Written by Jenny L. Bolivar-Medina, ITT WSU Extension, and Lee Kalcsits, Endowed Chair WSU Horticulture. August 2021. Reviewed July, 2022.
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What is sunburn?
Sunburn is a physiological disorder caused by the exposure of the fruit to excessive high solar radiation and temperatures. It is irreversible and can cause more than 10% of losses in WA State. Some cultivars like Honeycrisp are more susceptible to sunburn. Low wind speed and humidity also contribute to the susceptibility to sunburn under high light and temperature conditions. In general, air temperatures above 90 °F is when sunburn risk substantially increases and when sunburn mitigation plans need to be implemented. Under extreme conditions that were present at the end of June (air temperatures exceeded 100 °F from 10:30 am to sunset for four days), multiple sunburn mitigation practices should be used.
What are the risks?
Depending on the factor or set of factors that caused sunburn, there are three types of sunburn that have been characterized and are summarized in Table 1.
Table 1. Sunburn type and characteristics
Type of sunburn | Involved factor(s) | Characteristics | Example |
Sunburn Necrosis | High fruit surface temperature (FST): 52°C (125°F) for at least 10 minutes | Fruit with dark brown or black necrotic spots, as the result of cellular death.
Damage can be observed within 1 to 4 days after heat exposure. |
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Sunburn Browning | High solar radiation and high FST for at least 60 minutes.
High FST is cultivar dependent and can start at 46°C (115°F). |
Fruit with yellow, brown bronze or dark tan spots on the surface exposed to sun. |
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Photooxidative Sunburn | Sudden exposure of shaded fruit surface to full sunlight without prior acclimation. Produced by thinning, pruning, shifting of branches, etc. | Temperatures as low as 31°C (88°F) can cause white spots (bleaching) in areas of the fruit’s skin that were previously shaded and suddenly expose to sun light. White areas turn brown and later necrotic (cellular death) |
![]() https://treefruit.wsu.edu/360-rotator-photo-oxidative-sunburn/ |
What are the disorders produced or enhanced by sunburn?
Fruit disorders can be directly associated or enhanced by sunburn damage are lenticel marking, cracking, splitting, russetting, watercore, and bitter pit. Postharvest disorders influenced by sunburn are ‘Fuji’ stain and sunburn scald on ‘Granny Smith’.
Strategies to prevent sunburn.
Developing fruit are not able to dissipate excess radiation like leaves, hence fruit surface temperature (FST) can reach 10 to 15 °C (50 to 60°F) higher than air temperature. To prevent sunburn in the fruits, there are strategies that can be used and are described below (Fig. 4)
Fig. 4. Sunburn mitigation strategies. (Based on Racsko and Schrader, 2012). $ indicates estimated costs. $ = less expensive, $$$$ = most expensive.
Evaporative Cooling (EC)Over-tree irrigation should be applied in cycles to wet the fruit and then allow it to dry to use the latent heat required for evaporation to cool fruit. |
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Protective NettingHigh-density polyethylene nets use to cover tree canopy to provide shade. |
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Particle FilmsWhite particulates of kaolin, clay, calcium carbonate or talc applied directly to fruit. More information about particle films and concentrations in https://cpg.treefruit.wsu.edu/environmental-fruit-protectants/apple-sunburn/ |
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SunscreenCompounds with chemical absorbing agents and inorganic components able to dissipate high- intensity light, block, and scatter solar radiation. |
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References
Gindaba, J. and Wand, S. (2005). Comparative Effects of Evaporative Cooling, Kaolin Particle film, and Shade Net on Sunburn and Fruit Quality Apples. HortScience 40(30):592-596.
Kalcsits, L., Mussacchi, S., Layne, D., Schmidt, T., Mupambi, G., Serra, S., Mendoza, M., Asteggiano, L., Jarolmasjed, S., Sankaran, S., Khot, L., Zuñiga Espinoza, C. (2017) Above and below-ground environmental changes associated with the use of photoselective protective netting to reduce sunburn in apple. Agricultural and Forest Meteorology 237-238: 9-17. ISSN 0168-1923,
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.
Serra, S.; Borghi, S.; Mupambi, G.; Camargo-Alvarez, H.; Layne, D.; Schmidt, T.; Kalcsits, L.; Musacchi, S. (2020). Photoselective Protective Netting Improves “Honeycrisp” Fruit Quality. Plants 9 (12): 1708. https://doi.org/10.3390/plants9121708.
Contacts
Lee Kalcsits
Associate Professor
Endowed Chair
Tree Fruit Environmental Physiology and Management
WSU Tree Fruit Research & Extension Center
Wenatchee, WA
phone: 509-293-8764
email: lee.kalcsits@wsu.edu
Jenny L. Bolivar-Medina, Ph.D
ITT- WSU Extension- – Tree Fruit Horticulture
WSU-Irrigated Agriculture Research and Extension Center
24106 North Bunn Road
Prosser, WA 99350
phone: 509-786-9201
email: j.bolivarmedina@wsu.edu
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