Assessment of Hemileia vastatrix resistance in chemically mutagenized Coffea arabica L. leaf discs and the emergence of a novel resistance scale

Coffee leaf rust (CLR), caused by Hemileia vastatrix Berk. and Br, stands as a globally significant disease responsible for diminishing the grain yield and quality of Coffea arabica L. As such, the need arises for a practical and dependable method to quantify CLR disease severity and propose effective management strategies. The creation of a diagrammatic scale, featuring color photographs, proves instrumental in enhancing severity assessment and disease estimation. Mutagenesis serves as a potent tool for improving crop traits, notably resistance to pests and diseases, by inducing genetic variability, thus fostering phenotypic diversity. The primary objective of this study was to craft a novel diagrammatic scale for leaf discs, designed to gauge the severity of coffee leaf rust in C. arabica L. cv. Catuaí plants treated with sodium azide (NaN3) and ethyl methane sulfonate (EMS). The assessment encompassed a thorough examination of incidence, severity, incubation period, and latency in plant leaf discs inoculated with CLR isolates, all carried out under rigorously controlled laboratory conditions. The scale’s construction hinged on the frequency distribution of severity and Weber-Fechner’s visual stimulus law, which facilitated the determination of minimum and maximum limits, as well as intermediary levels. The resulting scale comprises seven distinct levels spanning a range from 0 to 52.15% of the afflicted leaf area. Employing conglomerate analysis (Wilks, p < 0.0001) unveiled four groups of progenies exhibiting significantly different (p < 0.05) resistance responses to CLR. Notably, Plant E154 displayed an incidence rate of merely 0.3% for CLR, while the remaining plants, including A123, E6, E64, A127, CES5, A107, A93, E29, E116, and A101, showcased an incidence rate of less than 10%. In conclusion, this scale represents a suitable, valuable, and dependable tool for visually estimating CLR severity in coffee leaf discs.