Uncinula necator (syn. Erysiphe necator) is a fungus that causes powdery mildew of grape. It is a common pathogen of Vitis species, including the wine grape, Vitis vinifera. The fungus is believed to have originated in North America. European varieties of Vitis vinifera are more or less susceptible to this fungus. Uncinula necator infects all green tissue on the grapevine, including leaves and young berries. It can cause crop loss and poor wine quality if untreated. The sexual stage of this pathogen requires free moisture to release ascospores from its cleistothecia in the spring. However, free moisture is not needed for secondary spread via conidia; high atmospheric humidity is sufficient. Its anamorph is called Oidium tuckeri.
This mildew can be treated with sulfur or fungicides; however resistance to several chemical classes such as Benomyl, the DMIs, and Strobilurins has developed. While synthetic fungicides are often recommended as applications around bloom, it is common to include sulfur in a tank mix to help with resistance management.