Written by Robert Blakey,Tree Fruit Extension Specialist, WSU Extension, Postharvest Information & Technology Transfer. May 22, 2017.

Types of Sunburn

Sunburn is a major cause of apple fruit quality loss caused by heat and light stress. There are three types of pre-harvest sunburn in apples: (i) Sunburn Necrosis, (ii) Sunburn Browning, and (iii) Photo-Oxidative Sunburn. Delayed Sunscald (Delayed Sunburn) is a post-harvest disorder with symptoms developing or worsening during storage.

Apple fruit are susceptible to sunburn because they have a much higher thermal mass (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 Necrosis: Damage occurs when the fruit surface temperature is greater than 125°F for 10min and exacerbated with low RH. The heat causes cell death. Photos are available at the online WSU Apple Defect and Disorder Guide here.

Sunburn Browning: A combination of UV-B radiation and high fruit surface temperature (115 – 120°F). Risk parameters for sunburn browning are maximum daily air temperature and mean maximum hourly temperature between 11am and 5pm. WSU DAS has a model for sunburn browning available. Sunburn browning is the major type of sunburn in Washington. Photos are available at the online WSU Apple Defect and Disorder Guide here.

Photo-Oxidative Sunburn: 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. Photos are available at the online WSU Apple Defect and Disorder Guide here.

Delayed Sunburn: Symptoms are similar to sunburn browning and can develop or worsen within the first four months of storage. Fruit that had pre-harvest sunburn that later covered over during color development are at risk of developing sunburn and are not suitable for long-term storage.

Sunburn-Related Disorders

Beyond sunburn symptoms already discussed, heat and light stress can cause other fruit defects and disorders. Related disorders are listed in the table below. Fruit physiology is also affected: ripening is affected (advanced or delayed depending on timing of the damage), starch clearing advanced, firmness increased, soluble solids increased, and titratable acidity loss during storage accelerated. On balance, fruit with sunburn damage have reduced storage potential.

External Disorders Internal Disorders
·     Cracking

·     Crinkle

·     Delayed sunburn (discussed above)

·     Fuji stain

·     Hot summer stress disorder

·     Lenticel marking

·     Russeting

·     Splitting

·     Bitter pit

·     Heat stress internal browning

·     Water core

Sunburn Control

Sunburn control requires a systems approach. Fundamental to this is good orchard management to balance vegetative growth and fruit and manage light and heat. Some factors that increase the risk of sunburn are:

  • High fruit:leaf ratio
  • Low leaf area index (≤1.5)
  • Water stress
  • Low leaf nitrogen
  • Fruit age
  • Selective picking

There are three main commercial strategies for active sunburn control:

  1. Evaporative Cooling,
  2. Protective Netting, and
  3. Protectant Sprays. Which are further categorized as:
    1. Kaolin-based particle films,
    2. Calcium carbonate particle films, and
    3. Wax matrices.
      Anti-oxidants (e.g. tocopherol) are also added to some protectant sprays. There are also some talc products available. Application guidelines for a number of protectant sprays are given in the Environmental Protectants section of the 2017 Crop Protection Guide for Tree Fruits in Washington

Bin covers are an option to prevent sunburn to harvested fruit.

Conclusion

Sunburn damage directly and indirectly affects fruit quality and physiology. Managing sunburn requires a systems approach, combining good orchard management and active sunburn management strategies that are timely and effective. We will be discussing these at the upcoming Sunburn and Delayed Sunscald Field Visit on May 25th 2017 – more details for the English session and the Spanish session.

Further Reading

  1. Racsko & L. E. Schrader. 2012. Sunburn of Apple Fruit: Historical Background, Recent Advances and Future Perspectives. Critical Reviews in Plant Sciences, 31:6, 455-504.

Contact

Blakey-Rob-wcRob Blakey, Ph.D.

Tree Fruit Extension Specialist, WSU Extension

Postharvest Information & Technology Transfer

509-786-9284

rob.blakey@wsu.edu