To identify characteristics for the selection of Verticillium lecanii isolates with high potential for biocontrol of Sphaerotheca fuliginea under glasshouse conditions, an exploratory study was performed on the effect of water limitation on the development of 14 isolates. The conidial germination, growth and sporulation of isolates of V. lecanii were studied in a tritrophic system on cucumber leaves and in a ditrophic system in Petri dishes. Their mycoparasitic ability was studied in S. fuliginea and Cladosporium cladosporioides . All characteristics were clearly affected by humidity. Four isolates showed good biocontrol potential. The performance of isolates on agar had less predictive value than on powdery mildew. The germination of isolates of V. lecanii was lower and the mycelial growth faster on agar than on mildewed leaves under corresponding humidity conditions. The results suggest that conditions in the phyllosphere differed from the set humidity in the surrounding air. A correlation was found between the lysis of C. cladosporioides growing in dual culture on agar with isolates of V. lecanii and the parasitism of powdery mildew on detached, rooted leaves. C. cladosporioides might offer a suitable substrate for testing isolates of V. lecanii for mycoparasitic potential under various environmental conditions. Conidial germination, growth and sporulation had limited predictive value.