Inoculum Potential ofCylindrocladium crotalariae: Infection Rates and Microsclerotial Density-Root Infection Relationships on Peanut

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Abstract
Numerous infections (1 to > 1000 per plant) caused by C. crotalariae were observed on asymptomatic taproots, lateral, and fine roots of peanut plants grown in naturally infested soils in the field or greenhouse. The majority of observed infections apparantly were not restricted to surface tissues, based on tests involving surface sterilization of roots with 0.1 and 0.25% NaCl0. In a time-course experiment (25.degree. C), the infection rate, Ir0, was 0.120, 0.162 and 0.199 observed infections/m of root per day per microsclerotium per gram of soil for the 1st, 2nd, and 3rd 21-day periods, respectively. The infection rate, Re, for estimated infections (log, [l/l.sbd.y], in which y is the proportion of plants with necroses) was 0.0017, 0.0038, and 0.0084 infections/plant per day per microsclerotium per gram of soil for the 1st, 2nd and 3rd 21-day periods, respectively. Based on the infection-rate curve, each plant had about 300 observed root infections when 50% of the plant population had root necroses. Regression line slopes of 0.98 (R2 = 0.94) and 0.99 (R2 = 0.94) were obtained for log10-log10 plots of microsclerotial inoculum density vs. the number of ohserved root infections per plant and per unit root length, respectively. Slopes of 21.4 (R2 = 0.95) and 2.3 (R2 = 0.95), respectively, were obtained for 1st-order regression lines in arithmetic plots of the same variables. Efficiency of inoculum for observed infection (percent of germinating microsclerotia that infect roots) estimates were high (near 100%), while efficiency of observed infection for necrosis (percent of infections that develop necroses, calculated from loge [l/l.sbd.y]) estimates were low (0.27-0.28%).