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Insects & Disease of Trees

 

Winter Injury to Trees and Shrubs

The frequency and severity of winter damage is determined by a number of factors, including the plant species or cultivar involved, the location and conditions under which the plant is grown, and the exact timing of weather extremes during the dormant period. Contrary to popular belief, plant damage is not generally caused by an unusually cold winter. Low temperature injury is more often associated with extreme temperature fluctuation than with prolonged cold weather.

TEMPERATURE FLUCTUATION

Acclimation to temperatures much below freezing results from exposure to slowly falling temperatures and other factors. Plants that are dormant but not fully acclimated can be stressed or injured by a sudden, hard freeze. Rapid or extensive drops in temperature following mild autumn weather cause injury to woody plants. Extended periods of mild winter weather can de-acclimate plants, again making them vulnerable to injury from rapid temperature drops.

LOW TEMPERATURES

Some species or cultivars of trees and shrubs are injured if temperatures fall below a minimum tolerance level. Plants most likely to suffer winter injury are those that are marginally hardy for the area or those already weakened by previous stress. Species such as rhododendron, holly, and some magnolias may survive several mild winters in the Chicago region before a more typical winter causes injury. Flower buds are often the most susceptible. If plants with marginal hardiness are used, they should be planted in protected sites, such as courtyards or sheltered areas. In general, low temperatures are much less damaging than rapid and extensive variations in temperature.

FROST CRACKS

Frost cracks, sometimes called radial shakes, appear as shallow to deep longitudinal cracks in the trunk of trees. They are most evident in winter at temperatures below 15°F. Frost cracks often, but not always, occur on the south or southwest sides of trees because this area experiences the greatest temperature fluctuations between day and night. A sudden drop in temperature causes the outer layer of wood to contract more rapidly than the inner layer, which results in a long vertical crack at weak points in the trunk. Once a frost crack occurs on a tree, it is likely to appear annually. Trees most susceptible to frost cracks include London plane, oak, Norway and red maple, horsechestnut, crabapple, walnut, linden, and willow.

SUNSCALD

An elongated canker found on the trunk of thin-barked trees, such as beech, maple, willow, white pine, and linden, is often referred to as "sunscald".  Sunscald often develops on the south or southwest side of trees following a sudden exposure to direct sun. In winter, the temperatures on the sun-side of the trunk may exceed air temperatures by as much as 20°F. This is thought to trigger de-acclimation of trunk tissue. The bark slowly darkens, turns reddish brown, and becomes rough. After a time, the callus tissue eventually cracks and falls away. Sometimes only the outermost cambium layer is damaged and a sunken area appears on the trunk. Affected trees often have sparse foliage, stem dieback, and stunted growth.

WINTERBURN ON EVERGREENS

A browning or scorched leaf tip on evergreen foliage in late winter and early spring is a form of winter injury. Browning usually occurs from the needle tips downward. Symptoms of winter burn are present on many narrow-leafed evergreens, such as hemlock, juniper, pine, and yew, and broad-leaved evergreens, such as boxwood and rhododendron.  Winterburn is usually attributed to desiccation or loss of water through leaf transpiration. Winter sun and winds dry needles. Water in the stems and roots is frozen and unavailable to replenish the loss. A rapid drop in temperature after a warm sunny day can also cause further injury to the plant. Applying an anti-transpirant, also called antidesiccant, helps reduce transpiration and minimizes damage to the foliage. At least two applications per season, one in December and another in February are usually necessary to provide protection all winter.

SPRING FREEZES

Once spring growth has begun, a late spring frost can cause damage to de-acclimated woody stems, blossoms, and new shoots. Frozen, succulent, new tissue turns flaccid, appears watersoaked, and withers within a short time. Though symptoms resemble blight diseases, freeze injury appears suddenly after a hard frost, while diseases such as bacterial fire blight, juniper blight, and pine tip blight are progressive over time.

ROOT DAMAGE

Root tissues apparently do not acclimate to temperatures much below freezing and can be killed or severely injured by soil temperature below 15°F. This is especially true for shallow rooted plants. Fortunately, the presence of mulch, leaf litter, or snow cover insulates most soils sufficiently to prevent soil temperatures from falling much below freezing. Plants with frozen roots may wilt and decline after growth resumes in the spring.

SNOW AND ICE BREAKAGE

Heavy snow and ice storms cause damage by bending and breaking branches. Multi-stemmed evergreens, such as yews, arborvitae, and junipers, are often the most prone to damage. To protect these plants from limb breakage prior to winter, tie branches together loosely with strips of cloth or coated twine. Remove in early spring. 

The branches of many hardwoods, such as Siberian elm, maples, and birch, may be seriously damaged in ice storms. Improper removal of ice or snow from the tree or shrub might increase damage. Heavy snow should be removed gently before it freezes to limbs and branches. Removing ice encased on branches can cause additional damage and should not be attempted. Instead, allow ice to melt off naturally.

SALT DAMAGE

Salts used for deicing pavements can cause damage to trees and shrubs. Symptoms of salt damage appear in spring and early summer and include browning of evergreens, leaf scorch, branch die back, and dead areas in turf. Branches and twigs can be killed from aerial deposits, and roots can be damaged from salt remaining in the soil. Salt will leach through well-drained soils, but damage can be extensive in poorly drained soils. Choose salt-tolerant species for sites where salt stress may be a problem.

GIRDLING BY ANIMALS

Mice and rabbits often damage young trees in the winter by feeding on the bark and girdling the trees. Damage occurs most commonly when there is prolonged, heavy snow cover, and food is scarce. Rabbits feed on the bark above the snow, while mice feed near the ground level. Mouse damage is usually more severe when the trees are surrounded by heavy grass, weed cover, or heavy mulch, so it is helpful to pull mulch away from trunks and branches. The most effective deterrent to girdling by mice or rabbits is to wrap the trunk and low branches of young trees with screen wire or hardware cloth from below the ground line to high enough above the possible snow line to prevent rabbits from reaching the trunk or branches. To help control mouse damage, maintain an area free of grass or weeds for a 1 to 2 foot radius around the base of the tree. Various chemicals are available to repel mice and rabbits, but are often not reliable in wet weather.

MINIMIZING WINTER INJURY:

    Select hardy species and cultivars.

    Avoid late-summer fertilization or pruning, which might stimulate new growth.

    Water trees and shrubs, especially evergreens, during dry periods until the ground freezes.

    Use mulch to conserve soil moisture and insulate the roots from cold temperatures.

    Protect evergreens from wind and salt spray with burlap screens.

    Apply anti-desiccant to evergreens starting in late fall, following label instructions.

 

What Is Thousand Cankers Black Walnut Disease?

Originally found to be killing trees in Colorado as early as 2003, this is a newly recognized disease (2008) of primarily Black walnut (Juglans nigra) and caused by a fungus, Geosmithia morbida, that is vectored into the tree by the walnut twig beetle (Pityophthorous juglandis). Thousand Cankers disease has produced widespread death of black walnuts in many western states during the past decade. Other species of walnut such as Arizona walnut, English walnut, and California walnut have all shown varying degrees of susceptibility to this fungus.

 

What are symptoms of the disease?

 

A newly recognized fungus, Geosmithia, kills a localized area in the phloem just under the bark in  >2cm wood after introduction by the walnut twig beetle.  These dead areas often overlap or coalesce from numerous strikes (35 insects per square inch of wood) causing nutrient disruption to foliage and thus leading to branch dieback.

The cankers rarely show any of the external symptoms that are associated with most canker producing fungi that affect trees. The affected area is shallow and confined to the phloem of the tree so that it can easily be missed if inspection cuts are made too deeply into the sapwood. Minor weeping may occur at points where walnut twig beetles enter the bark but often no symptoms area associated with the beetle attacks aside from minute entry wounds or star shaped cracks.


Early symptoms are yellowing of leaves and foliage thinning of the upper crown of the tree. As the disease progresses larger limbs are killed which may have dead, flagging leaves associated with them. In end stages the fungus may be introduced into the trunk and large cankered areas develop in the trunk. In susceptible hosts, such as black walnut (Julgans nigra), trees usually die within three years after initial symptoms are observed in the crown of the tree.

*Photo by Kathy Keatley Garvey

*Photo by Jim LaBonte, Oregon Department of Agriculture.

*Photo by Jim LaBonte, Oregon Department of Agriculture.

What Is Thousand Cankers Black Walnut Disease?

Originally found to be killing trees in Colorado as early as 2003, this is a newly recognized disease (2008) of primarily Black walnut (Juglans nigra) and caused by a fungus, Geosmithia morbida, that is vectored into the tree by the walnut twig beetle (Pityophthorous juglandis). Thousand Cankers disease has produced widespread death of black walnuts in many western states during the past decade. Other species of walnut such as Arizona walnut, English walnut, and California walnut have all shown varying degrees of susceptibility to this fungus.

 

What are symptoms of the disease?

A newly recognized fungus, Geosmithia, kills a localized area in the phloem just under the bark in  >2cm wood after introduction by the walnut twig beetle.  These dead areas often overlap or coalesce from numerous strikes (35 insects per square inch of wood) causing nutrient disruption to foliage and thus leading to branch dieback.

The cankers rarely show any of the external symptoms that are associated with most canker producing fungi that affect trees. The affected area is shallow and confined to the phloem of the tree so that it can easily be missed if inspection cuts are made too deeply into the sapwood. Minor weeping may occur at points where walnut twig beetles enter the bark but often no symptoms area associated with the beetle attacks aside from minute entry wounds or star shaped cracks.
Early symptoms are yellowing of leaves and foliage thinning of the upper crown of the tree. As the disease progresses larger limbs are killed which may have dead, flagging leaves associated with them. In end stages the fungus may be introduced into the trunk and large cankered areas develop in the trunk. In susceptible hosts, such as black walnut (Julgans nigra), trees usually die within three years after initial symptoms are observed in the crown of the tree.

*Photo by Kathy Keatley Garvey

*Photo by Jim LaBonte, Oregon Department of Agriculture.

*Photo by Jim LaBonte, Oregon Department of Agriculture.

 
 

Cankers

The beetles prefer wood larger than 2cm and feed on young branch tissue in the upper canopy early in the spring.  At these tunneling sites, cankers are diffus, brown to black and are often not visible until the outer bark is lightly shaved.  Later in the summer, adults move into the lower trunk to overwinter and continue to inoculate the phloem tissue with the fungus.  As the disease progresses, these cankers coalesce as well and can elongate to 2 meters in length.  In some cases, a brown or black stain will appear on the surface of these large trunk cankers.

Walnut twig beetle and associated staining around tunnel.

Coalescing branch cankers produced by Geosmithia.

Adult twig beetle tunneling on excised stem after 24 hours.

Large trunk cankers of black walnut.

*Photos courtesy of Colorado State University

 

 

Can Trees Survive This Disease?

Thousand cankers kills trees from the cumulative effects of numerous coalescing cankers that develop around individual entry wounds made by walnut twig beetles. Although the fungus does grow within the tree, the area infected is limited; it does not move systemically in the plant as do some other insect vectored fungi, such as the species involved in Dutch elm disease (Ophiostoma novo-ulmi). Instead, tree death results from disruption of phloem tissues transporting nutrients resulting in a progressive depletion of energy.
We do not currently know how long it takes to kill a tree once it has been initially colonized by walnut twig beetles and the Geosmithia associate. It is possible it may take many years – possibly sometimes a decade or more – to kill even a highly susceptible black walnut. However, observations of black walnut in the western states indicate that thousand cankers is ultimately fatal to essentially all trees of this species.
Trees that are well-sited and grow vigorously may resist, in part, the effects of thousand cankers disease. Furthermore, some Juglans species and hybrids appear to be more resistant to thousand cankers than is Juglans nigra (black walnut). The course of disease may be substantially slowed in such trees.
Theoretically, methods that can prevent tunneling by walnut twig beetles (e.g., certain insecticides) can prevent further spread of this disease. However, to date, effective spraying techniques to control the walnut twig beetle have not been identified.

?Does It Look Like??

Where is Thousand Cankers Now?

Confirmed populations are scattered throughout western states (Washington, Oregon, California, Idaho, Utah, Colorado, Arizona, New Mexico and now, Tennessee) and the disease is thought to be widespread. Cooperators from the impacted states believe that Thousand Canker Disease may be present wherever susceptible walnut species grow.

The most likely pathway for movement is raw wood (logs, burls, stumps, firewood, wood packaging material (WPM)). Other potential pathways include nursery stock, scion wood for grafting, and natural spread.  The beetle/pathogen complex is likely to enter the east with each entrance event, as follows:
 Movement of untreated walnut (logs, burls, stumps, firewood) across the country from the west into eastern states appears limited but it does occur and it is rarely documented. Low grade walnut may be utilized if bark is attached this could be an important pathway. Raw wood is the most critical pathway.  Campsites and sawmills in the Great Plains states may facilitate the eastern movement of Thousand Canker Disease.
 To date there have been no reports of infected trees in walnut production nurseries; however, if nurseries do become infected, this could become an important pathway.  Natural spread along riparian corridors is likely to occur.
We considered potential economic and environmental damage that could occur if the vector and pathogen were to become established in the eastern United States. Juglans nigra is a valuable timber and nut species in the east; production sectors that face negative impact include timber, furniture, nut and nursery stock. Exports could be affected. Homeowners may face the cost of tree removal. Additionally, because J. nigra is a hard mast producer, wildlife may be negatively impacted.


Map depicting TCD Distribution as of 11/2009

 

J. nigra (black walnut)
Native / Natural range extends throughout eastern U.S. and into Kansas and Nebraska; planted
throughout U.S. Timber, nut and ornamental tree; used as rootstock for English walnut grafts
Highly susceptible to Thousand Canker Disease

 

What Treatments are Available for Thousand Cankers Disease?

Currently there are no known insecticide sprays that reliably control this disease. Some techniques directed at the vector ultimately may prove to be useful in suppressing the rate of disease spread. However, it may be unlikely that effective treatments will be found that can control walnut twig beetles once tree attacks have begun. Control of walnut twig beetle by use of drenching trunk/branch sprays of insecticides (permethrin, bifenthrin) is a technique used successfully against some other bark beetles (e.g., mountain pine beetle, Ips beetles). However, infested black walnut trees that have received repeated insecticide spray treatments by arborists in Colorado are observed to continue to decline and die. This method appears to have only limited effectiveness, at most.  The use of soil applied systemic neonicotinoid insecticides (e.g., imidacloprid, dinotefuran, clothianidin) is a possibility for bark beetle control and there has been some attempts to use imidacloprid (e.g., Merit) for this purpose. Anecdotally these treatments seem to have had negligible effectiveness. Researchers report that the disease may be suppressed by soil applications of imidacloprid if they are applied before the disease has become well established in the tree; treatments made after symptoms begin to appear are ineffective.
Trunk injected fungicides combined with insecticides may be the most effective way to eliminate the beetle and the fungus. Additionally, injected fertilizers will assist in restoring the nutrients to the tree.

 

What can I do now?

Injection of fungicides and insecticides in combination with injected fertilizers may save the tree.

 

 

Acknowledgements

We would like to express our appreciation to the following individuals: For GIS-related data: Manuel
Colunga, Michigan State University; Frank Koch and Bill Smith, USDA Forest Service; and Dan
Borchert of USDA APHIS. For their assistance in data collection and updates on the current status of
TCD: Ned Tisserat, CSU; Mitch Nelson, Carolyn Pizzo, Tom Culliney, Mikell Tanner, USDA APHIS;
Bruce Moltzan, USDA FS; Mark Stirling, CDFA; Chuck Leslie and Steve Seybold, UCA-Davis; Malli
Aradhya, USDA ARS NCGR; Tim Ford, IPPFB; Marion Murray and Diane Alston, USU; Jim Hafferty;
Todd Morgan, UMT; Jay Pscheidt, OSU. We thank Bob Rabaglia and Bruce Moltzan of the USDA
Forest Service; and Heike Meissner, Andrea Lemay and Jim Smith, USDA APHIS for their willingness
to serve as reviewers.

 
 
 

 

 

 

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