Cytospora canker of spruce

Insect Infestations

Not a single disease, but a problem that can be caused or exacerbated by other illnesses, you must learn to recognize the signs and symptoms of an infestation in your tree. In conifer trees, there are several pests that can cause problems. These problems can range from damaging the tree and rendering it unsightly, to ultimately killing the tree.

Common enemies of fir trees in the United States include bark beetles, which are known to girdle even healthy trees as they lay their eggs, and Douglas-fir beetles. Douglas-fir beetles tend to seek out trees that have already been weakened by other diseases; they’ll move in and begin laying eggs. However, if there are no weakened trees available, these beetles will then seek out a healthier tree. Termites are another common pest species; you can recognize the signs of a termite infestation by the tell-tale sawdust tracks they leave scattered about your tree.

If you think your tree is infested by harmful insects, you may be able to eliminate them with a topical pesticide. However, this must be done with great care, because if it’s handled improperly you can harm the tree. As always, it’s best to contact your tree care professional if you are unsure.

Dwarf Mistletoe

The dwarf mistletoe is a small flowering plant that takes hold on pine trees, drawing away their nutrients in a parasitic relationship. While a small mistletoe plant is generally not harmful to the tree, they can overgrow and eventually develop large clusters, which over time can stress the tree to the point of death.

An infection of dwarf mistletoe begins innocuously enough, with a slight swelling of the bark at the site of infection. As the parasitic plant grows, you will notice small yellow, green, or brownish shoots protrude from the infected area. Over the course of a few years, the mistletoe will firmly take hold and can severely harm, or kill, the tree.

Pruning infected branches can be a possibility, however, in the case of a more severe infection, you may need to remove the entire tree to prevent the mistletoe from spreading to other nearby trees.

Sudden Oak Death

Certainly the entry on this list with the most alarming name, sudden oak death is caused by a fungus called Phytophthora ramorum. This pathogen infects the tree through the soil, causing a blight that can kill off the leaves and twigs of your fir tree. The name “sudden oak death” is certainly fitting when it comes to oak trees. Fortunately, if you have fir trees, the lethal action of the pathogen acts much more slowly, allowing you to work to solve the issue.

You will notice cankers forming on the trunks of your trees, as well as dieback of the needles and branches, if your tree has become infected by Phytophthora ramorum. If you notice any of these symptoms, contact a tree professional immediately because the fungus can easily spread to many vulnerable trees in your area and lay waste to them.

Tagged as: Douglas-Fir Disease, insects harmful to trees, Tree Disease

Drought Douglas-fir trees.jpg

Drought can damage and sometimes kill Douglas-fir trees. Dying trees have been spotted frequently in western Oregon this spring.

(Glenn Ahrens)

Many Oregonians have noticed widespread damage in landscape and forest trees this spring – and weather may be the culprit.

“Browning or dieback is often caused by weather-related stress, sometimes in combination with pests and diseases,” said Glenn Ahrens, a forester with Oregon State University’s Extension Service.

Stem canker fungi

Douglas-fir trees are the most common victims, he said, but stress due to weather is affecting many tree species and a variety of problems are showing up.

On some Douglas-firs, branches and tops are turning red or brown. Sometimes the entire tree dies. Older trees typically have milder symptoms.

“This sudden mortality or ‘flaring out’ of branches and tops is a classic symptom of drought in conifers,” Ahrens explained.

Possible stressors include last year’s long, dry summer ending with a hot period, followed by an early freeze in November and then a relatively warm winter, he said.

Drought-related injuries to the stems and leader are not always apparent when they occur, but often show up the following spring as the weather warms up and trees begin to grow. That seems to have begun with the warm weather of February and March, with symptoms becoming obvious in April.

Similar drought damage has occurred periodically over the last 15 years, most recently in 2013, according to Ahrens.

Heat and drought can kill trees outright or put the trees under severe moisture stress. Subsequent problems can happen when insects or diseases take advantage of a tree’s weakened condition.

Douglas-fir trees are most commonly affected, but similar problems occur with other conifers, including grand fir, noble fir, western redcedar and western hemlock. Grand firs around the Willamette Valley are notorious for health problems due to drought followed by secondary problems such as bark beetles and fungi.

Ahrens said drought-stressed Douglas-fir trees are often troubled by stem canker, normally caused by weak pathogens that become active in trees under stress. The cankers can coalesce to girdle branches or stems, and also can become sites of attack by bark beetles.

Insect pests that take advantage of drought-stressed trees include the Douglas-fir engraver and the pole beetle. Grand fir and noble fir are vulnerable to engraver beetles that attack true firs of all sizes.

Douglas-fir trees in some foothills around the Willamette Valley are afflicted with Swiss needle cast. The disease produces a pale overall appearance and sparse crown as individual needles turn yellow and drop.

“Swiss needle cast disease has been a problem in coastal Douglas-fir since the 1990s,” says Ahrens. “But last year we had increased reports of the disease in the Willamette Valley and we are seeing it again this year.”

Weather is also a contributing factor and the disease is most severe in years with a combination of a warm winter and abundant spring moisture, Ahrens said. Indicators of Swiss needle cast are progressive yellowing and shedding (casting) of needles, beginning with the older needles.

A healthy tree may carry four to five years’ worth of needles, while heavily infected trees may carry only one or two years’ worth. Although the disease is not generally fatal to the tree, it often has a significant impact on growth.

The Extension publication Forest Health Fact Sheet: Swiss Needle Cast in Douglas-fir in Oregon gives more information.

Ahrens offers the following tips for keeping trees healthy:

  • Prevent soil compaction caused by vehicle or animal traffic near trees. Compaction can damage roots, especially in clay soils.
  • Avoid direct damage to trees and roots by animals or machinery.
  • Reduce competing vegetation.
  • Irrigate landscape trees during dry spells. Apply water slowly over many hours; avoid frequent shallow watering. Apply mulch to maintain soil moisture.
  • Do not alter drainage near established trees (ditches, ponds, fill or removal of soil).
  • Plant trees that are well suited for the site. Where Douglas-fir mortality is occurring, consider planting Willamette Valley Ponderosa pine or hardwoods.
  • If insects or branch/stem cankers are evident, prune and destroy affected branches to reduce spread.
  • Do not fertilize during drought conditions; fertilization can increase a tree’s water requirements.

— Mary Stewart of Oregon State University Extension Service

Why are my Douglas fir turning brown?

For those of you growing Douglas fir Christmas trees or who have Douglas fir in your landscape – if you think you are seeing browning or bronzing of needles when looking at your trees from a distance, don’t assume its winter injury. Take a closer look because it might be the needle casting disease called Swiss needlecast, caused by Phaeocryptopus gaeumannii, or Rhabdocline needlecast, caused by Rhabdocline pseudotsugae. (Photo 1 and 2)

Photo 1. Field of Douglas-fir Christmas trees showing the browning
from Swiss Needlecast.

Photo 2. Douglas-fir in the landscape browning from Rhabdocline

These needle cast diseases can be infecting your trees for up to three years before you notice any symptoms; but whether it is the stress of winter, other stresses or a combination of events, we have seen this disease express its fruiting bodies with symptoms exploding on trees in early spring as the trees come out of winter. With Christmas trees these infections can limit the trees’ market potential and you may need to apply sprays for up to three years in order to mask the damage. So, take a closer look at those brown or bronzing branches on your Douglas fir.

Swiss needlecast symptoms first appear as browning on the tips of the needles. (Photo 3) These needles will eventually turn completely brown and fall off. Looking on the undersides of infected needles you will find two bands of little black specks along each side of the needle. A whitish, waxy looking plug normally sits where the black fruit bodies erupt through the needle’s surface. Both the white plug and the black fruit body are easily seen with a magnifying lens. (Photo 4) The white plugs are normal, but the black erumpent fruit bodies are a sign of the fungus indicating needle infection.

Photo 3. Typical browning of the needle tips from Swiss needlecast.

Photo 4. Needle on the left showing rows of black fruiting bodies
of Swiss needlecast.

Rhabdocline appears as brown to purplish-brown splotches on the needles. Usually just before budbreak, the banded areas will begin to swell and split open lengthwise on the undersurface of the infected needles. The fruiting bodies are spongy, orange in appearance, and will protrude from the undersurface when conditions are damp. (Photo 5)

Photo 5. Purplish-brown lesion and typical fruiting body (left) of
Rhabdocline needlecast.

With the appearance of these fruiting bodies, a new cycle of needlecast begins when the fruiting bodies are formed in the spring. To prevent these spores from continuing their infection cycle, fungicides should be applied when the new shoots are one-half to two inches long. A second application is usually required within two to three weeks. A third spray may even be necessary if we have a wet spring. The fungicide materials that work on Rhabdocline are effective on the Swiss needle cast pathogen; you will get control of two diseases for the price of one. Severely infected trees will need to be treated for several years to make them salable as Christmas trees. Always follow label instructions when spraying.

Cytospora canker

Scattered dead branches caused by Cytospora canker

How does Cytospora survive and spread?

Cytospora canker is caused by the fungus Leucostoma kunzei. This fungus is often present in healthy branches. Disease begins when the tree becomes stressed by insect feeding, snow or ice damage, drought or other factors.

A sunken canker forms on branches and is often coated in a thick layer of resin. It may take several years for the canker to girdle the branch.

Once girdled, the branch dies and the needles turn brown and fall off. The fungus quickly grows throughout the dead branch, but rarely grows into the tree trunk. The tree trunk can be infected through wounds.

Dark, raised, pimple-like, spore-producing structures form on infected branches. These structures release yellow tendrils of spores in wet weather. Spores are spread to new branches by wind or rain.

Cytospora canker rarely kills spruce trees, but it can severely deform them. Trees that have lost many branches to Cytospora canker are unattractive, do not provide privacy screening, or block sound or unsightly views. These trees are often removed and replaced.

A tree with lots of branch dieback from Cytospora canker.

Cytospora canker is a fungal disease that attacks and kills individual branches of primarily mature spruce trees, in particular the Colorado Blue Spruce. One by one the branches begin to discolor from the bottom of the tree upward until the otherwise striking pyramidal form of the tree is compromised.

Cytospora is caused by the organism Cytospora kunzei (imperfect form) or Valsa kunzei (perfect from). This fungus naturally occurs as a saprophyte on the dead bark of many conifers. The fungus only causes a problem when the health and vigor of the tree has been weakened by other causes such as drought or poor nutrition.

Trees which are susceptible to Cytospora include Black, Colorado Blue, Engelmann, Norway, Oriental, Red and White Spruces; Douglas-fir; Balsam fir; Eastern Hemlock; Eastern, European and Japanese Larch; and Eastern and White Pines. Infection in Wisconsin is most severe on Colorado Blue Spruce where it kills branches and results in excessive resin production.

As previously indicated, initial symptoms appear on the lower branches and progress upward. Occasionally however, symptoms may begin on the upper branches. Affected branches become off-color, taking on a purple cast which later fades to brown as the needles drop.

The fungus overwinters as fruiting bodies on the bark of infected tissue or as vegetative mycelium in cankers. Infection is suspected to occur during late winter and early spring. The site of infection may be natural wounds such as leaf scars or manmade pruning wounds. The spores of the fungus are dispersed to potential host trees through splashing rain.

Close-up of a bleeding branch.

The resulting lesions begin at the base of the branch near the branch collar and spread distally along the branch in an elliptical fashion. The cankers are reddish brown and slightly sunken. There is often an excessive amount of resin covering the cankers. Removal of the bark will disclose sapwood which appears normal in color. Upon close examination, one may find tiny, black, pimple-like pycnidia which represent the fruiting structures of the fungus. It is in these black pycnidia that spores are produced which will continue the disease cycle.

Cytospora canker will not result in sudden death of the infected tree but rather, several years or decades will pass before the trunk is girdled and an affected tree will die. Dead twigs may remain on the tree for many years. Typically, trees less than 15-20 years old are not affected. However, the fungus may attack small branches of young seedlings in nursery beds.

Because Cytospora normally attacks trees under stress, it is important to prevent or eliminate any stresses on susceptible trees. Selecting the proper location when planting spruce in the landscape may be the deciding factor as to whether or not a tree will succumb to Cytospora at maturity. Avoid overcrowding and drought. Take precautions not to injure the root system and do not prune except during dry weather. Disinfesting pruning tools in a 10% chlorox solution or 70% alcohol between cuts will prevent the spread or the disease to healthy trees or branches.

– Karen Delahaut, formerly of the University of Wisconsin – Madison 

Photos courtesy of Brian Hudelson, Plant Disease Diagnostics Clinic, UW-Madison  

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Cytospora Canker

Death of lower branches of Colorado blue spruce typical of Cytospora canker.

Brian Hudelson, UW-Madison Plant Pathology, UW-Extension
Revised: 3/11/2012
Item number: XHT1003

What is Cytospora canker? Cytospora canker is one of the most common fungal diseases of Colorado blue spruce. This disease can also affect Norway spruce (and less frequently other spruces) as well as Douglas-fir and balsam fir. Trees that are 15 years old or older and are at least 20 feet high most typically show symptoms of this disease.

What does Cytospora canker look like? Cytospora canker usually first appears on lower branches and progresses up the tree. Individual upper branches may show symptoms as well. Needles on infected branches turn purple, then brown and die. Diseased needles eventually fall off and the infected branches die. Infected branches often produce a bluish-white sap that oozes somewhere along their length.

Where does Cytospora canker come from? Cytospora canker is caused by the fungus Leucocytospora kunzei (also referred to as Leucostoma kunzei), which survives in infected branches. Spores of the fungus are spread by wind, rain splash, insects, birds and mammals.

How do I save a tree or shrub with Cytospora canker? Immediately remove and destroy any diseased branches, by pruning them using the 3-point method of pruning (see University of Wisconsin Garden Facts XHT1014 for details). Prune only in dry weather. Between cuts, be sure to clean your pruning shears by dipping them for at least 30 seconds in a 10% bleach solution or 70% alcohol (spray disinfectants that contain at least 70% alcohol can be used). This will prevent movement of the fungus from branch to branch, or from tree to tree during pruning. DO NOT attempt to use fungicide treatments to control this disease.

How do I avoid problems with Cytospora canker in the future? Perhaps the easiest way to avoid Cytospora canker is to avoid planting Colorado blue spruce. If you do plant blue spruce, allow adequate spacing between trees in new plantings. For established trees, judiciously prune branches to open the trees’ canopies. Proper spacing and pruning promote increased airflow, which leads to a less favorable environment for infection and disease development. In addition, minimize any stress to your trees. Prevent water stress by avoiding soil compaction, and by making sure there is adequate soil drainage. During dry periods, water your trees adequately (approximately one inch of water per week) using a soaker or drip hose. Proper mulching (one to two inches on a heavier, clay soil; three to four inches on a lighter, sandy soil) can help moderate your trees’ moisture levels. Prevent nutrient stress by properly fertilizing your conifers based on a soil fertility test. The University of Wisconsin Soil Testing Laboratories ( can assist with soil and plant tissue fertility testing.


Tags: canker, disease Categories: Tree & Shrub Problems

Colorado State University

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by W.R. Jacobi* (12/13)

Quick Facts…

Figure 1: Orange discoloration found in spring and early summer associated with cytospora canker.

Figure 2: Cytospora canker on three branches, each with scattered pycnidia.

Figure 3: Orange spores oozing from pycnidia.

  • Cytospora canker is caused by several species of Cytospora (sexual form Valsa and Leucostoma) fungi. The name comes from the asexual stage of the pathogen that is more commonly seen.
  • The disease occurs on woody shrubs and trees or parts of plants that are slightly stressed.
  • Many trees and shrubs are affected by this disease (apple, ash, aspen, birch, cottonwood, elm, maple, peach, spruce, willow).
  • The canker-causing fungi cause girdling of the plant, killing the plant above the canker.
  • To manage the disease, reduce stress on trees, use resistant plants, remove infected limbs, clean wounds and prune properly.

Cytospora canker is caused by various species of the fungus Cytospora (sexual genera of Valsa and Leucostoma). These pathogens affect many species of shrubs and trees in Colorado, including aspen, cottonwood, lombardy and other poplars, apple, cherry, peach, plum, birch, willow, honeylocust, mountain ash, silver maple, spruce, and Siberian elm. Some Cytospora species are host-specific while other species can infect several different tree species. For example, willow, cottonwoods, and aspen are susceptible to one species. The fungus attacks trees or parts of trees that are injured or in a weak or stressed condition. The fungus grows in the living bark (phloem) and wood (xylem) and kills by girdling the branch or tree. The fungus can attack tree bark during the fall-winter spring seasons when temperatures are warm but the tree is dormant and cannot defend itself. Trees affected by drought, late spring frosts, insect and fungi defoliation, sunscald, herbicides, or mechanical injury are susceptible to Cytospora infection. The disease especially affects trees with root damage, which are often found in areas under construction, or trees that recently have been transplanted. Stands of aspen that have been thinned and young aspen sprout stands may suffer from Cytospora canker.

Sexual and asexual spores of Cytospora species infect freshly wounded tissue. The spores are released after fruiting bodies have absorbed water during rain events. Conidia ooze out of the wet fruiting bodies and are dispersed by rain splash and blown by wind. Many times fruiting bodies are not formed since the cankered tissue dries out too rapidly in the dry western climates.


Cytospora species cause branch dieback and cankers on trees or shrubs. Cankers on stems and branches are often elongate, slightly sunken, discolored areas in the bark. Many times, however, the discoloration is not evident because the fungus killed the bark rapidly. The fungus grows so fast on stressed trees that there is no evidence of a sunken canker. Bark often splits along the canker margin as the tree is defending itself and callus formation occurs. The fungus may quickly girdle and kill twigs without forming cankers. Symptoms vary with host species affected and stage of disease development. Bark above infected cambium may appear sunken and yellow, brown, reddish-brown, gray, or black. Diseased inner-bark and cambium turns reddish-brown to black, and becomes watery and odorous as it deteriorates. Wood below the cambium is stained brown (Figure 1). Liquid ooze on aspen and gummy ooze on peach and cherry are common. Cankers, sunken dead areas of bark with black pinhead-sized speckling or pimples, may be evident (Figure 2). The pimples are the reproductive structures of the fungus. Under moist conditions, masses of spores (seeds) may ooze out of the pimples in long, orange, coiled, thread-like spore tendrils (Figure 3). Reddish-brown discoloration of the wood and inner bark also may be evident. Dead bark may remain attached to the tree for several years, and then fall off in large pieces.

On spruce trees, the disease appears as sunken, resinous areas surrounded by swollen callus, giving a gall-like appearance. Small black fruiting bodies may occur on the canker. Once the branch is girdled, needles may yellow or redden. The branch eventually dies. Large amounts of resin flow from infected areas, coating branches and stems. Unless you see sunken areas surrounded by swollen callus, resin flow on spruce may indicate that other stresses, diseases or insects are affecting the tree.


Because this canker disease usually occurs on a weakened host, the primary method of control is to prevent stress on the tree. Drought and oxygen starvation of roots by flooding soil with water are the two most common stresses that predispose trees to Cytospora infection. High temperatures seem to be related to Cytospora canker on our local alders.

To help a tree resist infection, prepare soil before planting, fertilize, water properly for winter and summer, prune, and avoid injury to the trunk and limbs. Proper care of recently transplanted trees also is essential to avoid stress and infection. See fact sheets 2.932, Environmental Disorders of Woody Plants, 7.211, Fall and Winter Watering, and GardenNotes 635, Care of Recently Planted Trees.

Wounds caused by lawnmowers and weed trimmers are prime targets for infection on trees in landscaped areas. Insects, such as oystershell scale, stress the tree and predispose it to Cytospora infection. Insects should be controlled to prevent mortality by the combined stress of the insects and Cytospora canker.

Help prevent cankers at pruning wounds on peach and cherry trees by applying labeled fungicides as wound dressings. Do not rely on the effectiveness of fungicides on wounds of other trees to prevent infection.

Another way to prevent Cytospora damage is to use species or varieties well adapted to the planting site conditions. These cultivars will be more likely able to resist the disease. Purchasing healthy nursery stock will decrease the possibility of infection. Once infection occurs, the best treatment is to increase plant vigor and sanitation. Remove all infected limbs and other areas. When removing branches, arborists and homeowners should make a smooth cut at the base of the limb, as near the trunk as possible, without damaging the branch collar (swollen area at base of branch). Jagged and rough cut surfaces promote infection. Once infection occurs, the best treatment is to increase plant vigor and sanitation. Remove all infected limbs and other areas. Clean wounds to avoid further spread of infection. Remove dead bark to dry out the diseased area and help the tree defend itself against insect and fungal attacks on the cankered area. Directions for proper wound and canker treatment are as follows:

  • Prune or cut trees only during dry weather.
  • Clean tools and wipe them with ethyl alcohol, Lysol or other disinfectant. Clorox may be used at a concentration of one part Clorox to nine parts water.
  • If a wound is fresh (one month old or less), use a sharp knife to carefully cut and remove all injured or diseased bark back to live, healthy tissue. If the wound is older, just remove loose bark pieces. It is important not to cut, remove or damage callus that may be forming at the canker edge. Callus will look like swollen bark growing across the dead area. Scrape the wound surface clean of loose bark.
  • Clean tools and disinfect after each cut.
  • Cleaned wounds should not have any sharp angles.
  • Do not apply any tar, oil-based paint or other wound dressing. The best method to prevent infection or decay is to allow the cleaned tissue to dry out.
Table 1: Some resistant species and cultivars.
Ash Most cultivars.
Aspen Resistant cultivars not commercially available.
Cottonwood Cultivars: Noreaster, Platte, Mighty Mo, Ohio Red. Avoid Lombardy, Bolleana, Sioux Land.
Elms Most cultivars
Hackberry Most cultivars
Honeylocust Most cultivars.
Junipers Most cultivars.
Lindens Big and little leaf.
Maples Most species and cultivars.
Pines Most species and cultivars.

1Colorado State University professor, bioagricultural sciences and pest management. 9/99. Revised 12/13.

Colorado State University, U.S. Department of Agriculture and Colorado counties cooperating. Extension programs are available to all without discrimination. No endorsement of products mentioned is intended nor is criticism implied of products not mentioned.

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Cytospora canker of conifers is caused by the fungal pathogen Cytospora kunzei (also known as Leucostoma kunzei).


Symptoms & Disease Cycle

Cytospora invades both young stems and large branches, creating small, eruptive cankers. Stem cankers can expand and coalesce, killing the cambium. While the underlying sapwood is colonized, it often shows no symptoms of infection. Infections lead to stunted growth of stems and needles, yellowing of needles, early needle drop and branch dieback. A prominent symptom of infection on spruce is the presence of hardened resin that has oozed from cankered branches. The cankers often have rough callus tissue on the margins of the infection site. Like many cankering fungi, Cytospora is an opportunistic pathogen, taking advantage of stressed hosts. Infections usually begin on older branches towards the base of the tree and gradually expand upward in the canopy. Shade and free moisture favor the pathogen and during extended periods of wet weather in late spring and early summer, Cytospora produces spores that are spread by wind and splashing rainwater. Bark wounds created by strong winds, heavy snow load, hail damage and insect feeding provide infection sites for dispersed spores. Stresses that facilitate infection include drought, needle cast infections, spider mite infestations, root compaction/severing, among other landscape stresses.


Chemical control for cankering pathogens can often have mixed results. Cankering fungi live within branch and stem tissue, which may or may be symptomatic at the time of treatment. Pruning and removal of the infected stems and branches is the best management practice. However, if chemical management is desired, copper-based fungicides (copper hydroxide and copper salts of fatty and rosin acids), mancozeb, thiophanate-methyl, azoxystrobin and Phosphorous acid may have utility against the pathogen. They should be applied as new growth is developing to help protect these sensitive plant parts from becoming infected. Maintaining high tree vigor helps prevent infections from developing. Maintain a large mulch ring and provide supplemental irrigation (if possible) to trees suffering from drought stress. Avoid needles wounding of the bark to limit the number of potential infection sites for the fungus.

Cytospora Canker of Spruce

Browning of needles and dying of the lower branches of affected trees are usually the first symptoms of Cytospora canker. As the disease progresses, it spreads to higher branches. Occasionally branches high in the tree are attacked even though lower ones are healthy. In time, affected trees become unsightly and lose their value for ornamental purposes. Needles may drop immediately from infected branches or the needles may persist. Eventually dry, brittle twigs remain in sharp contrast with unaffected branches.

The cankers produced are inconspicuous because the affected bark does not noticeably change color or become depressed. Frequently, amber, purplish white or white patches of resin appear on the bark in areas where cankers have formed. What color the resin may be depends on how much air mixes with the resin. Careful removal of a thin outer layer of bark in the area that separates diseased and healthy tissue will reveal tiny, black, pinhead-like fruiting bodies of the fungus in the diseased bark. These fruiting bodies contain minute spores which can be spread by rain, wind, or pruning tools. The development of trunk cankers may result in girdling and death of affected trees. Cytospora canker is most common on trees over fifteen years old, but may occur on younger trees as well.

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