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Effect of Disease Assessment Method on Ranking Potato Cultivars for Resistance to Early Blight. B. J. Christ, Assistant Professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Plant Dis. 75:353-356. Accepted for publication 24 September 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0353.

Significant differences among several potato cultivars (Solanum tuberosum) were observed for disease reaction to early blight, caused by Alternaria solani, in 1985, 1986, and 1988 field trials and in a greenhouse in 1986. Over the 3 yr, several assessment methods were used to characterize and categorize the disease reaction. Percentage of area of leaves from the middle third of the plant canopy that was covered with early blight lesions, assessed over several dates, provided the most information as well as a feasible method of assessing disease reaction. Cultivar ranking varied according to the assessment method. Regardless of assessment method, the late-maturing cultivars Katahdin and Kennebec were more resistant to A. solani than the early-maturing cultivars Norland and Superior but not necessarily any more resistant than the midseason-maturing cultivars Atlantic and Chieftain.