Validation of a diagrammatic scale to quantify the severity of corn leaf anthracnose

ABSTRACT The occurrence of foliar diseases in corn (Zea mays), caused by fungi, is easily diagnosed in the field. However, tools to obtain accurate information on disease quantification are necessary. The objective of this study was to develop a diagrammatic scale to estimate the severity of leaf anthracnose caused by Colletotrichum graminicola in corn. To elaborate the diagrammatic scale, 100 corn leaves showing different severity levels of the disease were collected and taken to the laboratory for selection and capture of images using Quant v.1.0.2 software. The images were analyzed by 22 experienced evaluators and by 13 inexperienced evaluators. Once the actual severity degree was known, regression analysis was used to determine the relationship between the actual severity degree and the estimated severity degree with the use or without the use of the scale. The accuracy of the evaluator was determined based on the coefficient of determination (R2) and variance. The proposed scale, with 18 distinct severities, showed to be an efficient tool to quantify the severity of anthracnose. The accuracy of all evaluators increased when the scale was used since, among the experienced evaluators, 14 out of 22 increased their accuracy and, among inexperienced ones, nine also had their accuracy improved by using the proposed scale. Therefore, the gains were higher for evaluators without experience, 69.2%.