Species of the genus Sclerotinia can function either as soil borne or airborne pathogens. Infections of above-ground plant parts result from ascosporic inoculum, whereas soil line infection may result either from ascospores or sclerotia. Below-ground infection, however, results from mycelial germination of soil borne sclerotia. Accordingly, the epidemiology of these two types of infections incited by Sclerotinia spp. is quite different and the effect of weather factors on their incidence and development differs considerably. Abawi and Grogan (1979) proposed that generally, the large sclerotia-type isolates of Sclerotinia (represented by S. sclerotiorum and S. trifoliorum) function primarily by producing apothecia and that mycelial germination from sclerotia contributes minimally, if at all, to the development of epidemics. In contrast, infections incited by the small sclerotial type isolates (S. minor) originate primarily through the eruptive mycelial germination of sclerotia, whereas production of apothecia under natural conditions occurs very rarely and thus is of minor importance. The different modes of infection exhibited by the large- and small sclerotia type isolates of Sclerotinia probably results from continued adaptation of these species to their ecological niches. Only a limited amount of detailed epidemiological information is available on a few important diseases. Furthermore, quantitative epidemiology data are essentially lacking (Abawi and Grogan, 1979). The information generated so far on epidemiology of different host-pathosystem is as follows.