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Relationships Between the Development of Root Systems of Tobacco and Infection by Phytophthora parasitica var. nicotianae. J. T. English, Former graduate research assistant, Plant Pathology Department, University of Florida, Gainesville 32611; D. J. Mitchell, professor, Plant Pathology Department, University of Florida, Gainesville 32611. Phytopathology 78:1478-1483. Accepted for publication 12 July 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1478.

Early infection of tobacco root systems by Phytophthora parasitica var. nicotianae was evaluated after plant growth in autoclaved and nonautoclaved field soils infested with a defined inoculum density of the pathogen. Incidence of plant infection and average numbers of infections per infected plant were similar for resistant and susceptible tobacco cultivars in either soil. Low efficiencies of inoculum for infection implied infrequent successful contact between susceptible root tissues and pathogen propagules. Root growth of both tobacco cultivars was not altered consistently by infection. Numbers and total lengths of elements of various root orders and branching ratios of healthy and infected plants differed significantly in autoclaved soil of one experiment only. The pattern of root system growth of Speight G-28 tobacco was more consistent than that of Hicks tobacco.