Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2001
Publication Date: N/A
Interpretive Summary: Sclerotinia stem rot (SSR) of soybeans is a major disease in the north central regions of the United States. There have been severe outbreaks in 1992, 1994, and 1996. One potential control of this disease is through the use of fungicides. The objectives of this study were to i) investigate the effect of different concentrations of several fungicides on fungal mycelial growth and evaluate several fungicides for control of the fungus on soybean plants. Azoxystrobin, benomyl, tebuconazole, thiophanate methyl, and vinclozolin were tested. Vinclozolin was the most effective in controlling S. sclerotiorum by inhibiting growth. Greenhouse grown seedlings treated with benomyl, thiophanate methyl, and vinclozolin prevented S. sclerotiorum from colonizing leaf tissue. This information is useful to growers, seed producers, and the chemical industry.
Technical Abstract: Sclerotinia stem rot (SSR) of soybeans, caused by Sclerotinia sclerotiorum, is a major disease in north central growing regions of the United States. One approach to control SSR on soybeans is with the use of fungicides. Azoxystrobin, benomyl, tebuconazole, thiophanate methyl, and vinclozolin were tested to control S. sclerotiorum in pure cultures on agar medium, inoculated soybean seedlings, detached inoculated leaves, and in the field. To evaluate the efficacy of five foliar fungicides in vitro, potato dextrose agar (PDA) was amended with different concentrations of fungicide and fungal radial growth in culture was measured. Vinclozolin was the most effective in controlling S. sclerotiorum by inhibiting mycelial growth at 1.0 ug a.i./ml. Mean mycelial growth of 91 isolates of S. sclerotiorum in the thiophanate methyl and vinclozolin amended agar was reduced 18-93% and 93-99%, respectively. Greenhouse grown seedlings treated with benomyl, thiophanate methyl, and vinclozolin prevented S. sclerotiorum from colonizing leaf tissue. Azoxystrobin and tebuconazole significantly (P<0.01) reduced disease severity ratings, but did not provide complete control. Detached leaves sprayed with thiophanate methyl and then inoculated with mycelial plugs of S. sclerotiorum did not become infected, and the area under disease progress curve was significantly less (P<0.01) than the nontreated control for leaves treated with azoxystrobin, benomyl, and tebuconazole. In field plots in 1997 and 1999, all plots had less than 1% disease, while in 1998, all fungicide treated plots had similar levels of disease as the nontreated control.