This is the fifth part of a series on summer tree diseases. This article examines canker diseases, and crown gall.
During the summer months, plants are in the midst of their development. While many plants flourish due to the warmer climate, others can be subject to infections from a slew of disease pathogens. The following describes some of the most common diseases to affect plants in summer, and how they impact their hosts.
Canker diseases are symptomatic of an injury associated with a plant that has become infected by a fungal or bacterial pathogen. Canker diseases frequently kill branches, and cause extensive structural damage to trees. Fungal infections rot the cambium, and decay the sapwood as much as three feet above and below the wound. This ability to create cankers, and destroy the cambium distinguishes these pathogens from fungi that affect the heartwood. Cankers are formed by the interaction between the host and the pathogen; they occur when the pathogen develops within the wood as the host simultaneously attempts to contain the growth.
Cytospora canker is one of the most common pathogens. It is caused by the fungus Leucostona kunzei, and can be found on spruce, pine, poplar, and willow. Phomopsis canker infects arbovitae, Douglas-fir, juniper, and Russian olive. It is caused by the fungus Phomopsis vaccinii. Nectria canker appears primarily on honey locust, oak, and maple, with red maple being the most susceptible. It is caused by two fungi, Nectria cinnabarina and Nectria galligena. Hispidus canker, caused by Polyporus hispidus, occurs on a variety of oaks, including willow oak and water oak. It may occasionally infect Nuttal oak, white oak, and hickory as well. Spiculosa canker, caused by the fungus Poria spiculosa, is most common on willow oak, water oak, honey locust, and hickory. Irpex canker is typically relegated to red oaks. It is caused by the fungus Irpex mollis.
Symptoms of Infection
Most cankers are brownish-red, and vary considerably in size. They typically range from oval to elongate in shape. They often appear as sunken, slightly discolored lesions on localized areas of dead bark. Lesions may also develop on the underlying wood of twigs, branches, and trunks. Cankered bark often splits between the diseased and healthy tissue. The inner bark turns black, sometimes exuding a pungent odor. As the pathogen reaches the sapwood, it disturbs the plant’s vascular system, hampering the transportation of water and nutrients. This often results in significant twig and branch dieback.
On infected branches, new leaves may be stunted. As the infection progresses, leaves transition from pale green to yellow or brown, often curling upwards.
- Prune and dispose of any infected twigs or branches. Pruning cuts should be administered at the branch collar. This will enable the wound to effectively callus over. Always remove any lingering branch stubs; these can die back, creating access points for infectious pathogens. Pruning tools should be sterilized between cuts with rubbing alcohol, or a solution composed of one part household bleach, and nine parts water.
- When planting, consider using hardy plants that are well adapted to the climate of the area. This will help minimize the potential for infection.
- 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 trees and shrubs. Mulching helps to improve the soil quality, moderate the soil temperature, and retain soil moisture.
- Prune trees and shrubs periodically to maintain their proper size and shape. Avoid pruning during wet conditions.
- Once a trunk becomes cankered, the tree or shrub may attempt to callus over the wound. Cankered trees should be culled to prevent infections from spreading to healthy trees.
Crown Gall (Agrobacterium tumefaciens)
Crown gall is a tree disease of many woody shrubs, and some herbaceous plants. The disease is caused by the bacterium Agrobacterium tumefaciens, a soil-borne bacterium that is normally associated with the roots of various plants. When a plant is infected, the size of the initial growth depends on the amount of inoculum present. If a large number of bacterium cells invade a plant wound, the disease will be more severe, and a large gall will develop at the infection site. Plants that are systemically infected will harbor the bacterium for extended periods of time. Injuries to the tissue that are induced by mechanical wounding, improper pruning cuts, or frost will elicit new infections, with numerous galls forming along the vascular system of the host.
More than 93 plant families are susceptible to crown gall. Infections can often be observed on woody plants, including fruit and nut trees. They can also occur on ornamental woody crops, such as roses, Marguerite daisies, and chrysanthemums. Infections are especially common in the rose family. Vines and canes, such as grapevines, blackberries, and raspberries, are vulnerable to infection as well. Marguerite daisies, chrysanthemums, and grapevines can become infected systemically. Occasionally, crown galls can be found on field crops, such as cotton, sugar beets, tomatoes, beans, and alfalfa.
Symptoms of Infection
Abnormal growths appear as galls on roots, and at the crown of woody plants. Young galls may be smooth, and somewhat spongy. Galls become rough, hard, and woody as they mature. Unlike insect galls, which are hollow in the center, crown galls are solid throughout. Galls vary in size: they can range from ¼ of an inch to just over an inch in diameter. After the first year, galls appear replete with cavities, and become friable. Insects such as earwigs commonly reside within these cavities.
Crown gall often results in a loss of yield on select trees and shrubs when seedlings are infected. Infected plants may be stunted due to the disruption of their vascular system, which impedes the flow of water and nutrients from the roots. Trees with chronically diseased root systems may lack vigor, and experience a reduction in foliage. When a mature tree becomes infected, secondary growths called epicormic sprouts will appear from the root system near the trunk. These growths are symptomatic of severe root infection.
- When planting, avoid selecting trees or shrubs that exhibit symptoms of gall formation near the crown, soil line, or graft union. Select resistant cultivars when planting in or near infested areas.
- Several transgenic crop plants, including grapevines and fruit trees, have been developed that express resistance to crown gall. They are available for commercial use in nurseries, and garden centers across the United States.
- When pruning susceptible plants, avoid creating wounds that are in close proximity to the soil.
- Remove severely infected plants, disposing of as many roots as possible.
- Avoid wounding of susceptible plants through cultivation practices, such as mowing, tilling, or weed removal.
- Control weeds, and limit populations of insects and nematodes that feed on roots.
- Some crown gall infections can be eradicated using chemicals, particularly creosote-based compounds, copper-based solutions, and oxidants such as sodium hypochlorite. The use of these chemicals is often labor intensive and costly. They are also ineffective against systemically infected plants. As such, chemicals are seldom used for the control of crown gall.
- Certain strains of the bacterium are sensitive to the antibiotic agrocin, which is produced in Agrobacterium radiobacter, a closely related soil-borne bacterium that does not infect plants. The agrocin can be used as a preventative treatment during planting.