2008)

97

found low-glucosinolate B. napus seed meal and Sinapis alba seed meal
amendments increase Pythium spp. populations, including those of known-
pathogenic Pythium species which cause damping-off. They assert that it is a
combination of ITCs directly damaging seeds / seedlings and increased Pythium spp.
populations that causes decreased germination and increased seedling mortality of
cash crops and weeds. Pythium spp. populations vary by soil type and location
(Hoagland et al. 2008). It is possible SWMREC soil is heavily infested with Pythium
spp. whereas that at HRTC is not. Hoagland et al. (2008) further note Pythium spp.
do not respond uniformly to treatments, perhaps explaining the inconsistency
producers experience when using Brassica cover crops to decrease weed
populations and also helping explain why germination at SWMREC was severely
inhibited while at HRTC it was not. Differing soil properties are another possible
explanation. The soil at SWMREC is sandy, while the soil at HRTC contains more
clay. Hoagland et al. (2008) state that clay and organic matter can act as buffers,
decreasing amount of cover crop substrate available to Pythium spp. and limiting
population increases. This same buffering (due to a high cation exchange capacity)
could also prevent lTCs from coming in to as much contact with seeds, decreasing
any direct effects on germination.

While Brassica cover crops have been shown to impact seed germination
and seedling survival, there were no such clear impacts beyond the seedling stage
in this study. There were no significant differences in individual vine weights
(Figure 4.1) or marketable harvest per vine (Figure 4.3). This concurs with the

findings of Haramoto and Gallandt (2005b) that a yellow mustard (S. alba) cover

98

crop neither suppressed nor encouraged green bean growth. While vine weights
did decrease over the course of the 30 d planting period, this was most likely due
to the increasing severity of downy mildew in the plot There was a significant
difference in total yield per vine: plants in oilseed radish and Oriental mustard
treatments produced 230.5 and 198.4 g fruit/vine, which was significantly
different than the 142.8 g fruit/vine produced by plants in the control (Figure 4.5).
While there was no significant difference in production of Grades 1, 2, and 3 fruit
per vine, there was a significant difference in the number of culled fruit per vine
(Figure 4.2). Plants in oilseed radish treatment produced 0.54 culled fruit per vine,
differing from those in the control which produced 0.36 culled fruit per vine. With
0.45 culled fruit per vine, there was no significant difference between Oriental
mustard treatment and the other two treatments. The majority of culled fruit were
culls because they were too large, not because they wererotten or badly
misshapen.

Cucumbers likely benefited from nutrient cycling and soil improvement
capacities of the cover crops; while the values were not statistically significant,
Figures 4.1, 4.2, and 4.3 support this assertion. Plants in oilseed radish treatment
consistently produced more fresh weight biomass per vine than those in the
control (Figure 4.1). Plants in oilseed radish treatment also consistently produced
more Grade 1, 2, and 3 fruit per vine than those in the control (Figure 4.2) and also
reliably produced more marketable fruit mass per vine than those in the control
(Figure 4.3). Finally, over the seven plant dates plants in oilseed radish and

Oriental mustard treatments produced more fruit mass per vine than those in the

99

control (Figure 4.5). The limited yield response of cucumber to the cover crops is
typical of most short cycle crops (Ngouajio, personal communication).

LABORATORY EXPERIMENT

Previous research indicates plants in the Brassica family can have an
inhibitory effect on seed germination and growth (Brown and Morra 1997; Oleszek
1987 ; Turk and Tawaha 2002 and 2003). Most of these studies focused on rape (B.
napus) and black mustard (B. nigra). Oleszek (1987) stated the impact of a Brassica
species on seed germination and seedling growth would vary depending on both
Brassica and crop species. This study allowed us to investigate impact of oilseed

radish on seed germination and radicle grth under controlled conditions.

The amount of oilseed radish biomass harvested was not as large as it
would have been had the plants been fully mature and beginning to flower
(Ngouajio, personal communication). Yield was about 5,866 kg/ha of fresh root
tissue and 19,968 kg/ha of fresh shoot tissue. This is important because oilseed
radish is usually tilled under after flower initiation but before seed set For the
above reasons, estimated field rates are likely underestimations of what would
happen in a normal situation where the cover crop is allowed to grow until the
flowering stage. Turk and Tawaha (2002 and 2003) found stronger concentrations
of B. nigra aqueous extracts were more inhibitory to seed germination and
seedling growth. Root tissue estimated field rate ( 0.2 mL or 6.7% of the non-
lyophilized full strength solution) falls between the 5 and 12.5% experimental

treatments; at these rates, germination inhibition was observed in cucumber and

100

honeydew, as was radicle elongation inhibition in muskmelon and cucumber.
Shoot tissue estimated field rate (0.7mL or 22.3% of non-lyophilized full solution
strength) falls between the 12.5 and 25% treatments; germination inhibition was
again observed in honeydew and cucumber at these rates, as was radicle
elongation inhibition in cucumber. Therefore, while many of the non-lyophilized
extracts treatments were more concentrated than the field rate equivalent, effects
were still seen at rates near the field rate equivalent and the stronger
concentrations help offset the smaller original biomass harvest (had the harvest
been of normal size and at the normal time, the field extract rate would naturally

be higher than it was).

The study conditions differ from field conditions in that in the field, organic
matter would degrade slowly and water soluble compounds would be released
over time. Also, due to soil dynamics, seeds would likely not come into contact with
such concentrated volumes of water soluble compounds. Compounds would be
mixed in with soil and with other chemicals in the soil, further diluting them and
potentially mitigating their impact on seeds. In addition, organic matter and clay in

the soil can act as buffers (Hoagland et al. 2008).

The EC of all three substances used in the solutions (root and shoot extract
and deionized water) were similar, making it unlikely any effects were due to
differences in osmotic potential. The pH of non-lyophilized root and shoot extracts
was similar and somewhat acidic (6.0 and 5.8, respectively), as was that of

lyophilized root and shoot extracts (6.8 and 5.6, respectively). An experiment was

101

performed to determine if pH was a potential confounding variable; water was
mixed with acetic acid to create a solution with a pH of 5.5. Muskmelon seeds
placed in Petri dishes with 3 ml of this solution had similar germination rates and
root lengths when compared to those in the control (deionized water) (Table
4.11). In the acetic acid solution, 97.5% of seeds germinated while 100.0% of those
in the control germinated; average root length in the acetic acid solution treatment
was 35.7 mm (with a standard deviation of 8.7), while in the control it was 36.3
mm (with a standard deviation of 9.9). Given the above information it is likely that
the observed results were caused by the treatments and not by differences in

extract pH or EC.

In this study both root and shoot non-lyophilized aqueous extracts
consistently inhibited germination of all three crops. This finding is in line with the
work of Brown and Morra (1996), which determined B. napus aqueous extracts
inhibit lettuce seed germination. It also coincides with the work of Turk and
Tawaha, who discovered B. nigra aqueous extracts inhibit germination of lentils
(2002) and wild oats (2003). Effects were far more pronounced with non-

lyophilized extracts than with lyophilized extracts.

Non-lyophilized root aqueous extract appears to be more inhibitory to
germination than shoot aqueous extract whereas lyophilized shoot extract was
more consistently inhibitory than the root extract The observation that non-
Iyophilized root extracts are more potent than shoot extracts is in contrast with

the findings of Brown and Morra (1996) that water soluble compounds from B.

102

napus leaves and stems inhibited lettuce seed germination, whereas those from
roots merely delayed germination. It is also in contrast with the work of Turk and
Tawaha (2002, 2003), which determined that B. nigra leaf extracts tended to be
more toxic than those from other plant parts including roots and stems. However,
Brown and Morra (1996) determined volatile compounds from B. napus roots
inhibited lettuce seed germination more than those from stems and leaves.
Brassica species vary in their chemical profiles (Kirkegaard and Sarwar 1998) and
these chemicals vary in their potency, depending on the crop involved (Brown and

Morra 1996).

Non-lyophilized root and shoot aqueous extracts likewise proved inhibitory
to radicle elongation. With the exception of the impact of root extract on
honeydew, as extract strength increased so did level of inhibition. Honeydew roots
were stimulated at 5 and 12.5% extract concentrations. Some previous work has
found low rates of Brassica residue stimulate growth (Mason-Sedun and jessop
1986), perhaps explaining this observation. Oleszek (1987) stated the impact of
Brassica cover crops varies depending on the cover crop and the cash crop,
perhaps explaining why honeydew grth was stimulated while muskmelon and

cucumber growth was inhibited.

As with germination rates, crop root elongation was less sensitive to
Iyophilized extracts than non-lyophilized extracts. When a significant effect was
present, in general, as concentration increased radical length decreased.

Differences in potency between lyophilized and non-lyophilized extracts probably

103

 

were due to varying levels of volatile versus water soluble compounds. While it is
likely that both types of compounds have inhibitory effects, volatile compounds
(specifically ITCs) are generally believed to be the major force behind biological
activity such as germination inhibition and pathogen disruption. Lyophilized
extracts likely contained less of these highly active compounds, and thus were

generally less potent

Even at low concentrations, most of the crops were sensitive to non-
lyophilized oilseed radish extracts. Crops were overall far less sensitive to
lyophilized extracts, typically beginning to show inhibition at the 2 to 4 g/L
concentrations. Cucumber and muskmelon root growth was inhibited at non-
lyophilized 5% root extract concentration. This observation coincides with Turk
and Tawaha (2002, 2003), who determined roots are particularly sensitive to
Brassica aqueous extracts. As with germination rates, radicle sensitivity varied by
crop, though differences were less pronounced. Muskmelon was more sensitive to
non-lyophilized shoot extract than honeydew and cucumber, while cucumber was
less sensitive to that extract than honeydew. Cucumber and honeydew were

equally sensitive to non-lyophilized root extract

CONCLUSIONS
Our field study demonstrates that at these levels of biomass production,
oilseed radish and Oriental mustard have limited inhibitory impact on cucurbit

germination under our specific field conditions. Differences between the results in

104

this study and those in Chapter 3 may be due to differing soil physical and
microbial properties.

The laboratory study demonstrates Brassica non-lyophilized aqueous
extracts can impact cucurbit germination and grth and confirms that oilseed
radish contains allelopathic chemicals. Inhibition was demonstrated even at
concentrations near the fairly low estimated field rate for non-lyophilized extracts.
Root and shoot extracts have varying degrees of toxicity, with non-lyophilized root
extract being generally more inhibitory of both germination and radicle
elongation. Further, the degree of inhibition varies by cucurbit crop. Muskmelon
germination and root growth were both less impacted by extracts than that of
honeydew and cucumber.

Lyophilized extracts were less potent than non-lyophilized ones. This
suggests the primary compounds involved in germination and growth inhibition
could be volatile in nature. More work should be done to identify these
compounds.

Laboratory work often provides more consistent results than field work
Discoveries in the laboratory, however, do not always have relevance in the field
due to the multitude of variables that can interfere with seemingly simple
interactions. Further work needs to be done in the greenhouse/field to determine
at what biomass levels Brassica cover crops might begin to impact germination
and growth and if this impact is practically significant More work also needs to be
done to determine how much interaction occurs between soil physical properties

(such as clay content), soil microbial communities (such as the presence of

105

Pythium spp.), and Brassica cover crops in the field. Finally, other commonly
recommended Brassica cover crops such as yellow mustard should be tested in the
laboratory to determine if they are likewise capable of inhibiting germination and

growth.

106

 

Table 4.1. Mean monthly and long term (8 year) average temperatures and
precipitation during cover crop and cucurbit growth in 2009 at HRTC Michigan.‘- ”

 

Monthly average temperature (°C) Monthly rainfall (mm)

 

8 yr average 2009 8 yr
Month Low High Low High average

May 7.9 20.5 7.4 19.9 109 106
lune 13.4 24.8 13.4 25.8 126 81
july 13.4 24.9 15.1 27.8 61 70
August 14.9 25.5 14.7 27.0 105 59
September 10.7 23.5 10.5 23.5 24 77
Ave/Total 12.1 23.8 12.2 24.8 425 393

 

‘ Cover crops were sown on May 8, 2009. Cucurbits were planted during the period
between June 24, 2009 and july 24, 2009.
“Raw data are from the Michigan Automated Weather Network (MAWN).

107

 

Table 4.2. Monthly and long term (8 year) average growing degree days (GDD)
during cover crop and cucurbit growth in 2009 at HTRC Michigan*. Base

temperature = 10°C.“

 

 

 

MonthlLGDD

Month 2009 8 yr average
May 163.8 144.6
lune 279.0 288.9
july 290.2 359.5
August 314.0 335.7
September 216.6 213.2
Total 1263.6 1341.8

 

‘ Cover crops were sown on May 8, 2009. Cucurbits were planted during the period
between june 24, 2009 and july 24, 2009.
“Raw data are from the Michigan Automated Weather Network (MAWN).

108

Table 4.3. Cover crop biomass production (dry weight) for field experiments
conducted in 2009 at the Horticulture Teaching and Research Center (HRTC), East
Lansing, Ml.*

 

 

Crop Ave no. of Dry weight Estimated
plants/ m2 (anZ) kg/ ha

Oilseed radish 68.0 (11.6) 476.8 (82.4) 4,677

Oriental mustard 165.2 (36.8) 525.2 (77.2) 5,250

 

*Two samples were collected in each treatment using a 0.25 m2 quadrat and final
data was converted to 1 m2. Values in parentheses are standard deviations.

109

Table 4.4. Emergence percentages of three cucurbit crops planted after oilseed
radish, Oriental mustard, or no cover crop. Each percentage is generally the
average of four replications, each planted with 24 seeds of each crop. OSR= Oilseed
radish, OM=Oriental mustard, CK=control. There was no plant date by cover crop
significance at the P=0.05 level with the exception of D0 cucumber.*

 

Day planted (days after cover crop incorporation)

 

 

Cucurbit Treat-
ment 0 5 10 15 20 25 30
MM cx 87.5 62.5 70.5 81.3 85.4 96.9 99.8
OM 77.1 54.2 80.2 82.3 85.4 89.6 96.9
osn 94.8 41.7 76.1 87.5 85.4 96.9 99.0
LSDo.os NS NS NS NS NS NS NS
HD cx 80.2 64.6 76.0 82.3 76.0 91.7 92.7
OM 63.5 69.4 82.3 73.0 74.0 90.6 85.4
0511 68.8 55.2 76.0 82.3 83.3 90.6 92.7
LSDo.os NS NS NS NS NS NS NS
Cuke CK 72.9 a 74.0 87.5 86.1 83.3 89.6 99.0
OM 73.6 a 74.0 69.8 81.3 80.2 90.6 86.5
058 45.8b 70.8 81.3 - 90.6 74.0 91.7 86.5
LSDo.os 18.7 NS NS NS NS NS NS

 

* Within each cucurbit crop and column, means were separated using the LSD at
the 5% level of significance. N8 is not significant

110

Table 4.5. Emergence percentages of three cucurbit crops planted after oilseed
radish, Oriental mustard, or no cover crop. Each percentage is the average of seven
planting dates, each consisting of four replications wherein each replication was
planted with 24 seeds of each crop. OSR= Oilseed radish, OM=Oriental mustard,
CK=control.

 

 

Cucurbit Treatment Average % emergence LSDo.os
Cucumber CK 84.6 a*

OM 79.4 ab

OSR 77.2 b 5.7
Muskmelon CK 82.8

OM 80.8

OSR 83.0 NS
Honeydew CK 80.5

OM 77.0

OSR 78.4 NS

 

* Means within column and crop followed by the same letter do no differ
significantly at the P=0.05 level. N3 is not significant

111

 

Table 4.6. Number of remaining vines left at harvest (out of 12) of cucumber
planted after oilseed radish, Oriental mustard, or no cover crop. Each value is the
average of four replications. OSR= oilseed radish, OM=Oriental mustard,

 

 

 

 

CK=control.
Treatment Day planted (days after cover crop
0 5 10 15 20 25 * 30
CK 7.3* 9.5 1 1.0 9.3 11.3 10.8 11.5
OM 7.0 10.0 10.8 9.8 9.5 11.8 10.5
OSR 7.0 10.3 10.5 11.0 9.8 11.5 11.8
LSD NS NS NS NS NS NS NS

 

*Within each column (planting date) NS indicates no significant difference at the p-
level of 0.05 for differences among cover crop treatments.

112

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113

Table 4.8. Oilseed radish harvest and lyophilized extract data and corresponding
field equivalent rates.

 

 

 

Ratios
Total
ma“ Area fresh Extract Extract/ Extract
source harvested d Extract/ area fl l d
biomass TY harvested fresh e
weight biomass rates“
In2 kg 8 .::/m2 glkg g/L
Roots 3.9 2.3 110.0 28.1 48.0 4.0
Shoots 2.9 5.8 181.0 62.4 31.3 8.9

 

*Estimated extract field equivalent rate calculation (Eq. [1]) **

** Field equivalent rate represents the maximum concentration assuming the
cover crop is incorporated 15 cm deep and that all allelochemicals are released at
the same time

114

Table 4.9. Oilseed radish non-lyophilized and lyophilized aqueous extract
treatments. Root and shoot concentrations were the same.*

 

 

N on-lyophilized Amount of non- Lyophilized extract
extract lyophilized extract concentrations
concentration % per Petri dish (ml) (g/L)
0.0 0.000 0.00
5.0 0.150 0.25
12.5 0.375 0.50
25.0 0.750 1.00
50.0 1.500 2.00
100.0 3.000 4.00
- - 8.00

 

*Each Petri dish received a total of 3 ml of solution.

115

 

Table 4.10. Properties of oilseed radish non-lyophilized and lyophilized root and
shoot aqueous extracts and deionized water. Values in parentheses are the
standard deviations; values are the average of 8 readings. Values are for the full
strength concentration (100% and 8 g/ L) of extracts.

 

Property Extract

Average
Non-lyophilized Lyophilized extract
extract

pH Root 6.0 (0.0) 6.8 (0.0)
Shoot 5.8 (0.0) 5.6 (0.0)
Deionized water 8.2 (0.1) 7.8 (0.1)
Vinegar solution - 5.5 (0.0)

EC (mv) Root 30.0 (0.5) 28.0 (0.0)
Shoot 25.4 (0.7) 25.1 (0.6)
Deionized water 28.1 (0.6) 28.6 (0.5)
Vinegar Solution - 117.0 (0.5)

 

116

 

 

Table 4.11. Effect of an acetic acid solution on muskmelon seed germination and
root length. Numbers in parentheses are standard deviations.

 

 

Treatment pH Average % Average root
Ermination“ length (mm)

Acetic acid solution 5.5 (0.0) 97.5 (5.0) 35.7 (8.7)

Deionized water 7.8 (0.1) 100.0 (0.0) 36.3 (9.9)

 

*Each value is the average of four replications consisting of ten seeds each.

117

Table 4.12. Germination rates of cucurbits exposed to seven concentrations of
oilseed radish lyophilized shoot aqueous extract Each value is the average of three
experimental runs, each consisting of three replications of ten seeds each.

 

 

 

 

Germination %

Treatment jL/L) Muskmelon Honeydew Cucumber
0.00 98.9 80.0 a* 82.2 a
0.25 97.8 58.9 bc 88.9 a
0.50 98.9 66.7 ab 88.9 a
1.00 97.8 50.0 c 80.0 a
2.00 98.9 48.9 c 75.6 a
4.00 97.8 53.3 bc 61.1 b
8.00 97.8 20.0 d 42.2 c
LSDo.os NS 14.4 14.0

 

* Means within a column followed by the same letter do no differ significantly at
the P=0.05 level. N5 is not significant

118

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Table 4.14. Radicle lengths (mm) of cucurbits exposed to seven concentrations of
oilseed radish lyophilized shoot aqueous extract. Each value is the average of three
experimental runs, each consisting of three replications of ten seeds each.
Averages were calculated using the number of seeds germinated, not the number
of seeds tested.

 

 

 

 

Root length (mm)

Treatmenug/ L) Muskmelon Honeydew Cucumber
0.00 42.6 27.0 24.2
0.25 60.8 30.4 28.5
0.50 30.2 27.2 26.0
1.00 28.1 29.8 24.9
2.00 28.8 26.2 24.0
4.00 26.3 29.3 23.1
8.00 19.9 21.2 18.6
LSDo.os NS NS NS

 

* Means within a column followed by the same letter do no differ significantly at
the P=0.05 level. N8 is not significant.

120

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300 —
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.5 150 -
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M
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<

50 - '

0 l l l l l r I
DO 05 010 015 020 025 030
Plant date

 

 

 

Figure 4.1. Individual fresh vine weights (g) at harvest of cucumber planted after
oilseed radish, Oriental mustard, or no cover crop. Each value is the average of four
replications. OSR= oilseed radish, 0M=Oriental mustard, CK=control. NS (not
significant) at the p-level of 0.05 for differences among cover crop treatments.

122

 

0'6 “ I OSR

Ave. fruit no. per vine

 

 

Grade 1 Grade 2 Grade 3 Cull

Fruit Grade

 

 

 

Figure 4.2. Average cucumber fruit produced per plant from seeds planted alter
oilseed radish, Oriental mustard, or no cover crop. Values are the averages of all
seven planting dates. OSR= Oilseed radish, 0M=Oriental mustard, CK=control. NS
at the P=0.05 level for differences between cover crop treatments with the
exception of the cull fruit. Different letters indicate significant difference; Means
followed by the same letter do not differ significantly at the P=0.05 level. N8 is not
significant; LSDo.os = 0.1 5.

123

 

on S H N
1 l l J

Marketable yield (3) per vine
b
O

 

 

 

00 DB 010 015 020 025 030
Plant date

 

 

 

Figure 4.3. Cucumber marketable yield (g) per plant from plants planted after
oilseed radish, Oriental mustard, or no cover crop. OSR= Oilseed radish,

0M=Oriental mustard, CK=control. NS (not significant) at the P=0.05 level for '
differences between cover crop treatments.

124

 

450 -

400 — +CK
-I-0M

e. ‘ +OSR

350 -
300 J
250 -
200 -

150 4

 

100 -

Total yield (3) per vine

50*

 

 

DO 05 D10 015 020 025 030
Plant Date

 

 

 

Figure 4.4. Cucumber total yield (g) per plant from plants planted after oilseed
radish, Oriental mustard, or no cover crop. OSR= Oilseed radish, 0M=Oriental

mustard, CK=control. NS (not significant) at the P=0.05 level for differences
between cover crop treatments.

125

 

250 -

g

H

U1

0
1

Average yield (3) per vine
H
s 8

 

 

O
I

Control Oriental mustard Oilseed radish
Treatment

 

 

 

Figure 4.5. Yield (g) per cucumber vine in each treatment averaged over the seven
plant dates. Bars with different letters are significantly different at the P=0.05
level. LSD = 43.35. Standard error bars are 5% of each value.

126

 

 

 

 

 

 

 

10° Root extract
90 +Muskmelon
80 -I-Honeydew
+Cucumber
3; 70
C
2 60
‘5 50
.E
E 40
i-
0
to 30
20 4
1o -
0 i 1 9“: I
o 20 4o ' 60 80 100
Extract concentration (%)

 

 

Figure 4.6. Germination percentages of three cucurbit crops exposed to six extract
concentrations of non-lyophilized oilseed radish root aqueous extract. Each

percentage is the average of three experimental runs, each consisting of three
replications of ten seeds each. i

127

 

 

 

 

 

10° Shoot extract
90 _ +Muskmelon
+Honeydew
80
+Cucumber
SE 70
c .
2 60
‘5 so -
.5
E 40 ~
3
u 30 -
20 4
10 '1
O l l | l l I
O 20 4O 69 80 100
Extract concentration (%)

 

 

 

Figure 4.7. Germination percentages of three cucurbit crops exposed to six extract
concentrations of oilseed radish non-lyophilized shoot aqueous extract. Each
percentage is the average of three experimental runs, each consisting of three
replications of ten seeds each.

128

 

 

b
O

35 Shoot extract
+Muskmelon

30 -l-Honeydew
+Cucumber

N
U1

Root length (mm)
a B

 

H
O
1

 

 

 

 

 

0 1 l l l r I r r wmi
o 10 20 30 4o 50 60_ 70 80 90 100
Extract concentration %

 

Figure 4.8. Radicle lengths (mm) of cucurbits exposed to six concentrations of
oilseed radish non-lyophilized shoot aqueous extract. Each value is the average of
three experimental runs, each consisting of three replications of ten seeds each.
Averages were calculated using the number of seeds germinated, not the number
of seeds tested.

129

 

 

 

 

 

 

40
Root extract
35 +Muskmelon
-I-Honeydew
30 +Cucumber
E 25
.5.
.c 20
5°
2 15
‘6
10
a?
5
0 I T IV
0 10 20 30 40 50 60 70 80 90100
Extract concentration %

 

 

 

Figure 4.9. Radicle lengths (mm) of cucurbits exposed to six concentrations of
oilseed radish non-lyophilized root aqueous extract. Each value is the average of
three experimental runs, each consisting of three replications of ten seeds each.

Averages were calculated using the number of seeds germinated, not the number
of seeds tested.

130

m I

I:-‘A.:

CHAPTER 5: Conclusions and Further Work

131

 

 

CHAPTER 5: Conclusions and Further Work \

These studies overall do not support the use of oilseed radish, yellow
mustard, or Oriental mustard as spring cover crops in an eggplant/muskmelon
short rotation if the cash crops are to be planted within two weeks of cover crop
incorporation. Furthermore, these cover crops should be used cautiously,
especially in the case of direct-seeded melons. The SWM REC field experiment
demonstrated these cover crops can inhibit muskmelon emergence. Although
inhibition was less of a problem for the re-seeded melons, the growing season in
Michigan is generally short and may not allow re-seeded plants to produce an
acceptable yield. Laboratory studies also showed oilseed radish to inhibit
muskmelon, honeydew, and pickling cucumber germination and radicle growth.
That cucumber germination was not inhibited by oilseed radish and Oriental
mustard in the HRTC field experiment suggests impact will vary by location /soil
type/microbial populations.

Laboratory experiments suggest impact of oilseed radish on cucurbit
germination is due at least partly to allelopathy, and the responsible compounds
are likely volatile in nature. Cucurbit germination and radicle elongation were
increasingly inhibited as non-lyophilized extract concentration increased. The one
exception was honeydew, whose grth was stimulated at low extract
concentrations. Inhibition was far less severe when lyophilized extracts were
involved. Non-lyophilized aqueous root extracts proved to be more inhibitory to
both germination and radicle growth than non-lyophilized shoot extracts, while

the opposite was true of lyophilized extracts. Species varied in their sensitivity, as

132

well: muskmelon was less inhibited than honeydew or cucumber with both types
of extracts.

Furthermore, the SWMREC field experiment provides some evidence that
these Brassica cover crops may be deleterious to eggplant growth. During the first
year of the experiment, plants in cover crop treatments (especially yellow mustard
treatment) were shorter on several consecutive data collection dates than those in
other treatments, including the fallow control. Eggplants in cover crop treatments
produced less fresh above ground biomass than those in other treatments during
the first year of the study. Differences were less pronounced/ not significant during
the second year of the study, perhaps due to decreased cover crop dry biomass
production.

The SWMREC study suggests while the Brassica cover crops may have some
impact on verticillium wilt incidence, it is not significant in a practical sense and it
is not reflected in the yields. Given negative impact on cash crop growth, the costs
are not worth the benefits under this production system.

One positive aspect of the use of these Brassica cover crops is the indication
in the SWMREC study that they do scavenge N, which is later released in to the soil
during decomposition. Oilseed radish and Oriental mustard treatment plots
showed significantly higher levels of nitrate during the growing season (June
and/or July) than those in other treatments. This finding suggests that Brassica
cover crops still have a place in sustainable production systems, just not as spring

cover crops immediately before a cash crop.

133

 

j {— twist-.5m. manna-J
A

These studies create more questions than they answer. Possible laboratory
and greenhouse research questions might include: do Oriental mustard and yellow
mustard also inhibit cucurbit germination; what impact do these cover crops have
on soil microbial populations; is the effect on cucurbit germination due to
allelopathy, microbial biological interactions, or a combination; what is the nature
of the potentially allelopathic chemicals (water soluble, volatile, or both); and what
is a safe plant~back period when growing Brassica cover crops? Field research
questions might include: do these Brassica cover crops impact germination of
other direct-seeded cash crops in Michigan when planted as spring cover crops;
how does soil type affect the impact of these cover crops on cash crops; and do
higher/lower amounts of Brassica cover crop biomass production have differing

impacts on cash crop health, growth, and yield?

134

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