Powdery mildew has become a common foliar disease of Cornus florida and other dogwood species in the eastern United States during the last several years. This study was conducted to determine the identity of powdery mildew fungi on C. florida and C. amomum. Ascocarps of Microsphaera pulchra and Phyllactinia guttata occurred singly and together on both C. florida and C. amomum leaves. M. pulchra ascocarps occurred at a higher density than P. guttata ascocarps on C. florida leaves, whereas P. guttata ascocarps occurred more frequently than M. pulchra ascocarps on C. amomum leaves. Histological studies, however, did not provide supplementary data of infection by the powdery mildew species that occurred less frequently on the leaves of each dogwood species. M. pulchra did not penetrate the cells of C. amomum, and likewise P. guttata did not enter through stomata of C. florida leaves. The presence of ascocarps of both species was not the result of infection of the dogwoods by both pathogens. The ascocarps of M. pulchra probably became airborne and then settled on the C. amomum leaves. Similarly, the ascocarps of P. guttata landed on C. florida leaves. These results emphasize the importance of correct pathogen identification using several criteria such as ascocarp morphology, host-pathogen relationships, distribution of the pathogen, conidial morphology, and histology.
Keywords: scanning electron microscopy.