Parasitism of Heterodera glycines by Hirsutella spp. in Minnesota Soybean Fields

Hirsutella rhossiliensis is a parasite of vermiform nematodes. In 1995, parasitism of the soybean cyst nematode second-stage juveniles (J2s) by this fungus was observed in a Minnesota field. A subsequent survey was conducted in 1996–1997 to determine the extent of infestation of soybean fields by this fungus in southern Minnesota. A total of 264 soil samples representing 237 fields in 27 counties was examined in three tests. In Test 1, 1000 J2s were added to 50 g of soil at days 0, 7, 14, and 21, and the nematodes were recovered at day 28. In Test 2, 2000 J2s were added to 50 cm3 of soil, and the nematodes were recovered at day 14. In Test 3, J2s were extracted from 50 cm3 of soil without addition of nematodes. The J2s were examined for fungal parasitism. Two species of Hirsutella, H. rhossiliensis and H. minnesotensis, were commonly encountered on the J2s. Combining data from all three tests, parasitism of J2s by Hirsutella species was observed in soil from 46% of the fields. Parasitism of J2s by H. rhossiliensis was observed in soil from 43% of fields and by H. minnesotensis in soil from 14% of fields. The highest percentage of J2s parasitized by Hirsutella spp. was 60%. Addition of J2s into soil increased efficiency of fungal detection.