COMPATIBLE AND INCOMPATIBLE RESPONSES IN ALFALFA COTYLEDONS TO RACE-1 AND RACE-2 OF COLLETOTRICHUM-TRIFOLII

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.