Sunburn Risk This Week in Pennsylvania Orchards: Mitigation Strategies

This week's forecast

A heat dome is moving into the eastern United States this week. South central Pennsylvania, including Adams County and the South Mountain Fruit Belt where most of the state's apples are grown, is in the forecast area. The National Weather Service is calling for the hottest stretch Wednesday through Friday, with highs around 99°F on Wednesday, 102°F on Thursday, and 101°F on Friday. Heat index values are expected to reach 100 to 110°F due to high humidity. Overnight lows will only drop into the mid to upper 70s. Hot, humid conditions are expected to continue into the Independence Day weekend.

These are the conditions under which apple sunburn becomes a real risk. In general, air temperatures above 90°F are when sunburn risk increases substantially, and multiple mitigation practices should be used when air temperatures exceed 100°F for several consecutive hours or days. This week's forecast meets all three risk factors: high temperature, multi-day duration, and high heat index. Low wind speeds during heat dome events add to the risk, since wind normally helps move heat away from the fruit surface.

How sunburn damages the fruit

Sunburn is caused by the temperature of the fruit surface itself, not air temperature alone. Apples cannot dissipate excess solar radiation the way leaves can, so on a calm, sunny day, the temperature of the fruit surface commonly runs 18 to 27°F above air temperature, and under still wind conditions it can run up to 30°F above air temperature. A 100°F afternoon can put sun-exposed fruit surfaces well above 120°F.

Research has identified two distinct types of heat injury in apple:

• Sunburn necrosis occurs when the fruit surface reaches approximately 126°F for as little as 10 minutes. At this temperature, epidermal and sub-epidermal cells die. Membrane integrity collapses, electrolyte leakage increases, and a necrotic, sunken spot forms on the sun-exposed side of the fruit. This damage is immediate and the fruit is unmarketable.
• Sunburn browning is a sublethal injury that occurs at slightly lower fruit surface temperatures, roughly 115 to 120°F, combined with UV exposure. It causes chlorophyll breakdown and pigment changes that leave a yellow, bronze, or brown blemish. This damage is not reversible, though anthocyanin development as the fruit colors up can sometimes partially mask it later.

A third form, photo-oxidative sunburn, can occur when fruit that has been shaded for most of its development is suddenly exposed to direct light, for example after aggressive summer pruning or limb repositioning during a heat event. This is one reason to hold off on canopy-opening work until this heat breaks: fruit that has not acclimated to direct light is more vulnerable to this type of injury.

This week is also next year's bud initiation window for cultivars such as Fuji

There is a second reason to keep trees well-watered through this stretch. This is also the period when next year's flower buds are initiated, and the timing is particularly important for biennial-prone cultivars like Fuji. Floral induction and initiation generally occur somewhere between 30 and 90 days after full bloom, depending on cultivar. Cultivars prone to biennial bearing tend to initiate earlier in that window. Fuji has more than half of its spur flower buds initiated by roughly 65 days after full bloom, earlier than consistently cropping cultivars like Gala, which peaks at 85 to 90 days after bloom. For most Pennsylvania orchards, with full bloom typically in late April to early May, that puts late June through July right now inside the floral initiation window for Fuji and other alternate-bearing cultivars such as Golden Delicious and Cameo. Heat and water stress during this window has been linked to reduced return bloom the following spring. Irrigation this week is not just about preventing sunburn. A heat-stressed tree now can also mean a lighter crop in 2027, which is one more reason to prioritize these blocks for water.

Mitigation options for growers in PA

Photoselective netting and overhead evaporative cooling are the two most effective sunburn countermeasures used in the Pacific Northwest, but both require infrastructure investment that most Pennsylvania growers do not have in place. There are still useful, lower-cost actions you can take this week.

Irrigate ahead of the heat

Trees that go into a heat dome already short on water cannot cool their canopy and fruit as effectively. Mature apple trees need roughly 1 to 2 inches of water per week under normal conditions; during a multi-day 100°F+ stretch, that need rises well above baseline.

Today and tomorrow: Run a full, deep irrigation set to recharge the root zone (top 12 to 18 inches) before Wednesday's heat arrives. A heavy set once temperatures spike is too late to help that day.

Wednesday through Friday: Switch to shorter, more frequent sets (daily or every other day) rather than your normal weekly schedule. Heat demand will draw down soil moisture faster than usual.

Irrigate early morning. This limits evaporative loss and avoids wet foliage overnight.

Don't stop as soon as the heat breaks. A multi-day deficit needs more than one irrigation set to recover. Keep frequency elevated for a few days after temperatures moderate.

Sprayable particle films and protectants

Please apply by this evening at the latest to ensure protective measures are in place.

• Kaolin clay particle films (e.g., Surround WP): These form a white, reflective film that scatters and reflects solar radiation, including UV wavelengths, away from the fruit and leaf surface.
• Calcium carbonate-based films (e.g., Eclipse, PurShade, and similar products): These work on a similar reflective principle to kaolin but bind less tenaciously to the fruit surface, meaning more frequent reapplication during a prolonged heat stretch (Bolivar-Medina and Kalcsits, 2021).
• Wax-based UV protectants (e.g., Raynox): Unlike the particle films above, these form a clear, carnauba wax-based coating that is not visible on the fruit. Raynox is formulated to reduce transmission of UV, visible, and infrared radiation reaching the fruit, and trials have found it to be among the more rainfast options, generally requiring three to four applications across the season (Bolivar-Medina and Kalcsits, 2021).
• Phospholipid-based cuticle supplements (e.g., Parka): Parka is a food-grade, lecithin/phospholipid-based product that supplements the fruit's own natural cuticle rather than coating it with a mineral film. It reflects UV radiation and helps reduce heat stress damage, leaves no visible residue, and is an option for growers who want to avoid the white residue that comes with particle films.

A few practical notes on using any of these products this week:

Coverage matters more than rate. Sunburn protection depends on how completely and evenly the product covers sun-exposed fruit, not just tank concentration. Pay particular attention to the south- and west-facing sides of the canopy, which take the brunt of the afternoon sun. Adding a small amount of an organosilicone surfactant (Silwet, etc.) to the tank mix can help: these strong spreading adjuvants reduce surface tension and droplet size, helping particle films and wax-based protectants spread evenly over the waxy apple cuticle instead of beading up. Check label compatibility first, since not all protectants are labeled for use with organosilicone surfactants.

Hold off on canopy-opening practices this week

As noted above, avoid summer pruning, hand-thinning that exposes previously shaded fruit, or limb tying and repositioning during the hottest days. Fruit that has developed in partial shade has a thinner, less acclimated cuticle and is more vulnerable to sudden, intense light exposure. This is the photo-oxidative sunburn injury that has been documented when growers combine canopy work with heat-wave timing.

Watch the temperature before spraying anything this week

Heat changes how safe and effective your normal spray program is, separate from sunburn protectants. A few things to keep in mind for any tank mix going on this week:

• As a general rule, avoid spray applications above 80 to 85°F whenever possible, not just for copper, which can be phytotoxic at high temperatures. Many tank-mix partners, oils, and adjuvants carry a higher phytotoxicity risk in hot conditions, since spray droplets dry faster, active ingredient concentrations on the leaf or fruit surface spike before they can be absorbed evenly, and heat-stressed tissue tolerates additional chemical stress less well. If a spray is not time-critical, wait for a cooler morning this week rather than running it midday.
• Hold off on systemic acquired resistance inducers during the hottest days. Products like Actigard (acibenzolar-S-methyl) or Regalia (Reynoutria sachalinensis extract) work by activating the tree's own defense pathways against disease, in effect signaling to the tree that it is under pathogen attack, so it mounts a defense response. That response has a cost. Plant biology research on the growth-defense tradeoff has shown that activating defense pathways diverts carbohydrates and energy away from growth and stress tolerance, since a plant has limited resources to split between the two. Asking a tree to mount a defense response on top of an already stressful 100°F+ heat event adds to the resource drain at the wrong time. Where possible, shift these applications to before or after this heat stretch rather than during it.

Bottom line

With highs near or above 100°F forecast Wednesday through Friday and little overnight relief, this week is a high-risk window for sunburn in Pennsylvania apples, particularly in more sunburn-prone cultivars like Honeycrisp. Without netting or overhead cooling, the most useful actions available this week are: get ahead of tree water stress with timely irrigation now, get a sunburn protectant on before Wednesday's heat builds, avoid opening up the canopy, and hold off on copper or systemic acquired resistance sprays until conditions moderate.

References

• Penn State Extension (2026). Penn State Tree Fruit Production Guide, 2026-2027. The Pennsylvania State University. 
• Bolivar-Medina, J.L., and Kalcsits, L. (2021). Sunburn in apple and strategies to mitigate it. WSU Tree Fruit, Washington State University. 
• Racsko, J., and Schrader, L.E. (2012). Sunburn of apple fruit: Historical background, recent advances and future perspectives. Critical Reviews in Plant Sciences 31(6):455-504.
• Schmidt, T. (2018). Apple Sunburn 101. WSU Tree Fruit, Washington State University.
• Schrader, L.E., Zhang, J. and Duplaga, W.K. (2001). Two types of sunburn in apple caused by high fruit surface (peel) temperature. Plant Health Progress. doi.org/10.1094/PHP-2001-1004-01-RS
• Schrader, L.E., et al. (2008). Scientific basis of a unique formulation for reducing sunburn of fruits. HortScience 46(1):6-11.
• Weather forecast and heat dome data: National Weather Service, as reported by Gettysburg Connection, June 29, 2026 (sourced from NPR and The Weather Channel).

This article addresses current weather conditions. Confirm forecasts with the National Weather Service and check current product labels before any spray application, since labeled rates, REI/PHI, and organic certification status can change.