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Effects of Monoculture with Susceptible and Resistant Peanuts on the Virulence of Cylindrocladium crotalariae. M. C. Black, Former research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27650, Current address: Texas A&M University Agricultural Research and Extension Center, Uvalde 78801; M. K. Beute(2), and K. J. Leonard(3). (2)(3)Professor, USDA, ARS, and plant pathologist, respectively, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Phytopathology 74:945-950. Accepted for publication 8 March 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-945.

Fifteen replicated populations of Cylindrocladium crotalariae were established in field microplots and exposed to a 3-yr monoculture of susceptible cultivar Florigiant or resistant cultivar NC 3033 peanuts (Arachis hypogaea). Twelve of the populations were composed of single isolates and three were composites of either six or 12 individual isolates. Six populations were studied for a fourth year. Mean inoculum density was lower following monoculture with NC 3033 than with Florigiant. Some populations were consistently high or low in virulence on both cultivars over years. In three populations, root rot relative to the cultivar year mean decreased over years on NC 3033. In one of these three populations, root rot severity also decreased over years on Florigiant. Root rot for one individual isolate (population 617) on NC 3033 was near the cultivar year mean over years 1- 2, but root rot severity over years 3- 4 was greater than the mean, indicating that resistance had been overcome. However, monoculture of resistant NC 3033 in microplots infested with three composite populations showed that quantitative resistance in peanut was stable for heterogeneous populations of C. crotalariae.