Which fungus causes powdery mildew in grapevines?

The fungus that causes powdery mildew in grapevines is Erysiphe necator.

Erysiphe necator is a host-specific pathogen of grapevines, including the wine grape Vitis vinifera. It was originally described in 1832 and was long known by the synonym Uncinula necator. The fungus is believed to have originated in North America, where native grape species evolved with greater resistance than many European grape varieties.

The disease appears as a gray-white, dusty coating made of fungal mycelium, conidia, and conidiophores. It can affect leaves, shoots, buds, flowers, and young berries. Infected fruit may grow poorly, split, or contribute to reduced crop yield and poor wine quality. The fungus obtains nutrients through haustoria, specialized structures that draw resources from plant cells beneath the epidermis.

Powdery mildew is easily confused with downy mildew, but the diseases have different pathogens. Downy mildew of grape is caused by Plasmopara viticola, one of the listed distractors. Erysiphe necator spreads through airborne conidia and can thrive without liquid water on the leaf surface; high humidity is often sufficient. Sulfur and fungicides are common control tools, although resistance can develop.