Many Kentucky producers are seeing losses in their soybean fields as a result of a pest they thought they had under control.

Soybean cyst nematode, SCN, a microscopic roundworm that feeds on soybean roots and robs the plants of nutrients, is again being found in large numbers in many fields where varieties thought to be resistant to the pest were planted. This phenomenon is happening in a number of other soybean-producing states as well.

Data collected thus far in Kentucky fields shows a troubling trend and indicates that many farmers who think they are successfully managing SCN are, in fact, not, said Don Hershman, plant pathologist with the University of Kentucky College of Agriculture.

“I was afraid this was the case, and now we have some data to back this up,” he said. “Similar data have been collected from Missouri, Illinois and Indiana. Populations are apparently shifting due to the overuse of resistant varieties which have the same source of SCN resistance in their background.”

Simply looking at a stand of soybeans will not indicate there is a problem with nematodes. Serious yield loss caused by the pest rarely is associated with obvious disease symptoms in Kentucky. This results in yield loss often being blamed on other causes, especially if farmers are using resistant varieties. Producers may never notice consistent, moderate damage since they may obtain adequate yields.

“I’ve been working on cyst nematode my entire career, and because this disease doesn’t usually cause symptoms you’ve got to wait for a teachable moment,” Hershman said. “In this case, the teachable moment is that these other states are finding out that all is not well. My particular interest is that Kentucky farmers require and should have their own information. So we are doing the testing to confirm that we’ve got the same thing that they’ve got going on in Missouri, Illinois and other states in our region.”

The Kentucky Soybean Promotion Board is funding a free SCN sampling program for 2006-2007. As part of this program, 45 soil samples were collected from seven counties this spring. Twenty-one samples had SCN populations above the damage threshold, Hershman said. 

“This is noteworthy because most of these fields are in a corn/soybean rotation and were in corn during 2005,” he said. “We have data that shows how corn rotation can lower SCN in a field.”

Eleven populations from Ballard, Carlisle, Lyon and Hopkins counties were sent to the University of Missouri Nematode Lab for HG testing (formerly called race tests). There were three different races found but none were Race 3, which historically has been the predominant race in Kentucky. All 11 SCN populations were successfully able to reproduce on the primary source of SCN resistance used in Kentucky and the Midwest, Hershman said. 

Hershman, with the help of county Extension agents, will be sampling fields again this fall. The samples will come from fields where soybeans were grown in 2005 and where corn was grown in 2006.

“We are generating data to prove there is a problem,” he said. “Farmers have a false sense of security in that they look at the seed tag; it says resistant to SCN and they move on because they’ve got a ton of other things to worry about, but that’s not good enough.”

In Kentucky, most available varieties contain resistance races 3 and 14, said Chad Lee, UK Extension grain crops specialist. However, there are very few varieties available in Kentucky with resistance to the races found in the latest round of testing. Resistance information usually is printed on the tag attached to each seed bag.

Within a few years, Kentucky farmers likely will have more resistance options available to them, but for now the best option to combat the problem is crop rotation, Lee said. 

Information garnered through UK and other universities will help seed companies in their variety development.

Hershman said growing another crop besides soybeans in a field may need to be done for more than a single year to significantly lower SCN levels in problem fields. 

Farmers who want to have their field checked, can contact their county Extension office.