Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/20/1999
Publication Date: N/A
Citation: Timper, P., Riggs, R.D., Crippen, D.L. 1999. Parasitism of sedentary stages of Heterodera glycines by isolates of a sterile nematophagous fungus. Phytopathology. 89:1193-1199. Interpretive Summary: The soybean cyst nematode (SCN) is one of the most damaging pests of soybean in the United States. The nematode is managed primarily by the use of resistant soybean cultivars and crop rotation. However, frequent planting of resistant cultivars has lead to the development of nematode populations that are damaging even to resistant cultivars. Furthermore, many growers are not willing or able to rotate out of soybeans for the three years necessary to suppress numbers of SCN to nondamaging levels. An effective biocontrol agent would provide growers with another tactic for managing SCN. ARF is a fungus that parasitizes several stages of this nematode. We studied different isolates of this fungus to determine why some are more effective than others in reducing nematode numbers. The most effective isolates parasitized more SCN juveniles, females, and eggs on soybean plants than did the other isolates. Moreover, after applying the fungus to soil, the effective isolates formed larger colonies in the soil than did the ineffective isolates. We hypothesize that ARF isolates forming large colonies in soil will have a greater opportunity for encountering soybean roots infected with SCN than those forming small colonies. When a colony comes in contact with SCN, it may parasitize the nematode and prevent it from reproducing. With our greater understanding of what makes an isolate of ARF effective, we can look for those traits in other potential biological control agents of SCN.
Technical Abstract: Isolates of a sterile fungus designated ARF can be separated into two groups, ARF-C and -L, which differ morphologically, physiologically, and in their ability to suppress numbers of Heterodera glycines on soybean. Our objectives were to determine whether the level of nematode suppression by an isolate was related to parasitism of juveniles, females, and eggs in the rhizosphere of soybean, saprophytic growth in soil, or production of a red pigment. The experiments were conducted in a greenhouse using soil infested with homogenized ARF mycelium. The ARF-L isolates parasitized more juveniles and young females than did the ARF-C isolates. Suppression of these stages was 67% for ARF-L and 12% for ARF-C isolates 14 days after nematode inoculation. When soybean plants containing gravid females were transplanted into fungus-infested soil, ARF-L isolates parasitized 55 to 98% of nematode eggs whereas ARF-C isolates parasitized 0 to 22%. The biomass of mycelial mats in soil, an indication of saprophytic growth, tended to be greater for the ARF-L than the ARF-C isolates. When u.v.-irradiated cultures, either lacking or over-producing a red pigment, were compared with the parent culture and a no fungus control, all of the altered cultures had lost their ability to suppress nematode numbers on soybean.