What Causes Downy Mildew of Grapes?

Downy mildew is a serious leaf and fruit disease.

Causes

The fungus overwinters in infected leaves of the previous year's crop on the ground. Spores infect leaves and fruit throughout most of the season. Leaf lesions typically appear as bright yellow spots on the upper surface of the leaf, with white fungal growth villi appearing almost on the surface below these lesions.

Characteristics

Fruit infection most often occurs on the lower half of the cob and first manifests as a firm brown rot that causes the berries to dry out and fall off prematurely.

Timing

This disease is generally more severe during the wet season and has the potential to cause complete defoliation in late summer, leaving the plant highly susceptible to damage from cold temperatures during the winter.