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As a basis for a prognosis model for the protection of sweet cherries against the cherry leaf scorch, caused by Gnomonia erythrostoma, this research investigates the formation and propagation of the spores of the disease between 1998 and 2001. G. erythrostoma forms perithecia with ascospores as sexual fruiting bodies and pycnidia with conidia as asexual fruiting bodies. It was observed that the beginning of the spore ejection differed considerably between the years concerned; in 1998 the first ascospores, which were released from perithecia formed in leaves during the previous year, were detected at the end of April, in 1999 at the end of March and in 2000 at the middle of March. The duration of spore maturation depends on the temperatures between autumn and spring. The higher the temperatures the faster the maturation and the earlier the spore dispersal. The ejection of spores takes place in the spring, only under the condition that the dead and dried leaves from the year before are thoroughly wet. The perithecia-covered leaves remain on the tree throughout the winter until summer, which makes the disease very distinct. These conditions for ejection occur usually only after sufficient precipitation or after nights with extremely high humidity (≥ 90%). The end of the spore dispersal depends on the frequency of precipitation and the amount of the inoculum. Regular rainfall results in short spore dispersal, longer periods without precipitation lead to a longer spore propagation. Two conditions have to be met for the infection to occur: first, the leaves have to be developed completely or the fruit has to be visible; and second, the leaves have to be thoroughly wet for at least six hours for the spores to germinate and successfully infect the leaves or fruits. An infection with conidia can be excluded due to their lack of germination ability.