DIAGRAMMATIC SCALE TO EVALUATE TARGET SPOT SEVERITY IN CUCUMBER PLANT LEAVES

Abstract

The lack of a diagrammatic scale to evaluate the cucumber target spot, caused by Corynespora cassiicola, led to the development of an assessment system for a faster and more reliable way to quantify this disease. So, a diagrammatic scale was elaborated considering the maximum and minimum severity limits of the disease observed in the field. The intermediate values followed logarithmic increments for the target spot symptoms (0.3%, 0.8%, 2%, 5%, 11.5%, 25%, and 46%). For validating the scale, eight raters quantified the disease severity on fifty leaves infected with different levels of the disease. Initially, the severity estimation was carried out without the aid of the scale. Then, the same raters used it to estimate the severity on the same leaves previously assessed. Assessments with the aid of the diagrammatic scale were more precise and accurate, according to most of the raters. The proposed diagrammatic scale was considered adequate to estimate the target spot severity in cucumber leaves, and it can be used in epidemiologic studies and in evaluation strategies for controlling this disease.