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.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. 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.Another way to prevent Cytospora damage is to use resistant species or varieties in new planting. 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. For more information, please see fact sheet No. 2.937, "Cytospora Canker."