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Consider Fall SCN Tests
Monday, October 15, 2018 6:08AM CDT
By Emily Unglesbee
DTN Staff Reporter

ROCKVILLE, Md. (DTN) -- The next time you head to the fields to check if they are harvest-ready, consider bringing a soil probe along to test for soybean cyst nematode (SCN).

The SCN Coalition, which first launched in 1997, is urging growers to put SCN back on their priority list -- and the first step is testing your soils.

Fall is an excellent time to get your fields tested for soybean cyst nematode, the tiny soil pest that causes big problems for the nation's soybean farmers, said Sam Markell, a North Dakota State University plant pathologist and SCN Coalition leader. Many growers do soil fertility testing at this time of year and adding an SCN test can fit right into that fall schedule, he said in a SCN Coalition press release.

WHY SCN IS STILL A PROBLEM

SCN has been confirmed in 31 states and two Canadian provinces. But the pest still flies under the radar, in part because the nematode infects soybean roots and doesn't cause many aboveground symptoms in low-to-moderate populations under good growing conditions, said Iowa State University Nematologist Greg Tylka. Once the problem is severe enough to cause visible yellowing and stunting, populations are dangerously high.

One 2017 study estimated the pest robbed growers of 617 million bushels between 2010 and 2014 -- more than twice any other soybean disease considered.

Many growers are under the impression that SCN-resistant soybean varieties on the market have solved the problem, said Melissa Mitchum, a nematologist at the University of Missouri.

However, 95% of soybean varieties still use a single source of resistance, PI 88788, according to the SCN Coalition. This has led to a rise in SCN populations that are able to reproduce on this type of resistant soybean -- a phenomenon called SCN virulence.

"Because there is a lot of genetic heterogeneity in SCN field populations, there are always a few individuals able to grow on [PI 88788] beans," Mitchum explained. "They infect, reproduce, create more progeny and then re-infect. Next year, they'll do better than the rest and slowly shift the population toward more individuals able to grow on an SCN-resistant variety."

Growers can break this cycle by planting other sources of resistance, such as PI 548402 (Peking) or PI 437654 (Hartwig), though both are scarcer in the marketplace.

But in order to manage virulence, growers must test their soils and discover what populations they are dealing with. Many labs can test not only for the presence of SCN but also for virulence on PI 88788. See the availability of labs here: https://www.thescncoalition.com/….

HOW TO TEST

Experts recommend using a soil-probe with a 1-inch diameter tube, according to the SCN Coalition. Aim to collect 15 to 20 core samples, 8 inches deep, for every 20 acres.

"We'd like cores collected either in a zig-zag pattern, or you can pull cores from high-risk areas in the field," Markell said in the press release. "Those include entry ways, high soil pH areas, low spots and areas that have previously flooded, as well as areas in the field with unexpectedly low yields that you can't explain."

See more details on how to handle the soil samples properly here: https://www.thescncoalition.com/….

Many states offer free SCN soil testing, including Arkansas, Michigan, Minnesota, Nebraska, Ohio, South Dakota, Tennessee, Texas, Virginia, West Virginia and Wisconsin. Check with your local state soybean board to see if your state also does this.

For those without the free option, tests can range from $1 to $2 an acre, Markell said.

"The yield loss potential is $10 to $20 per acre, so testing makes economic sense," he said. "You can have yield losses of up to 30% from SCN with no above-ground symptoms."

See more information about the SCN Coalition here: https://www.thescncoalition.com/… and the news release here: https://www.thescncoalition.com/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow her on Twitter @Emily_Unglesbee

(PS/CZ)


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