My Grape Leaves Are Turning Yellow in June

Identification

• Grapevines are susceptible to powdery mildew caused by Uncinula necator. The Cornell Cooperative Extension cites the disease as the most important fungal grapevine disorder in the world. Varieties of native American grapes are generally less prone to infection as compared to European and French grapes.

Damage

• Disease symptoms start on grapevines during summer with a dusty or powdery fungal presence on the upper sides of foliage and other areas of the vine. Affected leaves gradually turn yellow to brown and start to fall. As the disease progresses, infection spreads to the fruit. Grapes harden and brown and cease to mature. Small, black fungal fruiting bodies are seen on the fruit.

Management

• Grow grapevines in areas of less humidity, good air circulation and good light. Organic control options include the use of sulfur-based fungicides. Other recommended fungicides include azoxystrobin, myclobutanil or fenarimol.