Standard area diagrams to assess black crust (Phyllachora huberi) severity on rubber tree leaflets

A standard area diagrams set (SADs) was developed and validated to assess the severity of black crust (Phyllachora huberi) on rubber tree leaflets (Hevea brasiliensis). The proposed SADs include images of leaflets with six distinct disease severities (1, 3, 5, 12, 27 and 60%). The SADs was evaluated by 16 raters divided into two groups based on their experience in evaluating plant diseases. The severity was estimated without using the SADs, and then with the SADs. The statistical parameters upsilon, u, r, C-b, and rho(c) improved significantly when using the SADs. C-b (precision) was 0.66 and 0.96 without and with the SADs, respectively. Agreement (rho(c)) between the estimated severity and the actual severity was 0.54 without using the SADs and 0.89 with the SADs, which evidenced that the use of the SADs significantly improved the precision and accuracy. The inter-rater reliability also improved when using the SADs. For the inexperienced raters, the coefficient of determination (R-2) was 0.70 without the SADs and 0.76 with the SADs, and the intra-class correlation (ICC) was 0.63 and 0.84 without and with the SADs, respectively. For the experienced raters, improvements in the inter-rater reliability were more evident (without the SADs, R-2 = 0.71 and R-2 = 0.85 with the SADs, and ICC = 0.81 and ICC = 0.91 without the SADs and with the SADs, respectively). The SADs developed in this study were useful for obtaining more accurate, precise and reliable black crust disease assessments and could be used as support for evaluating diseases in epidemiological studies, breeding for resistance and monitoring for management practices.