INTERACTION OF PURPLE DEADNETTLE, LAMIUM PURPUREUM, SOYBEAN
CYST NEMATODE, HETERODERA GLYCINES AND ITALIAN RYEGRASS,
LOLIUM MULTIFLORUM
Thesis
Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in
the Graduate School of The Ohio State University
By
Bruce Austin Ackley
Graduate Program in Horticulture and Crop Science
The Ohio State University
2013
Committee:
Dr. Kent Harrison, Advisor
Dr. Mark Sulc, Advisor
Dr. Emilie Regnier
Copyrighted by
Bruce Austin Ackley
2013
Abstract
Soybean cyst nematode (SCN; Heterodera glycines) causes more economic
damage to U.S. soybean producers than any other soybean pathogen. Greenhouse and
field studies have shown that the winter annual weed purple deadnettle (PDN) is an
alternate host of SCN. Previous work has shown that Italian ryegrass significantly
reduced SCN populations in soil under greenhouse and field conditions, but the nature
and extent of this suppressive effect on SCN is not well understood.
My research focused on investigating the nature of the Italian ryegrass x SCN
interaction and the effectiveness of Italian ryegrass as a winter cover crop to suppress
SCN and PDN. The first study was conducted in the greenhouse and consisted of a
replacement pot experiment in which plant biomass and SCN reproduction was measured
from pots containing various ratios of Italian ryegrass (IR) and PDN, +SCN.
The second study was conducted at Waterman Agricultural and Natural Resource
Laboratory of The Ohio State University, Columbus, OH in SCN infested field
microplots and was designed to duplicate treatments investigated in the greenhouse with
the addition of other forage species. The studies focused on the dual ability of Italian
ryegrass to provide suppression of PDN and reduce SCN populations. A primary
objective of the research was to quantify the pest suppressive effects of Italian ryegrass
and propose methods by which to integrate it into a typical Ohio crop rotation. Dry shoot
biomass of IR or PDN did not differ between SCN-inoculated and non-inoculated
treatments. PDN had the competitive advantage over IR under greenhouse conditions.
ii SCN reproduction generally occurred to a similar extent regardless of the PDN:IR ratio.
Overall results of the field study showed that after two years of growing susceptible
soybean in heavily SCN-infested plots, all winter annual cover crops tested were
generally effective in preventing an increase in SCN population growth. Furthermore,
my research indicated that an IR cover crop planted in early autumn after soybean
significantly reduced SCN population density in soil and was significantly more effective
in reducing SCN egg population densities than oat or rye cover crops. Thus
incorporation of IR into soybean cropping systems as a winter annual cover crop has the
potential to be a useful SCN management tactic for producers.
iii Dedicated to my wife Marta and my daughter Iris, the ones who inspire and love. To my
father Bruce, who always gave me support and the gift of hard work. To my mother Dara,
the one who showed me how to dream. Life is the slow change of perspective over the
long course of time.
iv Acknowledgments
I would like to express my sincere gratitude and appreciation to my advisors Dr.
Kent Harrison and Dr. Mark Sulc for their patience, encouragement, and guidance. Both
have provided valuable time, effort, and energy helping me throughout the journey that
was my research and is this thesis.
v Vita
2001 – 2005.…………………………………………..B.S. Agriculture, The Ohio State University 2006 – 2008…………………………………………...Graduate Research Associate, Department of Horticulture and Crop Science, The Ohio State University 2008 – Present……………………………………….Extension Program Specialist, Weed Science, The Ohio State University FIELDS OF STUDY Major Field: Horticulture and Crop Science Weed Science vi Table of Contents
Abstract……………………………………………………………………………………ii
Dedication……………………………………………………………………...…………iv
Acknowledgments……………………………………………………………………........v
Vita…………………………………………………………………………………..........vi
List of Figures………………………………………………………………………..…viii
Chapter 1: Literature Review.….……………………………………………………….....1
Chapter 2: Interactions of Purple Deadnettle, Lamium purpureum, Soybean Cyst
Nematode, Heterodera glycines and Italian Ryegrass, Lolium multIflorum ……………18
Appendix A -­‐ Mean SCN egg population density change factor (Pf/Pi; final population/initial population) for various fall-­‐seeded winter cover crops following summer cropping with susceptible ‘Resnick’ soybean across three sampling times. A Pf/Pi value more than 1 represents an increase in SCN population density, Pf/Pi of 1 represents no change in SCN egg population density, and Pf/Pi less than 1 represents a decrease in SCN egg population density. Vertical bars represent +/-­‐ S.E……………………………………………………………………………………………………………………35
Appendix B - Soybean cyst nematode egg population densities measured in spring in
response to fall-seeded winter cover crops followed by SCN-susceptible ‘Resnick’
soybean in 2006, 2007, and 2008. Egg counts were measured after soybean removal but
before cover crop seeding each year……………………………………………………..36
Appendix C - Soybean cyst nematode egg population densities measured in fall in
response to fall-seeded winter cover crops followed by SCN-susceptible ‘Resnick’
soybean in 2006, 2007, and 2008. Egg counts were measured after soybean removal but
before cover crop seeding each year………………………...…………………………...37
Appendix D - SAS Program Statement Used To Test SCN Egg
Counts……………………………………………………………………………….…...38
Appendix E - SAS Program Statement Used To Test Plant Shoot Biomass and SCN Egg Counts…………………………………………………………………………………………………………39 References……………………………………………………………………………......40
vii List of Figures
Figure
Page
1.1
Shoot dry weights in pots containing various species mixtures of Italian ryegrass
and purple deadnettle in a greenhouse experiment conducted in Spring 2008. The
dotted lines represent theoretical yields of constituent monocultures at different
species mixtures. Vertical bars represent +/- S.E………………………………..28
1.2
Shoot dry weights in pots containing various species mixtures of Italian ryegrass
and purple deadnettle in a greenhouse experiment conducted in Fall 2008. The
dotted lines represent theoretical yields of constituent monocultures at different
species mixtures. Vertical bars represent +/- S.E………………………………..29
1.3
Soybean cyst nematode egg population densities in pots containing various
species mixtures of Italian ryegrass and purple deadnettle in greenhouse
experiments conducted in Spring and Fall of 2008. Means accompanied by
different letters within each experiment were significantly different according to
Fisher’s Least Significant Difference (P<0.05)………………………………….30
1.4
A.l
Mean SCN egg population density change factor (Pf/Pi) over three years in response to various fall-­‐seeded winter cover crops following summer cropping with susceptible ‘Resnick’ soybean. A Pf/Pi value more than 1 represents an increase in SCN population density, Pf/Pi of 1 represents no change in SCN egg population density, and Pf/Pi less than 1 represents a decrease in SCN egg population density. Vertical bars represent +/-­‐ S.E……34 B.1
Appendix B - Soybean cyst nematode egg population densities measured in spring
in response to fall-seeded winter cover crops followed by SCN-susceptible
‘Resnick’ soybean in 2006, 2007, and 2008. Egg counts were measured after
soybean removal but before cover crop seeding each year……………..………..36
C.1
Appendix C - Soybean cyst nematode egg population densities measured in fall in
response to fall-seeded winter cover crops followed by SCN-susceptible ‘Resnick’
soybean in 2006, 2007, and 2008. Egg counts were measured after soybean
removal but before cover crop seeding each year……………………………......37
Appendix A -­‐ Mean SCN egg population density change factor (Pf/Pi; final population/initial population) for various fall-­‐seeded winter cover crops following summer cropping with susceptible ‘Resnick’ soybean across three sampling times. A Pf/Pi value more than 1 represents an increase in SCN population density, Pf/Pi of 1 represents no change in SCN egg population density, and Pf/Pi less than 1 represents a decrease in SCN egg population density. Vertical bars represent +/ S.E……………………………………………………35
viii CHAPTER 1
Literature Review
1.1 Introduction
The soybean cyst nematode (SCN) consistently ranks as the most economically
important soybean pathogen in the United States and is responsible for greater annual
losses in U.S. soybean yield than any other pathogen (Wrather and Koenning 2006;
Wrather et al. 2003). Yield reductions caused by SCN can be as high as 30%, and vary
widely depending on SCN population density, crop cultivar, alternate hosts, soil moisture,
and temperature (Riedel et al., 1998; Schmitt, 1992, Schmitt and Riggs, 1989). The first
detection of SCN in an Ohio soybean field was reported in 1987 (Riedel and Golden et al.,
1988), and 25 years later, 68 of Ohio’s 88 counties contained fields infested with SCN
(Dorrance et al. 2012).
SCN is a microscopic, poikilothermic roundworm approximately 0.4 mm long
that feeds on roots of soybean or other compatible host plants. Encysted eggs of SCN can
remain dormant and viable for at least five years in the absence of a compatible host, and
SCN populations below the damage threshold can return to economically damaging
levels in a single year when a susceptible soybean variety is grown (Long and Todd et al.,
2001; Porter 2001). It has been recognized since the 1950s that variability in virulence
exists among populations of SCN, and the development of a virulence phenotype testing
and classification system began in 1970 (Golden et al. 1970). Current classification of
1 SCN virulence phenotypes is by “HG Type” (originally called “races”) and is based on
bioassay of a given phenotype’s ability to develop and reproduce differentially on seven
introduced soybean lines having different sources of SCN resistance (Niblack et al.,
2002). There are at least 16 known HG Types, and accurate virulence phenotyping is
important with regard to the proper selection of a resistant soybean variety in any SCN
management system (Niblack and Riggs et al. 2004).
One problem group of plant species associated with SCN is winter annual weeds
because a number of species have been identified as alternative hosts of SCN (Venkatesh
et al. 2000). Winter annual weeds have become increasingly prevalent in crop production
fields in recent years due to changing management practices such as no-tillage production
(Gibson et al. 2005; Nice and Johnson 2005), and research has shown that purple
deadnettle (Lamium purpureum) is the most compatible winter annual host of SCN in
Ohio among four winter annual weed hosts (Venkatesh et al. 2000). Current management
recommendations for SCN include control of weeds that serve as alternate hosts (Niblack
and Chen et al. 2004), rotation to non-host crops, and use of SCN-resistant soybean
cultivars (Faghihi and Ferris et al. 2006; Niblack 2005). A strategy that seems to hold
promise for both winter annual weed and SCN management is the use of an Italian
ryegrass cover crop, a non-host of SCN that can reduce SCN egg population density in
soil (Menke et al. 2004).
The objectives of my research were to evaluate the effects of an Italian ryegrass
cover crop on purple deadnettle growth and SCN reproduction in a controlled
environment, and to evaluate the effect of an Italian ryegrass cover crop on SCN
populations under field conditions and in comparison with other non-host cover crops.
2 1.2 Soybean cyst nematode - the problem
Soybean cyst nematode, Heterodera glycines, is a major yield-limiting factor in
most soybean-producing regions in the world (Wrather et al. 2003; Monson and Schmitt
2004). Soybeans are planted on more than 78 million acres in North America (USDA
2010). Currently, soybean cyst nematode (SCN) is widely distributed in all major
soybean production areas of the United States (Niblack et al. 2005) and is responsible for
greater annual losses in U.S. soybean yield than any other pathogen (Wrather and
Koenning et al., 2006; Wrather 2003). Recent estimates indicate that SCN accounts for
40% of total soybean yield losses attributable to diseases (Trick et al. 2007). Monetary
losses attributed to SCN were estimated at $784 million in 2002 (Wrather et al. 2003),
and U.S. producers lost more than 300 million bushels to SCN from 2003 to 2005
(Niblack et al. 2005). Recent yield loss estimates due to SCN in the U.S. exceeded 7.5
million metric tons, with much of this loss concentrated in the nation's principal north
central soybean production region where 50-80% of fields are infested (Trick et al. 2007).
The first detection of SCN in an Ohio soybean field was reported in 1987 (Riedel
and Golden et al. 1988), and to date virtually all of the counties in Ohio’s soybean
production region (68 out of 88 total counties) contained fields infested with SCN
(Dorrance et al. 2012). The magnitude of soybean yield loss attributable to SCN is
spatially variable within and among infested fields, and it depends on SCN population
density and its interaction with other factors including soil chemical and physical
characteristics, climatic conditions, soybean variety, and the presence of other crop pests
and diseases (Bond and Wrather et al. 2004; Schmitt 1992; Schmitt and Riggs 1989).
1.3 SCN Life Cycle
To better understand SCN it is necessary to explore its life cycle and reproductive
3 requirements. There are three major life stages of cyst nematodes: egg, juvenile, and adult
(Dorrance et al. 2012). A microscopic roundworm approximately 0.4 mm long, SCN is a
root-feeding pathogen that behaves as an obligate endoparasite, and at the end of its life
cycle the body wall of the dead female forms a durable cyst containing up to several
hundred eggs (Niblack et al. 2006). Encysted eggs of SCN can remain dormant and
viable for at least five years in the absence of a compatible host plant, and when a
susceptible soybean variety is grown, SCN populations once below the damage threshold
can return to economically damaging levels in a single growing season (Long and Todd
et al. 2001; Porter 2001).
When the nematode hatches from its egg as a second stage juvenile, or J2, its life
as a parasitic pathogen begins (Niblack et al. 2005). The hatched juvenile is referred to as
a J2 because the first and second juvenile stages are completed within the egg (Schmitt
and Riggs et al. 1989). What stimulates egg hatching is not completely clear, but it may
be caused by a combination of factors including soil temperature, host presence, or time.
In other words, some eggs may hatch when the soil temperature reaches an optimum
level, whereas others may hatch when the host plant releases certain exudates and/or after
some arbitrary amount of time passes (Yen et al. 1995). Once the juveniles have hatched
they search the soil for roots of a compatible host plant. The juveniles are limited to a
short distance in how far they can move through the soil before locating and entering a
root. If a root is not found the nematode starves and perishes (Dorrance et al. 2012).
Research indicates that J2 stage SCN is able to detect soybean roots through
chemolocation, and once the roots are located it produces cellulolytic enzymes in order to
penetrate the root tissue and move intracellularly to the vascular tissue (Davis et al. 2004).
4 The infective J2 nematode releases various secretions from its gland cells and stimulates
host root cells to initiate cell division and morphological changes (Noel et al. 2004). The
first infected cell and its neighboring cells are converted into a multicellular feeding site
known as a syncytium. The syncytium, which acts as a metabolic sink, is preserved
during the entire time the nematode is actively feeding (Niblack et al. 2005). During this
time the J2 transforms and begins swelling and molting into its third juvenile stage,
referred to as the “sausage-stage,” three to four days after infection. Six to seven days
after infection, the nematode transitions to the fourth juvenile stage and the final molt
occurs on day nine to 10 for females and day eight to nine for males (Niblack et al. 2005).
As females feed, they keep their heads embedded within the root while their posterior end
swells and erupts from the root surface and becomes visible with their characteristic oval
shape. Males detach from the root and regain a vermiform shape in order to fertilize the
swollen females (Niblack et al. 2005).
An individual female SCN can produce anywhere from 40 to more than 600 eggs
(Sipes et al. 1992). The cyst stage is the body of the dead female nematode filled with
eggs. The cyst serves as a protective structure for the developing eggs and young
nematode larvae and can withstand hostile environments for a number of years (Dorrance
et al. 2012). The SCN life cycle can be completed in 24 to 30 days under optimum
conditions, so it is possible for it to produce three to five generations in a single Ohio
growing season (Dorrance et al. 2012). Rates of SCN growth and development are
strongly dependent on soil temperature, but SCN hatching and root penetration can occur
over a moderately wide range of temperatures (Niblack et al. 2005). The rate of SCN egg
development increases linearly between temperatures of 15 C and 30 C (Alston and
5 Schmitt et al. 1988). The ideal temperature for SCN development is 25 C (Alston and
Schmitt et al. 1988), and temperature extremes outside of the 24 to 36 C range can be
fatal (Slack et al. 1972). However, SCN parasitism of soybean roots has been reported
to occur at temperatures as low as 10 C (Ross et al. 1958). Survival of SCN juveniles
inside host roots at temperatures below 10 C has not been reported.
1.4 SCN HG type
The SCN race test system developed in 1970 was originally used to evaluate the
abilities of different SCN populations to reproduce on resistant soybean varieties
(Niblack et al. 2002). Soybean breeders, agronomists, plant pathologists, and
nematologists developed the HG type test, which replaced the old race test in 2002 (HG
is the acronym for Heterodera glycines). The HG Type describes an SCN population that
is able to grow and reproduce on one or more soybean lines having a specific source of
resistance derived from soybean plant introductions (e.g., PI88788). The number or
numbers in the HG Type designation correspond directly to sources of resistance used in
available SCN-resistant soybean varieties (Dorrance et al. 2012).
The modern HG Type test is comparable to the old race test, but is easier to
interpret and contains more information (Niblack et al. 2005). The HG-Type test consists
of collecting an SCN population from an infested field and testing it for host
compatibility with the selected soybean lines under greenhouse conditions. Seven
soybean lines are used in the HG test and are referred to as “indicators” because they
indicate the virulence features of the SCN population. The seven indicator lines are each
inoculated with 1,000 eggs extracted from field soil samples. They are then grown under
optimal conditions for 30 days. The females are then removed, counted, and the relative
6 susceptibility is calculated based on comparison with SCN reproduction on a fully
susceptible soybean variety. At this point it is noted which indicator lines have elevated
SCN numbers (Niblack et al. 2002). Thus the HG Type applies to the nematode
population, not the soybean (Dorrance et al. 2012).
1.5 Winter Annual Weeds
Over 140 plant species are known to serve as alternate hosts of SCN (Riggs et al.
1992). A group of problem plant species associated with SCN is winter annual weeds, a
number of which have been identified as alternative hosts of SCN. The common winter
annual weeds purple deadnettle [Lamium purpureum (L.) LAMPU), henbit (Lamium
amplexicaule L. LAMAM), field pennycress [Thlaspi arvense (L.) THLAR], and
shepherd's-purse [Capsella bursa-pastoris (L.) Medik CAPBP] were confirmed as
alternate SCN hosts in Ohio (Venkatesh et al. 2000). Previous reports also identified
common chickweed [Stellaria media (L.) Vill. STEME] and small-flowered bittercress
[Cardamine parviflora (L.) CARPA) as SCN hosts (Riggs et al. 1992).
In recent years, winter annual weeds have become more widespread in crop
production fields of the U.S. Corn Belt (Gibson et al. 2005). Winter annual weed species
most often emerge in the fall, over-winter as small seedlings, and complete their life
cycles in the spring. They are capable of germinating from late summer through early
spring. One of the leading factors contributing to the spread of winter annual weeds has
been the increased adoption of conservation tillage practices (Wicks et al. 1994). In 1990
it was estimated that conservation tillage acreage was close to 70 million acres. As of
2004 conservation tillage acres had increased to over 110 million acres (Anonymous et al.
2005d). Reduction of soil disturbance associated with conservation tillage has created
7 conditions ideal for winter annual weed establishment and seed production (Wicks et al.
1994). Additionally, the widespread adoption of glyphosate-resistant crops and reduced
reliance on herbicides with soil residual activity is another development that may have
contributed to the upswing of winter weeds (Barnes et al. 2003).
Glyphosate resistant soybeans accounted for approximately 93% of the soybean
production acres in the United States in 2012 (Anonymous et al. 2012). The ease and
effectiveness of postemergence glyphosate application for weed control in glyphosate
resistant soybeans has led to the practice of postponing herbicide application until well
after crop emergence. This delay in the timing of weed control operations provides the
extra time needed for some winter annual weeds to complete their life cycles and produce
viable seed. Furthermore, reduction in the use of herbicides with prolonged soil activity
can be attributed to the increased use of post-applied glyphosate in soybeans. The soil
residual herbicides pendimethalin, imazethapyr, and imazaquin were each applied on 24
to 43% of the United States soybean acres in 1994, but by 2004 they were applied on
only 3 to 5% of the soybean acres (Anonymous et al. 2005b). Reduction in the use of
spring-applied herbicides with soil-residual activity has likely resulted in increased fall
emergence of winter annual weeds (Barnes et al. 2003). The relatively mild winters in
recent years may be an additional factor that has contributed to the increased survival and
abundance of winter annual weeds (Krausz et al. 2003).
A number of negative impacts on cropping systems can be attributed to winter
annual weeds. In the spring, dense populations of winter annual weeds can slow soil
drying and warming (Bruce et al., 2000; Dahlke 2001). Wet and cold soils can result in
delayed crop planting, which can potentially lead to reduced crop yield (Giieli and Smeda
8 et al. 2001). Winter annuals in conventionally tilled fields can also be detrimental due to
increased costs of fuel, labor, and tillage necessary for weed removal (Bruce et al., 2000;
Dahlke 2001). Another problem with winter annual weeds in no-till production fields is
that their advanced growth stage in late spring makes them less susceptible to herbicides.
In addition, high populations of winter annual weeds obstruct proper crop seeding depth
and crop establishment (Krausz et al. 2003).
Producers can achieve better control of winter annual weeds with herbicides
applied in the fall when the weeds are small, but the immediate incentive for producers is
low. Having no crop present and being able to fall back on spring tillage or preemergence
spring herbicides makes fall herbicide application a less appealing option. An alternative
management strategy is to plant non-host cover crops in the fall that suppress winter
annual weed infestations, while also reducing SCN levels and protecting the soil against
erosion.
In the northern regions of the United States, SCN has high overwinter survival
rates (Riggs et al. 2001). A survey conducted by Creech and Johnson (2006) assessed
Indiana fields with SCN and the level of winter annual weed infestation. Fields across
Indiana varied significantly in occurrence and density of weed species, although winter
annual weed hosts of SCN were found in 93% of surveyed fields and occurred at an
average density of approximately 150 plants m-2 (Creech and Johnson et al. 2006). A
number of fields had average field-wide densities exceeding 400 plants m-2 for the winter
annuals henbit and purple deadnettle, two highly compatible SCN hosts (Creech and
Johnson et al. 2006). Previous research has shown that SCN reproduction on fall-seeded
purple deadnettle resulted in three- to five-fold increases in egg population density in a
9 single year compared to weed-free control plots (Harrison et al. 2008). The same
research indicated that there was no increase in SCN egg population density in plots
where purple deadnettle was removed by four weeks after emergence.
The relationship between winter annual weed hosts and SCN in the field is greatly
affected by soil temperature. Field and greenhouse studies showed that completion of the
SCN life cycle on purple deadnettle was prevented if the weed was killed before the
accumulation of 380 soil growing degree-days within the 5 to 30 C range (Harrison et al.
2008). Soil temperatures below 10 C halt SCN growth and development (Alston and
Schmitt et al. 1988). In Illinois, Indiana, and Ohio, SCN completed reproductive cycles
on purple deadnettle in both the autumn and spring under field conditions (Creech et al.
2005, 2007). Furthermore, SCN reproduction on winter annual weeds in autumn was
higher than in the spring (Creech et al. 2007).
Purple deadnettle (Lamium purpureum) is one of the most common winter-annual
weeds of the eastern U.S. Corn Belt (Gibson et al. 2005). It behaves as an obligate winter
annual weed in Ohio, germinating from August to November and flowering in April and
May (Baskin and Baskin et al. 1986). Purple deadnettle is a member of the Lamiaceae
family and is a native of Europe, now found worldwide growing in almost any situation
or soil, but mainly in the temperate regions (DeFelice et al. 2005). It can reach heights of
40 cm. Cotyledons are oval, lack hairs and have crenate margins. Subsequent leaves all
have petioles, although petiole length and lamina size both decrease in an acropetal
direction. Leaves are opposite, triangular to heart-shaped, sparsely hairy and are coarse,
with rounded teeth at the margins. Leaves may be up to 2.5 cm long. Upper leaves appear
closely stacked, are overlapping and bend downward. Upper leaves are more purplish red
10 in color than the lower leaves, which tend to be deep green. Stems are branched at the
base, spreading and square in cross section. Purple to pinkish-purple flowers occur in
whorls in the upper leaf axils. Purple deadnettle reproduces by seed only, and a single
plant is capable of producing up to 27,000 seeds (Wilson et al. 1988). The seeds are oval
and less than 2 mm long, light brown to gray covered with white spots. Purple deadnettle
roots are shallow and fibrous.
Out of four winter annual weeds identified as SCN hosts in Ohio, purple
deadnettle (Lamium purpureum) was the most compatible with SCN and resulted in SCN
reproduction that actually exceeded that on a susceptible soybean variety (Venkatesh et al.
2000). As mentioned previously, populations of SCN are sustained or increased by purple
deadnettle (Harrison et al. 2008). In research conducted in southwest Indiana, Creech et
al. (2005) reported that SCN race 3 (HG Type 0) reproduction on purple deadnettle
occurred in the fall following soybean harvest. This information suggests that in the
southern Corn Belt, management of winter-annual weeds, specifically purple deadnettle,
might reduce SCN population density.
Juvenile-stage SCN was detected in purple deadnettle roots in a fall survey of
Indiana soybean production fields, but they were present at a time too late for them to
reach maturity the same year (Creech et al. 2007). The outcome of such juveniles, i.e.,
those inside roots as temperatures drop below the threshold of SCN development, has not
been reported. There is some evidence that temperature-dependent SCN hatching and
developmental responses may differ among host plant species (Harrison et al. 2008).
Chilly winter temperatures can cause nematodes to enter into diapause or outright perish,
which raises the possibility that winter annual weed hosts could stimulate nematode egg
11 hatching in the fall but prevent reproduction and raise the probability of SCN death by
freezing. Alternatively, winter annual weeds may provide a safe place for the nematodes
to overwinter and enter diapause as soil temperatures drop. In the latter scenario, growth
of SCN could resume in the spring when soil temperatures exceed the SCN threshold of
development, ultimately increasing the SCN population density (Creech et al. 2008)
1.6 SCN Management
Once SCN has been confirmed in a field, keeping its population as low as
possible is the best management option (Dorrance et al. 2012). Existing SCN
management strategies include use of SCN-resistant soybean cultivars and rotation to
non-host crops (Faghihi and Ferris et al. 2006; Niblack 2005), accompanied by control of
weeds that serve as alternate hosts (Niblack and Chen et al. 2004). Natural enemies of
SCN in soil have been reported in various soybean-producing regions around the world
(Morgan-Jones et al., 1981; Gintis 1983; Nishizawa 1986; Carris 1989; Liu 1996;
Mizobutsi 1999; Chen and Chen 2002; Chen 2004), and some soils in the United States
and China have been reported as being suppressive to SCN (Hartwig et al. 1981; Carris
1989; Liu & Wu 1993; Kim & Riggs 1994; Sun & Liu 2000; Chen 2004). A soil is
referred to as nematode suppressive if nematode populations in the presence of a
vulnerable soybean cultivar remain low compared to an average infection level in other
soils (Chen & Dickson et al. 2004).
Research in Minnesota has shown that SCN population densities can be greatly
reduced while maintaining higher crop yields when using SCN resistant cultivars
compared to SCN susceptible cultivars (Chen et al. 2001a). Furthermore, an increase in
these positive effects was shown with greater initial SCN population densities (Chen et al.
12 2001a). Fields infested with high SCN population densities typically have significant
yield loss even when a resistant cultivar is planted, although resistant cultivars usually
produce higher yields than susceptible cultivars (MacGuidwin et al. 1995; Tylka, 1997).
The North Central soybean-growing region of the United States continues to
experience the spread of SCN and increasing yield losses even though resistant cultivars
are abundantly available throughout the region. Part of this problem can be attributed to
the lack of diversity for resistance to SCN in soybean cultivars. In a recent survey, PI
88788 was the sole source of resistance for 94% of maturity group II-IV SCN resistant
soybean cultivars available for planting in the Midwestern United States (Shier et al.
2008). In addition, selection pressure is imposed on SCN populations by resistant
cultivars, which can result in shifts from one predominant HG Type to another (Young et
al. 1995, 1998). The most effective option for managing SCN is rotating host crops with
non-host crops. Populations in Ohio usually decline by 50 percent per year under nonhost crop conditions (Dorrance et al. 2012). The effects of SCN populations and crop
rotations on soybean yields have been reported in many studies (Ross et al., 1962;
Weaver 1988; Edwards 1988; Rodriguez-Kabana 1991; Weaver 1993; Koenning 1993;
Hershman and Bachi 1995; Koenning 1995; Howard 1998; Long and Todd 2001; Chen
2001c; Noel and Wax 2003). The effectiveness of crop rotation in reducing SCN
populations depends on three factors: 1) the host status of crop species 2) the number of
years of rotational non-host crops, and 3) the geographical location. In North Carolina,
for example, a one to two year rotation to a non-host crop is generally sufficient to lower
SCN population density to below damaging levels (Schmitt et al., 1991; Koenning 1993).
In contrast, in Minnesota a five-year non-host rotation along with SCN-resistant soybean
13 may be needed to reduce the SCN population density to a low level where a susceptible
cultivar can be grown without significant yield loss (Chen et al. 2001c).
How crop rotation affects SCN population dynamics is not completely understood.
It is possible that non-host crops, poor host crops, and crop residues that release toxic
compounds during their decomposition all reduce SCN populations. Examples of plants
that release chemicals toxic to nematodes as they decompose include Brassica spp., such
as cabbage, rapeseed, and mustard (Ellenby et al., 1945; Mojtahedi 1993; Donkin 1995).
In addition, cereal crops that contain phenolic acids (released during decomposition?)
such as wheat can reduce SCN population densities (Hershman and Bachi et al., 1995;
Blum 1996). Furthermore, SCN hatching may be stimulated by a weak host crop, which
may then lead to the nematodes not being able to reproduce well (Sortland and
MacDonald et al., 1987; Schmitt and Riggs 1991).
The principal production system of rotating corn and soybean is currently
practiced on over 20 million acres in the North Central region of the United States. The
foremost pest problem the past three decades in the corn-soybean production system is
SCN (Wrather et al. 2001; Monson and Schmitt 2004). A yield penalty occurs with a corn
following corn rotation, so a rotation sequence meant to lower SCN levels by boosting
the number of years of corn is not advisable (Crookston et al., 1991; Porter 1997; Porter
2001; Chen 2001c). Thus it is necessary for economically acceptable non-host crops to be
used in rotations with soybean for long-term effective SCN management.
1.7 Cover Crops and Italian Ryegrass
Greenhouse (Riga et al. 2001) and field (Rodriguez-Kabana et al. 1991) studies
have shown that non-host crop plants vary in their effects on SCN hatching and
14 reproduction. In one field study, sorghum was less effective than corn in lowering SCN
population densities at the end of the following soybean season (Rodriguez- Kabana et al.
1991). A more recent and extensive study investigating non-host or poor-host crops for
SCN management in Minnesota included alfalfa (Medicago sativa L.), barley (Hordeum
vulgare L.), canola (Brassica napus L.), corn, sorghum, oat (Avena sativa L.), pea, potato
(Solanum tuberosum L.), rye (Secale cereale L.), red clover (Trifolium pratense L.),
sugarbeet (Beta vulgaris L.), sunflower (Helianthus annuus L.), and wheat (Miller et al.
2006). Results indicated that leguminous non-hosts were most effective in reducing SCN
population density and that corn was one of the least effective crops.
The potential of non-host plant residues and plant root exudates to protect
soybeans from SCN was investigated by Riga et al. (2001). They found that soilincorporated residues from a number of plant species reduced nematode population
densities compared with residues of soybean incorporated alone. Italian ryegrass (Lolium
multiflorum Lam.) was more effective than other non-hosts in reducing infection of
soybean by SCN (Riga et al. 2001).
Italian ryegrass has leaf blades that are long and tapered, rough on the bottom and
glossy on top. Auricles are present and are narrow, long, and claw-like. The sheaths are
rounded and the collar is broad. The root system is fibrous and prolific, and the stems do
not root at the nodes. The seed head is a terminal spike. Italian ryegrass can tolerate close
mowing or grazing and is a highly competitive, clump forming annual grass (Uva et al.
1997). It can be grown as a fall seeded winter annual and is well adapted to the U.S. Corn
Belt (McCormick et al. 2004). Italian ryegrass in Ohio is typically planted in September
and can be used as a cover crop or for fall and spring grazing.
15 The use of Italian ryegrass as a cover crop is a strategy that seems to hold promise
for both the management of winter annual weeds and SCN. A study by Riga et al. (2001)
showed that Italian ryegrass was the most effective of several cereal and forage grass
species evaluated in reducing SCN populations under greenhouse and in vitro conditions.
The authors reported that Italian ryegrass increased SCN egg hatching in the absence of a
host plant, which resulted in a depletion of lipid reserves and death of the juveniles (Riga
et al. 2001). A field experiment conducted in Ohio showed that Italian ryegrass grown as
a winter cover crop following soybean reduced SCN and purple deadnettle population
densities by 60% and 55%, respectively, compared with no cover crop (Menke et al.
2004). A dense canopy is produced by Italian ryegrass that can suppress weeds (McKell
et al. 1969). This along with the emergence time of purple deadnettle and the life cycle
of SCN may create a simple solution for a complex and widespread problem. If IR is
planted in the autumn before PDN is established or directly after it has been treated with
a herbicide the SCN would not have a susceptible host for a prolonged period of time
which could cause a significant reduction of SCN population.
Specific Objectives
The objectives of my research were to evaluate the effects of an Italian ryegrass
cover crop on purple deadnettle growth and SCN reproduction in a controlled
environment, and to evaluate the effect of an Italian ryegrass cover crop on SCN
populations under field conditions and in comparison with other non-host cover crops. I
hypothesized that Italian ryegrass would reduce purple deadnettle growth and SCN egg
population densities, that a winter hardy ryegrass cultivar would provide greater SCN
suppression than a non-hardy cultivar, and that Italian ryegrass would have a greater
16 suppressive effect on SCN populations than rye (Secale cereale) or oat (Avena sativa)
cover crops.
17 CHAPTER 2
Interactions of Purple Deadnettle, Lamium purpureum, Soybean Cyst Nematode,
Heterodera glycines and Italian Ryegrass, Lolium multIflorum
2.1 Materials and Methods
Greenhouse experiment
A replacement series experiment was used to examine the competitive
relationship between Italian ryegrass (IR) and purple deadnettle (PDN) and how this
relationship was affected by the presence and absence of SCN. The replacement series is
designed to compare a plant’s growth in monoculture with its growth over a range of
proportions as part of a two-species mixture (deWit et al. 1960). Plant population density
is held constant so that inferences can be drawn about the relative competitive ability of
each species and insights can be gained about possible mechanisms by which species
interact when grown together (Connolly et al. 2001). Treatments consisted of the
following PDN: IR species ratios at a constant density of n=16 plants per pot, with and
without SCN inoculation: 100:0, 75:25, 50:50, 25:75, 0:100. Treatments were sown in
15-cm diameter plastic pots containing approximately 1600 cm3 of construction-grade
sand.
The replacement series experiment was conducted twice. The first experiment
was conducted from 03 December 2007 to 02 March 2008, and the second from 17
September to 28 November 2008. The experiment was arranged in a randomized
complete block design (RCBD) with 8 replications. The PDN was planted at an earlier
date and thinned weekly until a uniform stand of plants ranging from the 4- to 6-leaf
18 stage was established. The IR was planted when half of the desired PDN population was
emerged. This allowed formation of a well-established root system so that when the pots
were inoculated with SCN there would be an abundance of host roots available. Pots in
the first run were inoculated on 22 January 2008 with 45,000 eggs per pot. Pots in the
second run were inoculated on 1 October 2008 with 36,000 eggs per pot. The blocks
were arranged from east to west on the greenhouse bench. The individual pots within
each block were re-randomized every three to five days to reduce within-block location
effects on plant growth. Hand-harvested purple deadnettle seeds were collected from the
Waterman Agricultural and Natural Resource Laboratory at The Ohio State University in
Columbus, Ohio. Each May, mature PDN plants located in fields close to the
experimental site were collected, dried, and cleaned for seed. The seeds were then held
until September at room temperature (23C) to reduce dormancy by allowing afterripening (Baskin and Baskin et al. 1986). Pots were overseeded and thinned to the
desired treatment populations. Italian ryegrass (cv. Marshall) seed was obtained from a
commercial source in Columbus, Ohio. Seeds were evenly distributed across the sand
surface in the pots, and then covered with a thin layer of sand. Greenhouse temperature
was maintained at 24 C and high-pressure sodium lights provided approximately 220
µmol m-2 s-1 supplemental photosynthetic photon flux for a 16-h daily photoperiod. Pots
were placed in a large tray and sub-irrigated by ebb and flow two times daily to avoid
leaching out the vermiform SCN juveniles, and watering time was based on visual
assessment of soil water status. Irrigation water was maintained at a temperature of 30 C.
The plants were fertilized with a solution containing 400 mg/L N:P:K (20:20:20, w/w/w)
commercial fertilizer twice a week.
19 Harvesting was accomplished using cuticle scissors to cut the plants off even with
the soil surface. The two species were separated and fresh weight was recorded
immediately after harvesting. After recording fresh weights, the plant material was
placed by experimental unit and species into individual brown paper bags. The bags were
placed in a dryer at 50 C for 24 h and resulting dry weights were recorded. The pots
containing the sand and roots were stored in a cold room until the SCN cysts and
juveniles could be extracted and quantified.
Sand and roots were subjected to standard stacked-sieve extraction and staining
procedures to extract cysts and quantify SCN egg population densities (Willson et al.,
1996; Venkatesh 2000). Sand and roots from each pot were emptied into a plastic bucket
containing 4 L water and stirred vigorously for 20 seconds to suspend soil and dislodge
cysts from roots (Willson et al., 1996; Venkatesh 2000). The mixture was poured over
stacked sieves (840-mm over 250-mm mesh size) and roots were washed again with 250
ml water from a squirt bottle to dislodge cysts. The SCN cysts retained on the 250-mm
mesh sieve were rinsed with 100 ml water, and the rinsates containing cysts were
refrigerated until cysts could be extracted.
Cyst extraction involved rupturing the cysts with a Ten Bröeck glass tissue
grinder to release the eggs, and the cyst extract was poured through 60-over-200-over500-mm mesh screens. The SCN eggs and juveniles retained on the 500-mm mesh
screen were re-suspended in water, and a sample of the extract suspension was stained
with acid fuschin (Hussey et al. 1985). The SCN egg concentration in each sample was
quantified by counting eggs present in a 5-ml aliquots placed in a plate-count petri dish
and viewed under a dissecting microscope.
20 Analysis of variance (ANOVA) was performed on the plant shoot biomass and
SCN egg count data using the PROC GLM procedure of SAS. Differences among
treatment means were determined using Fisher’s protected least significant difference at a
significance level of α=0.05.
-
Field Experiment
A field study was conducted using square 3.3 m2 by 0.5 m-deep raised beds as
experimental units at the Waterman Agricultural and Natural Resource Laboratory of The
Ohio State University in Columbus, OH (40.0162°N, 83.0398°W; elevation 239 m). The
experiment was established in 2006 and plots contained sandy loam soil infested with HG
type 0 SCN (formerly classified as race 3) (Willson et al., 1996; Niblack 2002). Plots
were first sampled on 8 May 2006 before soybean planting to determine baseline SCN
egg population densities, and then in September 2006 after soybean removal but before
cover crop establishment. Plots were sampled similarly in 2007. Soil was sampled to a
depth of 15 cm with a stainless steel 2.5-cm-diameter probe. Ten soil cores were
collected randomly from the central 0.25 m2 of each plot and mixed thoroughly. Three
subsamples from each plot soil sample were subjected to standard stacked-sieve
extraction and egg staining procedures to quantify SCN eggs 100 cm-3 in the soil as
described previously (Willson et al., 1996; Venkatesh 2000). Results indicated that
baseline, pretreatment SCN egg population densities in May 2006 ranged from 4880 to
27,870 eggs 200 cm-3 soil.
Treatments were arranged in a completely randomized design with six
replications, and consisted of the following winter cover crop treatments: no cover crop,
‘Marshall’ Italian ryegrass (a winter hardy cultivar), ‘Gulf’ Italian Ryegrass (less winter
21 hardy), cereal rye, and oats. The cover crop seeding date varied somewhat depending
upon year-to-year field conditions, but generally occurred the last week in September.
The experiment was conducted in 2006 and repeated in 2007 with treatments assigned to
the same plots each year. All plots were maintained under no-tillage conditions. Cover
crops were seeded by hand at the following rates based upon forage planting
recommendations: Italian ryegrass at 11 g ha-1; rye at 45 g ha-1, and oat at 30 g ha-1. Plots
were mulched with a 5-cm thick layer of wheat (Triticum aestivum L.) straw immediately
after seeding and sprinkle-irrigated with approximately 5 cm water per square inch of
surface.
Glyphosate was applied at a rate of 1.5 kg ha-1 to all plots in late April, and SCNsusceptible ‘Resnick’ soybean was seeded at 490,000 seed ha-1 in rows spaced 38 cm
apart. All plots were kept weed-free during the cropping season by hand hoeing as
necessary. Soybeans were removed with a string trimmer fitted with a metal blade in late
September after soybeans had reached full maturity and were senescing.
Post-treatment SCN egg population densities may not accurately reflect true
treatment effects each year since pretreatment SCN population densities each year can
influence post-treatment population densities. To account for this covariance effect, a
population change factor (Pf/Pi) was calculated for each plot to more accurately
determine cover crop effects on changes in SCN egg population densities over time. The
population change factor was calculated by dividing the SCN egg population density at
each sampling period (Pf) by the initial baseline SCN egg population density (Pi) before
treatments were applied in the first year of the study (Chen et al. 2006). A Pf/Pi value of
1.0 indicates no change in SCN egg population density, whereas Pf/Pi values less than
22 1.0 or greater than 1.0 represent a net decrease or increase, respectively, in SCN egg
population density. The Pf/Pi data were log10(x+1)-transformed prior to statistical
analysis in order to approximate a normal error distribution (Chen et al. 2006).
Data were analyzed using the PROC MIXED procedure of SAS for repeated
measures (Littell et al. 2006). The MIXED procedure models the covariance structure of
repeated measurements taken on the same experimental units; thereby allowing more
efficient calculation of generalized least squares associated with time and treatment
effects (Littell et. al. 1998). The generalized linear model included year and cover crop
as fixed effects and plot within cover crop as a random effect. Plot within year was
designated as the subject on which repeated measures were taken, and heterogeneous
autoregressive covariance was designated in the model based on best fit according to the
Akaki Information Criterion, which is the best criterion for Type I error control according
to Littell et al. (2006).
Planned comparisons of treatment effects of SCN populations on Pf/Pi were made
using single degree of freedom contrasts implemented with the CONTRAST option of
SAS. The least squares means were back-transformed for presentation. A significance
level of α=0.05 was used for all statistical tests.
2. 2 Results and Discussion
Greenhouse Study
Dry shoot biomass of IR or PDN did not differ between SCN-inoculated and noninoculated treatments, so the data shown are the main effects of the mixtures on shoot
biomass composition (Figures 1.1 and 1.2). The first run of the experiment was started in
23 early December of 2007 and ran until the first week in March of 2008 (spring
experiment). In this study both species had poor germination and slow growth. Excluding
the 0:100 PDN: IR treatment, PDN accounted for the majority (>90%) of the total shoot
biomass among the remaining treatments and exceeded the theoretical yield of the PDN
monoculture. It appeared that PDN growth in the species mixtures benefitted by the
presence of the IR, yet the mechanism is not clear. Growth of the IR in mixtures stayed
at or below the theoretical yield of the IR monoculture. The relatively slow growth and
small amounts of total biomass produced over the course of the spring experiment were
likely due to low daily solar irradiance, and possibly lower temperatures in the
greenhouse. Results suggest that under cool, low irradiance conditions, purple deadnettle
was the dominant competitor in the IR: PDN mixtures.
The second run of the experiment was conducted in the fall of 2008 starting in
mid-September and running until late November of the same year. In the fall experiment
the germination and overall plant growth was much greater than observed in the spring
experiment, and total shoot biomass production among all treatments was approximately
seven- to eight-fold greater than corresponding treatments in the spring experiment
(Figure 1.2). The IR steadily decreased the biomass of the PDN as the ratio of IR to PDN
increased, but IR shoot biomass in the mixtures fell below the theoretical yield of the
constituent monoculture. In contrast, PDN shoot biomass in the mixtures exceeded its
theoretical yield in monoculture. These results support the findings of the spring
experiment in that PDN appeared to have the competitive advantage over IR under
greenhouse conditions. As an obligate winter annual species, PDN may be better
acclimated to attenuated light conditions and maintain higher rates of net photosynthesis
24 in the greenhouse than IR.
IR reduction of PDN growth
Although PDN may have had a competitive edge over IR in the greenhouse
experiments, Figure 1.2 illustrates that in the fall experiment, the amount of IR present
significantly reduced PDN growth. In the spring experiment, there was a minimal effect
of IR on PDN until the level of IR reached 75%. The way in which IR reduces PDN is
likely to be through the process of competition (Mock et al. 2012; Donald 2007). Under
field conditions, an IR winter cover crop reduced the PDN population density 55%
(Menke et al. 2004). Plants compete with each other for basic resources such as oxygen,
nutrient ions, water, sunlight, and space. Plants growing in close proximity to one another
will often deplete one or more of these growth factors and negatively impact the plants
around them (Donald et al. 2007).
Plant competition for water can simultaneously involve competition for mobile
mineral nutrients that become limiting. These limitations are more likely to occur under
field conditions than in container-grown plants in the greenhouse, since constant
fertilization and irrigation should minimize competition for those growth factors. Under
field conditions the IR can grow quickly in height and intercept more sunlight than the
PDN. This means the growth of the IR will exceed that of the PDN, and it will absorb
water and nutrients more quickly. This could leave the PDN at an increasingly greater
competitive disadvantage as less light, water, and nutrients become available (Thoden,
Korthals & Termorshuizen et al. 2011).
While there could be a wide number of resources for which the IR and PDN are
competing (Mock et al. 2012), possibly the most important of these under greenhouse
25 conditions is light. Plants often compete for light (Nelson et al. 2006), and in the case of
PDN under field conditions the plant is generally only five to 20 cm in height and could
be at a natural disadvantage when grown with a taller species. On the other hand, IR can
grow to be well over a meter high under field conditions and can outperform the PDN
with regard to capturing sunlight and casting shade on its competitor (Hooks et al. 2011).
In an outdoor environment, there is nothing that can be done to stop the IR from blocking
the sunlight that would otherwise be available to the shorter PDN. This effect could have
been reduced in the greenhouse where the lights were stationary above the plants and
minimized shading effects. Plants grown nearer to the equator can be less affected by
light competition due to the sunlight being more immediately overhead (Donald et al.
2007).
IR reduction of the SCN egg population density
In the spring experiment the SCN egg population density in all treatments
containing PDN ranged from 3300 to 3600 eggs 200 cm-3 and did not differ significantly
among treatments where PDN was present (Figure 1.3). In the 100% IR treatment, SCN
egg population density declined to 80 eggs 200 cm-3. In the fall experiment, SCN egg
population density was 18,400 eggs 200 cm-3 in the 100% PDN treatment and decreased
to approximately 8700 eggs 200 cm-3 in the species mixtures (Figure 1.3); however, there
were no differences in SCN egg counts among the three different ratios of PDN:IR. It
was originally hypothesized that the SCN population would decrease as the ratio of IR
increased (Nelson et al. 2006), but it appeared that SCN reproduction generally occurred
to a similar extent regardless of the PDN:IR ratio. Some studies have indicated that IR
was the most effective non-leguminous plant for reducing the SCN reproduction (Riga et
26 al., 2001; Hooks 2011), but it did not appear to greatly hamper SCN reproduction on
PDN in our experiments.
The egg counts were regressed on the proportion of total biomass composed of
PDN and IR respectively to determine if shoot biomass of each species was related to the
SCN egg population densities. Results of linear regression showed that for IR in the
spring y=3674-350.8(x). Which predicts that for each 10% increase in IR a 10%
reduction in SCN egg density will occur. With an r-square value of 0.41 in the spring
41% of the variation in egg counts is attributed to the increase in IR biomass. Results of
linear regression showed that for IR in the fall y=1712-177.0(x). Which predicts that for
each 10% increase in IR a 10% reduction in SCN egg density will occur. With an rsquare value of 0.59 in the spring 59% of the variation in egg counts is attributed to the
increase in IR biomass. While the mechanisms by which an IR cover crop reduces SCN
populations are not completely understood, three possible interactions have been
proposed (Riga et al., 2001; Donald 2007). One possibility is that IR may induce SCN
egg hatching in the absence of compatible host plants, thus causing starvation of the SCN
juveniles. Another possibility is that IR somehow depletes the lipid reserves of the
juveniles, causing starvation. A third possibility is that there is some degree of low
nematode parasitism on the IR, thereby reducing later SCN reproduction on soybean
roots (Hooks et al. 2011). It may also happen that in soybeans following a winter IR
cover crop, the IR residues may be releasing compounds that lower the level of the
soybean infection and/or reproduction by SCN on soybean.
The findings of the greenhouse experiments suggest that a future study could
involve investigating more PDN:IR ratios beginning with 25:75 and continuing to
27 increase the proportion of IR more gradually until there was no PDN present. This is an
approach that would likely be more sensitive in assessing the ability of IR to suppress
SCN reproduction in the presence of a compatible host (Nelson et al. 2006). Figure 1.1. Shoot dry weights in pots containing various species mixtures of
Italian ryegrass and purple deadnettle in a greenhouse experiment conducted in Spring
2008. The dotted lines represent theoretical yields of constituent monocultures at
different species mixtures. Vertical bars represent +/- S.E. 28 Figure 1.2. Shoot dry weights in pots containing various species mixtures of
Italian ryegrass and purple deadnettle in a greenhouse experiment conducted in Fall 2008.
The dotted lines represent theoretical yields of constituent monocultures at different
species mixtures. Vertical bars represent +/- S.E.
29 Figure 1.3. Soybean cyst nematode egg population densities in pots containing
various species mixtures of Italian ryegrass and purple deadnettle in greenhouse
experiments conducted in Spring and Fall of 2008. Means accompanied by different
letters within each experiment were significantly different according to Fisher’s Least
Significant Difference (P<0.05).
30 Field Study
Cover crop effects on SCN population growth
Results showed that over the course of the field experiment, all cover crops
caused significant reductions in the Pf/Pi when compared to the control with no cover
crop (P<0.001; Figure 1.4). The Pf/Pi values for ‘Marshall” IR, ‘Gulf’ IR, rye, and oat
were 0.78, 0.80, 1.03, and 0.99, respectively, compared to 2.47 for the control with no
cover crop. A single degree of freedom contrast indicated that Pf/Pi values were
significantly lower for the IR varieties compared to the other cover crops (P=0.02).
There was no difference in Pf/Pi between the two IR varieties (P=0.86), and no
difference between the rye and oat cover crops (P=0.84). In addition most of the
reduction in SCN populations from the ryegrass treatments showed up in the fall
sampling (see Appendix A and B for spring vs. fall sampling data).
Overall results of the field study show that after two years of growing susceptible
soybean in heavily SCN-infested plots, all winter annual cover crops tested were
generally effective in preventing an increase in SCN population growth (i.e., keeping
Pf/Pi values ≤ 1.0). In the absence of a cover crop, SCN populations with continuous
susceptible soybean more than doubled. Most importantly from a potential management
standpoint, the Italian ryegrass cover crops caused significant reductions in SCN egg
population densities. As stated earlier, the mechanism(s) by which Italian ryegrass
reduced SCN egg population density remains unclear.
31 Management Implications of Results
An important consideration when managing SCN is that it can use many plant
species as hosts on which to reproduce (Dorrance et al., 2012; Mock 2012). The usual
non-hosts of SCN used in crop rotations to manage SCN are wheat or corn. While the
SCN cannot use wheat or corn as a host, it can reproduce on sweetclovers, peas, crimson
clover, cowpeas, common and hairy vetch, green beans, and alsike clover. In addition to
these alternative hosts, there are different strains of SCN (i.e., HG-types), which may be
able to adapt to other types of hosts (Nelson et al. 2006).
The ability of SCN to adapt to other hosts makes controlling this pest with other
plants a complex problem (Thoden, Korthals & Termorshuizen et al. 2011). Plant
competition occurs throughout a field. An area that is devoted to soybeans may also have
other plants growing in it or nearby. These plants can interact with each other in complex
fashions and have unpredictable results with regard to SCN populations (Fisher et al.
2013).
Italian ryegrass has pros and cons as a potential SCN management tool. My
research shows that IR provides a significant reduction in SCN population compared to
rye and oat. While rye and oat cover crops did not allow a significant increase in SCN
population densities, IR reduced SCN population densities by about 20% (Fig. 1.4). All
cover crops were effective in preventing SCN increases when compared with the no
cover crop control treatment, which allowed the SCN population to more than double
(Fig. 1.4). Other benefits of IR include its ability to increase soil organic matter and
microbial biomass (Fae et al. 2009), and to mitigate soil erosion caused by wind and
water during the late fall, winter, and early spring. IR can also be used as a forage crop
32 for livestock grazing in the fall and spring, which provides the potential for a producer to
achieve multiple benefits from a secondary crop. Italian ryegrass can provide sufficient
forage dry matter yield and nutritive value to achieve optimal animal gains in
combination with mineral supplementation (Fae et al. 2009).
The disadvantages to incorporating IR in a rotation include challenges that can
arise when it comes to terminating its growth in the spring. Various spring burndown
programs can provide less than acceptable control, and IR escapes pose a competitive
threat to summer annual crops. In addition, some populations of IR have been found to be
glyphosate-resistant, creating another weed hazard for glyphosate-based cropping
systems (Christoffoleti et al., 2005; Jasieniuk 2008).
In conclusion, my research indicates that an IR cover crop planted in early autumn
after soybean harvest can reduce SCN population density in soil, and thus has potential to
be a potentially useful SCN management strategy for producers in a continuous soybean
cropping system. In regard to the concern of IR escaping spring control measures, the
recommendations of using a non-winter hardy IR variety, being prepared to scout early
and often, along with the possibility of making multiple spring herbicide applications
should not dissuade producers. Further research to examine blends of IR with other
forage crops would be recommended. The appeal of IR with its SCN suppression power
could also increase with the addition of a more robust grazing forage into integrated
cropping and livestock production systems.
33 Figure 1.4. Mean SCN egg population density change factor (Pf/Pi; final population/initial population) over three years in response to various fall-­‐seeded winter cover crops following summer cropping with susceptible ‘Resnick’ soybean. A Pf/Pi value more than 1 represents an increase in SCN population density, Pf/Pi of 1 represents no change in SCN egg population density, and Pf/Pi less than 1 represents a decrease in SCN egg population density. Vertical bars represent +/-­‐ S.E.
34 APPENDIX A
Figure A.1 Mean SCN egg population density change factor (Pf/Pi; final population/initial population) for various fall-­‐seeded winter cover crops following summer cropping with susceptible ‘Resnick’ soybean across three sampling times. A Pf/Pi value more than 1 represents an increase in SCN population density, Pf/Pi of 1 represents no change in SCN egg population density, and Pf/Pi less than 1 represents a decrease in SCN egg population density. Vertical bars represent +/-­‐ S.E.
35 APPENDIX B
Figure B.1 Soybean cyst nematode egg population densities measured in spring in
response to fall-seeded winter cover crops followed by SCN-susceptible ‘Resnick’
soybean in 2006, 2007, and 2008. Egg counts were measured after cover crop removal
but before soybean seeding each year. Vertical bars represent +/-­‐ S.
36 APPENDIX C
Figure C.1 Soybean cyst nematode egg population densities measured in fall in
response to fall-seeded winter cover crops followed by SCN-susceptible ‘Resnick’
soybean in 2006, 2007, and 2008. Egg counts were measured after soybean removal but
before cover crop seeding each year. Vertical bars represent +/-­‐ S.
37 APPENDIX D
SAS Program Statement Used To Test SCN Egg Counts. Data SCN_Box; input box rep trt SCNdate Pi Pf logPfPi; datalines; 1 1 2 1 14836 10880 -­‐0.134687929 2 1 5 1 12835 29840 0.366402946 3 1 1 1 7547 4337.5 -­‐0.240534862 4 1 3 1 11180 5720 -­‐0.291045775 5 2 3 1 13500 28960 0.331464789 6 2 1 1 11133 7400 -­‐0.177380489 7 1 4 1 11827 13400 0.054230202 8 2 2 1 16856.5 10980 -­‐0.186165065 9 3 3 1 9207 10220 0.045332753 10 3 1 1 8760 3560 -­‐0.391054108 ; proc sort; by box SCNdate trt; proc mixed data=SCN_Box; class SCNdate trt box; model logPfPi=trt/ddfm=kr; random box(trt); repeated/type=ARH(1) subject=box(trt) r rcorr; lsmeans trt/diff; contrast 'rye vs. oat' trt 0 0 1 -­‐1 0; contrast 'Marshall ARG vs. Gulf ARG' trt 1 -­‐1 0 0 0; contrast 'ARGs vs. non-­‐ARGs' trt 1 1 -­‐1 -­‐1 0; contrast 'rye+oat vs. control' trt 0 0 1 1 -­‐2; contrast 'ARGs vs. control' trt 1 1 0 0 -­‐2; contrast 'all cover crops vs. control' trt 1 1 1 1 -­‐4; run;
38 APPENDIX E
SAS Program Statement Used To Test Plant Shoot Biomass and SCN Egg Counts. Data; SCN_IR_GH; Input; rep season SCNlevel PDNpop IRpop pi pf pfpi logpi logpf logpfpi; datalines; 1 1 1 1 0 0 0 1.56 0.48 2 1 1 1 0 0 0 0.43 0.23 3 1 1 1 0 0 0 0.12 0.06 4 1 1 1 0 0 0 1.38 0.23 5 1 1 1 0 0 0 1.7 6 1 1 1 0 0 0 1.01 0.27 7 1 1 1 0 0 0 1.24 0.69 0.49 ; Proc sort; by rep season SCNlevel PDNpop IRpop; Proc mixed data=SCN_IR_GH; Class rep season SCNlevel PDNpop IRpop; Model logpfpi=season SCNlevel PDNpop IRpop SCNlevel*PDNpop SCNlevel*IRpop PDNpop*IRpop/ddfm=kr; Repeated/type=un subject=rep-­‐-­‐-­‐-­‐-­‐ Lsmeans season/diff; Lsmeans SCNlevel/diff; Lsmeans PDNpop/diff; Lsmeans IRpop/diff; Lsmeans SCNlevel*PDNpop/pdiff; Lsmeans SCNlevel*IRpop/pdiff; Lsmeans PDNpop*IRpop/pdiff; run; 39 REFERENCES
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