Development and validation of diagrammatic scale to assess target spot severity in cotton

Due to the emergence of target spot disease on cotton, caused by the fungus Corynespora cassiicola, the aim of this study was the development and validation of a diagrammatic scale to assess the severity of target spot on cotton leaves using Lin's statistic and linear regression models. For the design of the scale, 200 cotton leaves were collected from naturally infected plants. Severity was measured by APS assess image analysis software. The diagrammatic scale was developed with seven levels of severity: 1%, 2%, 5%, 9%, 19%, 37 and 53%. Validation was determined by severity estimates of 50 leaves by five experienced and five inexperienced evaluators. The first assessment was performance without a scale and the second with a scale aid. The data were analyzed by two methods: Lin's statistics and linear regression. The diagrammatic scale aid reduced the absolute and relative error and improved precision (R-2=0.79 and 0.91 without and with the scale, respectively). Evaluators overestimated the severity without the scale. Agreement (Lin's concordance correlation coefficient, p(c)=0.83 and 0.94, without and with the scale, respectively) and accuracy (Bias correction factor, C-b=0.93 and 0.98, without and with the scale, respectively) were improved with the scale. The agreement between experience evaluators was higher with the scale. The use of the proposed diagrammatic scale contributed to assessment of target spot in cotton leaves by improving inter- and intra-rater reliability and accuracy of experienced and inexperienced evaluators.