| General Recommendations: Cankers, diebacks and wilts are caused by pathogens that infect stems, branches and roots. Because these pathogens affect the water conducting tissues they are considered serious, and in many cases, life threatening. A canker is caused by either a fungal or a bacterial infection. It may appear as a localized lesion or as a discolored/distorted larger area associated with a stem, branch or trunk. Cankers often have discolored areas that become dried out and shriveled so that the outer tissue or bark is wrinkled. In other cases, cankers can appear similar to normal tissue, but show up as flat spots on woody branches or trunks. Cankers commonly enlarge to girdle stems, branches or trunks, killing the water-conducting tissues and causing the death of upper branches or terminal growth (top). This form of dieback can also be caused by root rot fungal pathogens that have invaded the roots or the crown of the plant at the soil line.
Diebacks and declines are often associated with large trees that have been under stress due to adverse environmental conditions such as drought, excess water, and temperature extremes. These stress factors, often combined with poor planting sites, are leading cause of tree mortality. Once dieback and decline are present corrective action may be difficult. Control is best focused on prevention and site improvement. Do not replant the same species into a location where a plant has died of a root problem without making significant site modifications. Ideally plant a different species that is better adapted to the site conditions.
Wilt diseases can be caused by either bacterial or fungal pathogens and result in the interruption of water and nutrient movement to the leaves. These are of the most serious plant disorders and may lead to death within weeks to months. Because pathogens become embedded in the host tissue, they are essentially protected from external control measures. Often there is very little that can be done to manage these problems except to lessen any water stress by improving drainage, or in the case of drought, applying water. Fungal wilt diseases are common in both woody and herbaceous plants. Bacterial wilts are more common for herbaceous plants.
Control of Canker, Diebacks and Wilt Diseases:
1. Prevent Wounds. Wounds allow bacterial and fungal pathogens to easily invade plant tissue.
Woody plants that are susceptible to canker and wilt diseases should not be pruned during the growing season except to remove diseased branches or stems. Pruning should be done during the dormant part of the season to allow time for the wound to heal over and provide a natural protective layer to resist infection. Tree and shrub roots should be left undisturbed as wounds caused by digging can invite soil-borne pathogens to enter and grow toward the main trunk. Do not till or dig under the canopy of trees nor within 10 to 20 feet of the drip line, as fine feeder roots are located at and beyond the outermost branches. Lawn mowers and string trimmers used too close to the trunk will also introduce wounds that are ideal sites for pathogen invasion.
2. Avoid stress. Extreme environmental conditions, such as drought and excess rainfall can cause plants to become stressed. When stressful conditions prevail for weeks, plants may lose their natural defenses against invasion by plant pathogens. Before planting, make sure the site is well drained or that the plant selected can tolerate conditions common to the area. If a water shortage occurs, apply one to two inches of water each week, and deliver it slowly so that the water moves into the soil and does not run off. Fertilize stressed plants in the correct season, which is typically fall or spring. Do not fertilize woody plants during the growing season. Maintain the proper fertilizing schedule for a normally healthy plant based on a soil test. Mulch the root system by applying 1 to 2 inches of leaf mold or compost and maintaining throughout the growing season. Mulch will buffer the soil against a rapid loss of water and subsequent water stress.
3. Remove infected plant parts. Inspect plants frequently. If cankers develop as evidenced by sunken, shriveled bark, dark discolored areas and/or branch dieback, remove the entire branch section. Cut at least six inches closer to the trunk than the cankered area. Do not apply a wound dressing. If the plant is generally healthy, healing will take place naturally. Sterilize your tools prior to pruning by dipping them in a solution of 1 part bleach to 4 parts water or you can use rubbing alcohol. Between cuts, be sure to re-dip your tools to prevent any transfer of the pathogen from one plant part to another.
4. Choose resistant varieties. Some varieties of herbaceous plants and vegetables have greater resistance to wilt pathogens. Select these when available. For woody plants, this strategy is not as effective because fewer resistant selections are available. If a particular plant is known to have canker or wilt disease problems, select a totally different plant. |
