Most Iowa soybean growers are aware of the economic threat posed by the soybean cyst nematode. Tremendous efforts have been made to develop and evaluate strategies to manage SCN and to provide Iowa growers with research-based information to maximize soybean yields and profits in fields infested with the pesky pest. "Knowing which of your fields are infested with SCN and growing high-yielding SCN-resistant soybean varieties that provide good control of the nematode are great first steps towards managing SCN," says Greg Tylka, an Iowa State University Extension plant pathologist and nematode expert. "But the battle doesn't stop there." SCN has the ability to slowly adapt to, or build up the ability to reproduce on, SCN-resistant soybean varieties, he explains. The buildup takes several years, not a single season, and it happens when soybean varieties with the same source of SCN resistance (the sources of resistance are Peking, PI 88788, Hartwig or PI 437654) are grown over and over again in the same field. Eventually, SCN egg population densities (numbers) start to increase or "creep" up, and the higher the egg number, the greater the chance of yield loss.
How would you know if SCN numbers were creeping up?
How would a person know if SCN egg numbers were creeping up in a field? "By comparing the results of soil samples collected this fall to results from samples collected several years ago," says Tylka. "If you don't have soil sample SCN egg numbers from several years ago with which to compare, then now's the time to get good numbers to use in comparisons in future years." He offers the following guidelines for collecting soil samples after harvest, so you can submit them to a soil testing lab to have an SCN analysis run.
Guidelines for collecting SCN soil samples after harvest:
• Collect 15 to 20 soil cores from no more than 20 acres (ideally); the accuracy of soil sample results increases as the number of soil cores increases and/or the area represented in a sample decreases.
• Follow a zig-zag pattern across the field when collecting soil cores.
• Soil cores should be 6- to 8-inches deep.
• Collect soil cores from the root zone of the soybean row. (Remember to do this when you collect soil cores in future years, too.)
• Mix the soil cores well, then fill a soil sample bag and send to a laboratory for SCN egg counts. The ISU Plant and Insect Diagnostic Clinic analyzes soil samples for SCN. The mailing address of the Diagnostic Clinic is 327 Bessey Hall, Department of Plant Pathology, ISU, Ames, IA 50011-1020.