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,
- Direct and reflected sunlight
- Heat radiated from the sun and radiated heat from and to the surroundings and the sky
- Heat movement from the sun side of the fruit to the core and the shade side of the fruit
- Water evaporating from the fruit
- Heat conducted from the fruit to the surrounding air
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 http://hort.tfrec.wsu.edu/pages/Sunburn
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.