Twigs are killed by winter temperatures. The last season's growth is killed back to a definite point. This occurs every year to some extent. It depends largely on whether or not the wood ripens properly in the fall.
Fig. 9. - Apple-tree showing frost cankers at crotch.
Roots suffer when the temperature is unusually low and the ground is bare. In some of the western states this is common and serious. It occurs more often on trees up to twenty years of age. The whole root-system may be killed, and the injury or death of tissues may extend to the crown. Ben Davis, Northern Spy and Wealthy suffer especially. Trees on crab - stock are said to be affected less than others.
The above described injuries on the roots, crown, trunk, crotch and branches are regarded as a winter type of Frost Injury. Late spring frosts are sometimes troublesome, and do damage to the fruit-buds, blossoms, leaves and young fruits. All parts of a bud are not necessarily killed. Generally only the floral parts are involved, so that the buds will open in the spring and the killing will not be readily observed. Injury to the blossoms is a common form of Frost Injury. Affected blossoms turn brown and die. It is likely that growers sometimes confuse loss of blossoms due to low temperatures with that caused by poor pollination. Young leaves are killed or injured at higher temperature than are old, mature leaves. Affected leaves crimp and curl; their upper surfaces are wrinkled and puckered and resemble peach Leaf Curl to a certain extent. Heavy frosts at a time when the leaves are partially unfolded bring about the injury. Frost bands on young fruits are familiar to all. Occasionally late frosts occur which do not destroy the set of fruit, but there results a peculiar russeting. In older apples, this appears as a band of varying width entirely around the fruit midway between the stem and calyx - ends.
It is held by earlier authorities that the various forms of Frost Injury arise in different ways. That is, it is thought that crown Rot and sun-scald are different. However, it is now generally agreed that crown Rot and sun-scald are essentially the same, differing only in appearance and in location on the tree. It may be assumed, then, that irrespective of the time of the injury or of the organ affected, low temperature acts the same in bringing about such injuries. The explanation of the action of frost in the production of injury is based on the principle that there is a limit to low-temperature endurance by the plant. When this limit is reached or exceeded, the part is frozen to death or is seriously injured. This phenomenon is essentially a desiccation - process. The plant cell contains, on the average, about 75 per cent water which is necessary to the life of the protoplasm. The permanent removal of water from the cell, which occurs in freezing, is detrimental and the effect is expressed in serious injury or death.
A consideration of the cause of Frost Injury involves a disposition of the many phases of the question of low temperature-action. To state that a given low temperature freezes certain tissues to death does not explain the manner of freezing. Among the more important phases in the process of injury the following may be noted: ice - formation in the tissues; slow versus rapid freezing; slow versus rapid thawing; bark tension and shrinkage of bark from wood; effect of snow and ice on the bark; alternate high and low temperatures.
It has been held in the past that ice-formation in the tissues takes place during the freezing-process. A warm, moist autumn, for example, offers conditions for excess water in the various tissues concerned. If winter should close in rather abruptly, the water in these parts would be frozen by the first period of low temperature. It was further believed that the formation of ice crystals in the tissues resulted in a tearing and rupturing of the cell walls. That ice is formed between the cells there is no doubt. Some, however, maintain that it is formed only when the freezing-process is very rapid. During such a process, it is thought, the water of the cell is withdrawn rapidly to the intercellular spaces; this rapid withdrawal of water from the cells results harmfully to them. At present the general opinion is that while the removal of water from the cells does act detrimentally, ice - formation itself is not directly responsible for the injury; it is only incidental and secondary.
The rate of the freezing has been a point of attention by many students of winter injury. Prominent writers are agreed, however, that it is the sudden or rapid temperature-fall that brings about the injury. But this is not the final cause of the injury or death of tissues. During the downward jump of the temperature, water is rapidly removed from the cell, and death follows. On a cold sunny day tissues exposed to the sun are warmed, while tissues not exposed remain at or near the temperature of the atmosphere. Toward evening with the withdrawal of the sun's rays the temperature of the sun-exposed area falls so rapidly the first part of the period as to produce death of the tissues on the southwest side of the trunk, while that on the northeast side, protected by the slower temperature-fall, remains uninjured. Thus, in brief, the uniform presence of Frost Injury on the southwest side of trees is explained. Localization of the injury at certain points, as, for example, the crown or the crotch, is due to the fact that the tissues at such places become mature more slowly than at other points. Perhaps, too, bark - tensions set up by rapid growth at these certain points also assist in localizing the injuries. And finally, rapid evaporation due to high winds may be a contributory factor.
Sudden thawing of frozen tissue is thought by some to be harmful in effect. In fact, one of the earlier theories advanced to explain the cause of sun-scald is that rapid thawing, subsequent to extreme temperatures, brought about by the action of the sun, produces the injury. This theory still has adherents. Those who hold to this explanation believe that during a rapid thawing the plant cell is unable to recover the water which has been removed during the freezing - process, and as a result the cell dries out and dies. But there are authorities who take the opposite view of the question and hold that rapid thawing is of no concern.
The question of crotch injury is often explained on the basis that snow and ice lodge therein, soften the bark on melting, and injury results. Here, it is maintained, the sun's rays are concentrated through the water acting as a lens and the intense heat injures the protoplasm.
It has been suggested that possibly tissues become more tender as a result of frequent alternation of freezing and thawing. While sudden changes in temperature are destructive in effect, yet it is generally agreed that the sudden fall is of vital importance but that the injury is independent of the rate of thawing.
In general, then, whatever the manner of killing, it may be said that when, during the freezing - process, the water withdrawal from the cell passes a certain limit, the cell is killed. Some lay emphasis on the effect of the rate of freezing and thawing; others lay emphasis on the role of winds, snow and ice in assisting the action of low temperatures to bring about the injurious effects.
Frost cankers, irrespective of their position on the aerial parts of the tree, are commonly inhabited by various fungi. The most common of these perhaps in the northeastern United States is the New York apple canker fungus, Physalospora Cydonice. This fungus may live saprophytically, or it may assume the role of a facultative parasite. In the latter case it enlarges the wound and does considerable damage.
The topography of the land is influential in its relation to winter injury. Trees on low, and consequently less-drained, land suffer more than others. The amount of water in the tissues at the time the low temperatures occur is a factor. The more water the cells contain, beyond certain limits, the more liable is the tissue to injury by freezing. On the other hand it is held that trees which have endured a drought during the growing season, will, if a severe winter follows, be seriously affected by low temperatures. A dry summer followed by a warm, wet fall makes a tree more susceptible to cold than it otherwise would be. Late cultivation and excessive nitrogenous fertilization forcing late succulent growth tend to an unripened condition of the trees, rendering them susceptible to early winter injury (crown Rot). Sun-scald, however, is not induced by these conditions. The topography, type of soil, and methods of orchard management are factors influencing the severity of Frost Injury.