Sunscald - 'SouthWest Disease'

Narrative and Photographs by
Curtis E. Swift, Ph.D.
Colorado State University Extension

Introduction

"Sunscald" is a condition that typically occurs on the south or southwest side of trees due to the heating and freezing of the bark tissue by the winter sun hence the common name "Southwest Disease". The direct rays of the sun can heat the surface bark tissue to 122 degrees Fahrenheit or higher. A layer of snow on the ground reflects even more heat onto exposed tree trunks. As these cells warm up they lose their resistance to cold. When the sun drops below the horizon, the drop in temperature in the cells result in their death.

This condition may even occur on the north side of trees if sun is reflected from light surfaces such as buildings or fences. To prevent this problem trunks of newly planted trees should be painted with a reflective paint or wrapped for at least the first two winters. These treatments are usually applied up to the lowest branch but can be applied higher if desired. The better protected the tree is the less damage will occur.

This photograph shows damage that occurred on the north side of a tree due to winter sun reflecting off a light-colored fence.

Trees are exposed to intense sunlight in the summer but in most cases, the leaves shade and protect the trunk from injury. When damage does result the trunk and unprotected branches will blister. This is especially true for species of trees that have smooth, thin bark. The bark becomes pinkish or reddish, and the surface tissue slowly roughens and darkens as patches of outer bark die, dry, and crack. This photograph is of a Golden Desert Ash (Fraxinus excelsior Golden Desert ®). This tree has thin, golden bark that is very susceptible to summer sun burn. Wrapping during the summer is critical.

Prevention

To prepare a reflective paint, use interior white latex paint diluted 1 part paint to 2 parts water. Apply this to the trunk and lower branches paying particular attention to the south and west exposures. Since paint will permanently discolor the bark gardeners concerned with the aesthetics of their trees should consider the option of using tree wrap.

Commercial tree wrap is available from most nurseries and garden centers. Tree wraps that are black on one side and white on the other should always be applied with the white side OUT. Do not use burlap, plastic sheeting, or other material not specifically designed for preventing "sunscald" as increased plant damage may occur.

The wrap should be applied about the first of November and removed around the first of April. If left on during the summer the wrap will provide an excellent habitat for disease pathogens and insect pests.

Avoid the use of black tree wrap as this absorbs heat	Do not apply the wrap from the top!

A commercial tree wrap made of white reflective plastic.	A commercial tree wrap made of corrugated paper

The wrap should be applied starting at the base of the trunk and spiral up the trunk and overlap in the process. The wrap can be attached to the trunk with tape. The use of twist-ties, wire, or string to hold the wrap in place is not recommended as these may result in girdling if the wrap is not removed in a timely manner in the spring.

Wrap should be applied starting at the base of the trunk overlapping it as you work your way up to and beyond the first or second structural branch. If there are small branches on the trunk apply the wrap right over those branchlets. Cutting off those branches to apply the wrap will open a wound through which moisture from the trunk can be lost. You can cut these small branches off next spring when you remove the wrap.

'Correcting' Sunscald Damage

When a tree or shrub is damaged by sunscald, the tissue below the damage dies. The tree will attempt to 'correct' this damage by covering it up with a new coat of wood (woundwood). This is evident from the roll of callus tissue that develops at the edge of the damaged area. This callus tissue develops around or over the injury and should eventually cover it by growing over the dead wood. You can help this process.

Remove any loose bark to reduce habitat for insects and other pests. Do not apply pruning paint as this may trap in disease organisms. Pruning paints have been reported to reduce callusing.

"Research indicates that wound dressings (materials such as tar or paint) do not prevent decay and may even interfere with wound closure. Wound dressings can have the following detrimental effects.
• Prevent drying and encourage fungal growth
• Interfere with formation of wound wood or
callus tissue
• Inhibit compartmentalization
• Possibly serve as a food source for pathogens"

Wrapping the new developing callus tissue during the winter helps prevent further damage. Maintaining plant health is critical. Unthrifty woody plants lack sufficient energy reserves for callus tissue development. Proper watering to include winter watering and proper fertilize applications are very important.

Evergreens

Evergreens typically don’t need to be wrapped as their needles shade and protect the trunk from sunscald. A couple of the cedars in the Mesa County Arboretum, however, have long bare trunks and those are wrapped to provide winter protection. Our bald cypress, a conifer that drops its needles in the fall, is also wrapped. In a couple of years these trees should have developed bark thick enough to reduce or eliminate the need for winter wrapping.

Further information regarding Sunscald can be found in the publication Environmental Disorders of Woody Plants

References:

Tree Wounds: Response of Trees and What You Can Do - University of Tennesee

Winter Watering

Fertilizing Trees and Shrubs

Placed on the Internet April 15, 1996
Updated September 10, 2009