Increased SCN Reproduction on Resistant Varieties Does Not Mean Failure

ISU plant pathologist says higher levels of SCN don't mean resistant varieties are failing.

Published on: May 11, 2011

When it comes to the management of the soybean cyst nematode, Iowa State University Plant Pathologist Greg Tylka says resistant soybean varieties are a great tool. Today soybean growers have access to hundreds of different varieties bred for SCN resistance. Not only do resistant soybeans typically keep nematode numbers in check, but Tylka says they also provide higher yields.

By scientific definition, resistant varieties are supposed to allow less than 10% reproduction of SCN, but according to Tylka there are reports of SCN populations reproducing on resistant varieties at higher levels. He says that doesn't necessarily mean resistant varieties are failing.

"Myself and other nematologists throughout the Midwest have been doing surveys to figure how much reproduction it does on the common source of resistance," Tylka said. "We are definitely seeing well above the standard amount of reproduction; 10% or less is what we would expect, but it's not uncommon to encounter SCN populations that reproduce to the order of 15%, or 20%, or 25% throughout the Midwest."

Tylka stresses that resistance is based on the amount of SCN that reproduces on a resistant variety, not on yield. In fact, he says yield is the silver lining of this problem.

"We conduct variety trials and we're still seeing very good, profitable soybean yields under the right conditions even in varieties that are having 15% or 20% reproduction of the nematodes," Tylka said. "So grower need to understand that yield is a function not only of the amount of nematodes reproducing on the resistant varieties, but also the general agronomics of those varieties, the growing conditions, the amount of moisture and so forth."

Before planting this season, there's not much growers can do. But once the crop is in the ground Tylka encourages growers to scout their fields for increased reproduction of SCN on resistant soybeans.

"Get out there and dig the roots and if a resistant soybean variety is growing in a field that has SCN, the grower shouldn't see many of the little white females on the roots," Tylka said. "If growers are digging roots and seeing dozens and dozens of little white SCN females that is probably a tipoff that that field's nematode population does have elevated reproduction on the source of resistance that is being grown."

At that point, Tylka says it's time to turn to a nematologist or plant pathologist at the local land-grant university to see about the need for an HG Type test or if there are other varieties the grower should look for.