Anthracnose of alfalfa (Medicago sativa L.), caused by the fungus Colletotrichum trifolii Bain & Essary, was identified as a potential problem of alfalfa in the United States in the late 1960s. Races 1 and 2 are known in the United States, and recently, race 4 was described in Australia (1). An additional race, race 3, had previously been reported in the United States, but isolates of this proposed race were not preserved and its status as a distinct race of C. trifolii is unclear. In June 2003, an isolate of C. trifolii was collected in Columbus, OH from a 4-year-old alfalfa stand of cv. 520. The isolate was obtained from typical anthracnose stem lesions. Stem sections with lesions were scraped with a sterile inoculation loop, and conidia were directly streaked onto acidified potato dextrose agar. Individual colonies were transferred to half-strength oatmeal agar. Morphological characters (conidia, acervuli, and setae) of this isolate, designated OH-WA-520, were consistent with those of other C. trifolii isolates and clearly distinguishable from C. destructivum or other Colletotrichum spp. that occur on alfalfa. The alfalfa differential cultivars Arc (resistant to race 1 and susceptible to race 2 of C. trifolii), Saranac AR (resistant to race 1 and race 2), and Saranac (susceptible to both races) were inoculated with conidia of isolate OH-WA-520 per the North American Alfalfa Improvement Conference standard protocol for determining anthracnose resistance (2). Isolate OH-WA-520 was avirulent on Arc but virulent on Saranac AR and Saranac. In each of three repetitions of the protocol, more than 65% of Arc plants survived, while less than 18% of Saranac and Saranac AR plants survived. These results indicate a physiological race inconsistent in reaction with C. trifolii race 1 or race 2, but similar in reaction to race 4 isolates previously described only from Australia (1). To our knowledge, this is the first report of a C. trifolii isolate virulent on Saranac AR but avirulent on Arc. This is also the first report of C. trifolii race 4 in the United States.
References: (1) J. M. Mackie et al. Aust. J. Agric. Res. 54:829, 2003. (2) N. R. O'Neill. Anthracnose resistance. Page D-1 in: Standard Tests to Characterize Alfalfa Cultivars. Online publication. North American Alfalfa Improvement Conference, Beltsville, MD, 1991.
