Amanda Green, Tree Fruit Specialist, OMAFRA
Temperatures are going to be hot this weekend and into next week. It will be hot enough to potentially cause sunburn damage. Sunburn browning occurs when the surface of the fruit reaches a temperature of 45-46˚C or greater for at least one hour (Schmidt, 2018). The fruit surface temperature (FST) can be 14-17˚C greater than the air temperature if fruit is in direct sunlight and there is a calm wind and low humidity (Schmidt, 2018). Therefore apples may be susceptible to heat stress when the air temperature is above 30˚C. Note that these are ambient air temperatures and not humidex temperatures. Humidity is actually negatively correlated with the FST (Schrader et al. 2003). For example, if the ambient temperature is 30˚C and the relative humidity is 50%, the FST will be lower than if the ambient temperature is 30˚C and the relative humidity is 25%.
Sunburn preventative measures include evaporative cooling with overhead irrigation (if you have the infrastructure) and sunburn protectant products like kaolin clay (ie Surround WP) or a calcium carbonate product. Keeping the soil moisture at optimum levels with irrigation will ensure that the trees have adequate moisture to transpire and help keep the fruit surface temperature from greatly increasing.
Schmidt, T. 2018. Apple Sunburn 101. WSU Tree Fruit. http://treefruit.wsu.edu/article/apple-sunburn-101/
Schrader, L., J. Zhang and J. Sun. 2003. Environmental stresses that cause sunburn of apples. Act Hort. 618