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Shade Tree Decline
Original Source:Kim Coder, Professor of Tree Biology & Health Care, Warnell School of Forestry and Natural Resources
September 9, 2011
Although irrigating trees during periods of drought is recommended, frequent and shallow watering contributes to shallow root development. This increases the chances for drought injury as well as the potential for winter injury during periods of extremely cold weather. When watering, be sure the moisture reaches depths of at least 5 to 7 inches. Water once every three to four days during periods of severe drought. Watering everyday may contribute to the decline of the tree because the activity of many parasitic and pathogenic organisms, like root rot, is stimulated by too much water. The amount of water to apply depends upon soil texture and potential size of the tree rooting area. Clay soils can be easily overwatered which destroys tree roots.
In urban areas, mechanical injury is a major cause of shade tree decline. In subdivisions and new housing developments, shade trees are often abused; roots are torn out of the ground, bark is bruised and the soil around trees is disturbed. Losses from such damage could be minimized or even avoided if people realized that trees may not survive such treatment and took precautions to avoid abusing them. Many utilities and municipalities are also guilty of tree abuse. When putting in gas and water lines or paving streets, workers damage or destroy roots which disturbs food production, growth control and the tree's top-root balance. Root loss contributes to the weakening and decline of a tree's crown. As with drought, these symptoms can often be delayed in appearance by 1 to 2 growing seasons.
Some of the worst things you can do to a tree are: add fill around the trunk, cultivate or remove soil from around the trunk, compact the soil (especially when the soil is wet), or damage the bark on the trunk. Each of these events leads to a weaker tree that can lead to other stress factors or pests injuring the tree further.
Chemical damage (pollution/pesticides) of trees is very common. Injury to trees from pollution as well as chemical application by homeowners and commercial applicators, are common occurrences. Pollutants are now a part of our urban and rural environment and ecology. Pollutants, such as ozone, sulphur dioxide, fluorides, sunlight-induced nitrates (PAN), road and sea salts and particulate matter (flyash, dust, cinders), all disrupt the life processes of trees. Some pollutants will be concentrated near roadways and factories that are their source. Other pollutants, like ozone, can disrupt tree growth a hundred miles downwind from a city. Pollution acts as one factor in a tree decline problem.
Chemical injury can be much more severe when trees are already weakened by other factors. The "spray and pray" concept (spraying a chemical and hoping it will control whatever the problem is) should be avoided. Chemicals are not always the answer and may actually create more problems. Good tree management should be practiced first. Use chemicals only as helpers after other management practices have been performed.
All of the factors already mentioned, and others not mentioned, weaken trees and make them more susceptible to pest organisms. Disease organisms are especially likely to take advantage of a weak tree.
- Tree Cankers: Many oak species are lost to Hypoxylon canker, a disease that is common in both urban and rural areas. This disease can be diagnosed by its grayish to brown felt appearance on the bark. Little can be done to control this disease since the fungus is actually growing into the wood of the tree. The removal of infected trees and pruning of infected branches will remove the fungus innoculum from the area. However, pruning will not solve the problem of low tree vigor. Careful management practices performed to increase tree vigor will encourage tree recovery.
- Twig Cankers: There are a number of canker-causing fungi which cause twig dieback in many shade tree species. Most are diseases that take advantage of trees that are in a weakened or declined condition. Prune dead wood and initiate management practices to help the tree recover. Fungicide applications generally provide little protection since tree health is the key to canker disease control.
- Leaf Spots: There are numerous leaf spot fungi which infect leaves and cause foliage loss from many urban shade trees. These diseases occur annually and may actually go unnoticed most of the time. During periods of stress, there may be an excessive amount of defoliation attributable to foliage diseases. Foliage diseases of large urban trees are not known to cause any permanent damage unless defoliation occurs several years in succession. Excessive defoliation often occurs when a tree is in a weakened condition.
- Slime Flux: Slime flux, called wet wood, is considered a disease of unthrifty or old trees. Symptoms of slim flux include oozing of either a white or brown, smelly substance from wounds, pruning scars and trunk crotches. The slime is toxic to the bark and may kill large patches when it stays on the bark for a long time. Rinsing twice a year may minimize the damage, but determine what weakened the tree initially and provide best management practices to improve tree vigor.
What can be done to prevent shade tree decline? The key to good health is tree vigor. Provide a site that is suitable for the species involved. Pick a strong species of tree. Provide construction protection for roots and trunks of trees to reduce accidental injury, soil compaction and to allow adequate room for tree growth. Plan ahead for future development. Street maintenance equipment often injures trees after roadways are widened. If twig dieback is observed, proper pruning will reduce disease susceptibility and improve the tree's appearance. Remove dead or dying branches. When roots are damaged or lost, continue to water and wait one growing season and then thin the crown. This helps the remaining roots sustain the health of the existing foliage. Water, fertilize and care for the tree only when needed. Do not "kill" your tree with kindness. Give your tree a chance to live a full, healthy life by helping when it has a bad year.
Shade tree decline is becoming much more prominent. Being able to recognize conditions which promote decline and taking steps to eliminate stresses before symptoms occur will save many urban shade trees.
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