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. 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 (Mulrooney 2011). During the 2009 survey, elevated reproduction on PI88788 was observed at rates of 44-80%. In 2020, samples were tested from the SCN research field at the Carvel and Research Education Center were found to have reproduction rates of 65%. Considering PI88788 once kept reproduction levels well below 10%, SCN is becoming increasing problematic in the region and across the US.

In 2019, the Koehler lab conducted nematode soil sampling across MD and DE to gather a baseline understanding of dominant species currently affecting soybean fields. Soil samples were collected from 60 organic and conventional soybean fields. Within sites sampled, SCN was recovered in 57% of fields, root knot nematode in 17%, lesion nematode in 60%, and dagger nematode in 17%. An additional 35 sites sampled in 2020 had SCN present in 51% of fields. The first soybean variety with resistance gene PI88788 was launched in 1978. This gene remained highly effective for many decades, but in the 2000’s elevated reproduction began to be reported. Today >90% of soybean varieties still employ PI88788. Other resistance genes have been discovered, but are more complicated to work with and incorporation into high yielding lines has been a challenge. Peeking is a resistance source currently available across a few maturity groups. Trials in Iowa indicated a 21 bu/a gain using Peeking over PI988788 (Tylka, 2020). The most recently released resistance source, PI89772, is currently only available in a 2.3 maturity, but holds promise. This resistance gene was first discovered in 1930, and 25 years of breeding efforts led to the release of the first commercial line in small quantities for the 2020 growing season. While this resistance source is currently only available in a maturity too low for our region, we are interested in observing nematode reproduction on this source to predict the efficacy of this resistance source as it becomes available in more maturity groups. Due to the immense interest surrounding this variety, it is possible we may not be able to acquire seed for the 2022 season; in this case, we will substitute with an additional variety of peeking resistance. Growers in the Mid-Atlantic and across the US are in need of more management options for SCN and other nematode species. The goal of this proposal is to generate a second year of local data on the effects of newly emerging resistance sources for SCN reproduction and subsequent yield potential. Nematodes are often patchy in their distribution
and a second year of observations will strengthen conclusions drawn from the project.

1. Compare performance of soybeans with resistance gene PI88788 to new resistance sources.
2. Assess resistance gene effects on field populations of soybean cyst nematode.
3. Share research findings through extension events and survey farmers regarding their concern towards soybean cyst
nematode population increases and knowledge of resistance gene sources.

Objective 1. Compare performance of soybeans with resistance gene PI88788 to new resistance sources.
Based on survey work and 2020 field trials, a field with SCN pressure has been established at the Carvel Research and Education Center in Georgetown, DE. Four hybrids with differing resistance genes will be set up in a randomized complete block design with five replications. Ideally, treatments will include PI89772, Peeking, and PI88788 resistance sources. Plots will be approximately 25 ft in length and 10 ft wide. After seed is planted, soybean seedlings will be monitored for stand emergence, height, and disease symptoms throughout the duration of the trial. Currently PI89772 is only available in a 2.3 maturity group bean. While it is unrealistic for this maturity group to be planted in our region, the goal is to assess nematode reproduction and the viability of this resistance gene so we have an idea about performance as it becomes available in maturity groups more appropriate to our region. Due to differences in maturites, plots may be hand harvested at appropriate timing for each maturity group, dried for 24 hours, and all samples will be processed through a small plot combine following the final hand harvest.

Objective 2. Assess resistance gene effects on field populations of soybean cyst nematode.
Within each of the 20 treatment plots, a plot-representative soil sample will be collected by combining 30-40 soil cores per plot at three time points throughout the growing season. Plots will be assessed for nematode populations at the time of planting, approximately 30 days after planting, and at harvest. Samples will be submitted to the North Carolina Department of Agriculture Nematode assay service to be processed. Population counts will be provided for SCN, root knot nematode, lesion, lance, ring, spiral, and stubby root nematodes. Additionally, plants will be destructively sampled approximately 30 days after emergence to count live female cysts on the roots.

Objective 3. Share research findings through extension events and survey farmers regarding their concern towards soybean cyst nematode population increases and knowledge of resistance gene sources.
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 2023 Delaware Ag Week. At University of Delaware hosted field days and commodity meetings, growers will be given a brief questionnaire to better understand concerns regarding nematode management and to investigate knowledge bases surrounding SCN resistance genes. In 2020, the United Soybean Board gave funding to the SCN Coalition to widen its reach to all states as data has shown that resistance gene PI88788, used in >90% of SCN-resistant soybean cultivars, is beginning to lose effectiveness. Dr. Koehler is a member of the SCN Coalition and questions used in this survey may include a set of questions released by the SCN Coalition to understand grower choices, perceptions, and
understanding of SCN host resistance.

Update:
A four treatment, 5 replication randomized complete block design trial was planted on 4/27/22 at 110,000 sd/acre. The seeds used represented 3 different resistance sources (PI89772, Peeking, and PI88788) across 4 relative maturities ranging from 2.3-3.9. Stand counts were collected after plant emergence, and plots have continued to be monitored. Plots are in 30 inch rows and now at full canopy cover. To monitor nematode levels, soil samples were collected at planting and again on 6/1/22. At the second soil sample collection date, 5 plants were destructively dug from row 1 of each plot and SCN females were counted from the combined root systems of each plot. A final set of soil samples will be collected in August and small plots will be harvested to compare yield effects.

Update:

View uploaded report

Soybean cyst nematode (SCN) (Heterodera glycines) is consistently ranked among the top destructive soybean pathogens across the United States and is the most significant nematode pest affecting soybeans in Delaware and the Eastern Shore of Maryland. SCN has been present in Delaware since 1979 prompting growers to rely on resistant varieties, primarily using the PI88788 resistance source. However, additional control strategies are needed as SCN populations have begun reproducing readily on these once resistant cultivars. A five-replication field trial was conducted in 2021 and 2022 to evaluate two additional resistance genes, Peking and PI89772, compared to lines with PI88788, for stand emergence, control of SCN, and yield differences. Soybean maturity group had the largest impact on yield, but the PI89772 resistance gene had the lowest numerical value of SCN at the end of the season in both years and numerically lower nematode reproductive factor than PI88788. If PI89772 is introduced to maturity groups appropriate for the region, this resistance source should be considered as an option to alternate with PI88788 in effort to reduce in-season nematode population increases.

Soybean cyst nematode (SCN) (Heterodera glycines) consistently ranks as top destructive pathogen of soybeans (Glycine max (L.) Merrill) across the United States (Allen et al. 2017). Nematodes often go undiagnosed and can be very damaging to soybean production, reducing both yield and quality. As the PI88788 resistance gene loses effectiveness, growers are in need of additional tools to manage nematode populations. A survey was conducted across DE and MD in 2019 that found SCN present in 57% of sampled fields. Continued survey work in 2020 identified SCN in 51% of fields sampled. In 2020, a field site was established at the Carvel Research and Education Center to conduct SCN related soybean research. Upon sample analysis, nematode populations in this field were found to have a 65% reproduction level on resistance source PI88788, this gene once kept reproduction well below 10%. The proposed project aims to conduct a second year of observations screening two new resistance gene sources compared to PI88788 to monitor nematode reproduction and populations over the course of the season. This project will fund 10% effort of a research technician to assist with trial implementation along with soil samples to provide insight on SCN populations and other nematode species present. This data will be complimented by a questionnaire distributed to farmers at extension events to gain insight on the concern for SCN population increases and awareness of soybean SCN resistance sources across the Mid-Atlantic. As we continue to deal with breakdown of resistance gene efficacy and subsequent increases in SCN populations, the goal of this project is to obtain data on resistance gene performance to expand potential management options for farmers.