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 occurs on woody plants or parts of plants that are weak or stressed.
- Many trees are affected by this disease, including aspen, birch, cottonwood, poplar, spruce, willow, ash, maple, elm, peach and apple.
- 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. These pathogens affect many species of 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. Based on recent studies at Colorado State University, some Cytospora species are host specific and will not spread to other tree species. Aspen and cottonwoods are attacked by the same fungus. Willow, green ash, alder and elm, however, are attacked by fungi that seem host specific.
The fungus attacks trees or parts of trees that are injured or in a weak or stressed condition. It can cause their death. Trees affected by drought, Insects, defoliation by fungi, 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 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 (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, 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 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.
Control
Because 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. Drought and flooding soil with water are the two most common stresses that predispose trees to cytospora infection.
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, and 7.833, The Science of Planting 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. They should be controlled.
Help prevent cankers at pruning wounds on peach and cherry trees by applying labeled fungicides as wound dressings. The effectiveness of fungicides on other trees is not known. Research on other diseases indicates effectiveness is probably limited.
Another way to prevent cytospora damage is to use resistant species or varieties in new plantings (Table 1). Remember, resistant does not mean the plant is immune, just better able to defend itself against the pathogen than some other tree. It is still important 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.
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 | All cultivars. |
| Aspen | Resistant cultivars not commercially available. |
| Cottonwood | Cultivars Noreaster, Platte, Mighty Mo, Ohio Red. Avoid Lombardy, Bolleana, Sioux Land. |
| Elms |
| Hackberry |
| Honeylocust | All cultivars. |
| Junipers |
| Lindens | Big and little leaf. |
| Maples | Most species and cultivars. |
| Pines |
|
