The Diagrammatic scales are important tools to standardize the severity assessments of plant diseases. The objective of this work was to develop and to validate a diagrammatic scale for severity assessment of leaves spot Phaeosphaeria on maize. The proposed scale had the levels of 1; 3; 6; 13, 25; 43; 63 and 79% of severity in the third medium part of the maize leaf. Scale validation was performed by ten raters (five inexperienced and five experienced), who estimated the severity disease in 50 maize leafs showing Phaeosphaeria leaf spot symptoms, with and without the use of the scale. Analyses of regression was used to evaluate the precision and accuracy of the estimates. The evaluation of the accuracy indicated that without the use of the scale seven raters had presented systematic shunting lines in the estimates, whereas with the use of the scale they was only two raters. The precision represented for coefficient of determination (r² ) of the regressions, varied of 0.86 the 0.95 for the inexperienced raters without scale and 0.91 the 0.98 with scale. The proposal diagrammatic scale improved the precision and accuracy of the estimates, independent of the experience of the raters.
Phaeosphaeria maydis; visual stimulus; Webber-Fechner; Zea mays
