Some microorganisms parasitize the inner bark or phloem and cause lesions called cankers. Many different fungi cause cankers on maples, and each has its own unique characteristics. However, as a group they have some common features:

1. They are most common on trees previously weakened by one or more environmental factors. Drought, freezing injury, toxic chemicals (i.e., deicing salt), and poor nutrition commonly predispose trees to canker diseases. Defoliation by insects or leaf pathogens also encourages increased incidence and severity of cankers.

2. Most microorganisms that cause cankers are not able to penetrate directly through dead outer bark to reach living phloem. However, they can quickly colonize wounds such as those that occur at weak branch junctions or that are inflicted by insects, birds, and other wild animals. Canker diseases can begin also in wounds caused by people.

The best way to control canker diseases is to prevent them from getting started. Take precautions to minimize wounding and to minimize the risk of infections in wounds caused by necessary pruning. Prune late in the dormant season, just before bud break. Be sure that shears, saws, or other tools are free of pathogenic microbes by swabbing blades with solutions of denatured alcohol (7 parts alcohol: 3 parts water) or household chlorine bleach (1 part bleach: 9 parts water) between each cut. Trees on dry sites should be watered weekly during the summer, and they should be fertilized at least once every two years if soil is deficient in nutrients.

If cankers do occur on maples, there is little that can be done to arrest them. Branches with cankers should be pruned back to their junction with another branch or to the main stem. Stem cankers can be excised with a sharp, sturdy knife, but the resulting wound allows entrance of wood decay fungi which may pose greater problems than the canker did. This surgery is best done by a professional arborist.

1) Nectria canker

These two closely related fungi cause diseases that look entirely different from each other, but both are called Nectria cankers. N. cinnabarina causes elongate lesions on stems and branches of weak trees (figure on the left). Its presence is characterized by bright, salmon pink (turning black with age) fungal pads that erupt from diseased or dead bark. These pads are 2-4 mm in diameter and fist appear in late spring. When moistened, they produce spores in slimy masses, and the spores are transported to new infection sites by splashing rain or by adhering to insects, tools, etc. A second spore-producing phase of this pathogen, characterized by clusters of tiny, red spheres on the bark surface, may be evident also. Both spore-producing bodies are illustrated in the figure in the center. Often, the second phase is so abundant that affected bark has a striking reddish hue when viewed from a distance. In addition to its role as a parasite of weakened trees, N. cinnabarina commonly attacks tissues killed by other agents.

N. galligena causes cankers that differ markedly from those described for N. cinnabarina When an infected tree is dormant, N. galligena advances into healthy bark, killing it. During the following growing season, the tree responds with a new layer of bark and undifferentiated wood (callus) to contain the pathogen. However, in the next dormant season the pathogen breaches that barrier and kills additional bark. Over the years, this seasonal alternation of pathogen invasion and host defense response leads to development of a "canker" with concentric ridges of callus tissue—a "target canker" (figure on the right). Other fungi also cause target cankers, but cankers caused by N. galligena are unique in that bark on their faces usually falls away as the cankers expand.

Close examination of crevices along canker margins may also reveal the presence of clusters of minute, red spheres that are the spore-producing bodies of N. galligena. However, these are often sparse and difficult to see; failure to find them should not rule out diagnosis of the disease.

N. galligena seldom kills trees outright, but it does expose woody stems to decay by other fungi. The disease is rare on landscape trees, but as cankers may be sites of eventual stem or branch breakage, infected trees near houses, driveways, etc., should be watched carefully.

Nectria cinnabarina widespread, all species	Spore producing bodies	N. galligena red, slv, sug, str, mtn, nwy

2) Eutypella canker
Eutypella parasitica red, sug, box, nwy, syc

Eutypella cankers are target cankers, and they often can be confused with cankers caused by N. galligena Features that help to distinguish the two are:

• Bark on faces of Eutypella cankers is usually blackened by reproductive bodies of the pathogen and it remains attached; bark is sloughed from Nectria cankers.

• Eutypella cankers often have greater quantities of host callus, and their margins are greatly expanded to resemble the head of a cobra when viewed from the rear. Cankers caused by N. galligena more closely resemble true "targets."

• When bark is removed from the upper or lower margins of Eutypella cankers, a beige mat of fungal tissue can be seen at the bark wood interface. Such a mat would not be present on margins of N. galligena cankers.

Eutypella cankers are sites of wood decay, and diseased trees often break during strong winds. Decay in such trees should be monitored carefully by a professional arborist if the trees are near homes or other buildings and are to be preserved.

3) Valsa canker
Valsa ambiens subsp. leucostomoides sug, red, nwy

This disease is characterized by presence of elongate, shallow cankers on stems and branches of host trees. Bark in centers of cankers usually contains many small (less than 1 mm diameter), gray to white, pimplelike reproductive bodies of the fungus (figure on the right). Valsa canker usually does not kill trees except following winters of unusually cold temperatures or summers of severe drought. Branches greater than 10 cm diameter rarely are affected.

4) Steganosporium canker
Steganosponum ovatum sug, red, nwy

This fungus is common as a decayer of maple bark killed by other agents. It is a pathogen only on trees that are predisposed by adverse conditions such as drought, defoliation, or winter injury. Affected bark often bears numerous discrete black fungal pustules on it. With time, the pustules may coalesce to form a crusty, black film on the bark.

5) Cryptosporiopsis canker
Cryptosporiopsis spp. red

Red maple cultivars 'October Glory' and 'Autumn Flame' are most susceptible to this disease. Young cankers are elongate, slightly sunken, gray lesions, and they occur often on stems less than 10 cm in diameter. Most cankers have a single, tiny (1 mm diameter) hole in their centers, and sap may ooze from these holes, especially in early spring. The holes are made by female narrow-winged tree crickets during egg laying. Apparently, the crickets introduce the fungus into these holes, and disease occurs thereafter. Callus usually forms around margins of cankers one year after infection, and the fungus is contained therein.

6) Bleeding canker
Phytophthora cactorum red, sug, nwy, syc

This disease, as its name implies, is characterized by sap oozing from bark fissures on lower parts of stems. Wood behind the wet areas is dead and stained reddish-brown. Infected trees may also display some dieback in their crowns.

Little is known about how the fungus that causes bleeding cankers spreads from tree to tree or how it gets into trees to cause disease. Thus, control measures are unknown.

7) Basal canker
Phytophthora spp. sug, nwy

In Wisconsin (and presumably elsewhere) several species of Phytophthora kill phloem and young xylem at bases of maples. Injury may result in death of entire trees. Diagnosis requires careful inspection of root collars of suspect trees; look for areas where bark is easily removed to expose dead wood. Laboratory cultures from samples of wood taken at margins of diseased and healthy tissue are needed for positive determination of the pathogen that is responsible for the damage.

How these fungi spread from tree to tree or gain entrance to healthy trees is unknown. However, it is likely that wounds are important routes for invasion, and care should be taken, especially when mowing near maples, to prevent wounding.

8) Galls and burls
Phomopsis spp. red, sug

Tumorous growths on red and sugar maple stems and branches are common, and they have been attributed to infection by a fungus in the genus Phomopsis. How ever, little is known about this disease. Even though affected trees often have multiple galls, the disease apparently does not spread easily from one tree to another.

Wood in galls is weaker than normal wood, and stems with galls are more likely to break in strong winds.