Interaction of Purple Deadnettle, Lamium Purpureum, Soybean

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

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.

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.

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

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 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

A.l 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 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

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

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).

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).

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

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

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

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

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

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

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

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.

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 non- host 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

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

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 soil- incorporated 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.

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

suppressive effect on SCN populations than rye (Secale cereale) or oat (Avena sativa) cover crops.

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

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 after- ripening (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.

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-over-

500-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.

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

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 SCN- susceptible ‘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

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 non- inoculated 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

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

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

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

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 r- square 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

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.

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.

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).

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.

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

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.

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.

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.

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.

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.

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 0.49 6 1 1 1 0 0 0 1.01 0.27 7 1 1 1 0 0 0 1.24 0.69 ; 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;

REFERENCES

Alston, D. G. and D. P. Schmitt. 1988. Development of Heterodera glycines life stages as influenced by temperature. J. Nematol. 20:366-372.

Anonymous. 2012. Acreage. 2012 USDA National Agricultural Statistics Service archive. http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=1 000

Anonymous. 2005b. Agricultural Chemical Use Database. U.S. Department of Agriculture, National Agricultural Statistics Service: Web page: http://www.pestmanagement.info/nass.

Anonymous. 2005d. National Crop Residue Management Survey. Conservation Technology Information Center (CTIC): Webpage: http:// www.conservationinformation.org/index.asp?site=1&action=crm.

Barnes, J. W., W. G. Johnson, K. A. Nelson, and R. A. Dewell. 2003. Impact of Spring applied, residual herbicides on winter annual weed populations after crop harvest. Proc. North Cent. Weed Sci. Soc. 58: 131.

Baskin, J. M., C. C. Baskin, and J. C. Parr. 1986. Field emergence of Lamium amplexicaule L. and L. purpureum L. in relation to the annual seed dormancy cycle. Weed Res. 26: 185–190.

Blum, U. 1996. Allelopathic interactions involving phenolic acids. J. Nematol. 28: 259– 267.

Bond, J. and J. A. Wrather. 2004. Interactions with other plant pathogens and pests. Pages 107–114 in D. P. Schmitt, J. A. Wrather, and R. D. Riggs, eds. Biology and Management of Soybean Cyst Nematode. 2nd ed. Marceline, MO: Schmitt and Associates of Marceline.

Bruce J. A., A. Moses, S. Mroczkiewicz, and M. Johnson.2000. Simazine: a new tool for winter annual weed management in corn. Proc. North Cent. Weed Sci. Soc. 55:98.

Carris L. M., Glawe D. A., Smyth C. A., Edwards D. I. 1989. Fungi associated with populations of Heterodera glycines in two Illinois soybean fields. Mycologia 81: 66-75.

Chen F.J., Chen S.Y. 2002. Mycofloras in cysts, females and eggs of the soybean cyst nematode in Minnesota. Appl. Soil Ecol. 19: 35-50.

Chen, S., D. L. Wyse, G. A. Johnson, P. M. Porter, S. R. Stetina, D. R. Miller, K. J. Betts, L. D. Klossner, and M. J. Harr. 2006. Effect of cover crops alfalfa, red clover, and perennial ryegrass on soybean cyst nematode population and soybean and corn yields in Minnesota. Crop Sci. 46:1890–1897

Chen S.Y. 2004. Management with biological control. Pages 207-242 in Schmitt D.P., Wrather J.A., Riggs R.D., eds., Biology and Management of the Soybean Cyst Nematode. Marceline, M.O.: Schmitt & Associates of Marceline.

Chen S.Y., Dickson D.W. 2004. Biological control of nematodes by fungal antagonists. Pages 979-1039 in Chen Z.X., Chen S.Y., Dickson D.W., eds., Nematology advances and perspectives. Vol. 2: Nematode management and utilization. Cambridge, MA: Tsinghua University Press and CABI Publishing.

Chen S.Y., Porter P.M., Orf J.H., Reese C.D., Stienstra W.C., Young N.D., Walgenbach D.D., Schaus P.J., Arlt T.J., Breitenbach F.R. 2001a. Soybean cyst nematode population development and associated soybean yields of resistant and susceptible cultivars in Minnesota. Plant Disease 85: 760-766.

Chen S.Y., Stienstra W.C., Lueschen W.E., Hoverstad T.R. 2001c. Response of Heterodera glycines and soybean cultivar to tillage and row spacing. Plant Disease 85: 311-316.

Chen, S.Y., W.C. Stienstra, W.E. Lueschen, and T.R. Hoverstad. 2001b. Response of Heterodera glycines and soybean cultivar to tillage and row spacing. Plant Dis. 85: 311–316.

Christoffoleti P, Trentin R, Nicolai M, et al. Alternative herbicides to manage Italian ryegrass (Lolium multiflorum) resistant to glyphosate at different phenological stages. 2005. Journal Of Environmental Science And Health. Part. B, Pesticides, Food Contaminants, And Agricultural Wastes 40:59-67.

Connolly, J., P. Wayne, and F. A. Bazzaz. 2001. Inter-specific competition in plants: How well do current methods answer fundamental questions? Am. Naturalist 157:107-125.

Creech, J. E. and W. G. Johnson. 2006. Survey of broadleaf winter weeds in Indiana production fields infested with soybean cyst nematode (Heterodera glycines). Weed Technol. 20:1066-1075.

Creech, J. E., J. S. Webb, B. G. Young, J. P. Bond, S. K. Harrison, V. R. Ferris, J. Faghihi, A. Westphal, and William G. Johnson. 2007. Development of soybean cyst nematode on henbit (Lamium amplexicaule) and purple deadnettle (Lamium purpureum). Weed Technol 21:1064-1070.

Creech, J. E., W. G. Johnson, A. Westphal, V. R. Ferris, J. Faghihi, T. J. Vyn, and J. B. Santini. 2008. Influence of winter weed management and crop rotation on weed growth, the weed seed bank, soybean cyst nematode (Heterodera glycines) 41

population density, and crop yield. Weed Sci. 56:103-111.

Creech, J. E., W. G. Johnson, J. Faghihi, V. R. Ferris, and A. Westphal. 2005. First report of soybean cyst nematode reproduction on purple deadnettle under field conditions. Online. Crop Management doi: 10.1094/CM-2005-0715-01-BR.

Crookston, R.K., J.E. Kurle, P.J. Copeland, J.H. Ford, and W.E. Lueschen. 1991. Rotational cropping sequence affects yield of corn and soybean. Agron. J. 83: 108–113.

Dahlke, B. J., T. A. Hayden, J. W. Leif, and C. R. Medlin. 2001. Fall application of imazaquin plus glyphosate (premix) for winter annual weed control in soybeans. Proc. North Cent. Weed Sci. Soc. 56:93.

Davis, E. L., Hussey, R. S., and Baum, T. J. 2004. Getting to the roots of parasitism by nematodes. Trends Parasitol. 20: 134-141.

DeFelice, M. S. 2005. Henbit and the Deadnettles, Lamium spp. – Archangels or Demons? Weed Technol. 19:768-774. deWit, C. T. 1960. On competition. Versl. Landouwkd. Onderz. 66:1-82.

Donald, P. A., Hayes, R., & Walker, E. 2007. Potential for Soybean Cyst Nematode Reproduction on Winter Weeds and Cover Crops in Tennessee. Online. Plant Health Progress doi:10.1094/PHP-2007-0226-01-RS.

Donkin, S.G., M.A. Eiteman, and P.L. Williams. 1995. Toxicity of glucosinolates and their enzymatic decomposition products to Caenorhabditis elegans. J. Nematol. 27: 258–259.

Dorrance A.E, B. Meiring, K. Harrison, N.J. Taylor, and T. Niblack. 2012. Soybean Cyst Nematode Fact Sheet. Ohio State University, Agriculture and Natural Resources AC-39-12.

Edwards, J.H., D.L. Thurlow, and J.T. Eason. 1988. Influence of tillage and crop rotation on yields of corn, soybean, and wheat. Agron. J. 80: 76–80.

Ellenby, C. 1945. The influence of crucifers and mustard oil on the emergence of larvae of the potato root eel worm Heterodera rostochiensis Wollenweber. Ann. Appl. Biol. 32: 67–70.

Faé, Giovani Stefani, Sulc, R. Mark, Barker, David J., Dick, Richard P., Eastridge, Maurice L., Lorenz, Nicola. 2009. Integrating Winter Annual Forages into a No-Till Corn Silage System Agron. J. 101: 1286–1296.

Faghihi, A. Westphal, and W. G. Johnson. 2007. Development of soybean cyst nematode (Heterodera glycines) on henbit (Lamium amplexicaule) and purple deadnettle

(Lamium purpureum). Weed Technol. 21: 1064–1070.

First report of soybean cyst nematode reproduction on purple deadnettle under field conditions. Crop Manag. http://www.plantmanagementnetwork.org/cm/.

Fisher, M. 2013. Choosing the path of least resistance. Crops and Soils 46: 4-10.

Gibson, K. D., W. G. Johnson, and D. E. Hillger. 2005. Farmer perceptions of problematic corn and soybean weeds in Indiana. Weed Technol. 19: 1065–1070.

Giieli, R. and R. J. Smeda. 2001. Soybean weed management with fall applied herbicides. Proc. North Cent. Weed Sci. Soc. 56: 98.

Gintis, B.O., Morgan-Jones G, Rodriguez-Kabana R. 1983. Fungi associated with several developmental stages of Heterodera glycines from an Alabama soybean field soil. Nematropica 13: 181-200.

Golden, A.M., J. M. Epps, R.D. Riggs, L.A. Duclos, J.A. Fox, and R. L Bernard. 1970. Terminology and identity of infraspecific forms of the soybean cyst nematode (Heterodera glycines). Plant Disease Reporter 54:544-546.

Harrison, S. K., R. Venkatesh, and R. M. Riedel. 2008. Purple deadnettle (Lamium purpureum) emergence and removal time effects on soybean cyst nematode (Heterodera glycines). Weed Sci. 56: 327–335.

Hartwig E.E. 1981. Breeding productive soybean cultivars resistant to the soybean cyst nematode for the Southern United States. Plant Dis. 65: 303-307.

Hershman, D.E., and P.R. Bachi. 1995. Effect of wheat residue and tillage on Heterodera glycines and yield of double crop soybean in Kentucky. Plant Dis. 79: 631–633.

Hooks, C. R., K. H. Wang, K. H., S. L. Meyer., M. Lekveishvili, J. Hinds, E. Zobel, A. Rosario-Lebron, and M. Lee-Bullock. 2011. Impact of no-till cover cropping of Italian ryegrass on above and below ground faunal communities inhabiting a soybean field with emphasis on soybean cyst nematodes. J. Nematol., 43:172-181.

Howard, D.D., A.Y. Chambers, and G.M. Lessman. 1998. Rotation and fertilization effects on corn and soybean yields and soybean cyst nematode populations in a no-tillage system. Agron. J. 90: 518–522.

Hussey, R. S. 1985. Staining nematodes in plant tissues. In B. M. Zuckerman, W. F. Mai, and M. B. Harrison, eds. Plant Nematology Laboratory Manual. Amherst, MA: The University of Massachusetts Agriculture Experiment Station. pp. 197–199.

Indiana production fields infested with soybean cyst nematode (Heterodera glycines). Weed Technol. 20: 1066–1075.

Jasieniuk M., Ahmad R., Sherwood A. M., Firestone J. L., Perez-Jones A., Lanini W. T.,

Mallory-Smith C., and Stednick Z. 2008. Glyphosate-Resistant Italian Ryegrass (Lolium multiflorum) in California: Distribution, Response to Glyphosate, and Molecular Evidence for an Altered Target Enzyme. Weed Sci. 56:496-502.

Kim DG, Riggs RD. 1994. Techniques for isolation and evaluation of fungal parasites of Heterodera glycines. J. Nematol. 26: 592-595.

Koenning, S. R., D. P. Schmitt, and K. R. Barker. 1993. Effects of cropping systems on population density of Heterodera glycines and soybean yield. Plant Dis. 77: 780– 786.

Koenning, S. R., D. P. Schmitt, K. R. Barker, and M. L. Gumpertz. 1995. Impact of crop rotation and tillage system on Heterodera glycines population density and soybean yield. Plant Dis. 79: 282–286.

Krausz, R. F., B. G. Young, and J. L. Matthews. 2003. Winter annual weed control with fall-applied corn (Zea mays) herbicides. Weed Technol. 17: 516–520.

Littell, R. C., P. R. Henry, and C. B. Ammerman. 1998. Statistical analysis of repeated measures data using SAS procedures. J. Animal Sci. 76:1216–1231.

Littell, R. C., G. A. Milliken, W. W. Stroup, R. D. Wolfinger, and O. Schabenberger. 2006. Pages 160–203 and 425–428, in SAS for Mixed Models, 2nd edition. Cary, NC: SAS Institute.

Liu X.Z., Sun M.H., Guo R.J., Zhang X.D., Xie Y.Q., Qiu W.F. 1996. Biological control of soybean cyst nematode in China. Pages 11-14 in Tang W, Cook R.J., Rovira A, eds., Proceeding of the international workshop on biological control of plant diseases, Beijing, 22-27 May 1996. Beijing: China Agricultural University Press.

Liu X.Z., Wu X.Y. 1993. Decline of soybean cyst nematode: A preliminary result. Proceedings of the Second International Workshop on Plant Nematology, University of Karachi, Pakistan, pp. 61-69

Long, J. H., Jr. and T. C. Todd. 2001. Effect of crop rotation and cultivar resistance on seed yield and the soybean cyst nematode in full-season and double-cropped soybean. Crop Sci. 41: 1137–1143.

MacGuidwin, A. E., Grau, C. R., and Oplinger, E. S. 1995. Impact of planting ‘Bell,’ a soybean cultivar resistant to Heterodera glycines, in Wisconsin. J. Nematol. 27: 78-85.

McCormick, J. S., R. M. Sulc, D. J. Barker, J. E. Beuerlein, and L. H. Rhodes. 2004. Forage yield and quality of winter hardy cereals and winter sensitive species for grazing. Amer. Forage Grassland Council Proc 13: 225-229

McKell, C. M., C. Duncan, and C. H. Muller. 1969. Competitive relationships of annual ryegrass (Lolium multiflorum Lam.). Ecology. 50: 653–657. 44

Menke, M. M., S. K. Harrison, and R. Venkatesh. 2004. Italian ryegrass as a potential tool to manage winter annual weeds and soybean cyst nematode. North Central Weed Sci. Soc. Abstr. 59: 142.

Miller, D.R., S.Y. Chen, P.M. Porter, G.A. Johnson, D.L. Wyse, S.R. Stetina, L.D. Klossner, and G.A. Nelson. 2006. Evaluation of rotation crops for management of the soybean cyst nematode in Minnesota. Agron. J. 98: 569–578.

Mizobutsi E.H., Ferraz S., Ribeiro R.C.F., Menezes M. 1999. Isolation of fungi from Heterodera glycines eggs collected in different soybean-producing areas of Brazil. Nematologia Brasileira 23: 69-75.

Mock, V. A., Creech, J. E., Ferris, V. R., Faghihi, J., Westphal, A., Santini, J. B., & Johnson, W. G. 2012. Influence of winter annual weed management and crop rotation on soybean cyst nematode (Heterodera glycines) and winter annual weeds: Years four and five. Weed Sci. 60:634-640.

Mojtahedi, H., G.S. Santo, J.H. Wilson, and A.N. Hang. 1993. Managing Meloidogyne chitwoodi on potato with rapeseed as green manure. Plant Dis. 77: 42–46.

Monson M, Schmitt D.P. 2004. Economics. In: Schmitt D.P., Wrather J.A., Riggs R.D., editors. Biology and management of the soybean cyst nematode. Marceline, MO: Schmitt & Associates of Marceline. pp 41-53.

Morgan-Jones G, Gintis B.O., Rodriguez-Kabana R. 1981. Fungal colonization of Heterodera glycines cysts in Arkansas, Florida, Mississippi and Missouri soils. Nematropica 11: 155-163.

Nelson, K. A., Johnson, W. G., Wait, J. D., and Smoot, R. S. 2006. Winter annual weed management in corn (Zea mays) and soybean (Glycine max) and the impact on soybean cyst nematode (Heterodera glycines) egg population densities. Weed Technol. 20:965-970.

Niblack T.L. and R.D. Riggs. 2004. Variation in virulence phenotypes. Pages 57-71 in D.P. Schmitt, J.A. Wrather, and R.D. Riggs, eds., Biology and Management of the Soybean Cyst Nematode, 2nd edition. Walsworth Publishing Co., Marceline, MO.

Niblack, T. L. 2005. Soybean cyst nematode management reconsidered. Plant Dis. 89: 1020–1026.

Niblack, T. L., K. N. Lambert, and G. L. Tylka. 2006. A model plant pathogen from the kingdom Animalia: Heterodera glycines, the soybean cyst nematode. Annu. Rev. Phytopathol. 44: 283–303.

Niblack, T. L., P. R. Arelli, G. R. Noel, C. H. Opperman, J. H. Orf, D. P. Schmitt, J. G. Channon, and G. L. Tylka. 2002. A revised classification scheme for genetically diverse populations of Heterodera glycines. J. Nematol. 34: 279–288

Niblack, T.L., and S.Y. Chen. 2004. Cropping systems. Pages 181–206 in D.P. Schmitt, J.A. Wrather, and R.D. Riggs (eds.), Biology and management of the soybean cyst nematode, 2nd edition. Walsworth Publishing Co., Marceline, MO.

Niblack. T. L., cd. 1999. Soybean Cyst Nematode Management Guide. North Central Soybean Research Program Publication. Columbia: University of Missouri Printing Services. 16 p.

Nice, G. and B. Johnson. 2005. Indiana’s top ten most problematic weeds. Purdue: Weed Science Bulletin June 22, 2005 University Weed Science Extension Bulletin. http://www.btny.purdue.edu/weedscience/2005/topten05.pdf.

Nishizawa T. 1986. On a strain of Pasteuria penetrans parasitic to cyst nematodes. Revue de Nematologie 9:303_304.

Noel, G. R. 2004. Soybean response to infection. Pages 131–151 in D. P. Schmitt, J. A. Wrather, and R. D. Riggs, eds. Biology and Management of Soybean Cyst Nematode. 2nd ed. Marceline, MO: Schmitt and Associates.

Noel, G. R. and L. M. Wax. 2003. Population dynamics of Heterodera glycines in conventional tillage and no-tillage soybean/corn cropping systems. J. Nematol. 35:104–109.

Porter, P. M., S. Y. Chen, C. D. Reese, and L. D. Klossner. 2001. Population response of soybean cyst nematode to long-term corn-soybean cropping sequences in Minnesota. Agron. J. 93: 619–626.

Porter, P.M., J.G. Lauer, W.E. Lueschen, J.H. Ford, T.R. Hoverstad, E.S. Oplinger, and R.K. Crookston. 1997. Environment affects the corn and soybean rotation effect. Agron. J. 89: 442–448.

Riedel, R.M., A.E. Dorrance, N.J. Taylor, P.E. Lipps, and S.K. Harrison. 1998. Soybean Cyst Nematode. Ohio State University Extension Fact Sheet. AC-39-98.

Riedel, R.M. and A. M. Golden. 1988. First report of Heterodera glycines on soybean in Ohio. Plant Dis. 72:363.

Riga, E., T. Welacky, J. Potter, T. Anderson, E. Topp, and A. Tenuta. 2001. The impact of plant residues on the soybean cyst nematode, Heterodera glycines. Can. J. Plant Pathol. 23: 168–173.

Riggs, R.D. 1992. Host range. p. 107–114. In R.D. Riggs and J.A. Wrather (ed.) Biology and management of the soybean cyst nematode. APS Press, St. Paul, MN.

Riggs, R.D., T.L. Niblack, R.A. Kinloch, A.E. MacGuidwin, A. Mauromoustaskos, and L. Rakes. 2001. Overwinter population dynamics of Heterodera glycines. J. Nematol. 33: 219–226.

Rodriguez-Kabana, R., D.B. Weaver, D.G. Robertson, E.L. Carden, and M.L. Pegues. 1991. Additional studies on the use of bahiagrass for the management of root-knot and cyst nematodes in soybean. Nematropica 21: 203–210.

Ross, J. P. 1958. Effect of soil temperature on development of Heterodera glycines in soybean roots. Phytopathology 54: 1228–1231.

Ross, J.P. 1962. Crop rotation effects on the soybean cyst nematode population and soybean yields. Phytopathology 52: 815–818. cyst nematode, Heterodera glycines. Phytopathology 53: 608–609.

Schmitt, D. I', and R. D. Riggs. 1989. Population dynamics and management of Heterodera glycines. Agric. Zool. Rev. 3: 253-269

Schmitt, D. P. 1992. Population dynamics. Pages 51-59 in R. D. Riggs and J. A. Wrather, eds., Biology and Management of the Soybean Cyst Nematode. St. Paul. MN: American Phytopathological Society.

Schmitt, D.P. 1991. Management of Heterodera glycines by cropping and cultural practices. J. Nematol. 23: 348–352.

Schmitt, D.P., and R.D. Riggs. 1991. Influence of selected plant species on hatching of eggs and development of juveniles of Heterodera glycines. J. Nematol. 23: 1–6.

Shier, M. 2008. Soybean varieties with soybean cyst nematode resistance. Available at https://netfi les.uiuc.edu/tjw/www/cover.htm. Univ. Illinois Extension, Univ. of Illinois, Champagne-Urbana, IL.

Sipes, B. S., Schmitt, D. P., and Barker, K. R. 1992. Fertility of three parasitic biotypes of Heterodera glycines. Phytopathology 82: 999-1001.

Slack, D. A., R. D. Riggs, and M. L. Hamblen. 1992. The Effect of Temperature and Moisture on the Survival of Heterodera glycines in the Absence of a Host. J. Nematol. 4: 263–266.

Sortland, M.E., and D.H. MacDonald. 1987. Effect of crop and weed species on development of a Minnesota population of Heterodera glycines race 5 after one to three growing periods. Plant Dis. 71: 23–27.

Sun M.H., Liu X.Z. 2000. Suppressive soils of soybean cyst nematode in China. Acta Phytopathologica Sinica 30: 353-356.

Trick, H 2007 RNA Interference: A Novel Approach for Soybean Cyst Nematode Resistance. Kansas State submitted to CRIS

Thoden, T. C., Korthals, G. W., & Termorshuizen, A. J. (2011). Organic amendments and their influences on plant-parasitic and free-living nematodes: a promising method for nematode management? J. Nematol. 13:133-153.

Uva, R.H., Neal, J.C., and DiTomaso. 1997. Weeds Of The Northeast. Cornell University Press. pp.. 248, 58.

Venkatesh, R., S. K. Harrison, and R. M. Riedel. 2000. Weed hosts of soybean cyst nematode (Heterodera glycines) in Ohio. Weed Technol. 14: 156–160.

Venkatesh, R., S. K. Harrison, E. E. Regnier, and R. M. Riedel. 2004. Purple deadnettle effects on soybean cyst nematode populations in no-till soybean. Proc. North Central Weed Sci. Soc. 59:56.

Weaver, D.B., R. Rodriguez-Kabana, and E.L. Carden.1993. Velvetbean in rotation with soybean for management of Heterodera glycines and Meloidogyne arenaria. J. Nematol. 25: 809–813.

Weaver, D.B., R. Rodriguez-Kabana, D.G. Robertson, R.L. Akridge, and E.L. Carden. 1988. Effect of crop rotation on soybean in a field infested with Meloidogyne arenaria and Heterodera glycines. Ann. Appl. Nematol. 2: 106–109.

Wicks, G. A., O. C. Burnside. and W. L. Helton. 1994. Weed control in conservation tillage systems. Pages 211-244 in P. W. Unger, ed. Managing Agricultural Residues. Boca, FL: Lewis Publishers.

Willson, H. R., R. M. Riedel, J. B. Eisley, C. E. Young, J. R. Jasinski, T. A. Wheeler, P. H. Kauffman, P. E. Pierson, and M. C. Stuart. 1996. Distribution of Heterodera glycines in Ohio. J. Nematol. 28:599–603.

Wilson, B. J., N.C.B. Peters, K. J. Wright and H. A. Atkins. 1988. The influence of crop competition on seed production of Lamium purpureum, Vioia arvensis and Papaver rhose in winter wheat. Asp. Appl. Biol. 45:71-80

Wrather, J. A. and S. R. Koenning. 2006. Estimates of disease effects on soybean yields in the United States 2003 to 2005. J. Nematol. 38: 173–180.

Wrather, J. A., S. R. Koenning, and T. R. Anderson. 2003. Effect of diseases on soybean yields in the United States and Ontario (1999–2002). Plant Health Prog. http://www.plantmanagementnetwork.org/php/default.asp.

Wrather, J.A., W.C. Stienstra, and S.R. Koenning. 2001. Soybean disease loss estimates for the United States from 1996 to 1998. Can. J. Plant Pathol. 23: 122–131.

Yen, J.-H., Niblack, T. L., and Wiebold, W. J. 1995. Dormancy of Heterodera glycines in Missouri. J. Nematol. 27: 153-163.

Young, L.D. 1995. Changes in the Heterodera glycines female index as affected by ten- year cropping sequences. J. Nematol. 26: 505–510.

Young, L.D. 1998. Influence of soybean cropping sequences on seed yield and female index of the soybean cyst nematode. Plant Dis. 82: 615–619.