Addressing the Gaps in our Knowledge of Grapevine Downy Mildew for Improved Forecasting and Management

The complex interactions of Plasmopara viticola with environment and host make grapevine downy mildew an ideal candidate for disease forecasting. However, a forecasting model is only as good as the knowledge used to build it, and DMCast is no exception. We addressed some knowledge gaps concerning this disease: (i) initial timing and span of primary infection; (ii) survival of the lesions and sporangia; and (iii) critical period of fruit susceptibility. Experiments revealed that, though emerging shoots are susceptible earlier than previously thought, primary infection frequently occurs near the confluence of a specific host phenological stage and certain weather conditions. Primary infection also may trigger new epidemics later in the season than was traditionally hypothesized. Lesions declined with repeated sporulation cycles but, contrary to prior reports, not age alone. Sporangia died within 8 h on dry, warm days but retained high viability on cooler days. With controlled inoculations, we determined that in the New York climate, fruit of several cultivars (Chardonnay, Riesling, Concord, and Niagara) become resistant to infection by 2 to 3 weeks post-bloom. These studies have clarified several knowledge gaps and long-held assumptions that have direct implications for improving disease forecasting and disease management. Accepted for publication 14 March 2007. Published 26 July 2007.