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An Index for Cultivar Resistance Based on Disease Progress Curves. K. B. McRae, Regional statistician (Atlantic), Agriculture Canada, Research Station, Kentville, N.S. B4N 1J5; H. W. Platt, Research scientist, Agriculture Canada, Research Station, Charlottetown, P.E.I., C1A 7M8, Canada. Phytopathology 77:1181-1186. Accepted for publication 25 November 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1181.

A new index for assessing cultivar resistance to infection compares the rate of disease progress in cultivars relative to that in a standard cultivar. Disease progress curves for cultivars are separated into components due to the disease response of individual cultivars and the underlying disease progression for the particular field conditions, as determined by the standard cultivar. The index has a special meaning if the logarithm of the percentage of healthy tissue in a test cultivar is directly proportional to that in the standard cultivar during the course of the epidemic. The proportionality constant for each test cultivar (relative to the standard) can then be estimated by a simple algorithm, even if the disease progression is monitored only partially. The index and related procedures are illustrated with data collected for assessing resistance of potato cultivars to Phytophthora infestans over four growing seasons. Although the proposed index and the index based on area under the disease progress curve gave similar separation among cultivar groups of similar maturities within years, the new index was more stable over years and has a clear meaning. Modifications to the conduct of trials for determining cultivar disease resistance using the proposed index are discussed.