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Etiology

Infection of Tobacco Callus by Phytophthora parasitica var. nicotianae. G. A. deZoeten, Department of Plant Pathology and USDA, ARS, Plant Disease Resistance Research Unit, University of Wisconsin, Madison 53706; G. Gaard(2), G. T. Haberlach(3), and J. P. Helgeson(4). (2)(3)(4)Department of Plant Pathology and USDA, ARS, Plant Disease Resistance Research Unit, University of Wisconsin, Madison 53706. Phytopathology 72:743-746. Accepted for publication 5 October 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-743.

Transmission and scanning electron microscopy was used to determine the response of tobacco callus tissue to inoculation with Phytophthora parasitica var. nicotianae. Callus resistant to Race 0 and susceptible to Race 1, or susceptible to Race 0 and Race 1, was used. Within 3 hr after inoculation, zoospores from compatible and incompatible races had encysted and germinated; then germ tubes of both races penetrated callus cells. Penetration often occurred in cells other than those on which the spores germinated. By 24 hr, when a hypersensitive reaction (HR) was clearly evident in the incompatible combination, almost all of the cells in the HR area had collapsed, whereas most cells in infected areas on callus showing the compatible reaction were still turgid. After 48 hr, hyphae of the compatible fungus had penetrated <50 cell layers, whereas those of the incompatible fungus were limited to 58 cell layers.