As summer continues with our commonly seen high temperatures throughout our apple growing regions, heat stress mitigation should be considered. Like us, when apples are exposed to prolonged high temperatures, damage can occur. Being aware of symptoms and mitigation strategies will help you make informed decisions. If you haven’t already, now is a good time to review orchard heat stress management practices that can reduce heat related injury in apple orchards.
Heat stress symptoms:
- Leaf wilting, marginal leaf scorch, and reduced shoot growth.
- Premature fruit drop due to increased ethylene production.
- Sunburn damage on exposed fruit.
- Increased risk of storage disorders and reduced fruit firmness after harvest.
Sunburn injury occurs when fruit surface temperatures exceed critical thresholds, often far above ambient air temperatures. Fruit exposed to direct sunlight can be 18–30°F warmer than the surrounding air. WSU AgWeatherNet has a fruit surface temperature model to help monitor orchard conditions. Common sunburn types include:
- Sunburn browning: Yellow to brown discoloration that can occur when fruit surface temperatures reach approximately 113–120°F. WSU DAS has a prediction model and .
- Sunburn necrosis: Cell death and dark, sunken lesions when fruit surface temperatures exceed about 126°F.
- Photo-oxidative sunburn: Bleaching injury that occurs when previously shaded fruit are suddenly exposed to intense sunlight.
Mitigation Strategies
Maintain Adequate Soil Moisture
Trees under water stress lose their ability to cool themselves through transpiration. During hot periods:
- Ensure soil moisture is adequate before heat arrives.
- Consider more frequent irrigation scheduling during prolonged heat events.
- Continue monitoring moisture levels for several days after temperatures moderate.
- If water is limited, choose to irrigate ground first before overhead cooling methods.
Protective Sprays
Reflective and UV-protectant materials can provide a lower cost strategy that can help reduce radiation stress and sunburn risk when applied before heat events. Good coverage of sun-exposed fruit is essential for effectiveness.
Examples include:
- Kaolin clay (e.g. Surround WP) is commonly used and works by reflecting solar radiation
- Calcium carbonate-based films (e.g. Eclipse, Diffusion, MicroCal) works similar to kaolin clay where it reflects solar radiation although it does not bind to the fruit surface, thus requiring frequent reapplication during prolonged heat events.
- Wax-based protectants such as Raynox reduce transmission of light radiation reaching the fruit. Do not apply when air temperature exceeds 85°F.
Cooling Methods: Evaporative Cooling, Hydrocooling, and Convective Cooling (Fogging)
These three cooling methods operate differently but activation threshold temperature is typically 85–90°F.
- Evaporative cooling involves overhead irrigation applied to the leaves and fruits of trees on a cycle of water application and evaporation. It cools through water evaporation and air movement. Often it operates in addition to the irrigation system for the orchard. Timing, frequency, and volume of overhead irrigation cycling will vary depending on conditions, time of day, wind, fruit temperature, tree canopy size, and other factors.
- Hydrocooling applications are the same as evaporative cooling, except it continuously operates during the hottest parts of the day. It cools through water runoff. This system is more effective than evaporative cooling in reducing orchard temperatures and can replace irrigation systems.
- Convective cooling or fogging is applied as a continuous fine mist that evaporates before reaching the leaves and fruit. As the name “convective cooling” implies it cools via circulation of cool air. This system requires a parallel irrigation system.
Netting Provides Additional Protection
Protective netting decrease incoming solar radiation, lowering fruit surface temperature by approximately 4.7-7.7°F compared to fruit exposed to full sunlight. Cooling effectiveness declines when air temperatures approach 105°F. Red color development can be reduced. To minimize impacts on color, consider retracting nets about 10 days before harvest. Industry standard is white netting with approximately 20% shade. While netting represents a significant investment, it can provide protection from not only sunburn but from hail and bird damage.
Other Considerations – Delay Canopy-Opening Activities
Avoid practices that suddenly expose fruit to direct sunlight during or immediately before extreme heat events, including:
- Summer pruning
- Limb positioning
- Aggressive hand thinning
These activities can increase the risk of photo-oxidative sunburn.
Looking Ahead
As climate projections indicate more frequent periods of extreme heat, evaluating long-term strategies such as orchard site selection, cooling infrastructure, cultivar choice, and protective netting may become increasingly important for maintaining fruit quality and profitability.
To tailor your mitigation method, accurate fruit surface temperature monitoring is crucial. AgWeatherNet Crop AI app paired with thermal-RGB cameras offer conservative estimates that isolate the hottest 20% of pixels on fruits that effectively identify localized “hot spots” of sunburn risk.
Keep an eye out for improved research-based actionable heat and cold mitigation management strategies from the SPARC project. This project brings together a team of scientists led by Dr. Kalcsits who are working to provide actionable solutions to pear and apple growers who are facing difficulties from changing heat and cold conditions.
Contacts
Lexie McDaniel
WSU Tree Fruit Horticulture Extension Educator
Alexa.mcdaniel@wsu.edu
Lee Kalcsits
Associate Professor and Endowed Chair for Tree Fruit Environmental Physiology and Management
Lee.kalcsits@wsu.edu
Additional information
- Amogi B, Khot L, and Sallato B. 2025. Managing Summer Heat for Better Color in Cosmic Crisp® Apples. Washington State University Tree Fruit.
- Bolivar-Medina, J.L., and L. Kalcsits. 2022. Cooling Mechanisms for a Tree Fruit Orchard. Washington State University Tree Fruit.
- Campbell T, Kalcsits L, and Moyer M. 2024. Managing Heat Stress in Apple Orchards. Washington State University Extension Publication TB103E.
- Mupambi, G., D. Layne, L. Kalcsits, S. Musacchi, S. Serra, T. Schmidt, and I. Hanrahan. 2019. Use of Protective Netting in Washington State Apple Production. Washington State University Extension Publication TB60E.
- Penn State Extension: Sunburn Risk This Week in Pennsylvania Orchards: Mitigation Strategies (June 2026).
- Thennakoon N, Bhalekar D, Sallato B, Khot L. From Invasive to Infrared: Tools for Quantifying Heat Stress in Apples. Washington State University Tree Fruit.
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