Nematodes in Michigan. I. Distribution of Heterodera glycines and Other Plant-parasitic Nematodes in Soybean.

In 1992, a detection survey for Heterodera glycines (soybean cyst nematode) was conducted in 16 counties in Michigan. The nematode was detected in 12 counties, with absolute frequencies ranging from 6 to 100%. A total of 149 samples was collected, and 53% were infested with H. glycines. Eighty-four growers participated in the survey, and 38 had samples collected from more than one field. Of the 38 growers, 42% had all samples positive for H. glycines, 18% had some positive and negative fields, and 39% had all negative. A risk index was developed to quantify three types of risks: short-term, long-term, and border risk from neighboring counties. Soybean yield was regressed on H. glycines population density and number of years of soybean. Thirty-one percent of the variability in soybean yields was explained by H. glycines cyst population densities. Total number of years in soybean over the last 20 year period explained 19% of the variability in yields. In addition, H. glycines frequencies and population densities were inversely related to Pratylenchus spp. frequencies and population densities.