Trees: Drought and Disease
|September 12, 2005|
In keeping our landscapes looking attractive, we naturally spend time and money providing supplemental water to plants. The drought situation has become severe in the 2005 growing season, and this is just one more appeal to spend time and money watering trees. |
Symptoms of drought, or lack of water, include wilting, leaf margin necrosis, leaf drop, and even death of plants. The symptoms may not be so noticeable on trees this season. Trees in prolonged water deficit, as we have experienced this summer, grow more slowly or may stop growing altogether. This can be seen as the amount on stem growth at the tip of the stems. Buds set this year will be small and will affect next year’s growth as well. Drought-stressed trees may have smaller than normal leaf size, less intense leaf color, fewer leaves, and possibly early fall color development. If the water deficit lasts a long time, stem tips die and we see typical dieback symptoms and branch decline. Some tree species can withstand the stress of drought more readily than others.
For this reason, some of your trees may be affected while others are apparent escapes. U of I Extension has a Web site called Hort Corners, http://www.urbanext.uiuc.edu/hort/index.html, with a section about trees, including a list of trees based on exposure. Trees are listed for sites that may be windy, wet, alkaline, compacted, exposed to salt or pollution, or dry. Trees recommended for dry sites can be found at http://www.urbanext.uiuc.edu/treeselector/ bytolerance.cfm?display=3. Barring other site stress, those trees would be expected to perform better in drought stress than trees recommended for wet sites.
Other factors can cause the same symptoms as drought stress. In a year like this, it is not too difficult to identify trees with drought symptoms; but keep in mind these other possibilities. Too much water, especially on poorly drained sites, can cause similar symptoms. Often these other factors involve trunk or root injury of some sort. Underground gas leaks, salt damage, wind, sun exposure, insect injury, construction injury, root compaction, and chemical injury are a few possibilities. Look for contributing factors on the site and changes that occur just prior to symptom expression. For example, newly planted trees are more susceptible to drought stress. The roots may take as many as three growing seasons to develop a root system able to with stand moderate drought stress.
Another major drawback to drought stress on trees is increased susceptibility to diseases. Research in Wisconsin has shown that pines growing in a site with water deficit are more likely to become infected with Sphaeropsis blight. The Hypoxylon fungus is known to infect weakened and dying oaks, causing dieback and decline. Armillaria root rot and many canker pathogens infect weakened trees.
Drought-stressed trees need to be watered now. Although most of the fine roots that absorb water for trees are in the top 8 in. of soil, these fine roots become less absorbent and may shrivel up in extended drought. A greater percentage of water is then necessary deeper in the root zone. It is difficult to give a cookbook method of watering trees because length of drought stress, soil drainage, soil type and texture, competing vegetation, exposure to sun, and other factors vary with each case. For most drought-stress situations in Illinois, water so that the soil is moist to about 10 in. The roots extend far beyond the drip line of a tree, sometimes as much as four times the distance from trunk to branch tips. Concentrate watering in the drip line and beyond. Watering a few inches from the trunk is not very helpful. In periods of drought, water once a week as described above.
If you cannot provide supplemental water to all your landscape trees, concentrate on small trees and most shrubs. Species to pamper include mountain ash, birch, dogwood, redbud, crabapples, tuliptree, rhododendrons, and azaleas. Keep up this watering during drought until the ground freezes. If water is not provided, expect to do extensive pruning and removal of dead wood next spring.
||Nancy Pataky |