This weekend, much of Wisconsin will experience a significant heat wave, with temperatures expected to reach 95–98°F (35–36°C) on Saturday and Sunday. These highs, combined with high humidity, will pose serious challenges for fruit crops across the state—from apples and grapes to strawberries, blueberries, and raspberries.

Under these conditions, plants have limited ability to cool themselves through transpiration. As a result, even well-watered orchards and berry fields can suffer from leaf scorch, sunburned fruit, poor photosynthesis, and increased disease risk. Below are strategies to help mitigate stress and protect your crops.

Practical Steps to Reduce Stress in Trees and Small Fruit Crops:

1. Prioritize Deep, Timed Irrigation

• Apply deep soil irrigation early in the morning (before 9 a.m.) or in the late evening to maintain root-zone moisture and avoid prolonged leaf wetness.
• Focus on young trees and vines, and recently planted berry fields, which are most vulnerable to water stress.
• Avoid overhead irrigation during peak humidity, which can increase disease pressure and offer little cooling benefit.

2. Enhance Soil Moisture Retention

• Apply or renew 2–4 inches of organic mulch around the base of trees and berry plants.
• Keep the root zone weed-free to reduce competition for water.
• Avoid cultivation that may disturb soil structure and expose roots to additional heat.

3. Provide Shade to Reduce Radiant Heat

• Use shade cloth or netting to cover sensitive fruit (e.g., Honeycrisp apples, raspberries, blackberries).
• If using temporary shade structures, ensure airflow is not restricted, as this can worsen humidity-related stress.
• In vineyards or trellised berries, leave enough canopy to shade fruit clusters during the hottest hours. Postpone shoot thinning in grapevines until the heatwave is over.

4. Delay Stress-Inducing Operations

• Do not prune, fertilize, transplant, or apply herbicides during this heatwave.
• These activities can increase metabolic demand or injury risk, exacerbating stress under high heat.

5. Monitor for Sunscald and Fruit Damage

• Watch for white, bronzed, or sunken patches on fruit, especially on west- and south-facing exposures.
• Remove or protect damaged fruit early to reduce further metabolic strain on plants.

6. Ventilate High Tunnels

• For berry crops grown in protected culture, ensure maximum ventilation and remove side panels if possible.
• Avoid misting inside tunnels, which can trap heat and humidity.

7. Scout for Disease After the Heat

• High humidity and warm nighttime temperatures create ideal conditions for fungal pathogens (e.g., leaf spots, fruit rots, downy mildew).
• It’s also important to note that certain fungicides may have limited efficacy or increased risk of phytotoxicity when applied during extreme heat.
  • Growers should monitor canopy conditions closely, adjust application timing to cooler parts of the day (e.g., early morning or late evening), and consult product labels regarding temperature precautions.
• Integrated disease management decisions should factor in both current crop stress and the likelihood of conducive conditions for fruit rot, fire blight, or other key diseases during this heatwave.

This article was posted in Disease, Other News and Resources and tagged Amaya Atucha, Josie Dillon, Leslie Holland.