A degree-day model to initiate fungicide spray programs for management of grape powdery mildew [Erysiphe necator]

Powdery mildew, caused by Erysiphe necator, is the most important grape disease in Quebec, Canada. Based on the premise that the production of secondary inoculum is a key factor in powdery mildew development, a model based on degree-day accumulation was developed and validated as a tool to initiate a calendar-based fungicide program. The Richard's model was used to describe the proportion of seasonal airborne inoculum as a function of degree-days (base 6 degrees C) accumulated since the Eichhorn-Lorenz grape phenological stage 7 (2-3 fully expanded leaves). The model explained 91% of the variation in proportion of seasonal airborne inoculum and 96% when validated against independent observations. Reliability of the model to time the initiation of a standard fungicide spray program was validated in experimental vineyards from 2004 to 2007. The following management schemes were compared: (1) no fungicides (control); (2) fungicides applied at fixed intervals starting at the 3-4 leaves growth stage; (3) a fungicide spray program initiated based on the degree-day model; and (4) a fungicide spray program initiated based on both the degree-day model and airborne inoculum concentration. Depending on years and cultivars, the use of the model reduced the number of fungicide sprays by 40% to 55%. The degree-day model could be used as a component of a risk management system for grape powdery mildew to estimate the need for fungicide sprays before bloom or to time the initiation of a fungicide spray program.