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Histopathology of Sweet Potato Root Infection by Streptomyces ipomoea. C. A. Clark, Associate professor, Department of Plant Pathology & Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center; Sharon W. Matthews, Research associate, Department of Botany, College of Basic Sciences, Louisiana State University, Baton Rouge 70803. Phytopathology 77:1418-1423. Accepted for publication 6 April 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1418.

Fibrous feeder roots and fleshy storage roots of sweet potato inoculated with the soil rot pathogen. Streptomyces ipomoea, were examined using light and scanning and transmission electron microscopy. Hyphal growth on the surface of fibrous roots was extensive, and the pathogen penetrated the root surface by either direct penetration of the periclinal wall by short lateral branches from the surface hyphae or by growing into fissures at the junctures between epidermal cells. Development on the surface of unwounded storage roots was minimal, and the hyphae did not penetrate the root. Postpenetration development within fibrous roots and wounded storage roots was similar. Hyphae ramified through tissue intracellularly and intercellularly. Localized degradation of host cell walls was observed in areas where hyphae either grew along the inner surface of the walls or penetrated the walls. Hyphae apparently penetrated the walls either by simply growing through extensively degraded sites or by growth of lateral branches through the walls. S. ipomoea did not produce specialized structures analogous to fungal penetration structures. Spore chains were observed within host tissue at late stages of infection.