Diagrammatic scale for assessment of late season leaf diseases in soybean

The occurrence of late season leaf diseases in soybean (Glycine max) caused by the fungi Septoria glycines and Cercospora kikuchii are easily identified in field. However, there is a need for more precise information to quantify damage and yield losses. The lack of a standardized visual method may lead to inaccurate estimates of severity and innaccurate conclusions. With the purpose of elaborating a diagrammatic scale to assess these diseases, leaves with different levels of severity were collected in the field. The area of each leaf and its correspondent severity were determined, and following the "Stimulus Law by Weber-Fechner", a scale was elaborated with severity levels: 2.4, 15.2, 25.9, 40.5 and 66.6%. Validation was carried out by nine appraisers, without previous practice in assessing late season leaf diseases, who estimated the severity on 30 leaflets of soybean (Glycine max) with disease symptoms. The variance found among appraisers (0.84<R²<0.65), and accuracy (0.00<a<3.40; 0.90<b<1.29), did not constitute systematic mistakes in over or underestimating the severity of disease. The developed diagrammatic scale is, therefore, appropriate to quantify late soybean season disease severity

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