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Effects of Inoculum Concentration and Temperature on Anthracnose Severity in Alfalfa. Ronald E. Welty, Research plant pathologist, Agricultural Research Service, U. S. Department of Agriculture, Oxford Research Station, Oxford, NC 27565, Present address of senior author: USDA ARS, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; John O. Rawlings, professor, Department of Statistics, North Carolina Agricultural Research Service, Raleigh 27650. Phytopathology 75:593-598. Accepted for publication 19 November 1984. 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, 1985. DOI: 10.1094/Phyto-75-593.

Three-week-old seedlings of eight cultivars of alfalfa susceptible or resistant to anthracnose were inoculated with four concentrations of conidia of race 1 or 2 of Colletotrichum trifolii. The severity of disease increased as inoculum increased; maximum severity was generally reached at 105 conidia per milliliter. In a second study, eight cultivars of alfalfa were inoculated with race 1 or 2 and incubated at 12, 16, 20, or 24 C. Disease severity was generally higher in seedlings inoculated with race 2, and in seedlings incubated at the highest temperatures. There were significant interactions among races, temperatures, and cultivars on disease severity. Disease severity increased in cultivars susceptible or resistant to race 1 as temperature increased. Disease severity among cultivars susceptible to race 2 was similar between 12 and 24 C; for cultivars resistant to race 2, disease severity increased as temperature increased. In a factorial study (races, temperatures, inoculum concentration, and cultivars) the level of resistance to race 1 or 2 in cultivars Saranac AN 4, Glacier AN 4, and Vernal AN 4 was influenced by temperature when the plants were inoculated with 106 conidia per milliliter. It is concluded that inoculum concentration and temperature should be controlled when managing a screening program for anthracnose resistance in alfalfa and screening and evaluation should be done at the most relevant temperature for field conditions in the area of adaptation. It is also suggested that the different genetic mechanisms controlling resistance in alfalfa to C. trifolii may be influenced by temperature.