THRESHOLD VALUES FOR CHEMICAL CONTROL OF POWDERY MILDEW (ERYSIPHE CRUCIFERARUM) ON BRUSSELS-SPROUTS

To establish control thresholds for chemical control of powdery mildew (Erysiphe cruciferarum) on Brussels sprouts, mildew intensity on leav es and buds was observed on the cultivars Lunet, Tardis and Asgard dur ing three years in unsprayed plots. Mildew infection on the leaves was observed from late August onwards, increasing to moderate or high lev els. In one year light infestation of the buds was observed, but no re duction in quality occurred. These preliminary results indicate, that from late August onwards the following levels of leaf injury by powder y mildew can be tolerated: T = 5 + 0.42(Julian date - 235), in which T is the tolerable leaf injury in percentage leaf area covered. When s ampling the crop to assess powdery mildew infection, care must be take n that leaves are sampled from all stem positions, as top leaves tend to be much less infected.