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Cellular and Histological Changes Induced by Phytophthora cinnamomi in a Group of Plant Species Ranging from Fully Susceptible to Fully Resistant. D. Cahill, School of Botany, University of Melbourne, Parkville, Victoria 3052, Australia; N. Legge(2), B. Grant(3), and G. Weste(4). (2)(3)Russell Grimwade School of Biochemistry, University of Melbourne, Parkville, Victoria 3052, Australia; (4)School of Botany, University of Melbourne, Parkville, Victoria 3052, Australia. Phytopathology 79:417-424. Accepted for publication 25 October 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-417.

The pattern of invasion and the histological changes in primary root tissues after infection by Phytophthora cinnamomi are described from the following species: Xanthorrhoea australis, X. resinosa, Themeda australis, Eucalyptus marginata, E. sieberi, and Acacia melanoxylon (susceptible); A. pulchella, E. calophylla, E. maculata, Gahnia radula, Juncus bufonius, Zea mays, and Triticum aestivum var. cappelle (resistant). Zoospores germinated on and penetrated the roots of all species, and lesions formed within 816 hr after invasion at 2024 C. Root growth ceased within 24 hr of inoculation but resumed within 48 hr in resistant species, usually from a lateral branch. In susceptible species, progressive symptom development included water soaking of tissues, lesion extension through the root to the hypocotyl, and root death. This was accompanied by wilting and chlorosis of the leaves, die-back of shoots, and plant death. Sporulation occurred between 24 and 72 hr after inoculation on all but the most resistant species. In resistant species, lesions were contained well before they extended to the hypocotyl. Deposition of phenolic materials, granulation of the cytoplasm, shrinkage of the protoplast, and cell wall distortion and disruption accompanied infection in all species. Lignification of cell walls, deposition of phenolics, and the formation of callosic papillae were more commonly observed in the resistant species but occurred in some susceptible species. No specific anatomical feature or histological change was consistently associated with resistance. Although the species examined were classed as either resistant or susceptible on the basis of their field response, examination of the anatomical and histological changes that followed infection showed a gradient in which the fully susceptible and most resistant types represent the extremes.

Additional keywords: histopathology, hyphal penetration resistance, tissue colonization.