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Conditions right for "conditional" canker diseases

By Dr. Glen Stanosz, Dept. of Plant Pathology, University of Wisconsin-Madison

Canker diseases are caused buy many different fungi that invade the bark, cambium and outer sapwood of the twigs, branches and main stems of trees. The killing of these tissues results in the symptom called a canker, which is the dead, often discolored, swollen or sunken, and cracked area of a stem. Some cankers expand rapidly up and down stems and grow around (or girdle) stems, resulting in death of all portions beyond the canker. Other cankers are localized and may be surrounded by healthy tissue called callus. Some canker pathogens aggressively attack and damage otherwise healthy trees. Many others are common on trees already subjected to factors that have adversely affected tree condition. Such "conditional" canker diseases are likely to become more prevalent and have severe impacts on street and landscape trees stressed by drought.

Conditional canker diseases (also referred to as saprobic canker diseases) often are caused by fungi that might normally be considered weak pathogens. In the otherwise healthy tree, these fungi might not cause disease, or produce only the most minor symptoms such as occasional twig death. When trees are grown outside their natural ranges, planted in poor sites, exposed to extremes in heat or cold, or subjected to defoliation or prolonged drought, however, they may be more aggressively attacked. The tree adversely affected by one of these conditions may be altered to become a better substrate for the growth of the fungus, or morphological and chemical resistance responses may be suppressed. Thus, the "weak" pathogen appears to become more aggressive and induces cankers that cause defect, dieback and even tree death.

A very large number and great variety of fungi associated with street and landscape trees are conditional canker disease pathogens. Several Cytospora and Fusarium species induce cankers of spruces and other conifers, poplars and willows, and maples, for example. Phomopsis species attack a wide range of trees from Douglas-fir to junipers to Russian olive. Diseases caused by numerous Sphaeropsis, Diplodia and Fusicoccum species cause diebacks of conifers and hardwoods, especially following drought or defoliation.

Identification of the cause of a particular conditional canker disease can be a challenge. Many conditional canker pathogens produce their fruiting bodies in abundance on portions of stems they have colonized and killed. These fruiting bodies are usually quite small, however, and may take considerable practice to notice in the field, even with a hand lens. Nonpathogenic fungi also very quickly colonize and produce fruiting bodies in dead tissues. Therefore, microscopic examinations of spores from these reproductive structures often is necessary to confirm the presence and identity of the pathogen. Contact your local government, university or private tree health expert for advice and assistance.

Regardless of the particular pathogen involved, several common measures can be taken to avoid conditional canker diseases and ameliorate their effects on trees. Treatments with fungicides are not usually practical or highly effective in preventing these diseases. But because the fungi that cause conditional canker diseases are responsive to tree conditions, choice of species or cultivar, site selection and planting preparation are very important. Trees should be well adapted to the region and particular location, and provided with adequate room for growth (especially root growth) in good soil. An adequate supply of water is especially important before, during and after transplanting. Protection from defoliating insects and diseases, and supplemental watering during prolonged droughts will avoid development of stress that stimulates conditional canker pathogens. Unnecessary nitrogen fertilization, which can stimulate activity of some conditional canker pathogens in trees and alter the balance between shoot growth and root growth, should be avoided.

If these diseases occur, sanitation pruning is necessary to eliminate the fungus and production of inoculum for further spread within the affected tree and to nearby trees. To avoid spread of fungal spores, pruning should be done in dry weather and tools should be surface-disinfected between cuts. Affected stems should be removed by cleanly cutting them at least six inches below any diseased wood and removed from the site for burning or burial.