Winter injury and south-west injury

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This disorder affects:

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Identification
Buds

Uneven or lack of bud break in the spring
Brown or black dormant bud tissue compared to green tissue of healthy buds
Scorching or drying of leaves

Trunk & Limbs

Injury occurs in the lower trunk, crown or roots near the soil surface
Large vertical cracks or splits on bark that may leak visible sap near the ground or snow line
Development of suckers at base of the tree
Damaged inner bark will turn brown, while healthy bark will appear greenish yellow
Injury become more apparent during the growing season; as the cambium dies, flattened, dark coloured areas will be apparent on the damaged portion of the trunk
Winter injury may cause tree death within one year. These trees may actually bloom and start to leaf out in the spring. Depending on the severity of the injury and the health of the tree the previous year, trees may die at any point during the first part of the growing season. Less severely injured trees may recover or they may decline for the next few years before dying.
Bending, splitting or breaking of limbs due to heavy snow or ice load

Often Confused With
Rootstock-scion incompatibility – bark tissue is discontinuous between rootstock and scion

Girdling – physical injury that prevents trunk growth

Poor drainage (water stress) – most severe in low-lying areas or where a tile is broken; puddling of water after rains

Nutrient deficiencies – confirm with tissue analysis

Drought- follows hot dry weather

Biology
Frequency and severity of winter injury depend on a combination of planting site, tree species, tree health and ultimately the severity of the winter. Winter injury can be caused by a number of factors, including excessive winds (desiccate trees), bright sun or high mid-winter temperatures when ground is frozen, alternate freezing and thawing of ground in late winter, heavy snow and ice cause bending and may snap roots, ice damage to trunks and branches, and mammalian feeding.

Buds and cambial tissue of fruit trees are the most sensitive to cold damage. Frequently, the damage is observed first in low-lying areas of the orchard or where natural airflow is restricted. Trees of low vitality, induced by poor cultural practices, inadequate insect pest and disease control, or heavy cropping are more susceptible to low temperature injury.

Desiccation occurs when water leaves the plant faster than it is taken up. Other cell contituents, like sugars, become more concentrated within the cell during dormancy, also contributing to dehydration. In severely cold weather, the ground may freeze to a depth beyond the extent of the root system, cutting off the supply of water. If the previous fall had been dry, there may be insufficient ground moisture to supply roots with adequate water. The risk of desiccation is greatest during periods of strong winds or sunny weather.

Freezing injury may occur when new growth is stimulated by late summer or early fall fertilization or pruning and may not have had time to harden off sufficiently to survive sudden drops to below freezing. Ice crystals form and rupture cell walls. This injury to the cells inhibits the movement of nutrients and water in the tree, causing damage, such as death of branch tips. If only some of the cells are killed, the tree will be able to survive in the spring when demands for water and nutrients are relatively low. In the summer, when such demands increase, the tree may show signs of delayed winter injury, because its damaged cells cannot move enough nutrients and water. Water may be plentiful at this time, but the plant will not be able to take it up as quickly as required. Symptoms of delayed injury include browning of leaves, flower and fruit loss. Freezing injury can also occur in the spring when sun or mild temperatures cause flower or leaf buds to break dormancy early. Freezing nights may kill these buds.

	Roots of cherries, peaches and nectarines are most susceptible to injury from soggy, wet soils.
	Pears, apples and plums are somewhat more tolerant but also can be injured by extended periods of wet soil conditions.

Southwest injury, or winter sunscald, occurs in the winter in, peach, pear, cherry, plum and apricot. Injury is usually confined to the southwest sides of the trunk and main scaffold branches due to mid-afternoon sun exposure when temperature is often highest.

Living cells just inside the outer bark (mostly phloem and cambium) are damaged by day to night temperature fluctuations during the winter months. Exposed bark warms up on sunny days and previously dormant cells within the tree become active in response to the warmth. These cells lose some of their cold-hardiness and are injured when temperatures drop below freezing during the night.

Generally, as the tree matures, it develops thicker bark and becomes less susceptible to injury. However, thin, dark colored bark like that of stone fruit may remain susceptible or even become more sensitive to southwest injury with age.

Period of Activity

	Dormant peach trees start to be seriously damaged (shoot death) when the temperature drops below -29°C, and fruit bud injury occurs once the temperature falls into the -23 to -26°C range.
	In pears, dormant freezing resistance of most cultivars is -30°C .

Scouting Notes
Injury will usually be more common in low spots in the orchard, areas of tree stress or where natural air movement is restricted resulting in areas where cold stagnant air accumulates.

Injured trees may become more susceptible to insect and disease during the growing season. Proper pest management will help to reduce tree stress.

Evaluate the extent of cold injury to fruit buds that may have occurred in their orchards. Dissect fruit buds vertically using a single-edge razor blade, and examine for browning of the flower parts. Undamaged buds will be uniformly 'greenish' throughout, while cold-damaged buds will be 'browned' to varying degrees in the center. Sometimes slightly browned buds will be OK, but more than likely, any significant browning indicates the flower bud is damaged and will not set fruit upon bloom. (Although the flower may actually 'bloom,' the reproductive parts are dead.) Usually, it is good to wait until warmer temperatures arrive (above freezing) to evaluate the damage, as the browning will be accentuated. Also, fruit buds should be sampled throughout the canopy to get an accurate estimate of percent injury.

Cambium injury in the trunk or limbs is also visible soon after the tissue thaws. Often trunk injury is most easily found near the ground or snow line.

Southwest injury is typically found on the portion of the tree with south or west exposure. Young trees with thin bark are most susceptible to southwest injury. Monitor closely when cold, still nights follow sunny days.

Management Notes
Make sure trees are fully hydrated in the fall before the ground freezes. A layer of mulch around the base of the tree may also help conserve soil moisture and moderate temperature.

Avoid heavy watering and fertilization in late summer or early fall as may encourage continued growth and limit chance for trees to harden off for winter survival.

Remove weak, narrow-angled, V shaped crotches.

Avoid late summer pruning as stimulates new growth and reduces supply of nutrients available to plant through winter.

Remove broken or dead branches in the spring and prune to stimulate new growth.

Carefully trim away loose bark at any crack until remaining bark is firmly attached to the tree. This trimming helps improve wound wood formation or callusing. Callus is cell tissue that forms to cover wounds.

Reduce stress on injured plants by fertilizing, watering during dry periods, controlling weeds and insects, mulching, and generally increasing plant vigour.

Southwest injury causes significant stress for trees and can make them highly susceptible to pests and disease entering the damaged areas. In stone fruit orchards, these injuries provide ideal sites for infection by peach canker or other canker diseases or infestation by tree borers.

Paint the trunk, crotches and lower parts of main scaffold branches with exterior white latex paint to minimize southwest injury. This helps to reflect light away and keep surface bark temperatures cooler, ensuring dormant cells remain inactive.

Do not use oil-based or latex paints that contain any oil. These products contain toxic materials that may injure or kill fruit trees.
October is the best time to apply the paint. Best results are obtained when trees are painted on days when the temperature is above 10°C and when paint will dry rapidly.
Apply the paint by brush or by spray. If you choose spray, use 1-2 L of water/4.5 L of paint, depending on the thickness of the paint and the air pressure available.
On young trees, paint the whole trunk. Only the south and southwest sides of older trees require paint. The whiter the bark after you paint, the greater the protection. Better quality paints are more durable.
Damage may eventually heal, but damaged areas should not be painted or filled with any sealing substance, including paint or tar.

Protection from southwest injury can also be accomplished by placing light-coloured spiral tree guards on newly planted, susceptible trees. Tree guards should fit loosely around the trunk for good air movement and should be periodically inspected for insects or excessive build up of moisture inside the wrap. They should be adjusted annually to prevent the guard from interfering with trunk expansion (growth).

Information included above excerpted from;

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Tender fruit

Winter injury and south-west injury