Southern Root-Knot Nematode Mapped

DECATUR, Ill. (DTN) -- Southern root-knot nematode (SRK) has become a nasty adversary in Southern soybean-growing regions. A new map shows the range of the pest may be broader than farmers realize.

To increase awareness about the pest, researchers have, for the first time, mapped the distribution of SRK in field crops (any field that might someday rotate to soybeans) by county across the continental United States. While SCN remains a bigger nematode adversary overall, SRK has nearly equal status as a troublemaker in Southern states such as Alabama, Arkansas, Delaware, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, Missouri, North Carolina, Oklahoma, South Carolina, Tennessee, Texas and Virginia. It prefers sandy soils and has also been identified in a few regions with the Midwestern states of Illinois and Indiana. Find the map here: https://cropprotectionnetwork.org/….

According to a recent news release from The SCN Coalition (https://cropprotectionnetwork.org/…), researchers estimated SRK cost Southern soybean farmers more than 13 million bushels in 2022.

The first step to active management is knowing where the problem exists. The SCN Coalition has been expanding its educational outreach to other parasitic nematodes soybean farmers face. This mapping is a starting place for farmers and their crop advisers.

CHECK THE MAP

"We're always trying to raise awareness about SRK. This (the map) is a visual way to do so," explained Travis Faske, a plant pathologist at the University of Arkansas System Division of Agriculture and a major player with the mapping. "The map gives the farmer or the agronomist a better idea whether SRK is a threat in their area, aiding early detection."

SRK survives in areas where the average temperature during the coldest month of the year stays above 37 degrees Fahrenheit. Changes in cropping systems and specifically limited rotation, have fueled SRK's spread in recent decades. Unfortunately, many crops, including corn and cotton, are hosts for the nematode.

"Cotton is the main reason why the Mid-Southern U.S. has a lot of Southern root-knot nematode and not the peanut root-knot nematode. We have only had peanuts a few years but cotton for decades. Nematodes, in general, follow the host crop. If you plant it, they will come," Faske told DTN.

Planting of earlier-maturing, more susceptible soybean varieties has also contributed to the spread of the pest. Ironically, some of today's more robust soybeans do, as well. "When we had soybean with a yield potential of 30 bushels per acre, the plants were small and roots too," Faske said. "With higher-yielding, 'healthier' plants, the root systems are larger and can support more nematodes." In other words, those that endure feeding better also support more nematodes, he added.

The map can help farmers determine whether they should be on guard against SRK. "By the time you have plants dying, I don't have any options for you other than next time plant a variety with better resistance against SRK," Faske explained. "Knowing what nematode threat you have early is critical. From there, you can come up with a plan."

A telltale symptom of SRK is the galls develop on the soybean roots during the growing season. "When soybean roots are developing, the second-stage juvenile migrates up to the root system to establish a feeding site," he said. "While the SCN adult females break through the root surface and attach to the exterior of the root, SRK enters and stays in the root and causes it to swell, which is called a gall.

"Each of these large galls acts like a rock in the middle of a small stream and reduces water uptake. That's why plants infected with SRKN die prematurely," he said. Cut open, the gall looks like root tissue, compared to soybean nodules which are a healthy pink and are much smaller than a gall at the end of the season.

CHART A PLAN OF DEFENSE

For the past six years, Faske has conducted field trials to determine what soybean varieties are susceptible or have a level of resistance to SRK. Find the most recent information here: https://arkansascrops.uada.edu/….

Results from the 2023 crop trials will be published later this year. "There are a few soybean varieties available that offer resistance to SRK," he said. "Seed-applied nematicides are also popular.

"A farmer focused solely on SRK management should bypass a cover crop, since most are hosts for SRK, with some less suitable hosts than others," he said. "If you can, plant peanuts, which are not a host, two years in a row if the field has a high SRK infestation. Next, plant an SRK-resistant cotton variety, and then return to soybeans."

Faske also recommends growing a later-maturing Group 5 maturity soybean variety rather than a Group 4 for farmers with heavy SRK pressure.

While peanuts are one of the few non-host crops for SRK, Faske acknowledges they aren't an option for everyone. "Arkansas' silt loam soil doesn't drain well, which is problematic for peanuts," he said. "Peanuts are also a much slower crop to harvest. And of course, proximity to a market also comes into play."

Additionally, weed control is extremely important when managing SRK. Faske pointed out, "Just about any weed can sustain a small population of SRK.

"Farmers can manage for SRK," he added. "They just have to adjust their production system to be able to accommodate it." Check out The SCN Coalition for more SRK management tips: https://www.thescncoalition.com/….

Find DTN's most recent writing on Soybean Cyst Nematode here: https://www.dtnpf.com/….

Pamela Smith can be reached at pamela.smith@dtn.com

Follow her on Twitter @PamSmithDTN

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