Grapevines are prone to several fungal diseases, but powdery mildew and black rot are the two a home grower is most likely to encounter.
Identifying the Problem
Powdery mildew appears as a white, dusty coating on leaves and young fruit, while black rot shows up as small brown spots on leaves that spread and eventually shrivel the grapes into hard, dark "mummies." Both fungi thrive in humid conditions with poor air circulation around the vine.
Cultural Controls First
Pruning to open up the vine's canopy, training it onto a trellis for good air circulation, and removing fallen leaves and mummified fruit at the end of the season all reduce the fungal spores available to reinfect the vine the following year. These cultural practices matter as much as any spray in controlling both diseases long-term.
Fungicide Treatment
A sulfur-based or copper-based fungicide applied starting at bud break, and continuing on a schedule through the growing season, is the standard preventive treatment for both powdery mildew and black rot. Once fruit is visibly infected, fungicide can only protect what's left healthy — it won't reverse existing damage.
