Development and validation of a standard area diagram to assess common scab in potato tubers

Common scab ( Streptomyces scabies ) is a severe disease of the potato ( Solanum tuberosum L.), which can seriously reduce the commercial value of tubers. More accurate and precise estimates of incidence and severity are required for its effective control. Standard area diagrams (SADs) were developed to aid the visual assessment of common scab severity in potato tubers. A set of 115 tuber images, with and without symptoms, was used to develop the scale, which contained 12 images of tubers with different symptoms. The SADs were then tested by 12 evaluators with no experience in appraising plant diseases, who assessed a set of 50 images 4 times, first without the aid of the SADs, then twice using the proposed scale, and then again using the scale of James ( Canadian Plant Disease, 51 (2), 39–65, 1971 ). The use of SADs increased the accuracy (coefficient of bias (Cb) = 0.99, 0.95 and 0.95 with SADs, without SADs and with the James SADs, respectively) and agreement (Lin’s coefficient (ρc) = 0.97, 0.90 and 0.88 with SADs, without SADs and with James SADs, respectively) of the severity estimates. Severity estimates were also more reproducible with the use of the scale (determination coefficient (R 2 ) = 0.93 using SADs and R 2 = 0.84 without the use of SADs). The results obtained with the scale were also repeatable (intra-rater determination coefficient (R 2 ) = 0.94). The proposed SADs have been shown to be useful in evaluating common scab in potatoes, increasing the accuracy, precision and reliability of disease severity estimates.