Effects of temperature and pentachloronitrobenzene on vegetative growth and development of Sclerotinia sclerotiorum (Lib.) de Bary were studied using Newton's synthetic liquid medium. Microconidial growth increased until the eighth day of incubation at 24.4° C., after which maximum dry weights were obtained. . Growth and development of mycelia and sclerotia was most rapid at 24° C., although a similar, but slightly less response was obtained at 20° C. On a dry weight basis, inhibition of mycelial growth and. development of S. sclerotiorum by PCNB was greater in shake culture than stationary culture at room temperature. PCNB was fungistatic at the concentrations used. Pathogenicity of soil-borne inoculum of S. sclerotiorum on Penn Lake lettuce seeds was affected by temperature, inoculum density, PCNB and a Trichoderma sp. The number of emerged seedlings decreased with increasing concentration of soil inoculum and decreasing soil temperature. PCNB applied to U.C. Soil Mix C as a drench treatment not only failed to control disease caused by S. sclerotiorum, but also following use of the fungicide, disease increased appreciably at all concentrations used. Trichoderma sp. reduced disease only at 15° C. and was ineffective at lower soil temperatures. Decreases in height of lettuce seedlings were observed with increasing inoculum density of S. sclerotiorum. / Land and Food Systems, Faculty of / Graduate