DIAGRAMMATIC SCALES FOR BEAN DISEASES - DEVELOPMENT AND VALIDATION

Diagrammatic scales were developed for anthracnose, rust, angular leaf spot, and Alternaria leaf spot of beans. Scales were developed accord ing to Weber-Fechner's visual acuity law, considering the lowest and h ighest limits of severity observed in the field. Thr maximum severin l imits represented in the scales were 22.5%, 24%, 30.4%, and 45%. for r ust, anthracnose, angular leaf spot, and Alternaria leaf spot respecti vely. Six levels of severity were represented for rust, nine for anthr acnose, nine for angular leaf spot, and seven for Alternaria in leaf s pot. Scales were validated by five experienced raters who utilized 70 to 90 leaflet samples with different levels of severity for each disea se. The scales permitted assessments to be accurate (intercepts of the regression lines between actual and estimated severity nor different from 0 and angular coefficients near I), and precise (90 % of the asse ssments with R-2 > 0.80) for ali diseases. The absolute error in estim ating severin, for any scale and rarer, was always lower than 15%. The reproducibility of the assessments, estimated by the coefficient of d etermination of regression lines between estimates of severity of the five raters (pairwise in ail combinations for all thr diseases), was h igh (R-2 > 0.76) for rust, anthracnose, and angular leaf spot. The sca les proved to be adequate Fur assessments of severin in the field and have been utilized to develop disease progress curves, and to determin e damage functions for bean crops.