Canker diseases are common, widespread, and destructive to a wide range of trees and shrubs. A ‘canker’ is really a symptom of an injury often associated with an open wound that has become infected by a fungal or bacterial pathogen. Canker diseases frequently kill branches or structurally weaken a plant until the infected area breaks free, often in a wind or ice storm. Some of the more common cankers are Cytospora canker found on spruce, pine, poplars and willows, Phomopsis canker found on juniper, Russian olive, Douglas-fir, and arborvitae, and Nectria canker found on honey locust, oak, and maple.
Cankers are usually oval to elongate, but can vary considerably in size and shape. Typically, they appear as localized, sunken, slightly discolored, brown-to-reddish lesions on the bark of trunks and branches, or as injured areas on smaller twigs.
The bark often splits between the diseased and the healthy tissue, and sometimes it may ooze sap or moisture. The inner bark turns black and sometimes gives off a foul odor. The newest leaves on affected branches are usually the first to show decline symptoms. Leaves may appear smaller than normal, pale green to yellow or brown, often curled and sparse. As the fungal pathogen invades bark and sapwood, the water-conducting tissues (vascular system) become blocked or dies, causing wilting and dieback to occur. Cankers are formed by the interaction between the host and pathogen. The pathogen grows within the wood and the host tree tries to contain the growth. Cankers can take months (or years) to enlarge enough to girdle twigs, branches, or trunks.
Canker and stem dieback diseases are most common on trees and shrubs under stress. Damage results when opportunistic, living (biotic), infectious pathogens (fungi or bacteria) enter a wound during a time of plant stress, such as transplant shock, drought, or winter injury. Other stress agents that provide opportunities for canker diseases include prolonged exposure to extremely high or low temperatures, flooding, summer or winter sunscald, hail, high winds, nutritional imbalances, soil compaction, mechanical injuries (lawn mower, vehicles), animal damage, pruning wounds, root rot, insect borers, and improper planting. Most cankers are caused by fungi, which invade bark tissue on current season wood. However, some colonize both bark and inner tissue causing canker rots that persist for years. All fungal cankers contain fruiting bodies that appear as pinhead-sized, black or colored (red-orange on Nectria canker) raised bumps embedded in the bark. When present these are an important diagnostic characteristic. Unfortunately, fruiting structures are not always present and many are not easily distinguished. The spores produced by these fruiting bodies serve as inoculum for new infections, mostly in wet or damp weather.
Cankers are difficult to control. No chemicals are universally registered for treatment of cankers. The best controls are preventative ones to keep plants healthy or to prune out the diseased plant parts when practical.
Grow only trees and shrubs that are adapted to the area and site, and select resistant varieties.
Keep plants healthy and vigorous through proper planting, mulching, watering, soil management, pruning, and winter protection practices.
Avoid all unnecessary bark wounds, because many pathogen’s main entry is through injuries.
If a canker infection occurs on twigs or branches, carefully remove the affected parts several inches behind the infection. Pruning cuts should be made at the branch collar and avoid leaving stubs.
Do not prune when the bark is wet, to reduce spread of the fungus. Pruning tools should be sterilized between cuts using rubbing alcohol or 10% household bleach.
Once a trunk canker develops, the tree may begin to seal off the area by forming a callus around the canker. Avoid cutting into such cankers because it may renew fungal activity and increase damage. Any type of trunk canker removal is best left to a professional certified arborist.
Information source: The Morton Arboretum
Lichens are a combination of a fungus with an algae or bacterium. They can be found on any non-moving substrate like rocks or woody plants.
Colors of lichens vary widely and include white, gray, red, green, yellow and black. The same wide variation is seen in shapes and sizes. Some lichens adhere to bark or rocks in a roughly circular flat crust, while others form raised lobes or branches.
Lichens reproduce when small pieces containing both organisms breaks off. These fragments can be carried by wind or water to a new location.
No management is necessary or should be done to reduce the presence of lichens on a tree or shrub. In fact, finding lichens on a tree or shrub in your yard is a sign that the air surrounding the tree or shrub must be clean, since lichens will not grow in areas with a smoky or polluted environment.
Information source: University of Minnesota Extension
One year old cankers are small, discolored areas that are flattened relative to adjacent bark, and only visible on thin-barked branches and stems. As a perennial Nectria canker infection grows, rounded, corky rolls of callus and bark develop a target-like pattern.
Perennial Nectria cankers rarely girdle stems several inches in diameter, but trees and shrubs exposed to strong winds or heavy snow load can fracture at the canker.
Initial infection occurs at leaf scars, branch stubs, cracks in branch axils, sunscald lesions, and other wounds to the bark that expose the cambium. Most spores are wind and rain splashed during the spring and the fall from nearby established cankers. The tree typically responds to a perennial Nectria canker by compartmentalizing the infection with, among other responses, a cork barrier. Once established in the cambium the Nectria fungus grows through the callus during the dormant season, killing the bark, cambium, and the outermost sapwood as it progresses. Clumps or individual red to orange fruiting structures can appear from autumn to spring on the surface of young cankers. The next growing season the tree responds to the breach in its defenses around the canker by forming another compartmentalization barrier. During the ensuing years as the tree and the Nectria fungus alternate with their respective growth responses, the series of callus ridges develop a target pattern.
Once perennial Nectria canker establishes itself in a tree, interventions focus on sustaining the vitality of the tree. Remove infected branches when the weather is too cold or dry for the fungus to infect the pruning wounds, and dispose of the debris away from the trees. Irrigate when conditions are dry, fertilize if soils are deficient in minerals, prune to preserve sound branch structure, avoid wounding the bark, and maintain 2-3 inches of composted mulch over as much of the root zone as possible.
Information source: Dan Gillman, University of Massachusetts Amherst
Frequently, tree limbs and leaves are covered by an unsightly, black, sooty growth called sooty mold. It may occur on any tree, shrub, or leafy plant.
Sooty mold is often found on or below plants infested with certain types of sap sucking insects, especially aphids and soft scales, which produce a sugary secretion called honeydew. This honeydew drips down onto leaves and branches providing a food base on which the sooty mold fungi can grow.
We do not recommend control of the mold itself. However, the presence of sooty mold is often an indication of insect activity that has the potential for causing damage. Proper identification of the insect is necessary to determine if management is warranted.
Light coverings of the mold will gradually disappear during dry weather when its nutrient source is eliminated. Sooty mold can be physically washed off small plants if desired.
Information source: University of Minnesota Extension