As the growing season progresses in Wisconsin vineyards, now is a good time to revisit an early-season disease that can have lasting consequences—Phomopsis cane and leaf spot. While this disease often goes unnoticed until leaf spots appear, it begins much earlier in the season, and its effects can carry over from year to year if not carefully managed.

Early Symptoms

While scouting your vineyard in spring or early summer, you may notice yellow spots on the edges of newly expanding grape leaves. These yellow lesions often develop small black centers, a clear diagnostic feature of Phomopsis leaf spot. As the disease progresses, shoot lesions can also develop, typically on basal internodes. These appear as black, elongated marks and can lead to cracking and weakening of the shoot.

Later in the season, Phomopsis can cause fruit and rachis infections, although these symptoms may not appear until just before harvest. These fruit lesions are sometimes confused with other fungal diseases, such as black rot. A key difference: black rot appears before veraison, while Phomopsis lesions tend to appear after veraison.

How It Spreads

Phomopsis is most problematic during wet spring conditions. The fungus overwinters in infected canes and produces spores that are spread by rain splash during wet periods between bud break and bloom. Infections are more likely during seasons with prolonged rain events following bud break. Once tissues mature, they become more resistant, so this early window is critical for both infection and management.

The spores don’t travel far, so infections often appear localized—especially around old wood that harbored the fungus over winter. Even dead wood can continue to produce spores for years, making sanitation an essential part of your disease control program.

Susceptibility to Phomopsis

Grape cultivars grown in Wisconsin—particularly those with Vitis riparia or V. labrusca parentage—are often susceptible to Phomopsis. High humidity and wet conditions in early spring, common in Wisconsin, make our vineyards especially vulnerable.

While fruit infections might not be visible until August or September, the damage was likely done much earlier, and management steps should reflect that.

Management Recommendations

Dormant Sanitation

• Remove and destroy infected canes during dormant pruning.
• Look for bleached, greyish canes with tiny black fruiting bodies.
• Reducing overwintering inoculum lowers disease pressure and improves fungicide effectiveness.

Early Season Fungicides

• Focus fungicide applications from bud break through bloom, the window when most spore release occurs.
• Effective products include mancozeb, captan, and ziram. Check pre-harvest intervals.
• Rotate fungicides to avoid resistance, and always follow label directions.
• Consult the latest Midwest Fruit Pest Management Guide for updated fungicide efficacy ratings.

Use NEWA Models

• The NEWA (Network for Environment and Weather Applications) grape disease models can help predict infection periods based on local weather conditions.

Take home

Phomopsis cane and leaf spot is a subtle but serious threat to vineyard health in Wisconsin. Because infections occur early—often weeks before symptoms show—preventive management is key. Focus on sanitation, apply fungicides during the bud break to bloom window, and monitor disease risk using local weather models. By taking action now, you can reduce the risk of yield loss and fruit rot later in the season.

Reminder: Mention of specific fungicides is for informational purposes only and does not constitute an endorsement. Always read and follow the label—it is the law.

This article was posted in Disease, Grapes and tagged disease, leaf spot, Leslie Holland, phomopsis cane, vineyards.