Sunburn damage costs apple growers tens of millions of dollars annually and is often the number one cause of fruit cullage for apples grown in the Pacific Northwest. Growers often lose more than 10% of their apples to sunburn unless they have used some means of protecting their fruit from sunburn damage. There are three types of apple sunburn: (i) Sunburn Necrosis, (ii) Sunburn Browning, and (iii) Photo-Oxidative Sunburn. Apple fruit are susceptible to sunburn because they have a much higher thermal mass ( the ability of a material to absorb and store heat energy) than leaves and are not able to dissipate this heat as well as leaves. It is important to remember that fruit temperature can be considerably higher (20 - 30°F) than the air temperature, for example, 125°F fruit surface temperature and 95°F air temperature in the afternoon.
Sunburn Browning is caused by a combination of UV-B radiation and high fruit surface temperature (115 – 120 °F, depending on variety). Risk parameters for sunburn browning are maximum daily air temperature and mean maximum hourly temperature between 11 am and 5 pm. WSU DAS has a model for sunburn browning available. Sunburn browning is the major type of sunburn in Washington. A yellow, bronze or brown spot develops on the sun-exposed side of the peel, but may not appear for a few days. In some varieties, unsightly sunburn-like blemishes develop during cold storage on fruit that was exposed to high light and temperatures during the growing season but still appeared normal at harvest. This disorder is known as delayed sunburn, storage sunburn, or sunscald. Delayed sunburn is usually fully expressed within 4 months of storage and can affect significant portions of unprotected fruit.
Sunburn Necrosis occurs when the fruit surface temperature is greater than 125°F for as little as 10 min and results in cell death. Development is exacerbated by low relative humidity.
Photo-Oxidative Sunburn is caused when the fruit experiences a shock exposure to visible light and the light bleaches the peel. This is temperature independent and can even occur when the air temperature is less than 64°F. Risk factors are hand thinning, selective picking, branch movement, summer pruning, and harvest and transit.
While sunburn obviously affects the apple's external appearance, recent research has revealed that internal fruit quality is also affected in apples with even slight sunburn. Increased flesh firmness and sugar content have been noted in apples with sunburn browning at harvest and during storage, but titratable acidity (TA) decreased as severity of sunburn browning increased, especially throughout the storage season. Since TA is not only directly related to tartness in the taste of apples, but also provides the metabolic fuel for the development of most flavor components, fruit with diminished acid levels are often considered to be bland and undesirable by consumers.
Effective strategies to mitigate sunburn damage and improve fruit quality are to reduce FST and/or UV-B light exposure to fruit. Growers in Washington have three basic options to achieve this goal: (i). Evaporative cooling (EC) (ii). Protective netting and (iii). Sprayable sunburn protectants. While these strategies have been proven to reduce apple sunburn incidence, none are 100% effective under extreme heat and light conditions. For maximum protection during severe weather periods, growers should consider a combination of strategies.
Evaporative cooling is very effective for lowering FST of apples, but EC alone does not adequately reduce damaging UV rays; thus, sunburn can occur even with EC. Protective netting may be deployed above the orchard canopy or draped directly over apple trees and has proven to be effective at reducing sunburn incidence, as well as conferring other benefits such as protecting against hail damage, reducing wind stress, and potentially excluding some invasive insect pests and birds.
Growers seeking immediate, temporary relief from sunburn pressure at lower up-front costs than installing an EC system or protective nets should consider the application of sprayable sunburn protectants. These products generally fall into one of three categories: (1) Kaolin clay-based particle films (e.g. Surround WP) (2) Calcium carbonate-based particle films (e.g. Eclipse, Diffusion, MicroCal), and (3) UV-blocking wax matrices (e.g. Raynox). When properly applied, most sprayable sunburn protectants can reduce sunburn symptoms by up to 50% in apple fruit; wax-based products like Raynox may be used in combination with EC to achieve even greater protection from sunburn than either strategy alone. Since sunburn incidence is highest in unshaded fruit that are exposed to direct sun (typically in the tops of trees), good spray coverage to the upper portion of tree canopies is critical.
Some particle films, particularly those comprised of kaolin clay, can be challenging to wash off the fruit surface during packing and growers should consult with their warehouse before using these products aggressively near harvest.
See General Recommendations for guidelines on table use. Read all product labels carefully.
Sunburn on apples: DO NOT mix RAYNOX and Surround in the same tank; they are physically incompatible. Do not apply RAYNOX and Surround in the same season on the same trees.
Diffusion: Apply prior to heat event and repeat every 2-3 weeks as needed. Residue removal on the packing line may be improved by acidifying rinse water to pH 5.5 or below.
Diffusion O: Apply prior to heat event and repeat every 2-3 weeks as needed. Residue removal on the packing line may be improved by acidifying rinse water to pH 5.5 or below.
Eclipse: Re-apply as needed to maintain adequate coverage as fruit grows.
Microcal: Re-apply as needed to maintain adequate coverage as fruit grows.
Raynox: Follow manufacturer's label, and apply 2.5 gal. RAYNOX in either 50 or 100 gal. water conditioned with RAYNOX water softener (according to label). To maintain good coverage of fruits as they expand, four applications should be made: first about 7 weeks after full bloom; 2nd 10 days later; third 3 weeks later; and fourth 4 weeks later. Do not apply when air temperature exceeds 85 deg F.
For RAYNOX AIR, mix 2.5 gal RAYNOX AIR in 15 gal. of conditioned water. Apply no less that 17.5 gallons of spray volume per acre. Four applications should be made as described above for RAYNOX.
For RAYNOX ORGANIC, mix 3.0 gal RAYNOX ORGANIC in 47 or 97 gallons of water (no RAYNOX water softener is needed). Four applications should be made as described above for RAYNOX.
Surround WP: Follow manufacturer's label. At least three applications are recommended. Do not apply any substance with or on top of particle film sprays that will increase the difficulty of removal.