Apple Growers Warned Of Heat Stress In The Coming Days

With hot, dry weather in the forecast across much of the region, Washington State University says now is the time to protect your apple crop from heat damage. Fruit exposed to direct sunlight can be much hotter than the air around it, increasing the risk of sunburn, fruit drop, and reduced storage quality.

School officials note heat stress symptoms include:

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.

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.
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.

Before the heat arrives, WSU advises that growers make sure orchard soil moisture is adequate and adjust irrigation schedules as needed. If water is limited, prioritize irrigating the root zone.

Consider applying reflective or UV-protective sprays before temperatures climb, and if your operation has evaporative cooling, hydrocooling, or fogging systems, be ready to use them when temperatures reach the mid-80s to 90 degrees.

Examples of protective sprays 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.

Shade netting can also help reduce fruit surface temperatures. 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.

WSU also advised to avoid summer pruning or other canopy-opening activities during extreme heat, since suddenly exposing fruit to direct sunlight can increase sunburn risk.