NEMATODES IN MICHIGAN .1. DISTRIBUTION OF HETERODERA-GLYCINES AND OTHER PLANT-PARASITIC NEMATODES IN SOYBEAN

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