Nectria canker is one of the most serious diseases to afflict hardwood trees. The disease is caused by several species of Nectria. Nectria is a genus of Ascomycete fungi. The most widespread and damaging Nectria fungus is Nectria galligena. Nectria magnoliae, Nectria coccinea, and Nectria coccinea var. faginata occur less frequently, but can be just as problematic. Several Nectria fungi target specific tree species. Nectria magnoliae affects magnolias and yellow poplar. Nectria coccinea invades sugar maple. Nectria coccinea var. faginata attacks American beech. Nectria coccinea var. faginata works in conjunction with a scale insect to cause beech bark disease. Another member of the Nectria genus, Nectria cinnabarina, causes the disease Nectria dieback. Nectria dieback is also referred to as coral spot Nectria or Nectria canker. Maples are especially vulnerable to this disease.
Distribution & Habitat
Nectria canker occurs throughout the temperate regions of the world. It is particularly common in North America and northern Europe. Distribution of the disease is generally influenced by various climatic factors. The disease favors high altitudes by exposed slopes with shallow and infertile soils, and lower altitudes by cold pockets with poorly drained soils.
Nectria canker occurs on over sixty species of trees and shrubs, including apple, ash, aspen, birch, dogwood, elm, holly, maple, pear, sassafras, sweet gum, and walnut. Trees that are frequently attacked by the disease include American beech, black walnut, red maple, sugar maple, sweet birch, paper birch, yellow birch, and largetooth aspen. White oak can be infected in certain localities. Nectria magnoliae infects members of the magnolia family. Nectria cinnabarina afflicts apple, ash, barberry, birch, boxwood, crabapple, elm, hickory, honey locust, linden, maple, pear, rose, and Japanese zelkova. Nectria fungi that cause cankers on one host species may readily infect other nearby tree species. Infections by proximity can occur on any susceptible tree, with the exception of magonlia, sassafras, and yellow poplar.
If conditions are favorable, Nectria canker may occur throughout the year. Infections are most common in spring and fall. Symptoms of infection are not always immediately apparent. The first visible symptoms generally appear in spring, as the host plant breaks dormancy, and resumes development. Dark red, or blackened water-soaked patches appear on the bark near wounds at the base of dead twigs, and branches. As the disease progresses, further discoloration of the bark occurs. Eventually, the bark fissures at the margins, and sloughs away, revealing the canker. In some instances, dead bark may remain in place, and conceal the canker. During rainy periods, small, creamy white or red to reddish-orange fruiting structures may appear on infected areas. These fruiting structures produce spores, which are expelled into the air, and disseminated by the wind or rain to create new infections.
Cankers may extend vertically for several feet. The diameter of each canker is determined by the width of the host plant. As cankers enlarge, they become gaping wounds in the host plant’s wood. Irregularities, or bulges in the bark can be detected, and generally indicate the presence of a canker beneath the bark surface. On fruit trees, infected fruit may blacken, and rot.
Affected twigs, branches, or entire plants may become devoid of growth. New growth often wilts abruptly. Extensive cankers may girdle larger branches and small trees, causing them to decline rapidly. Plants that are stressed, or have incurred wounds are especially vulnerable to infection. Infections may be intensified in fall and winter, when the host plant has entered dormancy. On most hardwoods, excepting sweet birch and yellow birch, infections become less frequent as the tree matures.
Nectria spores infiltrate the host plant through perforations in the outer bark that expose the inner cambium. They may also enter through natural openings, such as lenticels and leaf scars, or through plant wounds. In forested settings, the most common openings in trees are cracks occurring at the base of twigs and branches, which are often caused by sunscald, or loading from ice or snow. Additional plant wounds may be created by improper pruning cuts, insects, frost damage, and hail. If conditions are favorable, spores may germinate in a fresh wound, and become established within three to four hours. When a plant has become infected, a small germ tube arises from the fungal spores, and penetrates into the bark. The healthy bark is contaminated by the fungus, and killed. As the canker develops, and the bark cells die, water-soaked patches appear in the bark, enlarging as additional cells are killed. Bark at the center of the infection may become loose, and slough off, or remain held in place by the adjoining healthy bark at the periphery of the canker.
Each year, while the host plant is dormant, the fungus expands by ¼ of an inch to 1 ½ inches. As the host breaks dormancy in spring, the advance of the Nectria fungus is delayed, or halted as the host attempts to stave off the infection. During this period, the infection may be contained until the next dormant season, when the fungus renews its advancement. After a few years, a succession of surface convolutions appear that are shaped like targets. These are referred to as target cankers.
During rainy periods in fall, the fungus may begin to fruit. As the fungus fruits, it produces minute, creamy-white tufts of hyphae that grow through cracks or lenticels in the bark, along the outer callus ridges. These hyphae produce sexual spores that are expelled from the plant, and disseminated by the wind or rain. Nectria galligema produces small, red fruiting bodies called perithecia that occur individually, or in groups. These perithecia are usually found on dead tissue, or on the exposed wood at the center of the canker. When mature, the perethecia turn dark red, and produce ascospores. The ascospores are released from the perithecia, and dispersed by air currents to host plants, where they initiate a new disease cycle.
The disease cycle of the Nectria dieback fungus is similar to that of Nectria canker. Perithecia develop in spring or early summer. The perithecia vary in color, appearing cream-white, coral pink, pink-orange, or light purple. As the perithecia mature, they deepen in color, turning tan, brown, or black. Orange-red perithecia, which mature to dark reddish brown, are produced in summer and fall. Both fruiting structures release spores that are dispersed by air currents or rain, and invade susceptible tissue.
Effects on Hardwoods
Nectria cankers seldom kill their hosts. Occasionally, a canker may become so enlarged that it girdles, and kills the host plant. Birch and black walnut are particularly susceptible to girdling from cankers. Cankered branches are prone to breakages, which can distort the host plant, and stunt its growth. Cankered plants may be easily damaged by inclement weather. The incidence of cankering tends to be greater in areas where heavy snow and ice storms are common. Decay, and other disease organisms can infiltrate plants through Nectria cankers. The combination of diseases often causes the host to rapidly decline.
- Maintain plant vigor through sound cultural practices. Ensure that plants are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around the base of plants to improve soil quality, moderate soil temperature, and retain soil moisture. Many trees and shrubs also benefit from periodic fertilization.
- Select trees and shrubs that are well adapted to the climate of the area. This will minimize the potential for infection due to environmental stressors.
- Prune, and dispose of branch cankers during dry periods. Disinfect pruning tools between each cut using a solution comprised of one part bleach, and nine parts water. Avoid pruning in spring during moist periods.
- Minimize wounding due to pruning, transplanting, or maintenance activities to reduce the number of infection sites.
- In urban settings, severely cankered trees should be culled to prevent additional infections from occurring.
Photo courtesy of Missouri Botanical Garden.