Aspens Turning Orange? It’s Cytospora Canker

What is Cytospora Canker

Cytospora canker is a tree disease caused by several species of Valsa and Leucostoma fungi. The fungus attacks and kills the bark, causing dead or dying areas called cankers. In many cases, cankers can extend around an entire branch, cutting off water and nutrients to the rest of the branch and killing it. In severe cases, it can kill the whole tree.

Some fungi attack only specific tree species, while others can infect many different types of trees. For example, Valsa sordida primarily causes cytospora canker in Populus species, including our beautiful aspen trees, while conifers are infected by Valsa kunzei.

Trees Affected by Cytospora Canker

Cytospora canker disease affects a wide range of trees. Here in the Colorado foothills, we’re most likely to see it attack the following:

  • Colorado blue spruce
  • White spruce
  • Engelmann spruce
  • Douglas-fir
  • Aspen
  • Cottonwood
  • Lombardy and other poplars
  • Birch
  • Willow
  • Fruit trees, including apple, cherry, peach, and plum
  • Honeylocust
  • Mountain ash
  • Silver maple
  • Siberian elm

Be on the lookout for signs and symptoms of cytospora canker in trees that:

  • are older,
  • have been stressed (for example, by drought, late spring frost, defoliation by pests or disease),
  • have been injured (for example, storm damage, deer or elk rubbing, holes from boring insects or inappropriate pruning),
  • have root damage, such as newly (trans)planted trees and those in areas with nearby construction, and
  • are planted in windbreaks, on the south side of buildings (especially aspen) and ornamental plantings.
cytospora canker in spruce

Cytospora canker symptoms in a spruce tree.
Image by Joseph OBrien, USDA Forest Service,


The visible symptoms of cytospora canker vary depending on the type of tree and the disease stage, but generally involve dead or dying bark and branches.

The disease causes cankers – patches of dead or dying bark that are usually oval or diamond-shaped, discolored (usually yellow, brown, reddish-brown, gray, or black), and sunken. Cankers often ooze resin (in conifers) or orange-brown fluid that stains the surrounding bark.

As the canker progresses, the bark around it may die and fall off, and the inner bark beneath it darkens, deteriorates, turns spongy and starts to smell. In wet or moist conditions, you may also see yellowish, thread-like tendrils growing out of the infected areas.

Eventually, the canker girdles the branch (kills the bark all the way around the branch) and the branch dies.

Symptoms in Spruce

Older branches are more susceptible to the fungi, so symptoms are often noticed first in the lower parts of the tree. The most obvious signs are brown needles on the lower limbs that are shed during the winter. Over the course of several years, the disease progresses up the tree, causing more dead needles and branches.

If you look closely, you’ll also see cankers on the branches and sometimes on the tree trunk (mostly in Englemann and Douglas-fir). These sunken patches of dead bark usually exude a lot of clear, amber resin that forms a white crust around the edges of the canker.

While cytospora canker can cause extreme deformation to spruce trees, it’s rarely fatal.

Symptoms in Aspens

The most obvious sign of cytospora infection in aspens is patches of bright orange bark that (usually, but not always) ooze brown liquid. After a few years, the bark begins to fall off and you’ll see dark brown or black areas beneath it.

Unlike conifers, the disease can continue to spread throughout an entire aspen tree, girdling branches and, eventually, the trunk. Once most of the branches and/or the trunk is girdled, the tree dies.

fruiting body of cytospora canker on aspen tree

The orange fruiting body of cytospora canker fungi can often be seen in wet weather.

How does cytospora canker spread?

The fungi that cause cytospora canker typically enter susceptible trees through wounds or small openings in the bark. They also colonize dead or dying tissue, moving from there into healthy adjacent tissue. Recent reports indicate that they can also infect healthy bark and buds (where they don’t cause any symptoms), waiting until the tree is weakened or injured before quickly moving in to begin colonizing the tree’s tissues.

In wet conditions, fungal spores are released and carried to nearby trees by rain, water from irrigation sprinklers, insects, animals and wind. This typically happens in fall and spring, but can also occur during the July monsoon season when moisture levels increase.

Weakened trees are infected when the tree is dormant and new cankers usually being to develop when temperatures rise above freezing in spring; we often see cankers forming when buds begin to swell. Rainy, cold spring weather can delay the onset of canker growth but it tends to develop more quickly.

How to Prevent Cytospora Canker

bleeding aspen from holes made by poplar borers

Poplar borers have made holes in this aspen, causing extensive bleeding and making it vulnerable to cytospora canker.

Prevention is the best approach to avoiding tree damage or death due to cytospora canker. Because vigorous trees are less likely to be attacked, it’s important to maintain the health of your trees and avoid causing damage that could stress them.

Proper tree care practices to maintain tree vigor include:

  • Water during dry spells, including during the winter.
  • Fertilize to provide adequate nutrients for healthy growth. Do not over-fertilize as this will either damage the tree or produce excessive, weaker growth that’s more susceptible to attack.
  • Implement a Plant Health Care Program to prevent and/or quickly control pest and disease problems.
  • Prevent wounds from string trimmers or lawnmowers. Open wounds serve as entry points for the fungus.
  • Prune in late winter or early spring when the weather is dry and sunny.
  • Choose trees that are resistant to the fungus. Unfortunately, there are no aspen cultivars that are resistant.
  • Plant trees with enough space to grow and away from infected trees. Good air circulation helps prevent transmission of the fungus.
  • Avoid construction or landscaping activities that can damage roots of susceptible trees.
  • Repair wounds ASAP. Early or late storms (when trees have leaves) will cause limbs and tops to break, resulting in large, gaping wounds that allow canker to access the cambium. Pruning to repair these big wounds will reduce the chances of infection and increase the tree’s ability to heal the wound faster.

What to do if you suspect Cytospora Canker

Although cytospora canker is caused by fungi, treatment with fungicides is not effective. Instead, infected trees are treated to increase vigor, properly remove infected parts, and control the spread to other trees.

Once trees are infected, proper care by a Certified Arborist is your best bet for saving them. Even in aspens, about half can be saved if they’re healthy and well-maintained.

Here are some of the steps we take to manage trees affected by cytospora fungi:

  • Remove infected branches (the best option is to then burn them because the fungus can survive in cankered bark).
  • Remove dead bark to allow underlying tissues to dry out. This helps prevent the fungi from spreading and promotes healing.
  • Prune or cut trees only in dry weather and after the tree has flowered or leafed out (i.e., when it isn’t dormant).
  • Clean and disinfect tools between cuts to prevent fungal spread.
  • Make properly-placed, clean pruning cuts – no rough edges or torn bark.
  • Carefully clean out freshly-infected wounds, if possible.
  • Clear nearby plant material that could obstruct airflow or keep the tree wet.

Prompt diagnosis can help prevent the disease from spreading to adjacent trees. And proper tree care will protect your trees from infection in the first place.

If you suspect any of your trees are affected by cytospora canker, please give us a call at 303-674-8733 or request an inspection.

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