Grading seed potato lots to remove tubers with powdery scab damage may not eliminate the pathogen threat

Grading seed tubers is a common practice to remove visually diseased tubers and meet certification standards. This study sought to quantify Spongospora subterranea inocula, the causal agent of powdery scab in eight commercial seedlots prior to and following grading. Pathogen levels generally decreased although the extent of reduction was dependant on the mean disease level within each seedlot, prior to grading. Where mean seedlot disease incidence was high (20 to 40 %) grading was not effective, reducing pathogen levels by only 2-fold (P > 0.05) on the visually disease-free seed following grading which remained a high risk for disease in progeny plants. However, where mean seedlot disease incidence was low (5 to 7 %) grading substantially reduced S. subterranea inoculum (12–23-fold, P < 0.05) on the visually disease-free seed to low risk levels. In further pot and field trials, planting visually disease-free tubers graded from contaminated lines gave variable disease levels in progeny plants, although under conducive conditions resulted in root and tuber disease no different to planting diseased seed.