Research Highlights

Identifying and Managing Nematodes in the Atlantic Region

By Laura Temple

Nematodes are often called the silent yield robber because farmers frequently don’t know they are present. They cause few above-ground symptoms, notes Alyssa Koehler, Assistant Professor and Extension Specialist in Plant Pathology at the University of Delaware.

“We expect nematodes in the sandy soils of Delaware, Maryland’s eastern shore and other Atlantic region states,” she says. “Our crop rotations, which often include soybeans, corn and vegetables or, in Virginia, cotton, also create favorable conditions for nematodes, but famers need soil numbers to know for sure.”

To learn more about nematode populations in this region, the Atlantic Soybean Council is funding soil survey and seed treatment efficacy trial research led by Koehler. This project, funded for 2021 and 2022, builds on previous work she has done in Delaware. 

“A baseline understanding of nematode pressure in this area will help us determine what tools farmers need to better protect soybean yield,” she adds. “Growers in the Mid-Atlantic need more management options for nematodes, but until they know what pressure they face, they don’t know what to encourage the industry to develop.”

Soil Samples Reveal Nematode Levels 

To determine that baseline, Koehler and her team collected 135 soil samples from 100 farms in Delaware and Maryland in 2021. In fields with no knowledge of existing nematode pressure, they collected random samples. In fields with a known history of nematodes, they conducted diagnostic sampling of both problem and healthier spots.

Analysis of those soil samples found soybean cyst nematodes in 56 samples, a rate of 41.5 percent. Of those samples with SCN, 61 percent showed high populations above the threshold for damage. Just seven samples contained root-knot nematodes, but two of them, or more than 20 percent, had high enough populations to cause yield damage. Lesion nematodes were present in 105 samples, a rate of 78 percent, but only 10 percent of those samples had populations above the damage threshold.

“We expected to find SCN and root-knot nematodes, but we now know that we should be aware of lesion nematodes as well,” Koehler says. “We found more lesion nematodes that we expected, and they feed on both soybeans and corn, so they could become an issue in multiple crops.”

In 2022, Koehler will work with other researchers to take a more targeted approach and dig deeper into nematode populations. They will collect samples from fewer farms, but they will select fields with known nematode problems in New Jersey and Virginia, as well as Delaware and Maryland.

“We plan to do more intensive analyses to identify specific species of nematodes,” she explains. “For example, most of the root-knot nematodes we expect to find will likely be Southern root-knot nematode. We also plan to do HG Type tests on the SCN found in the samples.” 

Greenhouse HG Type tests determine how well the SCN reproduces on soybeans bred to have host-plant resistance. The test counts visible SCN females on the roots of resistant plant lines compared to standard varieties after 30 days. Some SCN now have a 65 percent reproduction rate or higher on soybeans with resistant genetics. Several years ago, when the resistance was more effective, the reproduction rate was only 10 percent.

Koehler also expects the 2022 samples to increase understanding of the impact of soil temperatures on nematode populations.

“My hope is that the colder winter of 2021-22 will knock down populations,” she says. “That’s just another layer to explore along with soil types, cover crops, and rotations.”

Exploring Management Options

This research is also screening two seed treatments, ILeVO and Saltro, for efficacy in reducing SCN yield loss in soybeans. Two 2021 trials located in Delaware and Maryland evaluated their effectiveness. One site had low SCN pressure and one had higher pressure. In 2022, another trial located in Virginia will also assess the value of seed treatments.

To monitor SCN populations, researchers sampled the soil at planting, 30 days after planting and at harvest. They also counted SCN females on soybean plants in each plot 30 days after emergence. 

“Our regional seed treatment observations in 2021 aligned with literature from other regions,” Koehler says. “We saw no statistical yield differences between treatments and the untreated control. We did see better emergence and crop canopy in treated soybeans, which could contribute to the numerical yield difference from treated soybeans in plots with pressure.”

She notes that the results reinforced that there was no difference in yield among treatments when SCN pressure is low. Repeating the trials in 2022 will provide more data and account for more variability in conditions. 

Koehler believes managing nematodes in soybeans is a long-term effort, and that farmers will need a patchwork of solutions to protect crops.

“All this information will raise awareness about nematode pressure in the Mid-Atlantic region,” Koehler continues. “Few soybean varieties with SCN resistance that fit this region and our maturity group sweet spot of high Group 3s and low Group 4s are available. With this research, farmers, agronomists and extension agents can encourage the industry to develop such varieties.”

To share what they are learning, the team hosted a nematode field day during the season and a webinar about nematodes in soybean production in November 2021. They plan to host another webinar that includes insights from 2022 research in November 2022.

Published: Sep 19, 2022