Technical Abstract: Powdery mildew is an important foliar disease of hydrangeas. For developing efficient integrated disease management strategies involving the utilization of resistant cultivars, it is essential to understand how the powdery mildew fungus infects hydrangea plants and how the host resists the parasitism by the fungus. This paper describes the infection process of Erysiphe polygoni and investigated the effects of hydrangea susceptibility to powdery mildew development using a leaf disk assay. Results of these experiments could be used to develop laboratory screening methods for evaluating hydrangea resistance to powdery mildew.