Effects of fluctuating soil temperature and water potential on sclerotia germination and apothecial production of Sclerotinia sclerotiorum

The effects of fluctuating soil temperature and water potential on sclerotial germination and apothecial production by Sclerotinia sclerotiorum were investigated in growth chamber experiments. In the temperature experiments, temperature fluctuations of 4, 8, 12, and 16 degrees C around a median of 20 degrees C, and a constant of 20 degrees C, were tested. Daily temperature fluctuations of 8 degrees C resulted in highest levels of sclerotial germination and apothecial production. The earliest appearance of apothecia occurred in the 8 degrees C fluctuation treatment, 24 days after the start of the experiment. Sclerotia in the 12 degrees C fluctuation treatment germinated last; its first sclerotium germinated 44 days after experiment initiation. For the soil water potential experiments, constant saturation (approximately -0.001 MPa) and three levels of soil water potential fluctuation from saturation - '' low '' (-0.03 to -0.04 MPa), '' medium '' (-0.06 to -0.07 MPa), and '' high '' (-0.09 to -0. 1 MPa) - were tested. Constant saturation yielded the highest number of germinated sclerotia and apothecia. All soil water potential fluctuations were detrimental to sclerotial germination and apothecial production, with sclerotial germination under fluctuating moisture conditions less than a tenth of that occurring under constant saturation. The first sclerotium in the constant saturation treatment germinated in 35 days,- however, 76 days were required in the high soil water potential fluctuation treatment.