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Pathogenic Variability and Host Resistancein the Colletotrichum trifolii/Medicago sativa Pathosystem. Nichole R. O'Neill, Research Plant Pathologist, Agriculture Research Service, U.S. Department of Agriculture, Soybean and Alfalfa Research Laboratory, Beltsville, MD 20705 .. Plant Dis. 80:450. Accepted for publication 3 January 1996.. This article is in the public domain and not copyrightable. It may be (freely reprinted with customary crediting of the source. The American Phytopathological Society, 1996. DOI: 10.1094/PD-80-0450.

Two hundred and fifty alfalfa cultivars currently grown in the U.S. were evaluated for their reaction to anthracnose caused by race 2 of Colletotrichum trifolii. Resistance was present in 30 cultivars, with seedling survival ranging from 20 lo 68%. Three unusually highly virulent isolates of Colletotrichum collected from three locations were identified to species and race by morphological and cultural characteristics, and by reactions on host differential cvs. Arc, Sa-ranac, and Saranac AR. Isolates Arl-NW and SB-2 were characterized as C. trifolii isolates of races 1 and 2, respectively. The morphology of isolate 57RR was distinct from that of C. trifolii and C. destructivum, and most closely conformed to the composite group C. gloeosporioides. Resistance to these isolates was evaluated in 40 selected and race differential cultivars. Resistance to isolate Arl-NW was highly variable, with seedling survival ranging from 0 to 77.8%. The isolate was classified as race I based on reactions with standard check cultivars. Isolate 57RR was highly virulent to anthracnose-resistant alfalfa clones and cultivars in needle inoculation assays, and was highly virulent to Saranac AR, Arc, and several race 2 resistant cultivars in seedling tests. Colletotrichum gloeosporioides is reported as a new pathogen of alfalfa and should be considered a potential disease problem. The variability of resistance reactions in commercially grown alfalfa cultivars to diverse isolates of Colletotrichum suggests that these cultivars will be useful sources for different types of anthracnose resistance.