The Relative Importance of Seed- and Soil-Borne Inoculum of Rhizoctonia solani AG-3 in Causing Black Scurf on Potato

Through a combination of controlled environment experiments and the monitoring of commercial potato crops, the relative importance of seed- and soil-borne inoculum of Rhizoctonia solani AG-3 in causing black scurf on potato tubers was investigated. Seed- and soil-borne inoculum of R. solani was quantified using an existing real-time PCR assay and related to subsequent incidence and severity of disease. In a controlled environment experiment, in the absence of soil inoculum, planting seed with >10% black scurf resulted in significantly (P 10%. The effectiveness of real-time PCR diagnostics to predict disease risk based on the amount of detectable soil inoculum is discussed in relation to detection thresholds and variable detection in different soil types.