By Blake Jackson
A heat dome is expected to bring extremely hot weather to the eastern United States this week, placing Pennsylvania apple orchards at greater risk for fruit damage.
According to Penn State Extension Assistant Professor of Tree Fruit Shanthanu (Shan) Krishna Kumar, temperatures in south-central Pennsylvania, including Adams County and the South Mountain Fruit Belt, are forecast to peak from Wednesday through Friday.
When air temperatures climb above 90°F, the likelihood of apple sunburn increases significantly. Fruit surface temperatures often rise 18 to 27°F above the surrounding air, and under calm conditions can exceed air temperatures by as much as 30°F.
As a result, apples exposed to direct sunlight may reach damaging temperatures above 120°F.
Researchers identify two primary forms of sunburn injury. Sunburn necrosis develops when fruit surfaces reach approximately 126°F, killing cells and leaving dark, sunken lesions that make apples unmarketable.
Sunburn browning occurs at slightly lower temperatures, between 115°F and 120°F, combined with ultraviolet exposure, producing yellow, bronze, or brown discoloration.
A third type, photo-oxidative sunburn, can occur when shaded fruit is suddenly exposed to intense sunlight after heavy summer pruning or branch repositioning.
High temperatures also coincide with a critical stage of flower bud development for next year's crop, particularly in biennial-bearing varieties such as Fuji, Golden Delicious, and Cameo.
Heat and drought stress during this period may reduce bloom potential and lower production in the following season.
Growers are encouraged to irrigate orchards before and during the heat event to help trees cool naturally and reduce stress. Adequate soil moisture becomes increasingly important during prolonged periods of extreme heat.
Protective spray products can also help minimize fruit injury. Options include kaolin clay particle films, calcium carbonate coatings, wax-based UV protectants, and phospholipid-based cuticle supplements.
Thorough coverage of sun-exposed fruit, especially on the south- and west-facing sides of the canopy, is essential for effective protection. Growers should also confirm compatibility before adding surfactants to spray mixtures to improve coverage.
Photo Credit: gettyimages-richard-7
Categories: Pennsylvania, Crops, Fruits and Vegetables, Weather