Trees, shrubs and other ornamental plantings represent a sizeable investment and asset to most homeowners. Keeping trees, shrubs and other woody plants healthy is the job of professional arborists. Deciding when to use fungicides and other plant health improvement techniques should be left to professionals with the knowledge and training to do the job safely and effectively.
Diseases of trees and shrubs may be caused by harmful bacteria, fungi and viruses. “Most often these diseases attack trees that have already been stressed by other factors,” notes Peter Gerstenberger , senior advisor for safety and compliance with the Tree Care Industry Association. “It is difficult to recognize tree diseases, because the organisms responsible are small and not easily detected in the early disease stages. Once a problem is recognized, the disease is often in an advanced stage, limiting an arborist’s treatment options.”
Some diseases can kill trees in a short period of time (oak wilt infections); others are long-term chronic diseases (nectria canker). Some diseases do not usually cause serious damage to trees (powdery mildew), while others can virtually eliminate an entire tree species (chestnut blight, Dutch elm disease). Harmful, parasitic bacteria cause a number of plant diseases. Damage from bacteria can be limited to leaves (shot hole disease) or the bacteria can infect and destroy the vascular system of the tree (bacterial leaf scorch), eventually causing tree death.
“In general, consumers will first notice leaves in a section of the crown showing signs of wilting or scorching,” explains Gerstenberger. “Arborists can pick up on more subtle clues. Often an upper portion of the crown shows signs first. Sometimes, such as in the case of verticillium wilt, one limb or group of limbs shows signs of a problem before the rest of the tree.”
Other symptoms to look for: trees of the same species that express themselves differently; wilting or scorching leaves; leaves smaller than normal in one section or the whole tree; early fall color in one section of the tree or the whole tree; dieback of twigs and limbs; reduced shoot growth; leaf discoloration; stress-induced production of numerous small cones or fruit of poor quality; and heavy resin or gum flows.
Extensive dieback and death can occur rapidly in the case of Dutch elm disease, oak wilt of red oaks, or blue-stain wilt of shortleaf and loblolly pines. In the case of bacterial leaf scorch, oak wilt of white oaks, and verticillium wilt, damage occurs more slowly and often can be controlled. When a tree has been infected, a professional arborist can assess treatment protocols and advise techniques for mitigating damage or avoiding a repeat of the infection.
Of course, the best way to avoid tree diseases is to keep trees healthy, making them less susceptible. Insects that spread diseases are attracted to trees that are stressed and have a weakened defense system. A professional arborist can assess and maintain a landscape, ensuring proper soil fertility and watering practices to promote plant health and minimize the effects of stress on plants. Arborists can also help diagnose and mitigate other causes of poor growth or decline, such as poor site conditions or mechanical, environmental, and chemical injuries. For example, witch’s brooms are common on small branches along highways where salt spray kills the tips of the branches every year.
What can you do?
Homeowners who would like a professional arborist to assess their trees should contact the Tree Care Industry Association (TCIA), a 67‑year‑old public and professional resource on trees and arboriculture. It has more than 2,000 member companies who recognize stringent safety and performance standards, and are required to carry liability insurance. An easy way to find a tree care service provider in your area is to use the “Locate Your Local TCIA Member Companies” program. You can use this service by calling 1-800-733-2622 or by doing a ZIP code search on the TCIA Web site, www.treecareindustry.org.
