A New Race of Colletotrichum trifolii on Alfalfa in Oklahoma. S. J. Allen, Graduate Research Assistant, Department of Plant Pathology, Oklahoma State University, Stillwater 74078. G. L. Barnes, Professor, Department of Plant Pathology, and J. L. Caddel, Associate Professor, Department of Agronomy, Oklahoma State University, Stillwater 74078. Plant Dis. 66:922-924. Accepted for publication 3 February 1982. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-922.

Pathogenicity of five Oklahoma isolates of Colletotrichum trifolii (OK1–OK5) and one Kansas isolate (KS1) were compared with Pennsylvania isolate PA1 (race 1) on alfalfa (Medicago sativa) in a greenhouse. Pathogenicity was determined by differential percentage of seedling survival of 10 alfalfa cultivars. Cultivars highly resistant to PA1 also had resistance to Oklahoma and Kansas isolates. One Oklahoma isolate (OK3) and the Kansas isolate (KS1) appeared to be race 1. Four cultivars were more resistant to four of the Oklahoma isolates than to PA1. No Oklahoma isolate was pathogenic on Arc, so none was considered to be race 2. Three Oklahoma isolates (OK1, OK2, and OK4) produced similar reactions different from race 1. Race 3 is proposed to designate this group. The fifth Oklahoma isolate (OK5) produced resistant reactions on all cultivars tested, but it appears to be discrete.

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