downy mildew, one of the most economically significant fungal diseases affecting vines, often called peronospora in parts of Europe. It is a particular problem in regions with warm, humid springs and summers such as many wine regions in northern Europe. The disease is caused by the organism Plasmopara viticola. This fungus is indigenous to eastern North America, and so some species of native american vines such as Vitis cordifolia, Vitis rupestris, and Vitis rotundifolia are relatively resistant. Commercially important varieties of vinifera, however, are highly susceptible.