Effects of Inoculum Concentration and Temperature on Anthracnose Severity in Alfalfa

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Abstract
Three-week-old seedlings of 8 cultivars of alfalfa susceptible or resistant to anthracnose were inoculated with 4 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/ml. In a 2nd study, 8 cultivars of alfalfa were inoculated with race 1 or 2 and incubated at 12, 16, 20 or 24.degree. 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.degree. 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 AN4, Glacier AN4 and Vernal AN4 was influenced by temperature when the plants were inoculated with 106 conidia/ml. Inoculum concentration and temperature should be controlled when managing a screening program for anthracnose resistance in alfalfa and screening an evaluation should be done at the most relevant temperature for field conditions in the area of adaptation. The different genetic mechanisms controlling resistance in alfalfa to C. trifolii may be influenced by temperature.