USE OF SPATIAL PATTERNS AND DENSITY OF INOCULUM OF CYLINDROCLADIUM-CROTALARIAE DURING FIELD-EVALUATION OF PARTIALLY RESISTANT PEANUT GENOTYPES

Three new peanut genotypes, NC Ac 18414, NC Ac 18416, and NC Ac 18417; susceptible cultivar, Florigiant; moderately resistant cultivar, NC 8C; and highly resistant genotypes, NC Ac 18016 and NC 3033, were evaluated in 1986 and 1987 for incidence of Cylindrocladium black rot in field experiments designed to take into account inoculum density and spatial patterns of propagules of Cylindrocladium crotalariae as well as genotype effects on disease incidence. Crop rotation and observation of previous black rot incidence were used to divide fields into quadrants with different average inoculum levels of C. crotalariae. Soil samples from each plot were assayed before planting each year to estimate inoculum density and to determine spatial patterns of inoculum. Estimates inoculum density were used as an experimental design factor such that genotypes were assigned to replicated plots representing similar ranges of inoculum density. Final disease incidence, area under disease progress curve, and indices relating performance of each genotype to that of Florigiant were used for comparison of the genotypes. Incidence of black rot in NC Ac 18417 was not significantly higher than that of NC 8C in 1986, but was in 1987. NC Ac 18414 performed only slightly better than Florigiant. NC Ac 18417 was chosen for release as moderately resistant cultivar NC 10C. Significant correlations between initial inoculum level and final disease incidence were detected in 1986 for all genotypes except the highly resistant line NC 3033. In 1986, NC Ac 18414 and NC Ac 18417 appeared to be more sensitive to increases in inoculum density than the other resistant genotypes, although performances of NC 8C and NC Ac 18417 were comparable at low levels of inoculum. Correlation of disease incidence with initial inoculum was not detected in 1987.