Cylindrocladium black rot (CBR) is a destructive root disease of peanut caused by Cylindrocladium crotalariae. New and quantitative approaches were developed to estimate the inoculum potential for this soil-borne pathogen under soil-temperature tank conditions favorable for CBR development. Using cultural plating methods, numerous Q. crotalariae infections (1 to > 1,000 per plant) were observed on asymptomatic root systems of peanut plants, susceptible and resistant to CBR, grown in naturally infested soils at 25 C. Regression-line slope values of log10-log10 plots for microsclerotial inoculum density versus the number of observed root infections per plant and per unit root length [0.98 (R2 =0.94) and 0.99 (R2 = 0.94), respectively] indicated direct proportionality between the respective 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 into necroses) estimates were low (0.27 to 0.28%). Observed infection observed infections per m root per day per rates, I 0 r (the number of microsclerotium per g of soil) were significantly lower (P = 0.001) for CBR-resistant 'Spancross• than for CBR-susceptible 'Florigiant•. The majority of observed infections did not appear to be restricted to outer cortical root tissues. A portion of observea infections near root surfaces resulted in subsequent colonization of the inner cortex and stele of plants for each cultivar. A significantly greater (P = 0.05) number of segments from steles of CBR-susceptible Florigiant taproots were colon~zed by Q. crotalariae than those from CBR-resistant Spancross. Using naturally infested peanut field soils, a dose of 7.4 )-lg NaN3 (sodium azide)/g soil was effective in reducing initial microsclerotial populations by 50%, based upon a highly significant dosage-response curve (R2 = 0.96, P = 0.0001). CBR development was reduced significantly (P = 0.05) for Florigiant plants grown in Q. crotalariae-infested soils treated with 7.5 fl' NaN3/g soil or higher, compared to plants grown in untreated soils. Sublethal doses of NaN3 combined with biological control agents may be useful in the development of integrated CBR control measures. / Ph. D.