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Ultrastructural Changes in Pepper Cells in Interactions with Phytophthora capsici (Isolate 18) and Monilinia fructicola. D. R. Jones, Postdoctorate Research Fellow, Agriculture Canada, Research Institute, University Sub Post Office, London, Ontario, Canada N6A 5B7, Present address of the senior author: Department of Plant Pathology and Agricultural Entomology, University of Sydney, Sydney, N.S.W. 2006, Australia.; W. G. Graham(2), and E. W. B. Ward(3). (2)(3)Electron Microscopist, and Plant Pathologist, respectively, Agriculture Canada, Research Institute, University Sub Post Office, London, Ontario, Canada N6A 5B7. Phytopathology 65:1409-1416. Accepted for publication 30 June 1975. DOI: 10.1094/Phyto-65-1409.

Three hours after inoculating fruit of sweet pepper (Capsicum frutescens) with Phytophthora capsici (isolate 18), hyphae with dense cytoplasm and functional organelles were observed in the outer walls of cells lining the fruit cavity. The second layer of cells was invaded within 5-7 hours and growth was arrested in the third layer 9-12 hours after inoculation. In invaded tissue, the cytoplasm of most hyphae and haustoria became vacuolated and degenerated between 7 and 12 hours. A reinvasion of previously uninvaded first layer cells by juvenile hyphae took place approximately 48 hours after inoculation and fruit tissue was rapidly colonized. The first few cell-layers were invaded in some areas by Monilinia fructicola 6-24 hours after inoculation, but little parasitic growth occurred subsequently, even though saprophytic hyphae were common on the inner fruit surface. In both interactions, pepper cells were rapidly reactivated and disorganized in a characteristic hypersensitive response. This involved the formation of lipid bodies, the vacuolation of the cytoplasm, the invagination and disruption of the tonoplast membrane, the formation of vesicles and their dispersion throughout the cell cavity. The extent of the reaction was limited to cells near to invading hyphae of P. capsici, but was widespread throughout the inner surface tissue after inoculation of M. fructicola.