Downy Mildew of Grape

Downy mildew is one of the most serious diseases threatening untreated grape foliage in the United States. The malady is caused by a fungus namedPlasmopara 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.