Comparison of methods to assess severity of common rust caused by Puccinia sorghi in maize

Abstract

Disease severity evaluation is an important decision support for adoption of strategies and tactics for disease control. The most commonly used method to assess disease severity is visual, but the problem is repeatability, due to subjectivity and imprecision of estimates. For Puccinia sorghi, a threshold of action of 1% severity was determined, so high precision is required in disease quantification. The aim of this study was to compare different assessment methods and analyze their association. Two diagrammatic scales were used to estimate severity, the Peterson and Amorim scales. Pustules were counted with the naked eye and with a 20x magnification hand lens. Software for disease quantification, Assess 2.0, was used to determine actual percentage area and lesion count. No significant differences were found between naked-eyed count and with magnifier. Lesion count with Assess 2.0 gave an imprecise result. Significant differences were found between diagrammatic scales. Compared with Assess 2.0, severity using Peterson was 2% higher, showing widely scattered differences (R2=0.48). Overestimation with visual scales was suggested, especially at low severity levels. Counting pustules was more objective, precise and reproducible. Thus, a calibration curve was constructed (R2=0.79), which will allow calculation of severity from counting pustules. Copyright by the Brazilian Phytopathological Society. Printed in Brazil.