Elaboração e validação de escalas diagramáticas para quantificação da severidade da mancha parda da mandioca e sazonalidade da doença no agreste meridional de Pernambuco

Abstract

Cassava (Manihot esculenta Crantz) plays an important role in human food. Among the diseases affecting this crop, the brown spot, caused by the fungus Passalora henningsii (Allesch.) R. F. Castañeda & U. Braun, is one the main diseases. The high intensity of this leaf spot, observed in the state of Pernambuco, together with the scarcity of information, demonstrate the need of further studies on this disease. Thus, the objectives of this research were: i) to develop, validate and verify the applicability of two diagrammatic scales to quantify the severity of brown spot on leaves and leaflets of cassava; ii) to study the epidemy of the disease in different areas in the Pernambuco state. To elaborate the diagrammatic scales, cassava leaves showing symptoms of the disease were scanned and the severity value was calculated using the Assess software. Two scales were developed, one to leaves and another to leaflets, each containing eight levels of severity (0.1; 0.6; 1.5; 3.0; 6.0; 11.0; 16.0 and 24.0%). The diagrammatic scales were validated by untrained evaluators, and both produced more accurate and precise estimates of the severity of brown spot of cassava. To assess the seasonality of brown spot in the Pernambuco state, 12 areas of commercial cultivation were selected during the season 2015/2016. The incidence was evaluated by counting the number of diseased leaves and the severity was measured with the aid of diagrammatic scale previously elaborated. This study showed that the disease occurs in all areas evaluated in the Pernambuco state, being mainly related to precipitation. Furthermore, it was found that some cultivars were more susceptible to disease.