During this period of extreme heat in Quebec and Ontario, apple growers are being advised to prepare for the extreme heat, the prolonged temperatures, above 30°C, can dramatically increase the risk of sunburn damage in apples.
Fruit exposed to direct sunlight often becomes much hotter than the surrounding air, with surface temperatures commonly rising dramatically. This can push fruit temperatures beyond 48°C, where damage becomes likely.
Researchers identify three types of sunburn injury. The most severe, sunburn necrosis, develops when fruit surface temperatures reach roughly 52°C, resulting in dead tissue and dark, sunken spots that make apples unsuitable for the market.
Sunburn browning occurs at slightly lower temperatures, typically between 46°C and 48°C, when combined with ultraviolet radiation, causing yellow, bronze, or brown discolouration.
Photo-oxidative sunburn can also occur if previously shaded fruit is suddenly exposed to intense sunlight after heavy summer pruning or branch repositioning.
The heat wave may also coincide with an important stage of flower bud formation for next year’s crop, especially in biennial-bearing varieties such as Fuji, Golden Delicious, and Cameo. Excessive heat and dry conditions during this period may reduce future bloom and productivity.
To reduce stress on trees, growers are encouraged to irrigate orchards before and throughout the heat event to maintain adequate soil moisture.
Applying protective products such as kaolin clay, calcium carbonate, wax-based UV protectants, or phospholipid-based cuticle supplements can also help shield fruit from sun damage.
Proper coverage of the sun-exposed sides of the canopy and checking spray compatibility before adding surfactants are recommended for the best results.