Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/1999
Publication Date: N/A
Interpretive Summary: Sclerotinia stem rot (SSR) caused by the fungus Sclerotinia sclerotiorum, is a newly emerged disease of soybeans grown in the north central region of the United States with severe outbreaks occurring in 1992, 1994, and 1996. Over long distances, the greatest potential of Sclerotinia spp. dissemination is by seeds. The objectives of this study were to determine if sclerotia, raisin-like shaped fungal bodies, can be produced from infected soybean and germinate to form their fruiting structures in the same season, and to determine if fungicide seed treatments can control sclerotia formation from infected seeds. When infected soybean seeds were placed in soil, a total of 553 sclerotia, 20 stems, and 10 fruiting structures were produced from 500 infected seeds. The fungicide, fludioxonil was the most effective fungicide for reducing radial growth of S. sclerotiorum in culture, while in seed lots containing infected seeds, captan + pentachloronitrobenzene + thiabendazole and fludioxonil completely inhibited mycelial growth. This report is signficant in that it shows that by planting infected seeds the fungus can multiply and have the potential to cause infection in soybean plants grown that same year. In addition, this report provides practical control measures of the seedborne infection using fungicide seed treatments. This information is useful to the growers, seed industry and chemical producers.
Technical Abstract: Seed coats were removed from soybean seeds infected with Sclerotinia sclerotiorum and the seed coats and cotyledons were surface sterilized with NaOCl (0.53%), rinsed in sterile distilled water, and placed on water agar. After 48 hours, mycelia of S. sclerotiorum were observed on both seed coats and cotyledons. Infected soybean seeds also were placed in aluminum pans containing field soil and placed in soybean fields near Urbana, IL and Clinton, WI. In 1997, a total of 553 sclerotia, 20 stipes, and 10 apothecia were produced from 500 infected seeds. In 1998, 201 sclerotia, and 22 stipes were produced, but no apothecia were observed from the 500 infected seeds. Fludioxonil was the most effective fungicide for reducing radial growth of S. sclerotiorum on PDA plates and suppressed 99% of the radial growth at 0.1 ug a.i./ml. When seed lots containing infected seeds were treated with several fungicides, captan + pentachloronitrobenzene + thiabendazole and fludioxonil completely inhibited mycelial growth from infected seeds; thiram and thiabendazole reduced infection by 90%. In a similar study in the field, thiram, fludioxonil, and captan + pentachloronitrobenzene + thiabendazole reduced sclerotia formation from infected seeds by more than 98%.