Spore Germination, Infection Structure Formation, and Colony Development of Erysiphe pulchra on Dogwood Leaves and Glass Slides

Flowering dogwood (Cornus florida L.) is used as an important ornamental tree in North America, Japan, and Europe. Since 1995, epidemics of powdery mildew caused by Erysiphe pulchra (Cooke and Peck) U. Braun & S. Takamatsu (syn. Microsphaera pulchra Cooke and Peck) have impeded production of flowering dogwood throughout much of the eastern United States. The fungus is an obligate parasite and grows mainly on the leaf surface. The disease can cause stunted, distorted growth or twig or plant death, which can be undesirable aesthetically. Using resistant cultivars is a major strategy in the management of powdery mildew on flowering dogwood in landscapes, and several resistant cultivars have been released (15,16). Conidia of most fungi causing powdery mildew start to germinate on leaf surfaces within 2 h after inoculation (hai; 4,11,13,14). However, patterns of infection structure formation differ in some pathosystems of powdery mildew. For most of the powdery mildew fungi, such as E. pisi DC. on sweet pea (Lathyrus odoratus L.; 13) and pea (Pisum sativum L.; 14), Sphaerotheca fuliginea (Schlechtend.:Fr.) Pollacci on melon (Cucumis milo; 12), and Oidium lycopersicum Cooke & Massee (Noordeloos & Loerakker 1989) on Lycopersicon spp. (10), a conidium germinates and produces the primary germ tube ending in a primary appressorium, and hyphae arise from the primary appressoria and the body of conidia. In contrast, for Blumeria graminis (DC.) E. O. Speer (syn. E. graminis DC. f. sp. hordei Ém. Marchal) on barley, the initial germ tubes do not form appressoria, but the secondary germ tubes form hook-shaped appressoria and hyphae develop from appressoria (1,8,11). Considered a new disease in the United States, little is known about the infection process of E. pulchra on dogwood. Understanding early stages of pathogen development on the host plant is essential to any future investigation of mechanisms of resistance to powdery mildew. The objective of the present study was to describe the infection process, including conidial germination, infection structure formation, and colony development, of E. pulchra on flowering dogwood.