WSU Tree Fruit Research & Extension Center

TFREC Computer & Web Resources

Good control of sunburn on apples returns benefits to the growers who would otherwise potentially lose over 12% of their crop. The best control growers have is over the skin temperature of exposed fruit (FST) using evaporative cooling, sunburn protectants, or both (see WSU Crop Protection Guide for Tree Fruits). However, this requires recognition of the factors contributing to FST.

Fruit, sun, and water

WSU-TFREC researchers have found that FST in excess of 113—115°F leads to sunburn. In the orchard direct sunlight can lead FST to exceed air temperatures by 25—30° F. This means that under very average weather conditions sunburn can occur.

Fortunately, as detailed in the WSU Crop Protection Guide for Tree Fruits, growers have a couple tools that can be used separately or together:

  • evaporative cooling by water
  • protectant sprays to reduce sunlight on the fruit


"Energy balance" model

The energy balance model brings out the best strategies to use with these tools. It's based on the relationship between FST and the balance of energy sources at the fruit surface.

Sources of energy, both to and from the fruit skin, are as shown in the figure,

  1. Direct and reflected sunlight
  2. Heat radiated from the sun and radiated heat from and to the surroundings and the sky
  3. Heat movement from the sun side of the fruit to the core and the shade side of the fruit
  4. Water evaporating from the fruit
  5. Heat conducted from the fruit to the surrounding air

The overall energy balance consists of the light coming to the fruit surface and the loss of energy from the fruit by either evaporating water from the fruit surface or the loss of heat to the surroundings.

On the sunlight side of the balance growers can use sunburn protectants to reduce the light reaching the fruit surface.

Wind and fruit size affect what's termed the "boundary layer." This is a layer of still air that builds up on the fruit surface. It insulates the fruit reducing the rate of energy transfer through both heat loss to the air and evaporation of water.

The smaller the fruit the more shallow the layer. Consequently, under similar conditions small fruit are better able to lose heat than large fruit. WSU researchers have collected data to indicate that sunburn damage before July may be uncommon in Washington.

On the other hand, there are some data that indicate larger fruit in September are at a higher risk than fruit under similar conditions in July and August.

Wind removes or changes the depth of the boundary layer. Data show that winds of a light breeze or more can reduce FST by five degrees or more. When the conditions for sunburn risk are otherwise marginal, a slight breeze can eliminate the need for evaporative cooling.

Evaporative cooling operates as long as there is water present on the fruit surface. Apple fruit have no stomates so they are not cooled by transpiration, the process by which leaves are cooled. It's worth noting that evaporative cooling is effective only during actual time periods of sunburn risk.

Considerable research has been done on the best way to apply water for evaporative cooling. Considerations to reduce disease potential and conserve water suggest that when possible water should be applied in cycles: water would be cycled on to wet the fruit surface, and then cycled off until the surface is dry. However, once the surface is dry, evaporation is no longer a source of cooling and FST can rapidly increase to temperatures resulting in sunburn damage.

Model calculators

A fruit skin temperature calculator is available for this model to illustrate the relative effects of different factors. However, it is not designed for orchard management decisions.

A more empirical version is currently available for testing at


Schrader, Larry, Jianguang Zhang, and Jianshe Sun. 2003. Environmental stresses that cause sunburn of apple. Acta Hort. 618: 397—405.

Evans, Robert. 2004. Energy balance of apples under evaporative cooling. Transactions of the American Society of Agricultural Engineers 47(4): 1029—1037.

Campbell, Gaylon. 1997. An introduction to environmental biophysics. Springer-Verlag, New York.

Ritenour, M.A., S. Kochhar, L.E. Schrader, T.P. Hsu, and M.S.B Ku. 2001. Characterization of Heat Shock Protein Expression in Apple Fruit Peel under Field and Laboratory Conditions. J. Amer. Soc. Hort. Sci. 126(5):564-570.