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Environmental Stress Management

Fruit trees grown in Washington state are frequently exposed to environmental stresses that impact their health and production. During the summer, solar radiation levels are extremely high, and often exceed levels that can be utilized by the trees. When coupled with high air temperatures, which also raise the fruit temperature, sunburn browning and other skin disorders can result making fruit unmarketable. For young trees that do not yet have an adequate canopy established, sunburn can also occur to the unprotected bark of south facing tree trunks.

In May of 2015, a drought emergency was declared by Gov. Jay Inslee for the state of Washington. A WSU drought alert website has been established. At sites where access to irrigation water is restricted or cut off, drought stress could not only impact cropping but also tree survival.

Cold temperatures can also result in damage – whether it is before the trees become completely dormant in the fall, during winter dormancy, or during early growth and flowering in the spring. Hail can cause cosmetic damage to fruit making it unmarketable and also cause physical damage to leaves, shoots, spurs and bark. The latter may produce wounds that could lead to disease infection and insect invasion.

Site selection to minimize temperature extremes is important. There are two important questions to ask: has the site had fruit grown successfully on it before? and, is the site known for consistent cropping year after year or not? Ideal sites are on gently sloping land with good air drainage, so that cold air can move down slope and not accumulate in a low spot or “frost pocket” causing bloom damage in the spring and subsequent crop loss. At sites that are known to be windy, planting wind-breaks can be helpful to reduce wind damage to fruit but still allow cold air to flow out of a tree fruit block. Overhead netting can be installed over trees to protect from hail and reduce sunburn. By placing protective covers over cherry trees, rain-induced cracking can be reduced.

The proper choice of scion cultivar and rootstock for your particular location is an important factor affecting the ultimate tree performance and fruit quality. The particular scion/rootstock combination will affect tree hardiness, bloom date, stress and disease tolerance.

The WSU apple breeding program led by Dr. Kate Evans, for example, is developing new cultivars that were bred for and selected in the stressful climate of central Washington. See video: Cosmic CrispTM WA 38 Field Days.  Performance of these new cultivars may be superior to those introduced from other parts of the world where the climatic conditions under which the cultivar was selected were quite different (e.g., Honeycrisp cultivar was selected in Minnesota).

Current research at the WSU-TFREC Tree Fruit Physiology lab in Wenatchee involves investigating changes in the physiology of apples under photo-selective anti-hail netting. The focus of their lab “has been understanding the complex physiological interactions between environment, genetics and horticultural management.” See Dr. Kalcsits’ website.

Stresses Relating to Sun and Heat

Sunburn browning of apples is caused by high fruit surface (skin) temperatures, and ultraviolet radiation. This is the most common form of sunburn. The temperature level that fruit must reach for sunburn browning to occur varies among different apple cultivars and ranges from 113° F  to 120° F. Sunburn browning symptoms appear as yellow, brown, or dark tan spots on the sun-exposed side of the apple. Spots may not show up until a few days after the damage has occurred. To view symptoms see: “Fruit Finish: Sunburn browning.”

Sunburn necrosis is the result of thermal death of cells that occurs when fruit surface temperatures exceed 126° F while the fruit is on the tree. A dark brown or black necrotic spot appears on the sun-exposed, affected area, and skin cracking is common. To view symptoms see: “Fruit Finish: Sunburn Necrosis.”

Photo-oxidative sunburn occurs on fruit previously growing in the shade that are suddenly exposed to full sun. (e.g., Apples exposed during thinning or pruning, or when a limb moves due to changing fruit weight.) This type of sunburn is caused by visible light. Symptoms include whitening of the skin on the sun-exposed side of fruit, appearing as a bleached spot. Necrosis may form on the bleached area later. To view symptoms see: “Fruit Finish: “Photo-oxidative Sunburn.”

Additional information about these three conditions can be found at Dr. Larry Schrader’s (WSU-retired) stress physiology webpages. There is also a sunburn prediction model here and at the WSU-DAS model explanation page.

The WSU Center for Precision & Automated Agricultural Systems (CPAAS) is currently researching methods to control sunburn. These projects include:

Other fruit skin disorders that are also sun and temperature-related include: Fuji stain, Lenticel marking, Fuji flecking, and sunburn scald of Granny Smith. More on all of these disorders can be found here. WSU-DAS also has an article about storage scald in apples and pears caused by high orchard temperatures.

Heat Stress – Trees transpire during high levels of heat and sunlight, to cool down. This lowers the moisture content in the tree, and can cause heat stress. Trees that are heat stressed during critical times, like bloom, will have reduced fruit set. Trees may photosynthetically shut down under heat stress. This can result in lower yields of smaller, poor quality fruit. Growers can cool orchards with Evaporative Cooling when orchard conditions causing heat stress are likely. For more information about managing heat stress see these articles:

Drought and Water Stress

As noted above, in May of 2015, a drought emergency was declared by Gov. Jay Inslee for the state of Washington. A WSU drought alert website has been established. Also, see details on state programs managing the issue.

When drought conditions persist, the amount of moisture in the soil will no longer adequately provide trees the water they require. Here are some things that growers can do to help fruit trees in drought situations:

  • Monitor soil moisture;
  • keep areas around young trees free of weeds;
  • use mulches to retain moisture in soil; improve irrigation efficiency;
  • use an irrigation scheduler;
  • and mow cover crops.

Moisture stress symptoms in tree fruits include dull leaf color, and leaf rolling without recovery by early to mid-evening hours. Critical growth periods for fruit trees are: Full bloom to 4 weeks after full bloom; fruit set; and the last 2 weeks to 30 days prior to harvest. It is especially important to supply adequate moisture during these times.

Drought spot of apple occurs during late fruit cell division and rapid cell enlargement in fruit. This condition causes bitter-pit like spots and tissue deformation near the calyx end of fruit. For more information on drought spot see: Drought Spot of Apple, G. Whitney, Chelan-Douglas County Extension article.

WSU has collaborated with UC Davis on a research project “Precision canopy and water management of specialty crops through sensor-based decision making.” See the project description or view the project website.

Additional Resources on Drought and Water Stress:

Management Practices to Protect Against Environmental Stresses Caused by Heat and Light

Evaporative cooling (EC) is one of the best techniques to lower fruit temperatures and reduce sunburn on apples. Overhead sprinklers, misters, and solid-set canopy delivery systems all deliver water to cool fruit before critical temperatures are reached. Evaporative cooling doesn’t protect fruit from UV light though, so EC is often used with other sunburn protection methods, such as protectant spray coverings. Food Safety and Modernization regulations regarding agricultural water that contacts edible portions of crops may require growers to sample water, depending on the source (i.e. surface or ground water.) The FSMA proposed rule for produce safety can be viewed here.  For more information on the use of evaporative cooling see these articles and report:

Protectant sprays are used to reduce sun damage on fruit skin, by reflecting or blocking light from fruit. A successful wax-based sunburn protectant developed at WSU is available (e.g., Raynox) along with sunblockers which are based on particle film technology (e.g., Surround WP). These particle film products cover the fruit with a white coat, which reflect and block light. For more information see: Protect Apples from Sunburn in the 2015 Crop protection guide for tree fruits in Washington.

Shade cloth/Netting for sunburn protection  is used in new high-density plantings which have narrow canopies, and less foliage to protect against environmental conditions like sunburn and hail. Overhead netting is used above fruit trees for protection from the elements. Growers in other countries have been using netting to cover tree fruit crops for protection from hail, sunburn, and insects for some time, but it is a fairly new practice in the U.S. Netting comes in different colors including pearl, yellow, red and blue. These colored nets transmit more sunlight, and shade the trees less than the traditional black nets. The results are more uniform skin color on the fruit, and improved size. WSU research is currently being conducted in Washington using colored netting for environmental protection of apple trees, with the aim to find out if there will be decreased tree stress and increased fruit quality, size, and production. There is the possibility that net use could eliminate the need for overhead evaporative cooling to protect from sunburn. For more information on the use of netting see these resources:

Trade Articles


Rain and Cherry Cracking

Most cherry cracking is caused by water absorption through the fruit skin. Fractures in the cherry skin cause more water absorption to occur.  For more information see the article: Cherry cracking: Causes and suppression, L. Schrader and J. Sun, Oregon Horticultural Society Annual Proceedings, 2006.

High water levels in the tree’s vascular system, and high humidity can also cause cracking. Growers use protective covers over cherry trees, applications of osmotic solutions, drying with blowers or helicopters, hydrophobic coatings, and high tunnels to protect cherries from rain cracking. For more information see: Protecting sweet cherries from rain cracking, in the 2015 Crop protection guide for tree fruits in Washington.

Resources on Cherry Cracking:

Scientists in Michigan and New York have been investigating the use of high tunnels for cherry production.  See articles on high tunnels for cherry production and protection from rain:

Wind Stress

Windy conditions are prevalent at times for many North Central Washington tree fruit sites. Planting lines of trees for windbreaks to protect fruit trees is a common practice. Windbreaks must be placed in positions that will allow cold air to flow out of the site so that frost protection can occur though. Vegetation, bins, and buildings will slow the movement of cold air out of an orchard, as well, depending on their location. Foliage, limbs and fruit can be damaged in windstorms. For more information see the article: Trees against the wind, D. P. Hanley and G. Kuhn, PNW 0005, 2003.

Low Temperature Stress

Cold Hardiness

Cold hardiness is the ability of trees to tolerate low temperatures. Cold hardiness may depend on different variables like how fast the temperature drops, what temperatures were before the temperature drop, and how long the severe cold lasts. Trees may be cold hardy in one location, and not in another, depending on site-specific variables. Cold hardiness of flower buds is genus and variety dependent.

Researchers at the WSU Irrigated agriculture research and extension center (IAREC) are developing a more accurate way to measure cold hardiness in apple and sweet cherry buds, to better help PNW growers protect their trees during frosts. The information on new apple and cherry variety cold hardiness will create a model for AgWeatherNet, to provide farmers with knowledge to help them make frost protection decisions. AgWeatherNet will send growers Cold hardiness warnings if critical injury temperatures for apple and cherry buds are reached. There are different variables that make trees more vulnerable to frost damage.

Resources for Cold hardiness:

Winter Injury

The term winter injury encompasses many symptoms of tree damage including: cambium injury on stone fruit, crown injury, winter sunscald (south west injury), trunk splitting, and shoot dieback. This is according to Jon Clements, Extension tree fruit specialist at the University of Massachusetts, describing some forms that fruit tree injuries can take in the Fruit Growers News article Cold injury to fruit trees is a big concern.

Buds and blooms, young shoots, and roots can also be injured during the winter, affecting crop load and fruit finish. Tree tissues are injured more often when there is a fast drop in temperature. Trees can exhibit poor growth, damaged/dead tissue, or die from injuries sustained from freezing.

As mentioned earlier, painting white latex paint on trunks of fruit trees protects the bark from splitting and cracking when exposed to freezing temperatures followed by thawing temperatures (sunscald of tree trunks). The paint reflects sunlight during the day, and keeps the tree warmer at night. See Fruit Growers News article, Painting tree trunks.

Also see Southwest crop injury: An After-effect of a cold winter, C. Herrick, Growing produce, May, 2014.

Frost/Freeze damage

Frost damage to fruit blossoms is a concern for growers early in the growing season during bloom, when frost occurs, and for fruit late in the season when temperatures get cold before harvest. Fruit tree tissues will freeze at certain temperatures, depending on the stage of tree development. When fruit trees come out of dormancy and buds start to become active, they are susceptible to freeze damage. The amount of damage incurred depends on the stage of development, the cultivar, and the length of time the freezing event lasts. See Frost injury to apple in the quick identification cards on the Cullage Assessment & Education website.

Apple fruitlets can be injured when freezing occurs during bloom time. When fruit develop and mature, freeze damage blemishes appear as russeted areas partly encircling the fruit or in irregular spots.

“Apple varieties are prone to freezing injury when temperatures reach 27.3° F to 29.4° F depending on variety. Ice crystals form between cells in an apple. Injury occurs when forces are exerted on the cells by ice crystals, and from chemical reactions in cells where excess water has been removed.”( excerpt from: C. Pierson et al., Market diseases of Apples, Pears, and Quinces Frost and Freezing Injury, C. Pierson, et. al. Agriculture Handbook No. 376, ARS-USDA, 1971.)

As mentioned above, buds and blooms, young shoots, and roots can be injured during freezing conditions, affecting crop load and fruit finish, and possibly killing trees. Trees can exhibit poor growth, damaged/dead tissue, or die from injuries sustained from freezing one or more years after the injuries.

Over-tree sprinkler and misting systems, evaporative cooling systems, and solid-set canopy delivery systems may be used for frost and freeze protection in orchards. Overhead sprinklers form clear ice, in which an endothermic reaction takes place, and the warmth of the plant is trapped inside. When water freezes, it releases heat to the surrounding environment. Growers also use the overhead sprinkling to slow down bud development during warm windows in late winter.  For more information see the article: Protecting your fruit from frost and freeze,  D. Sigler, Fruit Growers News, January, 2013.

Moist soil rather than dry soil has a greater ability to get and store heat energy during the day when it is warm, and release heat at night, helping to maintain air temperatures. Cold air is heavier than warm air, and settles to the lowest areas near the ground, so mowing tall grasses and cover crops is helpful in preventing freezing damage on trees.  For more information see these articles: Mist cooling for freeze protection, R. Lehnert, Good Fruit Grower, February, 2013; and Critical temperatures for frost damage on fruit trees, M. Murray, (WSU data), Utah State University Fact Sheet, February, 2011.

Other management practices used to protect fruit tree crops from frost and freezing conditions include:

    • Wind machines which are tall vertical fans (35 ft.), used for frost protection. They pull warmer air down to the orchard and mix it with the cold air near the orchard floor. These large fans can protect approximately 10 acres of fairly flat orchard. For more information see the article: Basics of frost and freeze protection for horticultural crops, K. B. Perry, HortTechnology, 8(1),1998.
    • Windbreaks near orchard blocks must be placed in positions that will allow cold air to flow out of a block, and frost protection to occur. Vegetation, bins, buildings, etc. will slow the movement of cold air out of an orchard.
    • Helicopters are sometimes used to move air for frost protection, but this option is very expensive.
    • Heaters are also used to warm orchards during frost conditions, but are not used as much today because of the cost of fuel, and they are not efficient at heating orchards. They are banned in some areas due to air quality issues. Propane heaters and diesel furnaces are still used in some areas for frost protection though. For more information see the article: Frost protection strategies, R. Lehnert, Good Fruit Grower, February, 2013.
    • Cryoprotectant chemicals generally have not proven successful at preventing freeze damage for blossoms, buds, or fruitlets when sprayed on foliage.
    • Growth regulators applied in the Fall, such as ethephon, have been shown to increase winter bud hardiness in peach.

Hail damages fruit, leaves, shoots, and the spurs for next year’s crop. Open wounds on fruit made by hail can create entryways for insects and diseases to enter the fruit.   Growers use protective netting over trees to prevent injury from hail. Netting has been used in fruit producing areas of the world to protect specialty crops against hail for a long time. The netting reduces tree stress, limits sunburn, and may exclude insect pests if the netting encloses the orchard (top and sides) and if the mesh gage is small enough. Colored hail nets may have additional benefits for fruit quality. For more information see these articles:

Air Pollution Stress

Whether from man-made sources or naturally occurring, (e.g., forest fires), pollutants in the air can also result in damage to trees. For more information see these resources:

Resources for Environmental Stress of Tree Fruit



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