Rhonda Ferree, Extension Educator in horticulture, says that trees can take three to five years to show symptoms from a severe event such as drought. Unfortunately trees under stress are less able to fight off insect and disease problems. Plant diagnosticians at the University of Illinois Plant Clinic describe the following diseases that take advantage of trees under stress.
Canker diseases are fairly common on stressed honey locust trees. Research has shown that honey locust cultivars vary in susceptibility to the canker fungi. Look for resistance ratings when purchasing new honey locust trees. The disease has been linked to drought stress in many cases. Where tree selection is not a choice, avoid injury, provide water in periods of drought stress, and help tree vitality by removing dead wood and fertilizing in the spring. As with most canker diseases, there is no rescue treatment that can be sprayed on the tree. When you see a canker problem, try to determine the cause of stress and take measures to alleviate that stress.
Drought-stressed elm trees are more attractive to the elm bark beetle, which may carry the Dutch elm disease fungus. With Dutch elm disease you will want to watch for yellowing of the leaves, followed by wilting and browning. Often this happens so quickly that the problem is first noticed when branches with brown leaves appear in the canopy seemingly overnight. It is generally too late to save a tree once it is infected, but an accurate diagnosis of the problem may help save nearby elm trees.
Pines under drought stress lack the ability to make resins to protect against pinewood nematodes and pine wilt disease. Pine wilt symptoms to watch for are gray-green foliage or needles that appear dull or off-color. The tree will not improve. Instead, as the summer progresses, the foliage will turn yellow and then brown and remain attached for a long time. Pine wilt occurs on any of the pines we grow in Illinois except for white pine. There is no cure for a tree infested with pinewood nematodes. Trees confirmed to be infected trees should be removed as soon as possible. Debris from the infested tree should be removed and burned or buried.
Symptoms of Verticillium wilt are more pronounced in drought, in part because drought inhibits the trees ability to wall off the fungus. Symptoms include wilt, branch death, and quick decline of plants. Hundreds of plant species, including trees, shrubs, groundcovers, vines, vegetables, fruits, herbaceous ornamentals, and flowers may become infected. Some trees that are frequently infected by this disease are maple, ash, and elm. There is no cure for Verticillium wilt.
The U of I Plant Clinic can test for diseases such as Dutch elm disease, pinewood nematodes, and Verticillium wilt. For additional information, please refer to the U of I Plant Clinic website: http://web.extension.illinois.edu/plantclinic/index.cfm