Development and validation of a standard area diagram set for assessment of plum rust severity

The aim of this study was to develop and validate a standard area diagram set (SADs) to aid assessment of plum rust severity. The elaborated SADs was composed of ten coloured images with different severities ranging from 0.3 to 35%. The SADs were validated in two steps (without and with SADs) by 10 raters with no experience, evaluating 50 leaves with different disease severity levels. The accuracy and precision of the estimates were determined using the Lin’s concordance correlation analysis and the reliability was measured using coefficient of determination (R2) and intraclass correlation coefficient (ρ). The SADs improved accuracy (Cb = 0.78 and 0.97, without and with SADs, respectively), precision (r = 0.87 and 0.93, without and with SADs, respectively) and Lin’s concordance correlation coefficient (ρc = 0.68 and 0.91 without and with SADs, respectively) of the severity estimates. There was greater reliability when SADs was used (R2 = 0.61 without and R2 = 0.79 with SADs, and intraclass correlation coefficient ρ = 0.45 without SADs and ρ = 0.88 with SADs). The SADs proposed here is a useful tool for improving accuracy, precision and reliability of visual assessments of plum rust severity.