Linear relationship between leaf incidence and severity of onion downy mildew
ABSTRACT Occurrence of onion downy mildew can reduce the productivity and compromise the bulb quality. The aim of this study was to evaluate the relationship between incidence and severity for the intensity of onion downy mildew. The experiment was conducted with cultivar EMPASC 352 – “Bola Precoce” at Catarinense Federal Institute/Rio do Sul Campus. The design was in randomized blocks, including four replicates and six treatments consisting of fungicide spraying according to the forecasting system proposed by Wallin (1962), with accumulation of 6, 8, 10, 12 daily severity values (DSV), compared to application at every 5 and 7 days and control without spraying, in order to generate a disease intensity gradient for 2014 and 2015. The disease intensity was weekly evaluated based on the severity and incidence. The obtained data underwent linear regression analysis. Using the equation that best fit in the two seasons and replacing the severity value of 1%, incidence of 5.7% was obtained. Severity estimated based on incidence and vice versa can be used by technical assistance in the monitoring of onion downy mildew. As the incidence evaluation is more rapid, more accurate and reproducible, according to the obtained data, severity can be estimated based on incidence, facilitating downy mildew quantification by technical assistance.