Using a leaf disk assay, eight cultivars or/and breeding lines in Cornus florida L., C. kousa (Buerger ex Miq.) Hance, five cultivars in C. kousa × C. florida, one cultivar in C. kousa × C. nuttallii Aud. and one cultivar in (C. kousa × C. nuttallii) × C. kousa were evaluated for resistance to powdery mildew [Erysiphe pulchra (Cooke and Peck) U. Braun & S. Takamatsu (syn. Microsphaera pulchra Cooke and Peck)]. Flowering dogwoods (C. florida) cultivars were more susceptible than the other species and hybrids with the exception of the C. florida × C. kousa hybrid ‘Ruth Ellen’. Resistance in kousa dogwoods and hybrids was manifested as restriction of hyphal growth and inhibition of sporulation. Although mildew colonies and sporulation were detected on leaves of all flowering dogwood cultivars, ‘Karen's Appalachian Blush’ and ‘Worlds Fair’ were more resistant than other cultivars. These dogwoods had significantly lower values of percent geminated conidia with branched hyphae, infection efficiency, sporulation and delayed latent period. The leaf disk assay provides a laboratory procedure to screen new cultivars and lines of dogwoods for resistance to powdery mildew.