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Cytology and Histology

Appressorium Formation over Stomates by the Bean Rust Fungus: Response to a Surface Contact Stimulus. Willard K. Wynn, Department of Plant Pathology and Plant Genetics, University of Georgia, Athens 30602; Phytopathology 66:136-146. Accepted for publication 16 August 1975. DOI: 10.1094/Phyto-66-136.

Germ tubes of bean rust (Uromyces phaseoli) formed appressoria in response to two types of topographical features: the surfaces of artificial membranes, and the stomates of bean leaves. The topography of collodion membranes together with their capacity to induce appressoria was altered by varying the procedures of membrane preparation. Appressoria were also induced specifically over wrinkles, craters, and scratches on membranes. Bean rust formed comparable numbers of appressoria on leaves and on plastic replicas of the leaves. On five bean cultivars, at least 92% of the appressoria were formed over stomates on both leaves and corresponding leaf replicas. On nonhosts, appressorial counts on replicas decreased similarly to those on leaves; total appressorium formation was less than 5% on leaves and replicas of two gramineous plants. The use of serial replicas demonstrated that no chemical stimulants were carried from leaves to replicas. Germ tubes were directed toward the stomates on bean leaf replicas since growth was predominantly at right angles to the ridges of epidermal cells and these ridges generally encircled the stomates. Evidence from scanning electron microscopy showed that the specific surface feature which induced appressorium formation on leaves was apparently the stomatal lips.