Development and Validation of a Standard Area Diagram Set of Leaf Rust Severity in Mulberry (Morus nigra L.)

Abstract

Standard area diagrams sets (SADS) are useful tools to improve accuracy and reliability of the estimates of disease severity in many pathosystems. This study developed and validated a SADS for quantifying the severity of mulberry leaf rust, caused by Cerotelium fici. The proposed scale is composed of eight images representing increasing levels of severity (1%, 3%, 5%, 10%, 15%, 25%, 35% and 45%). Twenty raters (10 experienced and 10 inexperienced) evaluated the SADS. The raters estimated the same set of 50 images twice, first without the SADS and then with the use of the SADS. The use of the SADS significantly improved the accuracy of the estimates. The bias correction coefficients (C_b) were 0.75 and 0.98 for inexperienced raters, and 0.99 in both situations for experienced raters, without and with the use of the SADS, respectively. The correlation coefficients (r) increased from 0.85 to 0.96 among inexperienced raters and from 0.94 to 0.96 among experienced raters. Lin's coefficients of concordance (ρ_c) increased from 0.64 to 0.94 for inexperienced raters and from 0.93 to 0.95 for experienced raters, also comparing the assessments without and with the use of the SADS, respectively. Without the use of the SADS, experienced raters performed statistically better than inexperienced raters in all variables analysed. However, with the use of the SADS, no significant differences were observed between the groups, demonstrating the potential of the tool to reduce the effect of prior experience on the accuracy of estimates. Furthermore, the coefficient of determination (R²) showed a significant increase with the use of the diagrammatic scale in both groups of raters. The SADS validated in this study constitutes a robust, accurate, and accessible tool for quantifying the severity of mulberry leaf rust, contributing to the advancement of epidemiological studies, the selection of resistant genotypes and disease management under field conditions.