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
Minnesota is reducing the use of salt (chloride) for deicing walks and roads in an effort to reduce the negative effect on our environment, especially our water. It can cause or aggravate winter injury and dieback of trees and shrubs through salt runoff from roads, and by salt spray from traffic and snowplows. Runoff leads to salt buildup in the soil that can injure roots and be absorbed by the plant, ultimately damaging the foliage. Salt spray can cause severe foliar or stem injury.
Information source: University of Minnesota Extension
Information source: University of Minnesota Extension
Varnish fungus rot, caused by the fungus Ganoderma lucidum and (unvarnished) fungus rot, caused by G. applanatum infect the roots and lower trunk (butt) of many deciduous trees and some conifers. They attack the lower heartwood, and at advanced stages damage the structural integrity of the host tree, often resulting in windthrow (the potential to be uprooted or broken by wind). Maples, oaks and honeylocusts are particularly susceptible, although ashes, elms and many other deciduous trees and some conifers can be attacked.
Trees affected by fungus rot may exhibit yellowing, wilting, or undersized leaves and dead branches. Tree vigor may decline as decay of the sapwood advances. The first visible sign of infection is often the formation of fruiting bodies (single or in clusters) on the lower trunk and exposed root areas. The fruiting bodies are conks: shelf-like in appearance and up to 14 inches wide. The upper surface of varnished fungus rot is typically red-brown with a white edge, shiny, and with a lacquered appearance. Conks of the unvarnished fungus rot are brown with a white edge weathering to grey. Both have a white, porous surface (when fresh) on the underside. Young trees as well as older, larger ones can be killed by this disease. Unfortunately, by the time the conks are noticed, it is too late to reverse the infection. The rate of decay can lead to death in as little as 3 to 5 years from the time of infection, and appears to be determined by tree vigor, which is often influenced by environmental stresses.
New spores released from the conks are dispersed throughout the summer during humid periods, and infect open wounds on root flares and lower trunk areas of susceptible trees. The spores germinate, and the infection advances to attack the sapwood of major roots and the lower tree trunk. The amount of decayed wood increases year after year, resulting in dangerously soft, spongy wood in the part of the tree that serves as its anchor. The conks are annual; new conks may be produced each summer and fall, after which they die and deteriorate.
Information source: Missouri Botanical Garden
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
Information source: University of Minnesota Extension
Information source: University of Minnesota Extension
What is Verticillium wilt? Verticillium wilt is a typically fatal disease that affects a wide range of deciduous woody ornamentals in Wisconsin. Trees most commonly and severely affected are maple and ash. Other trees and shrubs such as barberry, catalpa, elm, lilac, linden, smoke-tree and redbud susceptible. In addition, many herbaceous ornamentals, as well as vegetable crops, can be affected by this disease.
What does Verticillium wilt look like? The first signs of Verticillium wilt that you may notice are individual branches that suddenly wilt and die. Affected branches may occur on one side of the tree or may be scattered throughout the tree. If you carefully peel away the bark of these branches, you may see brown or green streaking in the sapwood just under the bark. Streaking is common in trees such as maple or redbud, but often is not visible in ash and lilac.
Where does Verticillium wilt come from? Verticillium wilt is caused primarily by two fungi, Verticillium dahliae and Verticillium albo-atrum. These fungi are commonly found in Wisconsin soils and in roots, branches and leaves of infected plants. These fungi enter trees and shrubs through their roots and grow in the xylem (i.e., the water-conducting tissue) of plants where they lead to blockage of water movement. This lack of water movement is what eventually leads to wilting.
How do I save a tree or shrub with Verticillium wilt? Trees and shrubs infected with Verticillium cannot be cured and will likely eventually die. However, you can extend the life of your plants by making sure that you water and fertilize properly. Make sure established trees and shrubs receive approximately one inch of water per week. If rainfall is insufficient, use a drip or soaker hose to apply supplemental water. Fertilize trees as needed, but be sure to base any fertilization on a soil nutrient test. To prevent competition for water and nutrients, remove lawn grass within the drip line of your trees and shrubs (i.e., the edge of where the branches extend) and replace it with shredded hardwood, pine or cedar mulch. On heavy clay soils, use three inches of mulch. On other soils, use three to four inches of mulch. Be sure to keep mulch two inches from the main trunks and crowns of trees and shrubs. In addition, prune out dead branches as they occur. Dispose of these branches by burning (where allowed by local ordinance) or landfilling them. DO NOT bury or compost these branches. Be sure to clean your pruning tools between cuts by dipping them for at least 30 seconds in a 10% bleach solution or (preferably due its less corrosive properties) 70% alcohol. Rubbing alcohol and many spray disinfectants contain approximately this alcohol concentration. Decontaminating your tools will help prevent spread of Verticillium from branch to branch, or more importantly from tree to tree, as you prune.
How do I avoid problems with Verticillium wilt in the future? The best way to avoid Verticillium wilt is to plant trees and shrubs that are immune or resistant. Resistant deciduous trees and shrubs include apple, aspen, azalea, beech, birch, butternut, crabapple, dogwood, flowering quince, ginkgo, hackberry, hawthorn, hickory, holly, honeylocust, katsura tree, mountain-ash, oak, pear, poplar, sweetgum, sycamore, walnut, and willow. Conifers (e.g., pines, spruces and firs) appear to be immune to the disease. Also, DO NOT use mulches that may have been produced from infected trees or that are of unknown composition. Finally, immediately collect and discard leaves that have fallen from symptomatic trees. Both mulch and leaves are potential sources of Verticillium.
Information source: University of Wisconsin-Madison