Diagrammatic scale to evaluate angular leaf spot severity in primary leaves of common bean

The aim of this study was to develop a diagrammatic scale to evaluate angular leaf spot (ALS) severity in primary leaves of common bean. A diagrammatic scale was developed and validated with nine levels of severity: 0 %; 0.1 to 0.5 %; 0.6 to 4.0 %; 4.1 to 7.0 %; 7.1 to 16 %; 16.1 to 26 %; 26.1 to 32 %; 32.1 to 38 % and 38.1 to 60 %. The use of this scale, associated with the early inoculation method, allowed rating of ALS severity in common bean leaves in the V2 stage of development that is, when the primary leaves are fully open. Validation of the scale was performed by 14 Evaluators. These Evaluators estimated the severity in 40 common bean leaves with different levels of severity, previously measured by the software QuantA (R). The accuracy and precision of each Evaluator were determined by simple linear regression, by comparing between actual severity, measured electronically, and that estimated by the Evaluator. Without the scale, most of the Evaluators overestimated the severity of the disease. With the scale, the Evaluators obtained better levels of accuracy and precision, with absolute errors less than 5 %. The diagrammatic scale proposed proved to be adequate for early performance of the pathogenicity test.