With this year's drought conditions, soybean cyst nematode (SCN) intensified the stress on soybeans in many fields. University of Illinois Plant Diagnostic Clinic and IPM coordinator Suzanne Bissonnette said the high levels of the pest can cause the yellow patches that growers may have seen in fields.
Although the symptoms were easy to see this year, Bissonnette warned that in more typical years, low to moderate levels of SCN may be present in a field without visible symptoms on the above-ground part of the plant. "To determine with certainty the SCN status of your soybean field, you must sample," she said.
Fall is an ideal time to do this. With overwinter survival of SCN at close to 100 percent, fall sampling provides a good estimation of the population that will be present at planting and gives time to prepare for variety selection for next year's soybeans.
Which fields should be tested?
"Easy." Bissonnete said, "All fields where soybean will be grown should be evaluated for SCN. This nematode is a survivor."
"Once present in a soybean field, SCN is nearly impossible to eliminate," she continued. "It survives even in the absence of a good host. As it is very difficult to reduce high SCN levels to a manageable population, SCN management should focus instead on keeping the SCN egg count low."
One sample should represent no more than 10 acres and comprise 20 to 30 subsamples, each taken at a depth of 8 to 10 inches. Mix the subsamples together in a bucket, remove enough soil to fill a 1-quart plastic bag, and mail the bag to a qualified laboratory for analysis. Results will be reported in either number of cysts, or in the number of eggs, per 100 cc of soil.
Once SCN is detected in a soybean field, the SCN egg count should be checked regularly to monitor the field population and assess management practices.
"The frequency of sampling will depend on the management protocol, the SCN egg count, and often, the type of SCN in the specific field population," Bissonnette said.
If consistent rotations with a non-host such as corn have been implemented and the SCN egg count has typically been low in the field, chances are the SCN level is well-managed, and sampling may be spaced to every third year that soybeans are grown. However, if soybeans are grown back-to-back, if SCN egg counts have been high, or if the SCN population is able to develop on resistant varieties, it may be necessary to sample more frequently.
Monitoring the SCN egg count is particularly important when soybeans, even resistant varieties, are consistently grown in an SCN-infested field. Often, a grower uses a soybean variety with resistance to SCN only to discover that the type of resistance was not effective against the specific field population, thus allowing the population to increase in that field. Moreover, this type of population growth causes the proportion of SCN that are capable of reproducing on resistant varieties to increase
"When SCN egg counts are moderate to high, and soybeans are to be grown, or if a resistant variety rotation does not seem to be managing the pest, then it's a good idea to perform a greenhouse assay called the 'SCN Type test' (formerly known as an SCN Race test)," Bissonnette advised.
This test evaluates the field population's development on the three sources of resistance found in varieties currently available to Illinois producers, thus providing information about the most effective type of resistance for a specific field. Variety selection can be narrowed to the varieties that will provide effective resistance and prevent increases in the SCN population. The actual soybean variety that will be or has been planted can be included in this test to assess its effectiveness.
Source: University of Illinois