Elaboration and validation of a digrammatic scale to evaluate soybean bacterial pustule

ABSTRACT Bacterial pustule, caused by the bacterium Xanthomonas axonopodis pv. glycines, is considered one of the most important bacterial diseases affecting soybean. The aim of this study was to develop and validate a diagrammatic scale for assessing the severity of soybean bacterial pustule. To elaborate the scale, soybean leaflets with different severity levels were collected to determine the maximum and minimum values of disease severity, while intermediate levels were established according to Weber-Fechner law of visual stimulus. The values proposed for the scale were 1.4%, 3%, 6%, 9%, 20% and 29.5%. To validate the scale, eight evaluators, with no experience in bacterial pustule evaluation, assessed 39 photos of leaflets with different severity levels, without and with the aid of the scale. The results of severity estimated by the evaluators were related to the actual severity values (determined with Assess 2.0 software from digital images of leaves with symptoms) and analyzed with simple linear regressions to assess the accuracy of evaluators. The precision of evaluators was determined based on the coefficients of determination and the variance in absolute errors. Estimates using the proposed diagrammatic scale indicated a significant increase in the coefficient of determination and lower variance in absolute errors, providing greater precision and accuracy of evaluations, which could be adopted to assess the severity of soybean bacterial pustule.