By Gary Pullano, Assistant Editor

Reprinted with permission from Fruit Growers News, April 2014 Edition Vol. 53, #4

The need for growers to understand the impact of cold injury to fruit trees in 2014, particularly to the more tender crops, made a series of presentations by Jon Clements, Extension tree fruit specialist with the University of Massachusetts, take on an urgent tone.

Clements spoke at the Ontario Fruit & Vegetable Convention in February and the North Jersey Commercial Fruit Growers meeting in early March. On both occasions, Clements addressed the impact of cold conditions on fruit production and what growers should look for in the way of potential damage to their orchards.

In the case of Ontario, where Clements said “they don’t call it tender fruit for nothing,” a cultivated plant is not found in its natural habitat. It’s bred for other qualities, but not necessarily for environmental adaptability.

“Cultural practices become important to augment the natural ability of the species to survive cold winters,” Clements said.

He described the various types of dormancy for fruit trees.

Ecodormancy, or quiescence. Buds are dormant as a result of external conditions unfavorable to growth. Examples of this are seen in late fall and early spring.

Paradormancy: This is represented by correlative inhibition, in which buds are dormant from the inhibitory influence of another plant. An example of this is seen in the dormancy of lateral buds because of dominance of the terminal shoot.

Endodormancy: Occurs at a rest stage. Buds are dormant because of internal physiological blocks that prevent growth, even under ideal external conditions. An example would be during chilling hours. This explains why mid-latitude temperate species subject to temperature fluctuation have evolved with long chilling requirements.

The initiation of dormancy is based on cold acclimation, Clements said. The process leading to development of freezing tolerance in plants can take place during warm days and cool, non-freezing nights. Exposure to sub-freezing temperatures leads to a maximum cold tolerance that follows exposure to temperatures approaching 0˚ F.

Cold hardiness allows plants to withstand winter cold, and is related to dormancy/winter rest (endodormancy). Plants gain hardiness in sub-freezing conditions and lose hardiness in warm weather. And they lose hardiness faster than they regain it.

Fruit tree shoots are generally more cold hardy. Water freezes in the shoots/bud scales. The ice pulls water from cells and concentrates a solution in the cells, which lowers their freezing point.

“Eventually, however, cells freeze and the structure/function is damaged,” he noted.

The cold hardiness of flower buds is genus- and variety-dependent: peach, minus 10˚ F; cherry, minus 15˚ F; apple and pear, minus 25˚ F to minus 30˚ F.

“There is not as much information on shoot/leaf hardiness,” Clements said. “Flower buds freeze individually, (displaying) a range of sensitivity. Terminal flower buds are more vulnerable than lower flower buds.”

He said cold hardiness is dependent on the time low temperature occurs (early versus mid- or late winter), how fast the temperature drops and what the temperatures preceding the cold temperatures were. The length of the sustained cold temperature also plays a role.

As an example, he said Siberian C peach rootstock is hardy in Ontario, where it sustains cold temperatures, but is not hardy in South Carolina, where it is exposed to fluctuating temperatures.

Winter injury most often occurs in extreme low temperatures, particularly when the low temperatures follow warm temperatures. In the fall, an early hard cold can be damaging if it occurs before plants are acclimated. A warm-up during winter can lead to a loss of cold hardiness. A cold snap after spring warm-up leads to a loss of dormancy and cold hardiness.

“Stressed plants can gain cold hardiness faster than normal – for example, drought stress,” Clements said. “But stressed plants have less sugars and cannot withstand as much cold. Heavy crops in particular can reduce reserves (sugars) available and reduce cold hardiness (particularly if there is an incomplete harvest).”

Winter injury on fruit trees can take many forms, including:

Blackheart: Fairly common. The pith is killed and heartwood darkened. Gumming occurs with cell death. It’s found in apple, peach, cherry, plum and pear. Young and nursery trees are more affected. Blackheart weakens the trunk and branches, but recovery can be rapid in healthy trees.

Cambium injury: Is most common on stone fruit, including peach, and is exacerbated by warm temperatures preceding and the fact that the cambium is last to harden off. It results in a weakening of the tree, secondary infection by fungi and canker. Gumming often is a symptom, but is not always a sign of cold injury.

Crotch injury: This may be the last area to harden off. Upright limbs with narrow crotch angles are the most likely to be cold injured. The injury may extend up and down the limb, and there can be cultivar susceptibility.

Crown/collar injury: This is represented in a winter killing of bark near the ground. It also may be late in hardening off. Apples, particularly Gravenstein and Northern Spy, may be more susceptible.

Winter sunscald: Commonly known as southwest injury, this occurs when the trunk heats during a sunny day, followed by a rapid drop in temperature with sunset. Peach with lower branches may be less at risk than apple. Use of white latex paint is advised.

Trunk splitting: More common on sweet cherry and apple, it occurs most often in late fall/early winter with a rapid temperature drop. Splitting/cracking can extend all the way to the pith. It may or may not close up and heal over. It represents “the beginning of the end” with stone fruit.

Shoot death/dieback: Mostly likely to occur with very cold weather when the tree has not fully acclimated/hardened off. It’s seen more often in young trees. Growers should watch their nitrogen fertilization and late pruning. It has an effect similar to a heading cut.

Injury to leaf and flower buds: In peach, expect to see some damage (10 percent) at minus 23˚ C; 100 percent damage at minus 29˚ C. During winter, the temperature required to damage buds may vary by as much as 5˚ C to 6˚ C because of acclimation/de-acclimation differences. Tree damage can take place at minus 35˚ C for apple, minus 32˚ C for apricot, minus 29˚ C for tart cherry, minus 26˚ C for sweet cherry and minus 25˚ C for peach.

Root death: This is less common than injury to the aboveground portion of the tree. Apple roots can be killed beginning at minus 4˚ C to minus 12˚ C. Bare ground exacerbates the condition.

Clements said the following horticultural practices can impact winter cold injury:

Site selection: Elevation, wetness/drainage issues and latitude all have an impact.

Irrigation/fertilization: Drought stress followed by a cold winter can be bad news. Too much water can keep trees growing later in the fall, reducing hardiness. High nitrogen levels lead to reduced hardiness. Growers should not fertilize too late into the fall.

Choice of variety/rootstock: Clements referenced an NC-140 peach rootstock trial, in which it looked like flower bud injury ranged from about 3 percent to 75 percent following temperatures of minus 11˚ F in early January. Redhaven was the variety, with 12 rootstocks examined.

Pruning practices: Pruning late decreases cold hardiness. Pruning’s impact can last well into midwinter. Late-pruned trees showed enhanced cambium activity into winter. Summer pruning of peach might impact cold hardiness in early winter. Peach pruned at pink might show reduced tolerance to frost during bloom.