Diagrammatic scale for rust severity assessment in broad bean (Vicia faba)

Abstract

Uromyces viciae-fabae has increased in economic importance in broad bean-producing regions of central Mexico. It is therefore convenient to have a standardized system to quantify its severity. The objective of this study was to generate and validate a diagrammatic scale to evaluate the severity of rust on broad bean leaves. We collected infected broad bean leaves with different degrees of damage from commercial crops in the Valle de Toluca and the southeastern region of Mexico State. One hundred and ten leaves representing the disease were taken and group discrimination was carried out to select the ranking visually. Then, the leaves were scanned and evaluated to obtain the real severity value for each leaf using the software ©ASSESS 2.0. A six-class diagrammatic scale was generated: 0 (0.0), 1 (>0.1-6.0), 2 (>6.1-12.0), 3 (>12.1-24.0), 4 (>24.1-56.0) y 5(>56.1-<100), and the exactitude, precision, and reproducibility of its estimations were verified. Fifty-eight leaves were evaluated by 20 evaluators without previous knowledge of the disease; the results obtained were analyzed using simple linear regression. In the initial selection evaluation, average r² values of 0.738 were obtained, without scale, while for the evaluation where the proposed scale was used, the average was 0.93, which confirms adequate levels of accuracy and reproducibility that the proposed scale can provide, for this disease.