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Infection of Avocado and Other Species of Persea by Phytophthora cinnamomi. H. H. Ho, Visiting Research Associate, Department of Plant Pathology, University of California, Riverside, CA 92502, Present address: Department of Biology, State University of New York, College at New Paltz, NY 12561; G. A. Zentmyer, Professor, Department of Plant Pathology, University of California, Riverside, CA 92502. Phytopathology 67:1085-1089. Accepted for publication 8 March 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1085.

Zoospores of A1 (Pc97) and A2 (Pc40) mating types of Phytophthora cinnamomi were attracted to roots of avocado (Persea americana), P. indica, P. borbonia, P. pachypoda, camellia, and mandarin orange. Zoospores encysted rapidly on the root surfaces, most commonly just behind the root tip, on cut ends and around wound sites. Cyst germination was stimulated by the presence of roots; germ tubes extended from the side of cysts closest to the root and then grew toward it. Repeated emergence of zoospores was not observed. Germ tubes of Pc40 and Pc97 penetrated the root epidermis and colonized the cortical tissues of both resistant and susceptible plants with no difference in the mode of penetration and post-penetration development of the fungus. Hyphal swellings (vesicles) and chlamydospores were produced by both mating types in the cortex, but oogonia with oospores were found only in Pc40. No anatomical differences were found to account for host resistance. Persea americana, P. indica, and to some extent, very young seedlings of P. borbonia were susceptible to Pc40 and Pc97 whereas P. pachypoda was resistant. Pc40 was consistently more virulent than Pc97.

Additional keywords: zoospore behavior, scanning electron microscopy, development of root rot.