Sclerotinia sclerotiorum (Lib.) de Bary is a facultative parasite to which the cultivated sunflower is highly sensitive. The fungus attacks the root, stem and leaves, head discs and seeds, causing root rot, stem rot and head rot forms, respectively. According to the survey, the root rot and head rot were the major forms in China.In Jilin Province, the root rot was more serious, in Nong An county (in 37 fields) the incidence was between 0 and 17.5% over 5 years, i.e., the average incidence was 5.13%, which was related to the amount of sclerotia in seeds and soils. For economic purpose of controling the diseases, forecasting is important. This paper reports the results of studing the root rot forcast and its application.