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Resistance

Characterization of Induced Resistance to Anthracnose in Alfalfa by Races, Isolates, and Species of Colletotrichum. Nichole R. O’Neill, Germplasm Quality and Enhancement Laboratory, Beltsville Agricultural Research Center, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705; James H. Elgin, Jr.,(2), and C. Jacyn Baker(3). (2)Deputy area director, Plant Sciences Institute, Beltsville Agricultural Research Center, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705; (3)Microbiology and Plant Pathology Laboratory, Beltsville Agricultural Research Center, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705. Phytopathology 79:750-756. Accepted for publication 11 January 1989. 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, 1989. DOI: 10.1094/Phyto-79-750.

The phenomenon of induced resistance and its biochemical and physiological basis is being investigated in the anthracnose disease of alfalfa (Medicago sativa). By prior inoculation with race 1 of Colletotrichum trifolii, race 1-resistant, race 2-susceptible cultivars such as Arc can be protected against subsequent infection by race 2. In this study we investigated the effectiveness of race 1 inoculum concentrations on induced resistance, the effectiveness of protection against high race 2 disease pressure, and the association of pathogen virulence with the capacity to induce resistance. Protection of Arc seedlings against race 2 was highly correlated with increasing spore concentrations of race 1. Arc has no specific genes for resistance to race 2, and a gene for resistance to race 1 present in 65 to 75% of the plants of this cultivar permits a similar level of survival in inoculated seedlings. An inducing concentration of 2 × 10⁶ race 1 spores/ml followed by challenge inoculation 24 hr later with race 2 permitted 55% seedling survival, whereas all unprotected seedlings were killed. Race 1 protection was not diminished by high disease pressure from concentrations of race 2 inoculum of up to 4 × 10⁶ spores/ml. Relative virulence to resistant host genotypes varied significantly among isolates of race 1 and was associated with their ability to induce protection in Arc against race 2. Of other species of Colletotrichum tested, an isolate of C. malvarum and one of C. gloeosporioides induced significant protection against anthracnose.