Occurrence of Race 2 of Colletotrichum trifolii in North Carolina and Resistance of Alfalfa Cultivars and Breeding Lines to Races 1 and 2. Ronald E. Welty, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Oxford, NC 27565. Ramzy Y. Gurgis, Agronomist, Department of Plant Pathology, North Carolina Agricultural Research Service, Raleigh 27650, and Dennis E. Rowe, Research Geneticist, Department of Crop Science, North Carolina Agricultural Research Service. Plant Dis. 66:48-51. Accepted for publication 27 March 1981. 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, 1982. DOI: 10.1094/PD-66-48.

Moderate to severe damage from anthracnose (Colletotrichum trifolii) is widespread on alfalfa in the eastern and southeastern United States. During July 1979, anthracnose lesions were collected from eight fields of alfalfa in five counties in North Carolina. Sixty-three isolates of three species of Colletotrichum were recovered. Nineteen isolates with conidia nearest the average length and width of C. trifolii were selected for testing. Seedlings of Arc and Saranac AR were inoculated with conidia from single-spore cultures of each of the 19 isolates and with conidia from cultures known to be race 1 or 2. All 19 isolates were race 2; 17 had been obtained from plants in 1- to 3-yr-old stands of Arc alfalfa in Iredell, Rowan, and Davidson counties. Resistance to race 1 (isolate PA) and race 2 (isolate NC-4) in 31 alfalfa cultivars and breeding lines was evaluated. Resistance to race 2 was found in Saranac AR, Vangard, and breeding lines with Saranac AN 4 or Vernal AN 4 in their pedigree. Mycelial growth of race 1 (PA) was compared with that of three isolates representative of race 2 (NC-4, D-3-9, M-8) at 4–36 C in four-degree increments. Optimum temperature for growth for all isolates was between 20–28 C, and minimum and maximum temperatures for growth were 8 and 32 C, respectively.

Keyword(s): disease resistance, Medicago sativa, pathogenic races, virulence.