SCN OFF THE RADAR?
Soybean cyst nematodes (SCN) are adapting to the most common source of plant resistance, PI88788. “We clearly have a developing problem,” says Don Hershman, a University of Kentucky plant pathologist, but for many soybean growers, “it doesn't make the radar screen.”
Many growers mistakenly assume that if they plant SCN-resistant varieties, they're protected, he says. And that has led them to quit monitoring their fields for SCN.
Very few Kentucky growers soil sample for nematodes, he says, even though the testing is free, and most are already sampling for fertility and could easily split the soil samples.
A University of Missouri grower awareness survey confirms that. In 2005, researchers collected soil samples from 47 Missouri counties. More than 80% of farmers taking part in the awareness study grew SCN-resistant varieties — many for 10 years or more, says Robert Heinz, coordinator of the University of Missouri nematology lab in Columbia. Lab tests showed that 61% of producers had SCN egg counts above the damage threshold for Missouri, he says. “Yet 62% of them did not believe they had any yield loss due to SCN,” and 64% had never sampled for SCN, he reports.
Just 6% of Missouri farmers surveyed in 2005 had sampled for SCN within the last five years, Heinz says. Even with higher soybean prices the past two years, he's not getting any more samples these days. “This seems to indicate that SCN was considered a problem in the past, but is not seen as a yield-reducer today,” he says.
In informal surveys at grower meetings, University of Illinois Nematologist Terry Niblack found that nearly two-thirds of Illinois farmers plant SCN-resistant varieties on three-fourths of their soybean fields. But fewer than 20% of farmers actually test for SCN.
Iowa State University Nematologist Greg Tylka found a similar response among Iowa growers. About one-third of growers attending 2007 Extension meetings said they regularly tested for SCN, he says. The same was true in Indiana, where a 2005 Purdue University survey found that about 35% of growers checked their fields for SCN.
“What makes this dangerous is that you can't always see the damage,” Niblack says. SCN can cut yields by up to 30% without obvious symptoms, she says. And on susceptible soybean varieties, nematode numbers “can go from ‘barely there' to ‘oh my gosh!' in one season,” says Hershman.
“That's why we're still focused on soil sampling,” Niblack says.
To preserve the long-term soybean productivity of your land, start growing resistant varieties as soon as you discover SCN in a field, Tylka advises. Rotating crops and genetic sources of SCN resistance helps keep nematode numbers in check.