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 < 0.05) higher incidence of black scurf on progeny tubers (93%) than planting seed with less severe symptoms (<25%). When inoculum was added to the soil, the incidence (≥80%) and severity (c. 2 on a scale 0–5) of disease on the progeny tubers was similar to that resulting from high levels of seed inoculum. There was no additional increase in disease incidence or severity when both soil and seed inoculum were present. When 108 commercial potato crops were monitored, the health status of the commercial seed stocks planted (often chemically treated) was found to be relatively less important than soil-borne inoculum in causing black scurf. However, in 50% of the 28 crops where no inoculum was detected on either seed or in soil, some black scurf was recorded on progeny tubers; in 11% of these crops (3 out of 28), the incidence of disease was >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.