Soybean cyst nematode (Heterodera glycines) is considered the most damaging pathogen of soybeans across the US and is the most significant nematode pest affecting soybeans in Delaware and the eastern shore of Maryland. Following closely is root knot nematode (Meloidogyne incognita). SCN females establish permanent feeding sites on roots and turn into cysts that each contain hundreds of eggs. There may be several generations within a single growing season and recalcitrant cysts and eggs can persist in the soil for long periods of time. SCN has been present in Delaware since 1979 and Maryland since 1980. Growers have relied heavily on resistant varieties, primarily using the PI88788 resistance source. Long-term exposure to this resistance gene can select for SCN populations that are able to overcome this source of resistance. Detailed SCN surveys were conducted across Delaware and Maryland in 1993 (Sindermann et al. 1993). Surveys in DE were conducted again in 2009 and it was observed that populations shifted from predominately race 3 to
race 1. During the 2009 survey, elevated reproduction on PI88788 was observed at rates of 44-80%. Surveys were continued in 2019-21 and high populations with reproduction rates >75% were observed. In these surveys, RKN populations ranging from 4,000-9,000 nematodes per 500 cc of soil were also observed, particularly in fields with a history of vegetable rotation. A high level of infestation is considered >170 RKN per 500 cc, meaning many local fields are dealing with populations 25-53 times above threshold. Fumigants such as methyl bromide and ethylene dibromide once provided excellent control to soilborne pathogens like RKN, but were phased out due to human and environmental health and safety concerns. Safer, non-fumigant chemical control options have not been as effective and integrated approaches to nematode management are needed. Many growers are already planting winter cover crops, so there are consistently questions regarding the effect of these crops on nematode populations the following season.

Cover crops are commonly used as a method to slow erosion, control weeds and pests, and improve water availability and soil health during the non-growing season for future crops. A common question regarding cover crops is their ability to suppress nematode population densities by breaking pest cycles or via suppression. Cover crops such as winter rye, Daikon radish, and annual ryegrass have reduced SCN populations when used in consecutive years (Acharya et al., 2021). Greenhouse trials comparing the use Daikon radish, annual ryegrass, and crimson clover as cover crops, followed by SCN susceptible soybeans also demonstrated reduced SCN females on soybean roots (Harbach and Tylka, 2021). There was no evaluation for the effects of these cover crops on Root Knot Nematode, a common species within the region. In a previously funded project, Dr. David Owens has established plots to examine the role of cover crops on slug populations. Leveraging these already planted plots, this proposal aims to sample for nematodes within the cover crop choices of rye, barley, crimson clover, or tillage radish and winter rape. This will provide an opportunity to monitor nematode populations in grower fields and increase observations of nematode response to cover crops in our region. Much of the work conducted in the Midwest is in soils with high organic matter. In sandier, low organic matter soils, nematode damage can be more severe and interactions with cover crops can differ. The ability to use common cover crops within the region as a tool to manage nematodes would be very helpful, but little is known regarding these complex interactions at this time.

Objective 1. Utilize rhizoboxes to visualize economic damage thresholds for Soybean Cyst Nematode (SCN), Root Knot Nematode (RKN), and the interaction of SCN and RKN in local soil types.

Objective 2. Track in-season nematode populations following the use of winter cover crops.

Objective 3. Share research findings through extension events and use findings to inform future management trials.

Objective 1. The Koehler lab received funding to build rhizoboxes to aid in extension demonstrations. These boxes have a clear Plexiglas front that allows for visualization of root systems. Two soybeans will be planted into each box. Using RKN populations maintained at the Carvel Research and Education Center Greenhouses, we will extract RKN eggs from roots. We will allow eggs to hatch and use infective stage J2 nematodes to inoculate. Inoculations will be at various thresholds to visualize symptoms and signs observed at low, moderate, and high populations. Once soybeans are planted into our SCN trial field for 2022, we will extract SCN females from 20-30 day old soybean plants for inoculation of SCN boxes and for mixed population boxes. SCN females will be extracted following a root blast protocol where females are washed from the roots and collected in a sieve for enumeration. Photos and time-lapse videos will be captured to preserve visualization of symptoms.

Objective 2. This portion of the trial will be conducted in field trials set up by Dr. David Owens as part of a previously funded DSB proposal “Cover Crop Selection and Termination Implications for Slugs”. Briefly, large field plots will be planted on two cooperator farms in the same location of the field in 2021 and 2022. At each location, 4 cover crops (rye, barley, crimson clover, tillage radish and winter rape), were planted in October in 50’ x 100’ plots. In the spring, plots will be subdivided with half being terminated early and half terminated late along with vertical tillage. Four plots will be left bare ground as a no-cover control. When plots are sampled in the spring and fall for slugs, select nematode soil samples will also be collected to represent a range of cover crop and tillage combinations at the field site with highest base levels of nematodes. The cash crop will be assessed in season for nematode damage.

Objective 3. Findings from this project will be shared through the University of Delaware’s Weekly Crop Update, which reaches over 700 growers, consultants, and stakeholders and provides a platform to discuss disease concerns and other production issues. Data will also be shared through training events and extension presentations such as Mid Atlantic Crop School, and the 2022 Delaware Ag week.

Updated July 29, 2022:
The first test with rhizobox inoculation is currently under way. We plan to set up additional boxes to trial through the fall. Soil samples were collected from various cover crop treatments on 5/25/22 and samples will be collected again in August from the main crop. Preliminary data from this project will be shared at a nematode field day to be hosted on August 18 and through articles in the Delaware Weekly Crop Update.

Updated January 2, 2023:

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Soybean cyst nematode (SCN) (Heterodera glycines) and Root knot nematode (Meloidogyne spp.) consistently rank as top destructive pathogens of soybeans (Glycine max (L.) Merrill) across the United States. In surveys conducted across DE and MD from 2019-2021 SCN and RKN were widely prevalent across the state, particularly in Sussex County. Nematodes often go undiagnosed, but can be very damaging to soybean production reducing both yield and quality. Visualizing stunting and other silent symptoms can assist recognition of unnoticed yield impacts. The PI88788 resistance gene once effectively managed soybean cyst, but populations are now able to reproduce at high levels, reducing the effectiveness of this resistance source. Growers are in need of additional tools to manage nematode populations and often ask questions about the impact of cover crop decisions on nematode populations. Project objectives included: 1) Utilize rhizoboxes to visualize economic damage thresholds for Soybean Cyst Nematode (SCN), Root Knot Nematode (RKN), and the interaction of SCN and RKN. 2) Track in-season nematode populations following the use of winter cover crops. 3) Share research findings through extension events and use findings to inform future management trials. In rhizobox trials, effect of SCN and RKN on reduced growth and pod set were visualized and optimizations to the rhizobox system will be made to continue to develop photo and video resources at varying nematode population levels. Nematode soil samples were collected from different cover crops to provide insight on the nematode species present and their relative abundance among plots after winter cover crops. No notable nematode populations were recovered in any of the plots in the spring, fall results are pending analysis and final conclusions will be updated upon their arrival.

Soybean cyst nematode (SCN) (Heterodera glycines) and Root knot nematode (Meloidogyne spp.) consistently rank as top destructive pathogens of soybeans (Glycine max (L.) Merrill) across the United States (Allen et al. 2018). Nematodes often go undiagnosed, but can be very damaging to soybean production reducing both yield and quality. Visualizing stunting and other silent symptoms can assist in recognizing yield impacts that might be occurring without realization. The PI88788 resistance gene once effectively managed soybean cyst, but populations are now able to reproduce at high levels, reducing the effectiveness of this resistance source. Growers are in need of additional tools to manage nematode populations and often ask questions about the impact of cover crop decisions on nematode populations. In surveys conducted across DE and MD from 2019-2021 SCN and RKN were widely prevalent across the state, particularly in Sussex County. This project aims to use rhizoboxes to visualize nematode symptoms at economic threshold levels and to track nematode populations following the use of various cover crops. This project will fund two months of support for a M.S. student focusing on diseases of soybeans to conduct greenhouse rhizobox trials and collect soil samples to provide insight on the nematode species present and their relative abundance among plots after winter cover crops. This data will be shared at multiple extension events to help farmers make informed decisions regarding nematode management.