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Effect of Host Resistance on Spread of Phytophthora parasitica var. nicotianae and Subsequent Development of Tobacco Black Shank Under Field Conditions. H. D. Shew, Assistant professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; Phytopathology 77:1090-1093. Accepted for publication 27 January 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1090.

Four flue-cured tobacco cultivars representing different levels of quantitative resistance to the black shank disease were planted in small plots in a field with no history of tobacco production. After the last cultivation, a single plant in each row was inoculated by inserting 3 ml of an aqueous suspension of Phytophthora parasitica var. nicotianae into the stem at the soil line. The field was not reentered until the end of the growing season, when disease incidence was assessed. In the average-rainfall growing season of 1982, spread of the pathogen was detected in rows of all cultivars. Spread was detected in 78, 56, 17, and 17% of the rows with inoculated plants and percentage of total plants expressing symptoms was 38, 15, 10, and 8% for the susceptible cultivar Hicks, low-resistance cultivar Coker 319, moderate-resistance cultivar Coker 411, and high-resistance cultivar NC 82, respectively. Symptomatic plants were not continuous down a row from the point of inoculation, but P. p. var. nicotianae was recovered from soil and root assays from all intervening asymptomatic plants, indicating that pathogen spread was continuous. Movement of the pathogen from the inoculated plant was detected in 75% of the rows that contained no symptomatic plants except the inoculated plant. In the dry summer of 1983, spread was detected only in plots of the cultivar Hicks, with spread occurring in 20% of rows that contained an inoculated plant.