Why is a follow-up greenhouse test necessary for samples with low SCN egg counts?
At the ISU Plant Disease Clinic, a follow-up greenhouse test is performed on samples with less than 500 eggs per 100 cc of soil to confirm that the sample is truly infested with SCN. When a soil sample is tested for SCN, 100 cc of soil is processed but the amount of the sample that is observed with a microscope for the presence of SCN eggs is 1 percent of the sample. Consequently, an egg count of 200 means that two eggs were observed under the microscope. With very low egg counts, it is possible that an egg from a previous sample may have been carried over on the processing equipment. Also, it is possible that a few eggs of non-plant-parasitic nematodes may have been recovered from soil particles during processing. To double check that a sample with low egg counts is truly infested with SCN, a seedling of a SCN-susceptible soybean variety is transplanted into the soil remaining from the sample after the initial 100 of soil has been processed. The seedlings are grown under controlled te
