Standard area diagrams to aid assessments of the severity of blight leaf spot symptoms on cassava leaves

A standard area diagram (SAD) set to assess the severity of blight leaf spot (BliLS) caused by Passalora vicosae on cassava leaves was developed and validated. The proposed SAD set includes diagrams illustrating six leaves with different severities of blight leaf spot (5, 10, 20, 30, 40 and 50%). The SAD set was validated by ten raters with no previous experience in disease assessment. Lin’s concordance correlation analysis of estimated versus actual disease severity showed that the statistical parameters (υ, u, r andρc ) were improved when raters were aided by the SADs. Overall agreement measured by Lin’s concordance correlation coefficient (ρc) increased from 0.70 to 0.82, without and with SADs, respectively. Similarly, by analysing the coefficient of determination (R²) and intra-class correlation coefficient (ρ), the estimates of severity were more reliable by using the SADs. The proposed SAD set increases the accuracy and reliability of estimates of the severity of BliLS in cassava and therefore is a valuable aid for research on the epidemiology, breeding for resistance, fungicide screening and pathotype characterization.