Development and validation of a standard area diagram set to estimate severity of leaf rust in Coffea arabica and C. canephora

A standard area diagram (SAD) set to aid assessment of the severity of coffee leaf rust (Hemileia vastatrix) was prepared and validated, and the characteristics of the rust lesions on Coffea arabica and C. canephora leaves were compared. The results indicated that the lesions were similar, so one scale could be used to evaluate the severity of rust on both species. The proposed SAD set contains illustrations of leaves with six disease severities (2·5, 5, 10, 20, 40 and 80%). The SAD set was validated by 10 raters with no previous experience of disease evaluation. The severity of rust was first estimated without using the SADs, and the same raters assessed the leaves again using the SADs. Regression analysis and Lin’s concordance correlation (ρc) analysis of estimated against actual disease severity showed precision and accuracy was significantly better using the SADs, for most raters, with improvements in both the slope and intercept of the regressions, and the values of u, υ (components of accuracy and precision of the ρc) and ρc improved to 60, 70 and 90% of raters, respectively, when the SADs were used. The absolute error was ≤38% without the SADs, and ≤22% with them. Severity estimates were more reliable using the SADs (R2 = 0·70–1·00 unaided, and R2 = 0·80–1·00 using the SADs). The SAD set improved rater assessments for the estimation of arabica and conilon rust severity, and can be used for assessing rust severity for many purposes, including plant breeding, fungicide screening and pathotype characterization.