Soybean cyst nematode (SCN) is a widespread and serious pest of soybeans in Iowa. But the nematode often does not cause obvious aboveground symptoms, at least not until population densities become extremely high.
One fairly consistent, albeit somewhat indirect, symptom of SCN parasitism that is apparent at this time of the year is early senescence of the soybeans. The early senescence of soybean caused by SCN is illustrated in the diagram on page 227. The squares in the map in the diagram represent 3-foot by 3-foot (9 ft2) square areas of the field. Three 1-inch diameter, 8-inch deep soil cores were taken from each square in May.
The aerial photograph shows the sampled area on September 10. The map of egg population densities has an "M"-shaped pattern of higher egg counts that corresponds fairly well with the "M"-shaped pattern of maturing plants in the September 10 aerial photograph.
Growers and agronomists should consider checking for the presence of SCN in fields that have areas that mature earlier with no apparent reason. To check fields in the fall for the presence of SCN, soil samples should be collected and sent to a qualified laboratory for analysis. Soil samples should be comprised of well-mixed soil obtained from soil cores collected from 15 to 20 different locations within an area of 20 acres or so. Each core should be collected from a total depth of 6 to 8 inches. Large fields should be divided up into smaller areas from which a 15- to 20-core soil sample should be collected.
Soil samples can be tested for SCN by the Iowa State University Plant Disease Clinic. Samples should be accompanied by a completed Plant Nematode Sample Submission Form (ISU Extension publication PD 32). Currently, there is a $15 per sample charge for processing each sample from Iowa, $20 per sample for out-of-state samples.
This article originally appeared on pages 226-227 of the IC-496(24) -- September 18, 2006 issue.