Development and Validation of a Diagrammatic Scale for Estimation of Cashew Blight for Epidemiological Studies

Abstract

Cashew (Anacardiumoccidentale L.) blight is caused by the fungal pathogen Cryptosporiopsis spp. The objective of this study was to develop a scale based according to Weber-Fechner’s visual acuity law, considering the lowest and highest limits of severity observed in the field. A diagrammatic scale with six levels of disease severity (0.1-3.0; 3.1-6.0; 6.1-12.0; 12.1-25.0; 25.1-50.0; 50.1-75.0%) was developed to assess cashew blight severity, caused by the fungus Cryptosporiopsisspp. Scale validation was performed by six raters (three inexperienced and three experienced), who estimated the severity of 48 cashew leaves showing blight symptoms, with and without the use of the scale. Precision and accuracy were determined by linear regression, relating the assessments using the scales to actual severity. Actual severity was assessed with the software Scion image. With the scale, the raters obtained better accuracy and precision levels, although the tendency to overestimate was maintained. Experienced raters were more accurate and precise than inexperienced raters, and assessment improvements with the use of the scale were more significant for inexperienced raters. Two evaluations were performed on the same set of leaves, but in a different sequence order, by the same raters. Using the diagrammatic scales, some raters were able to improve precision and accuracy. Therefore, we could conclude that the diagrammatic scale presented here was suitable for evaluating cashew blight severity.