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Assessment of Alfalfa Cultivar Reaction to Anthracnose in Controlled and Field Environments. C. R. Grau, Department of Plant Pathology, University of Wisconsin-Madison 53706. D. C. Arny, and S. L. Nygaard. Department of Plant Pathology, University of Wisconsin-Madison 53706. Plant Dis. 73:167-170. Accepted for publication 21 September 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0167.

Fifty-two alfalfa cultivars were evaluated for reaction to Colletotrichum trifolii (anthracnose) by a standardized seedling assay and during natural epidemics. An anthracnose index was calculated for each cultivar by dividing the number of stems with foliar symptoms per square meter by the number observed for Saranac (the susceptible standard cultivar) and multiplying by 100. Cultivar ranks based on this anthracnose index were significantly correlated with rankings based on percent resistant seedlings determined in the standardized seedling assay. There was a significant and inverse linear relationship between percent resistant seedlings and the ln(x + 1) transformation of the anthracnose indices for cultivars at all locations. Anthracnose indices from the three field assessments were significantly correlated with one another and effectively differentiated cultivar reactions to C. trifolii. Anthracnose indices recorded for the cultivars Maris Kabul, Vista, Thor, Trumpetor, and Vertus were higher than expected based on results from the seedling assay and lower than expected for the cultivars WL-313, Apollo II, and Baker. The latter phenotypic reaction is suggestive of a type of field or adult-plant resistance not measured by the current protocol of the seedling assay. These results establish the importance of evaluating the reaction of alfalfa cultivars to anthracnose under both controlled conditions in the seedling stage and in the field.