Parasitism of the nematode Heterodera glycines by the fungus Hirsutella rhossiliensis as influenced by crop sequence

The effect of crop sequence on parasitism of second-stage juveniles (J2) of Heterodera glycines by Hirsutella rhossiliensis was investigated. Data wer e collected from plots of a long-term crop rotation experiment established in 1982. Crop sequences included (i) continuous monoculture of corn and soy bean; (ii) annual rotation of the two crops; and (iii) 1, 2, 3, 4, or 5 yea rs of each crop following 5 years of the other crop. The nematode J2 densit y and percentage of J2 parasitized by the fungus were determined at plantin g, midseason, and end of season in 1997 and 1998. A significant effect of t he crop sequence on parasitism of J2 was observed at midseason in both year s and at end of season in 1998. In plots of first-year soybean following 5 years of corn, fungal parasitism increased from an undetectable level at pl anting to 2% and 4% of J2 parasitized by ends of season in 1997 and 1998, r espectively. Fungal parasitism was similar in plots of second-through-fifth -year soybean after 5 years of corn and in plots of soybean monoculture. Pa rasitism of J2 in the soybean plots in annual rotation with corn increased from undetectable and 2% at planting to 6% and 23% at midseason in 1997 and 1998, respectively The effect of crop sequence on the fungal parasitism of J2 may be attributed to a density-dependent relationship between the paras ite and its host. Season also affected the fungal parasitism; percentage of J2 parasitized by the fungus was the highest at midseason and the lowest a t planting.