Scanning Electron Microscopy, Histological, and Tissue Culture Studies of Powdery Mildews Infecting Dogwoods

Powdery mildew has become a common and widespread foliar disease of dogwoods. Microsphaera pulchra Cooke and Peck, and Phyllactinia guttata (Wallr.: Fr.) Lev. are two causal agents of powdery mildew on dogwood species in eastern North America. A clear understanding of the host-pathogen relationship between these fungi and their host must be obtained to advance research in disease management and resistance breeding. A necessary step in studying a disease is to identify the causal agent. Leaves of C. florida \u27Cherokee Sunset\u27 and C. amomum (silky dogwood) were observed under a compound microscope to determine causal agents of powdery mildew based on ascocarp morphology. Ascocarps of two different species of powdery mildew, M. pulchra and P. guttata, occurred together on \u27Cherokee Brave\u27 and C. amomum leaves. Microsphaera pulchra ascocarps occurred at a higher density on \u27Cherokee Sunset\u27, whereas P. guttata ascocarps were more prevalent on C. amomum leaves. Scanning electron microscopy (SEM), paraffin based histology, and low magnification light microscopy revealed that the ascocarps in lesser density may have landed on the leaves after dissemination and likely were not formed on the leaves. Both flowering dogwood and silky dogwood appeared to have only one powdery mildew pathogen. A reliable inoculation procedure is necessary to study powdery mildew disease processes. Three inoculation techniques were compared to determine the most efficient method to transfer conidia from diseased tissue to an uninfected leaf surface. The inoculation methods were as follows: (1) infected leaves were shaken over non-infected leaves, (2) conidia were deposited by wiping an infected leaf over the surface of a non-infected leaf, and (3) conidia were deposited onto non-infected leaves with a camel-hair brush. Inoculation with a brush was the least damaging method to deposit suitable numbers of viable M. pulchra conidia on the surface of dogwood leaves. Microshoots and callus tissue were inoculated with conidia of M. pulchra in attempts to establish powdery mildew colonies in vitro. Sporulating colonies were formed on microshoots. Scanning electron microscopy and histology indicated that disease processes in microshoots were similar to those in intact \u27Cherokee Sunset\u27 leaves. The establishment of powdery mildew colonies in vitro through tissue culture techniques would provide a continuous source of inoculum, a possible means to screen for resistance, and the means to study powdery mildew disease in a controlled environment