The influence of different nutrition media (PDA and Carrot agar), temperature (15, 22 and 30 °C) and origin (mycelium/sclerotium) on growth and development of mycelium and sclerotium formation of Sclerotinia sclerotiorum was researched under laboratory conditions. Carrot, tomato and cucumber fruits were artificially infected with mycelium and sclerotium of S. sclerotiorum in order to evaluate the development of disease in daylight conditions and thermostat chamber. On average, regardless of the temperature and origin, the mycelium diameter was significantly higher on PDA (75.8 mm) than on Carrot agar (48.5 mm). The temperature had significant influence on mycelium development so the growth of mycelium was lower at 30 °C compared to 22 °C and 15 °C on both substrates. The difference in the development of mycelium from sclerotia and mycelium depended on the nutrition media and temperature. The development, number and size of sclerotium on PDA depended on temperature, where the temperature of 22 °C favored the development of a greater number of smaller, and the temperature of 15 °C a smaller number of larger sclerotium. The pathogenicity of the isolate in the artificial infection of carrot, tomato and cucumber fruits depended on storage conditions (thermostat chamber/daylight), origin (sclerotium/mycelium) and host. Carrot fruits completely collapsed after two weeks, and the most pronounced symptoms were observed when they were infected with mycelium and kept in the thermostat chamber. Nine days after infection the rot has spread to more than 50% of the tomato fruit surface and mycelium and sclerotium formation was observed. Cucumbers infected with mycelium collapsed regardless of the storage conditions, while the disease has not developed from sclerotium.