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Past Issues

Drought Stress and Tree Diseases

June 26, 2007

Most of Illinois has been without significant rain for the latter part of May and the first 3 weeks of June. Trees are suffering from drought stress. Even with relief from a few storms, symptoms may persist. Undoubtedly, the problem will appear again in August. Here are some tips on spotting this injury and watching out for your trees.

Drought symptoms may be chronic or acute. Acute symptoms appear within hours or days of drought stress and are usually recognizable.

Wilting of foliage and stems and scorching of leaves are acute symptoms that may indicate drought stress. We usually recognize both as such and respond by watering the symptomatic plants.

Chronic symptoms, more difficult to recognize, appear over days to months. Slow plant growth, reduced leaf size, less-intense leaf color, leaf-spotting on species such as tuliptree and callery pear, early fall color (red maple), early defoliation (tuliptree), dieback of stems (linden, poplar, hickory), bark cracking, trunk bleeding, and predisposition to disease and insect problems may be chronic symptoms of drought stress.

Diseases can also cause some of these same symptoms, so how can you distinguish an infectious disease problem from drought stress? One tip is to look at the pattern. Drought, as with other environmental stress factors, usually affects groups of trees—all in the same soil type, all exposed to the same environmental conditions. If your maple is showing drought stress, it is likely that other maples in your town will appear similar. Generally, infectious diseases do not affect all plants in a group at once. Even in a group of Scotch pines, a disease like pine wilt may occur on only one or two trees.

Still, disease and drought stress go hand in hand. Drought-stressed trees (especially the defoliated) are more susceptible to stem-invading pathogens. Many fungi that cause cankers are prime examples. Botryosphaeria canker is more common on drought-stressed trees. Drought-stressed spruce trees are more susceptible to Cytospora canker. Severe drought weakens trees, so they are predisposed to infection by opportunistic fungi, including most of the canker fungi. Because drought can cause bark cracking, it may also provide a wound for canker fungi to enter the tree.

There are several ways in which drought predisposes plants to infection. For example, consider Armillaria root rot. Defoliation and drought cause the release of chemicals that stimulate Armillaria growth. Pines under drought stress lack the ability to make resins to protect against pinewood nematodes and pine wilt disease. Symptoms of Verticillium wilt are more pronounced in drought, in part because drought inhibits the tree’s ability to “wall off” the fungus. Insects are involved in this complex as well. Drought-stressed elm trees are more attractive to the elm bark beetle, which may carry the Dutch elm disease fungus.

As you may have guessed, the take-home message here is to keep trees watered in extended periods of drought. Watch plants for symptoms, and respond to their needs. Maybe you have some indicator plants that show symptoms first and let you know it is time to water: for example, doublefile viburnum, azalea, dogwoods, forsythia, Japanese maple, redbud, and hydrangea. It also helps to control weeds and grasses that compete with trees and shrubs for water. Organic mulches over the root systems of trees (but not touching the trunk) help maintain soil moisture. When watering annuals and perennials in your garden, don’t forget about your trees.