Development of sclerotia and apothecia of Sclerotinia sclerotiorum from infected soybean seed and its control by fungicide seed treatment

Field and laboratory studies were done to evaluate the development of sclerotia and apothecia of Sclerotinia sclerotiorum from soybeans and its control with fungicide seed treatment. Soybean seed infected with S. sclerotiorum produced mycelia on both seed coats and cotyledons after 48 h on potato dextrose agar (PDA). Obviously infected soybean seed also were placed in aluminum pans containing field soil and placed in soybean fields near Urbana, Illinois and Clinton, Wisconsin. 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 μg a.i./ml. S. sclerotiorum was recovered from 2% of soybean seed lots containing infected seed. When this seed lot was treated with several fungicides, captan + pentachloronitrobenzene + thiabendazole and fludioxonil completely inhibited mycelial growth from infected seed; thiram and thiabendazole each reduced recovery of S. sclerotiorum by 90%. In the field, 754 sclerotia and 10 apothecia were produced from 1,000 infected seeds over a two-year period. When evaluating fungicide control in the field, thiram, fludioxonil, and captan + pentachloronitrobenzene + thiabendazole reduced sclerotia formation from infected seed by more than 98%.