WSU Tree Fruit Research & Extension Center

Interactive sunburn model

Thursday, June 29, 2017

Model description

This model is designed specifically for prediction of sunburn browning in apple fruit for up to one week.  The model is based on several years of research on apple fruit sunburn at the WSU Tree Fruit Research and Extension Center, Wenatchee, WA. Sunburn browning occurs when the fruit surface (peel) temperature exceeds a threshold temperature. This threshold varies among different apple cultivars from 113 to 120 °F. NOTE: This model will not predict the occurrence of photooxidative sunburn which is not dependent on air temperature and occurs when previously shaded apples are suddenly exposed to high light.  The first symptom is whitening of the sun-exposed side and can occur during thinning or summer pruning.  For further information go to ( ). 

Many factors including solar radiation, air temperature, and windspeed influence peel temperature, which can exceed air temperature by 18 to 27 °F on a calm, clear day when relative humidity is low. It is the interaction of several factors that determine whether sunburn browning occurs on a given day. These interactions serve as the basis for this predictive model. The current model is designed to predict sunburn browning incidence of ten cultivars from June to October. Users need to select the cultivar, fruit diameter, and canopy density from pulldown menus. Meteorological data based on weather forecasts for the users' site can then be entered to predict whether or not sunburn browning will occur.

If the model predicts that "Sunburn browning is expected", users should employ necessary remedial measures before sunburn occurs (for example, use evaporative cooling or apply a particle film).

If the model predicts that "Sunburn browning will probably happen!", user should check the weather status from 11 AM to 2 PM of that day. During this day, there is a high risk of sunburn incidence if

  1. sky is clear (average solar radiation is higher than 700 W/m²; available from AgWeatherNet);
  2. average ambient temperature is higher than 86 °F (30 °C);
  3. average relative humidity is lower than 26%; and
  4. average windspeed is less than 3 mph.

The user will probably want to implement remedial measures to protect the fruit.

Sunburn Prediction Calculator

Please note: the following model is under development and currently being tested. At this time, it should not be used for commercial orchard management. However, your comments are requested!

Please select and input data


Fruit diameter

Canopy density

 Air temperature °F 
DayMaxMinSkyWind (mph)HumiditySunburn


Model developed by Jianguang Zhang and Larry Schrader. Send comments to Dr. Larry Schrader (

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

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