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

The aim of this study was to develop and validate a standard area diagram set (SADs) to assess the severity of peach rust, caused by Tranzschelia discolor. The proposed SADs includes ten images of leaves with a range of severity (0.1, 0.5, 1.0, 2.0, 5.0, 10, 15, 20, 25 and 30%). The SADs was validated by 14 raters who had no experience in plant disease severity estimation. In the first step of the validation, the raters made severity estimates of 50 leaves with a range of rust severity without using SADs. In the second step, the same raters estimated severity of rust on the same 50 leaves using the SADs to aid estimation. Lin’s concordance correlation analysis showed that both precision and accuracy improved when the raters used the SADs compared to the assessments made without SADs. Accuracy, as measured by the coefficient of bias (Cb) improved from 0.70 to 0.98, without and with SADs, respectively, and precision measured by the correlation coefficient (r) improved from 0.85 to 0.90, without and with SADs, respectively. Overall agreement, measured by Lin’s concordance correlation coefficient (ρc), improved from 0.59 to 0.88 without and with SADs, respectively. Furthermore, estimates were more reliable when using SADs: the coefficient of determination (R2) was 0.60 without and 0.73 with SADs; and the intra-class correlation coefficient (ρ) was 0.72 without, and 0.86 with SADs. Thus, the use of SADs improved the precision, accuracy and reliability of visual estimates of severity of peach rust.