To develop a quantitative evaluation of grapevine resistance to powdery mildew and of pathogenicity of the causal agent (*Erysiphe necator*), a spot inoculation method was developed using detached leaves of potted plants. The percentage of inoculating spots leading to a colony and the mean diameter of colonies were determined to assess the host-pathogen relationships. Significant differences were found between host cultivars and their ranking was associated with that observed in the vineyard. There was a significant interaction between cultivar and replicate during the whole experiment indicating that the physiological state of detached leaves is important. Aging of tissues was accompanied by a gain in resistance that was considerably more marked in resistant cultivars. Only partly expanded leaves of resistant cultivars that stopped expansion on the agar medium supported the development of the fungus. Significant differences between *E. necator* isolates were also demonstrated, but these variations were less marked than those due to host cultivar and leaf position. Preliminary results obtained with different isolates from the two European genetic groups (A and B) indicate that, on average, group A is less pathogenic