Fungal parasitism of soybean cyst nematode, Heterodera glycines (Nemata: Heteroderidae), in differing cropping-tillage regimes

Abstract

A 2 year field study was designed to determine if soybean-wheat double-cropping and reduced or no tillage altered the soil mycoflora antagonistic to the soybean cyst nematode, Heterodera glycines. Six treatments were sampled monthly: disc 10 cm deep, followed by roller harrow; chisel plow 20 cm deep, followed by disc and roller harrow; moldboard plow 15 cm deep, followed by disc and roller harrow; non-tillage in the previous year's soybean stubble (NT-Soy); non-tillage in standing wheat cover crop (NT-1); non-tillage after wheat cover crop (NT-7). Each treatment had been applied to the same plots for up to 7 years. At appropriate monthly intervals, nematode cysts, females, and eggs were collected and assayed for the presence of fungi, which were identified to species-level taxa. Effects of cropping-tillage regimes on numbers of infective juveniles, numbers of eggs within cysts, and vertical distribution of cysts were determined. Similarities of egg and female mycofloras in this study to those of previous studies were examined. A total of 61 species-level fungal taxa were identified, along with 20 non-sporulating isolates distinguished from one another by appearance, growth pattern, and color. Fusarium solani was the most frequent of 47 species infesting cysts. Fusarium solani and F. oxysporum were the most frequent of 20 taxa isolated from field-collected females; in females produced on soybean plants grown in field soil in the greenhouse, F. oxysporum was the most frequent of 21 taxa. Paecilomyces lilacinus was the most common fungus among the 38 species isolated from eggs. For all fungi, percentages of parasitized females and eggs were similar in all treatments, but some treatments affected the parasitism rates of Paecilomyces lilacinus and Verticillium chlamydosporium. Parasitism of females was relatively low in July and August, but tripled in September. Egg parasitism was generally below 10% regardless of sampling time. Vertical distribution of cysts had no discernible effect on sampling precision, since cysts were concentrated in the top 25 cm in two dissimilar treatments (moldboard plow, NT-7). No consistent treatment effects were seen on densities of soil-borne infective juveniles in winter or spring, nor on average numbers of eggs per cyst. Based on similarity indices among this study and four previous studies, a biogeography of H. glycines mycofloras is proposed.