Sun Scald Meets Science (Ilya Kotovich Upends the Apple Cart)
By Mark Weaver – December 2013
It is difficult to grow fruit trees in Alaska without sooner or later encountering damage caused by sun scald. Typical damage appears in the spring on the southern, sunny sides of trees. Sometimes it consists merely of roughened, discolored areas on the bark, that may seem no more serious than a sunburn on a human. More extreme cases appear as severe flaking and large dead patches, usually developing on the trunk of the tree above snow line. These deepen over time and are a sign of serious, often lethal, injury.
Literature published by various nurseries and extension services in the lower 48 is rife with “common sense” explanations of what sun scald is, when and why it occurs, and how to prevent it. Usually the litany reads like this:
“Rapid freezing of fruit tree tissue after thawing, such as occurs on the south sides of trees when the clear, sunny afternoons of late winter and early spring turn into clear, frosty nights, is a significant cause of tree damage.”
“Repeated freeze-thaw cycles caused by solar warming of trees causes the warmed tissues to withdraw from quiescence and lose hardiness. Thus they become increasingly vulnerable to evening frost damage.”
“Young trees should be whitewashed on their south sides, or wrapped with heat reflective material. This prevents sunscald by reducing solar warming and subsequent re-freezing after sunset.”
“Because the barks of some trees have evolved so as to reflect sunlight, they are not susceptible to sun scald. Birch trees are the most obvious example, but some apples, like Dolgo and Siberian crab, are also naturally protected.”
Perhaps I have not been alone in accepting such platitudes as gospel. Yet I have also wondered why one sunny spring seems to wreak havoc on trees whereas another proves totally benign. In Southcentral Alaska, March and April almost always include a large number of clear, sunny days and correspondingly frigid nights. Nonetheless, damage from sun scald does not always occur.
At long last, a Russian agronomist, Dr. Ilya Kotovich, has made some sense of this mystery. His treatise “Sun Scalds of Fruit-Trees” (St. Petersburg, 2009) details his more than fifty years of observing orchards and testing trees—mostly apples—in the far northern areas of the former Soviet Union and Russia. Kotovich’s specialty is biophysics, and he reports some simple but startling discoveries:
(1) Rapid freezing during spring evenings in the far north is NOT a problem for properly winter-hardened trees. Kotovich’s measurements show that freezing rates vary dramatically with branch size: the fastest freezing occurs in the smallest branches. But even on days of unusual extremes, temperatures in small branches simply do not fall fast enough to cause tissue injury. The rate of temperature-fall in tree tissues rarely reaches 30°F per hour and is usually less than10°F per hour. Laboratory tests indicate that temperature-fall rates of more than 45°F per hour are required before tissue damage occurs. Such precipitous rates simply do not occur in orchard trees under natural conditions.
(2) Winter and spring sunshine has little warming effect on small trees—that is, on trees with exposed trunks and limbs of an inch or less in diameter. The trunks and branches of nurslings and even 2- to 3-year-old trees have very small mass relative to surface area. Even in bright sunshine, they tend to stay close to ambient air temperature. Thus, whitewashing or putting reflective wrapping on young trees has little effect on their temperature cycles.
(3) Trees with thick trunks and limbs ARE warmed significantly by winter and spring sun, but this does NOT in itself appear to be injurious to the tissues of a healthy tree or to cause it to lose hardiness. Kotovich recorded more than 95 freeze-thaw cycles during a single winter in apple orchards near St. Petersburg, yet none of the tree buds developed signs of injury. Similarly, frost-sensitive buds of a peach tree endured 49 cycles without damage of any kind.
“Fascinating,” you may say, “ but my trees still get south-side injury. So what is happening?”
Kotovich’s answer is that two very different processes are at work. One is caused by temperature sensitivity, the other by light sensitivity. Both, he says, are triggered only by severe cold.
Thermal Sun Scald:
Kotovich found that sun scald injury in mature trees is a thermal (warming) phenomenon that occurs when trees have already been injured. Freezing and thawing cycles cause previously damaged cambium cells to release harmful toxins called polyphenols. These toxins spread by diffusion and poison adjacent healthy cells. Under the influence of repeated freeze-thaw cycles, newly poisoned cells die and release their own toxins, and a chain reaction begins. The injury always begins in the cambium layer and works its way outward. Thus, on a succession of sunny winter days, even a small pre-existing injury on the southern side of a mature tree can enlarge exponentially as each afternoon thaw contaminates a new and larger ring of damaged tissue. On the cooler, northern side, previously injured cells remain frozen, no toxins are released, and the injury stays small.
Kotovich notes that thermal sun scald is always worst on the southwestern side of a tree (in the northern hemisphere) and that any injury to the cambium is sufficient to start the process of unilateral poisoning (another reason not to weed-whack our trees!). In fact, he discovered the process only after noticing that the tiny pin-prick holes left by his temperature-sensing probes caused large dead spots to grow on the sunny sides of trees, while the holes on the shaded sides remained small and inconsequential. But Kotovich contends that the most common cause of thermal sun scald in northern orchards is…frost damage!
Kotovich’s compilation of weather records in northern Russian orchards shows that massive occurrence of sun scald damage on apple trees always followed winters with severe temperature drops in early winter, drops on the order of -27°F and below. Also, scald damage was always greatest in varieties known to be marginally hardy, whereas the hardiest varieties came through the spring undamaged. Thus, he reasons, the problem typically originates in years when temperatures plummet before trees have hardened to their full tolerances, and the early cold damages immature sapwood from the previous year’s growth.
Kotovich explains that early winter frost damage may be slight and outwardly invisible. However, when spring comes and sun warms the cambium on the southern side of a frost- damaged tree, the dead cells on that side thaw and release toxins, so that the south-facing portions of the injury expand. As a result, the injury appears to be unilateral—that is, limited to only one side of the tree—when in fact it originated on all sides. A cross-section of the trunk of such a tree, he says, will reveal a darkened ring in the sapwood all around the tree where the frost damage originally occurred. Tellingly, Kotovich notes that in all his years of dissecting growth rings affected by sun scald, he has never found a case in which tissues on the southern side were dead while those on the northern side were completely uninjured and healthy. Moreover, his studies show that frost-damaged trees tend to withdraw from quiescence and lose hardiness, thus becoming exceptionally vulnerable to further injury from cold spells before spring arrives.
Kotovich says that hardiness is the real reason why trees like birch and Siberian crab never seem to incur freeze-thaw injury. Their immunity has nothing to do with reflective qualities in their bark; rather, it results from the fact that they are extremely winter-hardy, northern-adapted trees that harden off early. Once quiescent, they are virtually immune to cold. Laboratory experiments have shown that the tissues of these trees, properly hardened off, are able to withstand immersion in liquid hydrogen at -423°F; they simply are not damaged by periods of severe frost that injure more tender cultivars. Because they remain healthy throughout the coldest parts of winter, they are not affected by freeze-thaw cycles in late winter and spring.
Enough about temperature-inflicted sun scald. Kotovich says that there is another process of sun scald at work, a process that damages young trees that in some years become light sensitive.
The bark of immature fruit trees is thin and semi-transparent. Also, one of the layers of inner bark, the phelloderm, contains chlorophyll. After periods of severe winter cold (-20°F and below), cells containing chlorophyll become vulnerable to a process called photo-oxidation, in which exposure to the blue-violet spectrum of sunlight in the presence of oxygen turns lethal. Death occurs in the spring, when temperatures are at or just above freezing. The colder the previous winter, the more likely photo-oxidation is to occur. Kotovich’s laboratory tests show that prior exposure to temperatures of -20°F leads to modest injury, with 10% to 15% cell fatality. Chilling cells to -40°F, however, increases cell fatalities caused by later blue-light exposure to 70-80%.
Kotovich notes that young trees are especially vulnerable to sun scald by photo-oxidation in winters that follow an overcast autumn. Apparently, sunlight during the growing season causes the bark of young trees to produce anthocyans—color pigments—and these pigments help to block blue-spectrum light, thus protecting trees. During a cloudy fall, few anthocyans are produced, and the inner bark is left with little natural protection. Damage is not immediate, because the trees have not yet been exposed to cold temperatures. Also in the far north, early winter is dark and the sun is low in the sky. However, as temperatures drop and frost damage occurs, vulnerability increases. In late winter, with the onset of longer days, higher temperatures, and higher sun angles, exposure to blue wavelengths increases, and serious injury begins. Photo sun scald tends to be centered on the southeastern side of the tree (unlike thermal sun scald, which develops on the warmer, southwestern side).
Fortunately, sun-scald injury caused by sensitivity to light is limited to young trees. Kotovich explains that as a tree ages, its outer bark thickens, and damaging blue-spectrum light can no longer penetrate to the vulnerable, chlorophyll-containing tissues.
Although the processes of sun scald damage are obviously complex, Kotovich’s message for northern fruit growers is clear:
- Sun scald is the result of prior frost damage.
- During normal winters, trees of regionally hardy varieties can endure frosts and subsequent days of solar radiation without injury.
- In winters of severe cold, the key to keeping trees healthy is to protect them from initial damage due to frost in early and mid winter, and from secondary damage due to solar radiation in late winter and spring.
Before asking “How does Kotovich expect a grower to “protect” trees from severe frost? Acres of visqueen? Smudge pots? Electric blankets with long extension cords?”, please consider some of his related observations and recommendations:
First, although the potential winter cold tolerance of a cultivar is largely dependent on the genetics of that cultivar, it is often the time when a tree hardens off that proves critical. Kotovich’s studies show that in northern Russia, fruit trees usually reach optimal hardiness in January. Frost injuries tend to occur in the earlier part of winter because trees have not reached full quiescence. Obviously, the orchardist should not exacerbate this problem by delaying quiescence through untimely fertilizing, irrigating, or pruning during late summer or fall. (Anything that promotes tree growth late in the growing season tends to delay the onset of hardiness and should be avoided!)
Additionally, Kotovich observed that during periods of severe cold, significant temperature differences develop between the top and the bottom of a tree. His measurements showed that the coldest zone is usually right at snow line; temperatures only 6 feet higher are about 10°F warmer. Wrapping the trunks of mature trees loosely with reflective, expanded polyethylene (so as to form an oversized tube in which air can flow) raised winter trunk temperatures significantly (and also provided shade that minimized solar thawing in the spring). Moreover, pruning a tree so that the lowest limbs were quite high on the trunk, well above the snow-line, elevated the limbs above the zone of coldest temperature. A tree trunk with high limbs could be wrapped continuously from the ground to those limbs, and thus gain a significant amount of protection from periods of extreme cold.
Other protections discussed by Kotovich are also noteworthy. Because even mild frost damage will leave a tree vulnerable to later, more serious injury by sun scald, it makes sense to shade marginally hardy trees from late winter and early spring sun. However, care must be taken when a tree is whitewashed, even if water-based dilutions are used. Whitewash is most appropriate on mature trees to reduce freeze-thaw cycles, but should be applied ONLY to the south side of the tree. Whitewashing all sides stunts bark growth and alters fruit distribution. Additionally, in order for water-based whitewash to be durable, it should be applied during summer, when temperatures are about 50°F or above.
Kotovich found that different approaches work better for young trees. Planting nurslings immediately to the north side of a 2-inch-wide wooden stake offers effective protection from photo damage, and the stake can be removed when the tree has reached its fifth or sixth season. By then, the bark has thickened and is no longer transparent. Also, although whitewashing can be used on young trees, painting with a blue dilution is more effective because this color reflects the wavelengths that cause photo-oxidation but admits those that stimulate anthocyan production. Thus, young trees can continue to develop their own natural protective mechanism beneath the blue wash (see accompanying photo). Kotovich recommends that blue wash be used only until the tree is about 5 or 6 years old. After that, white wash is preferable because thermal rather than light injury causes damage to mature trees.
Benefits of Raised Beds:
Kotovich discusses some recommendations unrelated to sun scald. Perhaps the most interesting is that fruit trees be planted on raised beds or hills. He found that in the St. Petersburg area (latitude 60°N), trees planted in depressions had a life expectancy of only 10 to 15 years, trees planted on raised beds one meter square by 0.5 meters high lived 15 to 20 years, and trees planted on raised beds one meter square by one meter high were still alive and producing at 25 years. Kotovich believes that raised beds are beneficial in the St. Petersburg area because excessive moisture there causes poor soil health. His studies demonstrate that raised beds help to keep the soil properly aerated and so improve microbial activity and tilth, as long as the trees are mulched so the root zone does not dry out during hot summers.
(Note: Kotovich does not speculate about other possible benefits from planting trees on raised beds, but as many vegetable gardeners can attest, raised beds encourage growth in cool, short-season areas by warming soils quickly during the critical period of spring and early summer. Raised beds might also help a tree to harden off quickly by cooling the root zone in the fall. Finally, it should be noted that planting trees on raised beds both elevates trees and creates drainage paths that channel cold air down and away from tree trunks. If Kotovich’s temperature studies are accurate, raised beds should probably provide several additional degrees of winter frost protection to an Alaskan orchard.)
Although the remainder of Kotovich’s findings are beyond the scope of this article, Kotovich’s lifework has been to explore and develop solutions to problems that afflict fruit trees in the far north. Many of the practices he recommends seem likely to provide a practical improvement in tree survival and health. Nonetheless, his studies make clear that weather extremes in the sub-arctic pose daunting challenges to fruit growers.
Reading Kotovich’s publication has strengthened my conviction that lack of hardiness caused by dependence on marginal cultivars is the most serious limitation on fruit growing in Alaska. Lack of hardiness is, of course, the chief cause of winter frost injury and, according to Kotovich, the cause of all consequent complications of sun scald.
There is a bright spot in this assessment for Alaska growers. The potential for hardiness in fruit trees is for the most part genetically determined, and some trees, including various crabs and a few cultivars of standard apple, obviously have genes that produce hardiness quite adequate for Alaskan conditions. Developing additional, productive, super-hardy cultivars should be a rather simple—if time-consuming—exercise in plant breeding. To this end, northern breeding programs like those at The University of Saskatchewan in Canada and at the Swedish University of Agricultural Sciences in Balsgard may be a useful source of new, improved varieties suitable to Alaskan orchards. If and when truly hardy fruit trees replace the many marginal cultivars now being tried, frost damage and subsequent damage from sun scald may no longer be a concern.
(Note: Ilya Kotovich’s “Sun Scalds of Fruit-Trees” (St. Petersburg, 2009) was retrievable as of Dec. 25, 2013 at http://www.stabilen.ru/files/sun_sclads_en.pdf. It consists of an 81 page, downloadable pdf and includes numerous tables and photographs.
The text of the document indicates that Kotovich was born about 1931 and pursued graduate studies at the Leningrad Agricultural Institute of the former USSR (the institute has recently been renamed The St. Petersburg State Agricultural University). The text appears to have been translated into English, presumably from original Russian, and made available on the internet either by Kotovich himself or by a person or persons unidentified. Although the document offers a wealth of information and experience useful to Alaskans and indeed to anyone trying to grow fruit in cold climates, the translation and organization are in some places confusing. I have tried to accurately summarize points that would be most useful to local growers, and to add annotation based on my reading of the entirety of Kotovich’s discussion and my own experience. Nonetheless, I may have misunderstood or misapplied some points, and readers are encouraged to study Kotovich’s publication for themselves while it is available.)