Coffee leaf rust assessment: comparison and validation of diagrammatic scales for Coffea arabica

Coffee leaf rust (CLR) is the most important disease in coffee and is caused by Hemileia vastatrix. The use of a diagrammatic scale helps people more effectively evaluate rust severity and improves disease measurement by evaluators. Our goal was to develop a new scale with colored pictures and seven disease levels (0; 0.1-1.0; 1.1-2.0; 2.1-5.0; 5.1-10.0; 10.1-25.0; >25.0%) and then compare the severity results of CLR on arabica coffee leaves with those based on three other scales. Three evaluations were performed by ten different evaluators. The first assessment was performed without a scale. On the same day, raters performed four assessments with each of the four diagrammatic scales. The third evaluation was performed at seven-day intervals. We analyzed the statistics with linear regression and Lin's concordance correlation. The evaluators using the proposed scale improved the precision, accuracy and reproducibility of the evaluations and reduced residual distribution when compared to the evaluators who did not use the proposed diagrammatic scale or who used the other scales. Overall, the proposed diagrammatic scale is a tool that can assist users in producing a disease estimate close to the real value of CLR on arabica coffee leaves.