Effects of tillage and crop sequence on parasitism of Heterodera glycines juveniles by Hirsutella SPP. and on juvenile population density

The effect of tillage on parasitism of Heterodera glycines second-stage juvenile (J2) by the nematophagous fungi Hirsutella rhossiliensis and/or Hirsutella minnesotensis and on J2 population density was investigated at four sites in soybean-corn rotation and one site in soybean monoculture in Minnesota. Soil samples were taken at planting, in midseason, and at harvest in 2002, at planting, 1 and 2 months after planting, and at harvest in 2003, and 2 weeks and 2 months after planting, and at harvest in 2004. Heterodera glycines J2 were extracted, and the number and percentage of J2 parasitized by Hirsutella were determined. No significant differences in die percentages of parasitized J2 were observed between conventional tillage (CT) and no-tillage (NT) at all sampling dates, except that the percentage of J2 parasitized in NT was significantly higher than in CT during the soybean-growing season in 2003 at one site in the soybean-corn annual rotation. The percentage of J2 parasitized by Hirsutella was higher in soybean fields than in corn fields, and the percentage of J2 parasitized in midseason was generally higher than that at planting and harvest. Growing a resistant soybean cultivar in the preceding year reduced the percentage of J2 parasitized by Hirsutella in the following season. Conventional tillage increased the mid-season J2 population density during 2002-2004 soybean-growing seasons as compared with NT at two of the five sites.