Problem type: Disease
Name of problem: Stem Cankers
Plant name(s): All woody trees and shrubs
Symptoms / Characteristics:
Cankers are dead or diseased areas on stems and branches of trees and shrubs and are usually seen as sunken lesions on woody tissue. Cankers are caused by a number of pathogens including bacteria, fungi and frost, which attack a wide range of plants, and symptoms vary from one host to another. Generally, cankers cause decline or dieback on a plant depending on where the canker is formed and what pathogen caused it. Any growth above the canker will usually show signs of decline. Cankers kill the bark in a particular area and kill the transport systems of the tree, therefore girdling the branch or stem, causing decline of the portion above that point.
Signs of decline may first be evident in leaves, shoots or flower clusters which may become discolored, shriveled or wilted. Most fungal and bacterial pathogens enter plants through wounds caused by frost, sunscald, pruning wounds, wind or other factors that may have stressed or wounded the plant to make it weak and susceptible. Yard equipment such as lawnmowers commonly injure the base of trees, resulting in the development of a canker. Mechanical injuries can eventually girdle a tree, especially if done repeatedly and open an entryway for wood rotting organisms.
Some tree species more readily develop cankers than others. Russian olive trees are commonly attacked by a fungus, which can kill small trees or cause dieback and cankers on larger trees. Young cankers on smooth-barked branches or stems of Russian olive usually exhibit a reddish brown to black color, with the underlying wood being reddish brown. Gum may exude from these cankers and harden as darker blobs on the branch or stem. Larger limbs may have a darker, depressed area where bark may begin to split.
Poplar trees are quite susceptible to a number of canker causing pathogens, most of which are fungi. Poplar cankers generally appear as circular to elongated sunken areas, sometimes encircling the entire stem or branch. Cankers may appear swollen and develop gray black margins throughout or around the canker. Cracks usually develop exposing the underlying wood.
One common fungus causes the disease Cytospora canker in spruce trees and other tree and shrub species. Another common canker occurring on many trees, including elms, is slime flux or wetwood and is caused by a bacterium.
Cankers can be spread by wind, rain, pruning tools, insects and animals and commonly invade through wounds or openings.
Control / Preventions:
There is no chemical control for fungal or bacterial cankers.
The only way to control them is to prune branches back well into healthy wood. Trees having cankers on the main stem should be monitored closely and may have to be removed.
Properly pruning an infected plant will help to reduce canker problems. This should be done in the fall or spring, or shortly after leaves have expanded. Remove dead, diseased or weak branches, and any branches that are rubbing against each other, which may create wounds. Clean cuts should be made leaving no stubs, and making as small a wound as possible, which can then close naturally.
Maintaining plants in a healthy, vigorous state will help them battle invading pathogens. Drought stress is probably the most common factor contributing to unhealthy and stressed plants. Proper planting site, providing ample space for proper root and crown development will promote the development of a healthy tree. Sufficient moisture and nutrients should be in the soil to maintain health and vigor.
Cankers can be avoided or the chance of becoming a problem can be reduced by planting less susceptible plant species.