Downy mildew is one of the most serious diseases threatening untreated grape foliage in the United States. The malady is caused by a fungus named Plasmopara viticola, which occurs in humid and rainy climates where temperatures are moderate to cool. As evidenced by this micrograph, combining phase contrast microscopy with classical histological staining techniques often yields enhancement of cellular features.

The mildew fungus attacks green portions of the grape vine and leaves, causing rot and defoliation. Fruit obtained from infected plants typically has a low sugar content and is unsuitable for commercial or domestic use. All common species of cultivated and wild grapes are attacked by the fungus, including those from Europe and Asia.