Soybean producers who have unexplained yield losses shouldn't dismiss soybean cyst nematode as the possible culprit, even if they're planting SCN-resistant varieties, a University of Missouri researcher said.

"Many farmers think the SCN problem is licked (with resistant soybeans)," said Bob Heinz, coordinator of the MU Nematology Laboratory. "I would think it's licked, too, if I didn't see the high samples come in."

During the 2005 season, Heinz, in cooperation with producers and assistance from MU Extension regional agronomists, collected 122 soil samples from 47 Missouri counties and tested them for soybean cyst nematode, a parasitic roundworm that feeds on the roots of soybeans and can cause up to 30 percent yield loss.

Participating producers also were asked about their use of SCN-resistant soybean varieties and their perceptions about the impact of SCN on their operations.

Analysis revealed that while 61 percent of the producers' samples contained SCN egg levels that exceeded the economic damage threshold, 62 percent of the producers did not believe they had any yield loss attributable to SCN.

"We asked how long they had been planting resistant soybean varieties, and we found that 85 to 92 percent of producers currently plant SCN-resistant beans," Heinz said. "Producers are losing yield, but they're not attributing the loss to SCN because they are growing resistant beans and don't think they have a problem."

Heinz explained that just as bacteria can become tolerant of antibiotics and weeds can become tolerant to herbicides, soybean cyst nematodes can overcome the defenses of a once-resistant soybean variety.

"Around 90 percent of all SCN-resistant beans in Missouri derive their resistance from the same source," he said. "By only having one source of resistance, you're just asking for trouble because it could eventually break down. A farmer thinks he's growing resistant beans, but really he's developing a population of nematodes that may grow well on his resistant line."

Heinz said scientists are uncertain why resistant lines seem to remain effective for one producer, while the same resistance is overcome by nematodes in another producer's fields.

"It'd be nice to make generalizations for everyone to use, but we don't fully understand all the genetic and environmental variables in the different field populations," he said. "The best action producers can take is to monitor their fields by sending in a soil sample for an egg count at least once every three years."

Heinz added that researchers such as MU plant pathologist Melissa Mitchum are studying how soybean cyst nematodes break resistance and are seeking new strategies for resistance through biotechnology.

Of those producers participating in the 2005 survey, 64 percent had never submitted a sample for an SCN egg count test. Only 6 percent of producers had submitted a sample within the past five years. "If you have a problem field, a $15 SCN egg count can buy a lot of peace of mind," Heinz said.

He recommended sampling fields that have slipped in yield or have had a history of high egg counts. Proper sampling technique will insure an accurate count.

For those who do find high egg counts in fields where resistant lines have been grown for years, the lab also offers an HG Type race test that indicates the race of SCN in the field, Heinz said. Producers also can include up to two soybean varieties with the test, providing them with information on how those varieties fare against the SCN types in their fields and helping them choose a soybean variety with a different source of resistance.

"Races can shift and decrease the effectiveness of resistant soybeans, but the process of races shifting and the breakdown of resistance may vary greatly from one farm to another," he said. "If a field is not yielding up to par, a farmer's first step should be to get an SCN egg count test. If the test reveals a problem, then a HG Type race test may be needed. However, unless a field has a high egg count, the SCN race isn't very important."

More nematology lab information including submission forms, sampling techniques and fee structures can be found online at http://soilplantlab.missouri.edu. Submission forms also are available at local extension offices throughout the state, he said.