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Cytospora canker is caused by various species of the fungus Cytospora.  These pathogens affect many species of trees in Colorado including aspen, cottonwood, lombardy and other poplars, apple, cherry, peach, plums, birch, willow, honey-locust, mountain ash, silver maple, spruce, and Siberian elm.

The fungus attacks trees or parts of trees that are injured or in a weakened or stressed condition and may cause their death.  Trees affected by drought, insects, defoliation by a fungi, sunscald, herbicides, or mechanical injury are susceptible to cytospora infection.  The disease especially damages trees with root damage, which often are found in areas under construction, or trees that have been recently transplanted.  Stands of aspen that have been thinned and young aspen sprout stands may suffer from cytospora canker.

Symptoms

The symptoms of this disease are yellow or orange-brown to black discolored areas on the bark of the trunk and branches.  Oozing of liquid on aspen and oozing of gums on peach and cherry are common.  Cankers, sunken dead areas of bark with black pinhead-sized speckling or pimples, may be evident.  The pimples are the reproductive structures (pycnidia) of the fungus.  Under moist conditions, masses of spores (seeds) may ooze out of the pimples in long, orange, coiled, thread-like spore tendrils.  Reddish brown discoloration of the wood and inner bark also may be evident.  Dead bark may remain attached to the tree for several years, then fall off in large pieces.

On spruce trees, the disease appears as sunken areas surrounded by swollen callus giving a gall-like appearance.  Small black pimples (pycnidia) may occur on the canker.  Once the branch is girdled, needles may yellow or redden with eventual death of the branch.  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 other stresses, diseases, or insects are affecting the tree.

Control

Since this canker usually occurs on a weakened host, the first and foremost method of control is to prevent infection by preventing stress on the tree.  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.  Wounds caused by lawnmowers and weed eaters are prime targets for infection on trees in landscaped areas.  Insects (such as oyster-shell scale) stress the tree and predispose it to cytospora infection and should be controlled.

Prevention of cankers at pruning wounds on peach and cherry trees is helped by Benlate or Mertect340F wound dressings.  The effectiveness of fungicides on other trees is not known but research on other diseases indicates effectiveness is probably limited.  For further information on diseases and controls, especially on fruit trees, see the Colorado State University Cooperative Extension publications, Colorado Tree Fruit Pest Management Guide and Ornamental and Turf Spray Guide.

Another way to prevent cytospora damage is to use resistant species or varieties in new plantings.  Remeber, resistant does not mean the plant is  immune, just better able to defend itself against the pathogen than some other tree.  You still need to keep all trees healthy.  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, 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.

Wounds should be cleaned 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:

1. Prune or cut trees only during dry weather.

2. Clean tools and wipe them with ethyl alcohol, Lysol or other disinfectant.  Clorox may be used at a concentration of 1 part Clorox to 9 parts water.

3. If a wound is fresh (one month or less) use a sharp knife3 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.

4. Tools should be cleaned and disinfected after each cut.

5. Cleaned wounds should not have any sharp angles.

6. Do not apply any tar or oil-based paint, or other wound dressing.  The best method to prevent infection or decay is to allow the cleaned tissue to dry out.

Courtesy of the Colorado State University Cooperative Extension (Revised 12/88)

    by W.R. Jacobi