Numbers of colonies and spores of powdery mildew formed by Erysiphe graminis f.sp.hordei on 2 cm x 5 cm barley leaf segments. Seedlings were inoculated either with an avirulent, inducer isolate then a virulent isolate, or with the virulent isolate only. Three sets of four near-isogenic lines, with different race-specific genes, were used. Values are the number of colonies per leaf segment (left) or number of spores per lesion (right) (from Martinelli et al. 1993).