Standard area diagrams for assessment of powdery mildew severity on tomato leaves and leaflets

Two standard area diagrams (SADs) to assess the severity of powdery mildew (Leveillula taurica) on tomato leaves and leaflets were developed and evaluated. The SADs are composed of two sets of images of leaves and leaflets with six distinct percentages of diseased areas (1, 5, 10, 20, 40 and 60%). Diagrams were evaluated by 16 raters divided into two groups, based on their former experience in assessing plant disease intensities, using the two sets of images, first without SADs and then using the proposed SADs. Lin's concordance correlation analysis of estimated vs. actual disease severities showed that precision and accuracy were clearly improved when raters were aided by the proposed diagrams, for both leaflets and leaves. Based on coefficients of determinations (R2) and on the intra-class correlations (ρ), the estimates of severity were consistently more reliable when using SADs. Furthermore, when the scales were adopted, the performance of the inexperienced group improved to the same level of that of the experienced group. The diagrams improved subjects' ability to accurately, precisely and reliably estimate tomato powdery mildew severity, and as such can be used to assess severity for studies in epidemiology, quantitative host resistance and in the evaluation of management practices in this important pathosystem. •More accurate field assessments of tomato powdery mildew are now possible.•Diagrams for tomato powdery mildew on leaves and leaflets are made available.•Scientifically-tested diagrams for disease assessment improve raters' performance.