If southern root-knot nematodes are robbing yield from your Mid-South soybean fields, you might consider selecting a cultivar with lower susceptibility and terminate a cover crop ahead of your planting date before nematode populations can increase.

Black-hulled oats, known to slow down nematode reproduction, earned an impressively low reproduction factor (RF) rating among six cover crop species tested in a research study on the susceptibility of various cover crops, says Travis Faske, plant pathologist, University of Arkansas System Division of Agriculture.

Faske reported study findings during his presentation on southern root-knot nematodes and their interaction with cover crops at the 64^th annual Tri-State Soybean Forum held recently in Dumas, Ark.

"We placed individual cover crop seeds in pots, let them establish a root system and placed 6,000 southern root-knot nematode (RKN) eggs into the pots. We pulled out 4.8 times more nematodes than we put in on black-hulled oats. Barley allowed the highest population, garnering an RF of 75.8."

Faske admitted there is very little research on the reproduction of southern root-knot nematodes in cover crops but said any RF over 1 means the cover crop acts as a host for southern root-knot nematodes.

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You might want to avoid wheat, barley, Austrian winter peas, and certain clovers in fields with high populations of root-knot nematodes.

"There is a wide range in RKN susceptibility in cover crops, with additional variability among varieties within some cover crops like clovers and mustards," Faske says.

Control options

Nematodes cannot survive cold weather without a host. "By March or April, 80% to 90% of nematode populations will die off without that host plant," Faske says. "One Georgia cotton/cover crop rotation study shows that RKN can complete two life cycles on a susceptible cover crop in a moderate winter."

Root-knot nematode populations matter little until they start affecting soybean yield. "I advise growers to select a moderately-resistant cover crop and always monitor soil temperature. Their reproduction is severely limited when the soil temperature is below 50 degrees," Faske says. "Terminating a cover crop ahead of planting can avoid the 'green bridge' where pests survive on healthy plant material."

Field study, recommendations

Each year, Faske solicits recommendations from various seed companies for varieties exhibiting low susceptibility to nematodes. In 2019, he conducted a field study with several of those varieties in a field infested with southern root-knot nematodes and evaluated them for galling at the R4 to R5 growth stage. "There is no percentage standard for root system galling. I try to compare varieties within each trial rather than making judgments across trials," Faske says. "As a general rule, soybean varieties exhibiting the lowest gall rating had the greatest yield."

Faske equates each gall to a cafeteria for nematodes feeding inside the root. "They impede the plant's ability to take up water and nutrients, stealing those nutrients and restricting development," Faske says. "Delta Grow 4940 has looked good the last few years, so I made that my resistant control variety."

Faske says growing a resistant or moderately resistant cultivar in a nematode-infested field will deliver greater yield and reduce nematode reproduction compared to a susceptible cultivar. "Though some cultivars are more tolerant to nematodes by delivering more yield than others with a similar degree of galling, they do nothing to lower RKN populations for subsequent crops," Faske said.

A complete list of varieties in the study, their percent galled rating and yield harvested can be found at www.arkansasrowcrops.com.