Powdery mildew or odium in English and oïdium in French is a fungal disease organism of fruit (grapes) and foliage (leaves, stalks) caused by biotic factors. It is capable of ruining wine and, ultimately, destroying the vine. The causal agent is Uncinula necator (Oïdium tuckerii). The main method of controlling powdery mildew is via sulfur-based sprays. See also propolis, and roses. Preventative measures include ensuring leaf canopies are not too dense.
Nicolas Belfrage MW, From Brunello to Zibibbo—The Wines of Tuscany, Central and Southern Italy (2nd edition, London, 2003), p.464.