Effects of Monoculture with Susceptible and Resistant Peanuts on the Virulence ofCylindrocladium crotalariae

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Abstract
Fifteen replicated populations of C. crotalariae were established in field microplots and exposed to a 3-yr monoculture of susceptible cultivar Florigiant or resistant cultivar NC 3033 peanuts (A. hypogaea) [North Carolina, USA]. Twelve of the populations were composed of single isolates and were composites of either 6 or 12 individual isolates. Six populations were studied for a 4th yr. 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 3 populations, root rot relative to the cultivars .times. year mean decreased over years on ''NC 3033''. In one of these 3 populations, root rot severity also decreased over years on ''Florigiant''. Root rot for 1 individual isolate (population 617) on ''NC 3033'' was near the cultivar .times. year mean over years 1-2, but root rot severity over years 3-4 was greater than the mean, indicating that resistance was overcome. Monoculture of resistant ''NC 3033'' in microplots infested with 3 composite populations showed that quantitative resistance in peanut was stable for heterogeneous populations of C. crotalariae.