Development and validation of a set of standard area diagrams to estimate severity of potato early blight

Abstract

Standard area diagrams (SADs) to assess the severity of potato early blight (Alternaria grandis) on leaves of potato (Solanum tuberosum L.) were developed and validated. The proposed SADs include images of leaves with 10 distinct disease severities (0.1, 1, 3, 5, 10, 20, 40, 60, 80 and 100 %). The SADs were validated by 12 raters who had no previous experience in evaluating plant disease. Lin’s concordance correlation analysis of estimated vs. actual disease severity (based on image analysis) showed that precision and accuracy improved for most raters using the SADs, compared to assessments made without the SADs. The SADs improved accuracy (coefficient of bias, C b = 0.97 and 0.99, without and with SADs, respectively) and agreement (Lin’s concordance correlation coefficient, ρ c = 0.91 and 0.98 without and with SADs, respectively) of the estimates of severity. Severity estimates were more reliable when using SADs (coefficient of determination, R 2 = 0.80 unaided and R 2 = 0.95 with SADs, and the intra-class correlation ρ = 0.86 without SADs and ρ = 0.97 using the SADs). The SADs improved raters’ ability to accurately, precisely and reliably estimate potato early blight severity, and as such can be used to assess severity for several purposes, including breeding for resistance, fungicide screening, and pathotype characterization.