Proposal and validation of diagrammatic scale for assessment of orange rust of sugarcane

The aim of this work was to elaborate and validate a diagrammatic scale to quantify the severity of orange rust of sugarcane and to compare its efficiency in assessing the disease with the scale of brown rust. For the design of the scale, 120 sugarcane leaves were collected from a susceptible cultivar and the highest and the lowest severity founded were selected as extremes of scale. The intermediate levels were calculated according to the law of Weber-Fechner. The scale was proposed with nine levels of severity: 0,06; 0,14; 0,36; 0,89; 2,17; 5,18; 11,87; 24,92 and 45,00%. The validation was accomplished by eight appraisers who estimated the severity of 120 leaves in three assessments: without scale, with scale of brown rust and with the proposed diagrammatic scale. The scale proposed was more efficient than the scale of brown rust to assess the severity of this disease, providing an average coefficient of determination (R-2) of 0,91 and absence of constant and systematic errors. With the scale of brown rust, the average R-2 was 0,84 and three appraisers committed constant errors. Moreover, with the scale proposed, approximately 92% of the estimates of the appraisers had been in the range of variation of 5% of the value of real severity, while in the assessments without scale and with scale of brown rust, approximately 78% and 84% of the estimates were in that range, respectively.