Sunburn doesn’t only happen to people or livestock. It happens to crops, too. Sunburn in apples – the term sunscald is often used interchangeably – not only affects their appearance; it goes deep down to the cellular level, initiating changes that impact fruit quality, and cause economic loss for growers.
There’s more than one factor that makes fruit susceptible to sunburn. Ultraviolet radiation (UV), light and heat all play a role in fruit sunburn. Because fruit has a high thermal mass, it can store heat more efficiently than leaves, and it can’t readily cool itself down. As a result, fruit temperatures can be much higher than the actual air temperature. Keeping fruit cool – both by having healthy orchard trees and by taking proactive measures when conditions are prime for sunburn to occur – can prevent many sunburn concerns.
“There are four types of sunburn: sunburn browning; sunburn necrosis; photo-oxidative browning; and delayed sunscald/sunburn,” Rob Blakey, tree fruit Extension specialist, Washington State University Extension, said. “There are other associated defects, too.”
Knowing when fruit is most at risk, and acting to prevent damage, is the key to successfully reducing fruit loss due to sunburn damage. Mitigating tree stress through the management of water, nutrients and leaf area, or applying protective coverings to keep light and heat away from fruits, are all tools to manage sunburn in orchards.
“Leaves are very good at absorbing and dissipating heat,” Blakey said. “There needs to be a balance between leaves and fruit, and the leaf area index (total leaf area: ground surface area of the tree). Fruit needs to be exposed to sunlight to develop color and sugars, but it is a balance between color development, sugar development and sunburn.”
Leaf coverage depends upon nitrogen management. Too much, and the leaves thrive at the expense of the fruit, with bitter pit often the result. Too little nitrogen, and the tree isn’t vigorous. Its lack of vegetative growth puts fruit at high risk for sunburn concerns.
Water, too, matters. Trees transport nutrients via water and use water for transpiration, which serves to cool the tree. But a tree will greatly reduce transpiration if it is losing too much water during heat stress events, and fruit will suffer unless some other means of keeping cool is provided.
Types of burns
Although fruit does need to keep its cool, it also needs to avoid exposure to intense light, no matter the air temperature. If fruit is exposed to intense sunlight, it is at risk for developing photo-oxidative sunburn. This type of burn can happen at any time in the fruit life cycle, and events such as pruning, hand thinning, selective picking and harvest and transport increase the risk of exposure to the sun’s rays. The peel will whiten from the sun exposure, then brown, depending on severity.
When the fruit surface itself reaches temperatures greater than 125 degrees Fahrenheit for at least 10 minutes, sunburn necrosis occurs. Days with a low relative humidity bring the highest risk. Skin cell death happens, and the heated area of the fruit later develops a brown or black spot, normally with cracking.
Maintaining a healthy orchard through nutrient and water management and keeping that balance between vegetative and fruit growth in check can go a long way in minimizing sunburn concerns. But when conditions are ripe, taking preventive action is warranted.
Evaporative cooling is one method used to keep fruits cool. Water is applied and allowed to evaporate and cool down the fruit, in a cycle that is repeated. Due to the high latent heat of water, it can absorb a lot of heat prior to transitioning to vapor, thus removing the heat from the tree, Blakey said.
“Evaporative cooling becomes effective by depositing water on the fruit and leaf surface, which evaporates and reduces the surface temperature. The skill is in not applying too much water, or too late, which will saturate the soil; or too little, too late, which won’t be effective in sunburn control,” he said. “Getting the cycle correct, because fruit can burn very quickly – in minutes – once the water has evaporated” is imperative.
Protective films include kaolin clay, talc or calcium carbonate, all of which work by reflecting sunlight off of the fruit. Raynox, a sunscreen developed by Professor Larry Schrader at Washington State University, is a wax that absorbs the sunlight, preventing it from damaging the fruit.
Another method is the use of photoselective netting. “Netting principally reduces the amount of light and heat reaching the tree, and certain types can also diffuse the light,” Blakey said. “By reducing the amount of light and heat reaching the tree, you are reducing the stress on the tree. By diffusing the light, the sunbeam is scattered in different directions, so the energy is not concentrated in a particular area.”
Because netting helps prevent trees from becoming overly stressed, they can continue to transpire longer under stressful heat conditions. This enhances tree vigor, which is particularly valuable during the establishment years. And, it means less intervention via evaporative cooling for the grower. Netting also protects from hail and wind. Protecting fruit from sunburn is like putting on your hat and sunblock. It takes time and effort, but the outcome is worth it.