VIEW ARTICLE

Resistance

Compatible and Incompatible Responses in Alfalfa Cotyledons to Races 1 and 2 of Colletotrichum trifolii. Nichole R. O’Neill, Soybean and Alfalfa Research Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705; James A. Saunders, Soybean and Alfalfa Research Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705. Phytopathology 84:283-287. Accepted for publication 14 December 1993. 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, 1994. DOI: 10.1094/Phyto-84-283.

Specific race interactions between Colletotrichum trifolii and cultivars of resistant and susceptible alfalfa were examined in nonwounded excised cotyledons. Cotyledons in incompatible and compatible interactions exhibited visible differential reactions within 7–10 days after inoculation. C. trifolii infected alfalfa by directly penetrating the epidermis via a penetration peg from the appressorium. Appressoria matured within 20 h on resistant and susceptible cotyledons, but primary hyphae did not develop on resistant cotyledons. Phenylalanine ammonia-lyase and the pterocarpan phytoalexin medicarpin began to increase in resistant tissues after 24 h. Cotyledon tissues apparently became resistant prior to significant increases in medicarpin. Medicarpin concentration in resistant cultivars was highest at 72 h and declined to baseline levels by 120 h. Secondary spores were produced in susceptible tissues after 96 h. Cotyledons appear to be ideal tissues to use in investigations of defense expression in the anthracnose disease of alfalfa.