Resistance Induced by Race 1 of Colletotrichum trifolii to Race 2 in Alfalfa Resistant to Race 1. Stanley A. Ostazeski, Research Plant Pathologist, USDA, Field Crops Laboratory, Plant Genetics and Germplasm Institute, Beltsville Agricultural Research Center, Beltsville, MD 20705. James H. Elgin, Jr., Research Agronomist, USDA, Field Crops Laboratory, Plant Genetics and Germplasm Institute, Beltsville Agricultural Research Center, Beltsville, MD 20705. Plant Dis. 68:285-288. Accepted for publication 23 September 1983. 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, 1984. DOI: 10.1094/PD-68-285.

Alfalfa plants resistant to race 1 but susceptible to race 2 of Colletotrichum trifolii could be protected from race 2 when needle-inoculated with mixtures of races 1 and 2. Dilution was not a factor because race 2 alone at 1×10⁶ conidia per milliliter was pathogenic but the race 2 component in the mixture at 4 × 10⁶ conidia per milliliter was relatively, or usually, completely nonpathogenic. In spray-inoculation experiments, most cultivar Arc seedlings were protected from race 2 by prior inoculation with race 1. The survival rate of Arc seedlings inoculated with a mixture of races 1 and 2 was much higher than that of seedlings inoculated with race 2 alone. Analysis of plants capable of being protected from race 2 in mixtures, or by protective sprays with race 1, demonstrated race 1 resistance was a requirement for the race 2 protection phenomenon to be operative.