Mycofloras in cysts, females, and eggs of the soybean cyst nematode in Minnesota

During 1996-1997, mycofloras in cysts, females, and eggs of the soybean cyst nematode (SCN), Heterodera glycines, were investigated in 45 fields from 26 counties across southern Minnesota, USA. A total of 4500 cysts, 4500 females, and 45,000 eggs were examined, and 62 fungal species were isolated and identified from the nematode. Fungi colonized 55% (29-89%) of cysts with Cylindrocarpon destructans, Fusarium solani, Pyrenochaeta terrestris, Fusarium oxysporum and Cylindrocarpon olidum the most common. The frequency of fungi encountered in cysts was positively correlated with egg density in fields. Only 3.4% of females were colonized by fungi with Oidiodendron cerealis, F. solani, F. oxysporum, C. destructans, and P. terrestris the most common. Only 1% of eggs yielded fungi with C. destructans, F. solani, P. terrestris, Exophiala pisciphila, and F. oxysporum the most common. Average fungal egg-parasitic index (EPI at a scale from 0 to 10 to measure fungal parasitism of eggs) in the fungus-colonized cysts was 2.0, and average EPI in females was 1.8. The EPI was positively correlated with percentage of cysts colonized by fungi and egg density in the fields. The Simpson's diversity index (at a scale from 0 to 1) was low (average 0.25), indicating that fungal community in the cysts was highly diversified and no single species was dominant. Average similarity index (at a scale from 0 to 1) for mycofloras in cysts between fields in Minnesota was 0.53 (range 0.13-0.85). The study demonstrated that the mycofloras in the cysts and females from the Minnesota fields might have limited antagonistic effect on the SCN population.