A standard area diagram set for severity assessment of botrytis leaf blight of onion

This work aimed to develop and validate a standard area diagram set (SADs) to estimate the severity of botrytis leaf blight of onion caused by Botrytis squamosa. For this purpose, a SADs with nine levels of severity (0.3; 1.0; 2.5; 5.0; 10; 20; 30; 40; and 57.7%) was developed from 193 symptomatic leaves collected in the field. The SADs was validated by 16 raters with no experience in evaluating plant diseases. Both precision and accuracy improved when they used the SADs. The statistical parameters for the 16 raters were: coefficient of bias - C-b (no SADs = 0.592, with SADs = 0.988); correlation coefficient - r (no SADs = 0.864, with SADs = 0.921); and Lin's concordance correlation coefficient - rho c (no SADs = 0.513, with SADs = 0.909). In addition, estimates were more reliable: coefficient of determination - R-2 (no SADs = 0.681, with SAD = 0.796); intra-class correlation coefficient - rho (no SADs = 0.729, with SADs = 0.876). The SADs proposed here is a useful tool for improving visual assessments of severity of botrytis leaf blight of onion.