Sunburn damage costs apple growers tens of millions of dollars annually, as sunburn is often the number one cause of fruit cullage for apples grown in the Pacific Northwest. Growers often lose 10% or more of their apples to sunburn unless they have used some means of protecting their fruit from sunburn damage. Of the three types of apple sunburn identified at WSU, sunburn browning is the most important commercially. It requires both high air temperatures and high solar radiation, which affect fruit surface temperature (FST). On most summer days, FST is at least 20 °F higher than air temperature, and FST can be up to 30 °F higher.
Sunburn browning occurs when FST is between 115 and 120 °F, depending on the apple variety; damaging ultraviolet (UV-B) rays are also required. A yellow, bronze or brown spot develops on the sun-exposed side of the peel, but may not appear for a few days after the damage is done. In some varieties, unsightly sunburn-like blemishes develop during cold storage on fruit which was exposed to high light and temperatures during the growing season, but still appeared normal at harvest; this disorder is known as "delayed" or "storage" sunburn. Delayed sunburn is usually fully expressed within 4 months of storage and can affect significant portions of unprotected fruit.
While sunburn obviously affects the apple's external appearance, recent research has revealed that internal fruit quality is also affected in sunburned apples. Increased flesh firmness and sugar content has 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 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 reduce FST and/or UV-B light exposure to fruit. Growers in Washington have three basic options to achieve this goal: 1. Evaporative cooling (EC) 2. Protective netting and 3. Sprayable sunburn protectants. While each of 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 have 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.
Growers seeking immediate, temporary relief from sunburn pressure at lower up-front costs than installing an EC system or protective nets should consider application of sprayable sunburn protectants. These products generally fall into one of three categories: 1. Kaolin clay-based particle films (i.e. Surround WP) 2. Calcium carbonate-based particle films (i.e. Eclipse, Diffusion, MicroCal) and 3. UV-blocking wax matrices (i.e. 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 exposed directly to the sun (typically in the tops of trees), good spray coverage to the upper portion of tree canopies is critical; in some cases, helicopter applications may be superior to tractor-pulled sprayers for effective supression of sunburn.
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.
|Except where noted, rates are amount per acre (amount per 100 gallons in dilute sprays)|
|Diffusion||2-4 gal (2-4 gal)|
|Diffusion O||2-4 gal (2-4 gal)|
|Eclipse||2.5-3 gal (2.5-3 gal)|
|Microcal||2.5-3 gal (2.5-3 gal)|
|Raynox||2.5 gal (2.5 gal)|
25-50 lb (25-50 lb)
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 °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.