Ergosterol biosynthesis inhibitors (EBIs) have a remarkably broad spectrum of antifungal activity. They belong to the commercial fungicides which exhibit the highest activity known to date. Resistance to EBIs was found in vitro, but the level of resistance and the decreased fitness of resistant strains led to the hypothesis that resistance to EBIs was rather unlikely to develop in practice. Despite the fact that EBIs were introduced as early as 1972, up to 1980 only data on EBI-resistant laboratory strains were available. However, gradually there appeared reports on decreased sensitivity to EBIs in practice. This made it of interest to investigate the potential development of resistance to EBIs in vivo.In this study the pathogen used to gather the desired information was Sphaerotheca fuliginea, as this fungus is widespread in cucumber glasshouses in the Netherlands, intensively controlled with fungicides and resistance-prone. This thesis contains five papers describing several aspects of resistance to fungicides in S.fuliginea.In the first paper it has been shown that S.fuliginea is present on cucumber plants all year round. A gradual increase in the number of infected crops, in the form of S-shaped curves, was observed from planting until May. In the districts with the highest crop density (Pijnacker), mildew was generally observed early in the growing season, while the apparent rate of increase of infected crops was higher than in other districts. In the district with the lowest crop density (Northern Netherlands), mildew was observed late in the growing season and the apparent rate of increase of infected crops was low. Early in the growing season the pathogen is probably dispersed by transportation of infected planting stock and by man. When the disease pressure increases, inoculum is probably dispersed by wind. This implies that inoculum is highly mobile and that, in the case of development of resistance to fungicides, the resistant strains will disperse to crops where the spray regime did not cause ...