by Tamara Scully
It may be winter, but keeping cool should be on the mind of orchardists. Sunburn or sunscald on apples, pears, plums and other tree fruits causes changes internally, in the cells of the fruit, resulting in damage which goes deeper than any cosmetic symptoms. There are several types of sunburn conditions which impact fruits. The key to managing all of these is to keep fruits cool.
Sunburn necrosis occurs if the fruit surface reaches a critical temperature for a given time period. The exact parameters depend upon the fruit. For apples, temperatures greater than 125 degrees Fahrenheit for at least 10 minutes will result in cell death. The “hot spot” on the fruit will later brown and crack.
Similarly, sunburn browning is caused by heat, but also involves exposure to ultraviolet — specifically UVB — rays. Fruit exposed to conditions where the maximum daily temperature and the mean maximum hourly temperature are above the tolerance threshold can show yellowing or browning.
Photo-oxidative sunburn results in bleached skin, which may also cause browning. This type of sunburn doesn’t require high temperatures: Intense exposure to sunlight at any temperature is the cause.
Delayed sunburn occurs when fruit’s external sunburn damage disappears and the fruit regains its color. However, in storage, the internal damage to the fruit becomes apparent as symptoms reappear.
Rob Blakey, Tree Fruit Extension Specialist, Washington State University Extension, has an informative fact sheet on sunburn symptoms and causes at http://treefruit.wsu.edu/article/sunburn-management-in-apples.
Protection from sunburn
Trees naturally protect fruit from sunburn via their leaves, which provide shade for the fruit, preventing it from overheating. Leaves also release water taken up by the roots, and this transpiration serves as a cooling mechanism for the tree. But if transpiration is causing too much water loss and threatening the tree’s survival, the stomata will close and cooling will slow down, resulting in hot fruit.
Those fruit temperatures can be significantly higher than the ambient air temperature. So even if you are cool, your fruits may not be. Temperatures on the fruit surface can be more than 25 degrees warmer than the air temperature.
If a fruit tree is exposed to too little nitrogen, its ideal leaf-to-fruit ration will be altered. There simply won’t be enough leaf coverage to adequately protect fruit from the sun’s light and heat.
Water management can become crucial when transpiration slows and cooling is reduced under hot temperatures. The tree is trying to stay alive and cooling fruit isn’t a critical function for the tree’s survival. But keeping fruit cool is imperative for the grower and fruit-cooling procedures might be necessary.
“Managing water is fundamental to fruit production,” Blakey said. “Water management can become crucial when transpiration slows and cooling is reduced. It is then that evaporative cooling becomes effective by depositing water on the fruit and leaf surface, which evaporates and reduces the surface temperature.”
By applying water to the leaf canopy, the evaporative process will occur and will cool fruit much as transpiration does naturally. But if the water is applied too late, or if too little is utilized, the fruit can still burn.
“The principle of evaporative cooling is that water is applied to the fruit and leaves in on-off cycles during a hot day,” Blakey said. “Water is applied, turned off, and allowed to evaporate and cool the tree — and the fruit, especially. The cycle is repeated until the risk of sunburn has passed.”
Protection from sunburn can also involve films of kaolin clay, talc or calcium carbonate. These products reflect light off of the fruit, preventing damage. There is also a wax which absorbs sunlight and provides protection.
But just as with humans, applying suntan lotion isn’t the only way to stop sunburn. Covering up so fruit is not exposed to the sun’s light and heat involves photoselective netting. Applied after pollination and remaining in place until snowfall, netting in various colors, which can also be used as anti-hail netting, is being studied as another route to sunburn protection.
“Putting shade nets over the top of orchards is used to reduce sunburn in other apple-growing regions of the world,” Dr. Desmond R. Layne, professor of pomology, Department of Horticulture, Washington State University, said in a video introducing a research trial being conducted on Honeycrisp apples. “Another advantage of the netting is that it is designed to protect the trees to damage due to hail.”
Researchers at WSU are conducting a three-year trial involving blue and red photoselective anti-hail netting. The Honeycrisp trees in the study were planted in a V-trellis system, on Bud 9 rootstock.
Even though the netting did not reduce air temperatures, it did reduce wind speed, slowing too-rapid transpiration, and it reduced light intensity by 20 percent. This resulted in higher soil moisture levels and decreased soil temperatures. Fruit surface temperature was significantly reduced. Fruits grown under the canopy were of larger size than those in the uncovered control. Sunburn incidents were reduced by approximately 20 percent.
The netting alters the entire growing environment and many factors which impact fruit quality and tree health are affected by the netting, although some of these effects are cumulative and will take time to evaluate. Photoselective netting, no matter the color, allows modified light as well as some unmodified rays, to pass through. The micro-climate below the netting is exposed to both types of light, with the mixture depends upon the precise characteristics of the net.
The color of netting changes the growing environment underneath as well. Some of the differences seen with netting of various colors, due to the manner in which light is scattered as well as the actual light spectrum which is allowed through the netting, include changes to fruit size and color, timing of fruit set, and days to harvest. Blue netting was found to decrease soil temperatures significantly over red, as did the pearl netting. Workers feel cooler under blue netting. Fruit has more color under red nets.
Black shade nets, commonly used in nurseries, do not modify the light which penetrates, but only reduce the amount of light which passes through the net. Clear nets increase light scattering, but do not modify the light spectrum.
Bird damage has also been seen to be reduced under the netting canopies, although birds are able to enter and move about freely. Pest and diseases, soil microbial communities and the microclimate under the net are all potentially altered by the photoselective nets. Because photoselective netting also reflects light off of its surface, insects in the vicinity can be impacted, too. Research into these issues is ongoing.
Costs of the photoselective netting are approximately $10,000 per acre. For high-value cultivars such as the Honeycrisp, netting just might make sense. Taking steps to protect your fruit from sunburn is the cool thing to do.