Following significant rain across much of Wisconsin in the past couple of weeks—and more in the forecast—grape growers need to be on high alert for early signs of two destructive fungal diseases: black rot and downy mildew. Both pathogens thrive in warm, wet conditions, and early infections can lead to serious crop loss if not identified and managed promptly.

Recent weather has provided ideal conditions for infection:

• Frequent rain = leaf wetness and spore dispersal
• Warm temperatures = rapid fungal development
• Vine growth = high volumes of susceptible tissue

Both black rot and downy mildew require moisture to infect, and young fruit and leaves are especially susceptible. Infections during this time can go unnoticed until they severely impact yield and fruit quality.

Black rot infections typically start on leaves and quickly spread to fruit clusters. Rainfall triggers spore release from overwintered mummies and canes.

Early Symptoms:

• Leaves: Small, tan or reddish-brown spots with dark borders (“frog eye” lesions)
• Shoots/Petioles: Sunken, dark brown to black lesions
• Fruit: Brown sunken lesions that expand and eventually mummify the berries

Where to Look:

• Basal leaves and lower canopy
• Clusters and rachises
• Areas with poor air circulation

Downy mildew favors wet foliage and humid nights. Look for signs especially following overnight rain or extended leaf wetness.

Early Symptoms:

• Leaves: Pale yellow, oil-spot lesions on the upper surface
• Undersides of leaves: White, downy growth (especially in early morning)
• Shoots or clusters: Brown, water-soaked lesions

Where to Look:

• Shaded areas
• Lower canopy and dense foliage
• Leaves close to the trellis wire or ground

Management Strategies

Scout Immediately and Often

• Walk blocks after rain
• Focus on susceptible cultivars (e.g., Brianna, Marquette, Frontenac, La Crescent)
• Use a hand lens for lesion identification
• Tag or mark infected vines for monitoring

Black Rot Management

• Apply protectant and systemic fungicides ASAP if symptoms are present or weather models indicate infection events
• Effective products:
  • FRAC 3 (tebuconazole, myclobutanil)
  • FRAC 7 or 11 (SDHIs or QoIs, but avoid back-to-back use to prevent resistance)
  • Captan or Mancozeb for broad-spectrum protection

Downy Mildew Management

• Apply fungicides before or immediately after rain when infection risk is high
• Effective products:
  • FRAC 40 (e.g., Revus, Zampro)
  • Phosphonates (e.g., ProPhyt)
  • Mancozeb or Ziram for contact activity
  • Avoid copper on sensitive varieties like Brianna, Foch, or Leon Millot

Cultural Control

• Remove infected leaves or clusters when practical
• Manage canopy:
  • Shoot thinning
  • Leaf pulling
  • Improve airflow and sun exposure
• Remove mummies and infected debris during winter pruning

Use Disease Forecasting Tools

• Log in to NEWA (https://newa.cornell.edu/) and select your local station
• Use the Grape Disease Models
• These models help time fungicide applications based on temperature, leaf wetness, and rain

Fruit is most susceptible to black rot for about 6–7 weeks after bloom, and downy mildew can remain active through mid to late summer during wet periods. With more rain predicted, continued vigilance is key.

Additional disease concerns

Unlike many other grape diseases, powdery mildew does not require free water to infect and is less common during extended rainy periods. However, high humidity—especially in shaded, poorly ventilated areas of the canopy—can still promote its development. Growers should continue to monitor shaded interior leaves and lower canopy zones, where conditions remain humid and infections may persist or initiate despite the rain.

Although Phomopsis spore dispersal typically declines after bloom, many vineyards are now seeing significant symptoms from earlier infections, especially on shoots and rachises. These symptoms are a result of infections that occurred during cool, wet conditions earlier in the season and are now becoming more visible. While new infections are unlikely at this stage, severe symptoms can still impact fruit quality and yield, so continued monitoring is important.

Don’t let early infections compromise your harvest. Timely scouting and strategic fungicide use now will reduce the likelihood of cluster infections, fruit loss, and disease pressure later in the season.

This article was posted in Disease, Grapes and tagged Black Rot, disease, Downy mildew, Grapes, Leslie Holland.