Development of grape downy mildew (Plasmopara viticola) under northern viticulture conditions: influence of fall disease incidence

In Eastern Canada, several of the grape (Vitis spp.) cultivars susceptible to downy mildew (Plasmopara viticola) are hybrids (e.g. 'Chancellor' and 'Vidal') that are protected with soil or geotextile fabric during the winter months. This practice, although useful for protecting vines from winter injuries, provides shelter for P. viticola oospores during adverse winter conditions. It is thus expected that a large proportion of P. viticola oospores will overwinter. The objective of this study was to investigate the relationship between fall downy mildew incidence and disease development in the following spring. This relationship was established using data collected from 2008 to 2011 in plantings with the cultivars 'Chancellor', 'Vidal', and 'Seyval Blanc' in plots with fall mildew incidence of 0 % to 2.5 %, > 2.5 % to 5 %, > 5 % to 10 %, and > 10 % to 20 %. Downy mildew severity was assessed weekly from bud break until harvest, and the proportion of leaf area diseased at 25 % bloom and at harvest, the area under the disease progress curve, the time to 50 % maximum disease, the rate of disease progress, and the yield were estimated. Regardless of the cultivar and year, fall mildew incidence had a significant effect on mildew progress. Higher fall mildew incidence was associated with earlier development of symptoms and higher severity of mildew at bloom in the following spring. This information could be used to determine the most appropriate strategy to manage downy mildew during the following spring. However, more research is needed to determine how management strategies should be adapted to the various levels of risk.