Development and Validation of a Standard Area Diagram Set for Assessment of Target Leaf Spot Severity on Handroanthus serratifolius

Target leaf spot, caused by Corynespora cassiicola, is a common disease on Handroanthus serratifolius (yellow ipe) in Brazil. This study aimed to develop and validate a standard area diagram (SAD) set to estimate the severity of target spot on H. serratifolius leaves. For this purpose, a SAD set with 10 levels of severity (1%; 3%; 5%; 10%; 20%; 30%; 40%; 55%; 70% and 85%) was developed. The SAD set was validated by 20 raters with no experience in evaluating plant diseases. Both accuracy and precision improved when the proposed SAD set was employed. The statistical parameters evaluated were bias coefficient factor - C_b (no SAD set = 0.898, with SAD set = 0.994); correlation coefficient - r (no SAD set = 0.926, with SAD set = 0.979); and Lin's concordance correlation coefficient - ρ_c (no SAD set = 0.835, with SAD set = 0.974). In addition, estimates were more reliable as showed by the intra-class correlation coefficient - ρ (no SAD set = 0.792, with SAD set = 0.962). The SAD set proposed here is a useful tool for improving visual assessments of target spot severity on H. serratifolius leaves.