Many flowerbulb crops can be infested by fungi of the genus Botrytis. Especially the aerial parts of the plants are vulnerable to attack. Generally, specialized species of Botrytis are involved, e.g., Botrytis tulipae in tulips (Beamont et al., 1936; Price, 1970; Doornik and Bergman, 1974), B. elliptica in lilies (Ward, 1888; McLean and Shaw, 1949) and B. gladiolorum in gladioli (Timmermans, 1941).Under conditions of high humidity and moderate temperature conidia of Botrytis spp. germinate and enter plant tissue through stomata or by penetration of the cuticule (Coley-Smith et al, 1980). Often, infected leaves or stems at first develop small, dark-coloured to necrotic spots, that are supposed to be the result of a hypersensitive response of the invaded plant tissue. Dependent on various conditions, e.g., humidity and susceptibility of the plant genotype, some of these spots develop into expanding lesions (‘fire’), on which abundant spore formation can take place. The spores may be spread by wind, rain or other mechanical means and start a new infection cycle. Dependent on the time of initial appearance of blight symptoms and final disease severity, bulb growth is affected. Next to other cultural measures, e.g., adequate crop rotation and removal of plant debris, bulb growers spray fungicides on the crop weekly or fortnightly to prevent bulb yield losses caused by Botrytis spp. (Koster and Meer, van der, 1993). Such a fixed spraying schedule does not take into account the infection conditions and, therefore, needless sprays are often applied.In The Netherlands, a long-term policy was formulated that aims at a reduction of the use of chemical pesticides. Needless sprays should be avoided. Therefore, a Botrytis disease-forecasting model is being developed to predict the infection periods and to plan necessary crop sprays with appropriate fungicides.