WSU Tree Fruit Research & Extension Center

Stress Physiology

Thursday, June 29, 2017

Dr. Schrader's research has focused on understanding the causes of several stress-induced disorders in apples and cherries (reports available in PDF format) and then developing practical solutions for growers' use to mitigate losses from the disorders.

Tree fruit crops grown in Eastern Washington are frequently exposed to severe environmental stress. Solar radiation is extremely high and often exceeds what can be utilized by trees. The excess light energy is converted to heat energy, especially in the fruit. High air temperatures are prevalent many days during the growing season and also heat the fruit. Consequently, the fruit surface temperature can exceed air temperature by up to 30 °F.

Sunburn Research. For apples, high light and high temperatures lead to several skin disorders such as sunburn. Sunburn is usually the major cause of cullage with losses averaging about 10% in Washington State if no protective measures are used. In the subsection titled "Sunburn Research" the results of several years of research are summarized. Three types of sunburn have been identified and characterized. In research to find ways to protect apples from sunburn, a sunburn protectant called RAYNOX® was invented, and is now the leading sunburn protectant for apples. Evaporative cooling was also studied. Pigment changes that occur as a result of sunburn are described. Changes in internal fruit quality that occur as severity of sunburn increases are also elucidated.

Sunburn Predictor. A predictive model for sunburn browning has been developed, and is available to growers for predicting the risk of sunburn browning occurring under the prevailing conditions in their orchards. Weather forecasts and specific information about cultivar, canopy density, and fruit size can be entered to obtain a prediction about the likelihood of sunburn browning for up to 7 days.

Other Skin Disorders. Several other skin disorders of apple are described in another subsection. Some of these disorders (e.g. ‘Fuji' stain and "sunburn scald" of ‘Granny Smith') appear postharvest during cold storage on apples that were sunburned earlier. Lenticel marking appears before harvest on the sunburned area.

Sweet Cherry Cracking. Another environmental stress is rain that causes sweet cherry cracking. Occasionally, an entire crop is lost to rain . A protectant called RainGard® was developed to help protect cherries from cracking.


Contact Info

Larry Schrader, Horticulturist/Plant Physiologist

Email Address:

Office Location:
Tree Fruit Research & Extension Center, Wenatchee
Office Ph: 509-663-8181 x265
Office Hours: by appointment

Laboratory staff

The contributions of the following former colleagues are gratefully acknowledged: Visiting Professors Dr. Jianshe Sun, Dr. Jianguang Zhang, Dr. Jizhong Xu (all from China) and Dr. Jeong-Hak Seo (from Korea); graduate student David Felicetti; and several Associates in Research who provided excellent technical support. 

Tree Fruit Research & Extension Center, 1100 N Western Ave, Washington State University, Wenatchee WA 98801, 509-663-8181, Contact Us