The clinic has had many complaints of browning of tree leaves in the past few weeks. Symptoms vary from edge burn to necrotic spots to curling and twisting of leaves and even leaf drop. It seems (based on the calls) that such injury is occurring sporadically over much of Illinois. There are several problems that might be involved.
One possibility is leaf scorch, a noninfectious, environmental scorch that occurs each year when water cannot be translocated to the foliage as rapidly as it is lost. The causes vary and might include root injury, root rot, poor soil conditions, high winds, transplant shock, flooding, drought and so on. The possibilities are explained more completely in Report on Plant Diseases No. 620. Symptoms of scorch include browning of the leaf margins as well as tissues between veins. If you have been inspecting trees regularly, you will notice that injury appears first on the newest, most tender growth, which still has thin cuticle tissues. If you have not been as diligent, you may not notice anything until the entire tree is affected. Often injury is worse on the south and west sides of the tree that are more exposed to wind and sun. Badly affected leaves will drop from the tree, but most scorched leaves hang on and become tattered and torn as the wind whips the scorched areas. The newest growth may turn entirely black, but this will extend only an inch or two into the newest stem growth.
Anthracnose diseases are early season fungal diseases that may also cause spotting and edge burn on leaves. Many of the anthracnose-infected leaves will drop from the tree. You may want to refer to issue no. 3 of this newsletter for more details on tree leaf diseases. Anthracnose fungi generally cause spotting and blighting of leaves. This is particularly true on maple, sycamore, oaks, and walnuts. The anthracnose disease of ash will cause an edge burn that may look like scorch, but it will not uniformly affect all sides of the leaves as would scorch. Ash anthracnose may also produce some minor leaf curling and cupping. Oak anthracnose may also cause irregular edge burn and minor curling of the leaves around the necrotic areas. Because the causal fungi grow best in moist environments, anthracnose will be worse on the lower or inner parts of the tree. It will be less intense on the south and west sides of the tree—a fact that distinguishes it from environmental scorch.
Many calls have concerned curling and cupping of leaves. These symptoms always suggest the possibility of herbicide-drift injury. That is not something we can diagnose at the Plant Clinic, but we can rule out other problems and make some helpful suggestions concerning what to look for on your trees. Chemical injury from the plant growth-regulator herbicides will cause cupping and curling of growth, especially new growth. Often the tips of leaves are stretched out and much more pointed than usual. Leaves affected by growth-regulator herbicides are usually thicker than normal and stiff or distorted. Look for a pattern of injury that seems worse on the side where the chemical may have originated. Also look for other broadleaf plants in the path of the source of the chemical. Do they too have symptoms? Consider a lawn herbicide application as a possible source of injury as well. Cupping, curling, and distortion of the foliage without necrotic areas on the leaves probably indicate herbicide drift injury has occurred. Look closely for insects in the affected tissue though, because there are some insects that cause these symptoms as well.
None of the problems we have discussed is likely to kill a tree, especially in one season. Help the tree recover by watering in periods of drought stress, fertilizing in fall, and removing any dead wood that is present. To assess the tree’s ability to refoliate, look for live buds on the twigs. A bud is alive if it is green and fresh inside. Pick off a few buds and look at the base of them to make this observation. Also try scraping the newest twig growth with your thumbnail. If the wood is green and fresh when scratched, then it has a good chance of producing more leaves. If there are no live buds and internal wood tissue is dead, then a more serious problem has affected your tree.
The Plant Clinic cannot diagnose chemical injury on ornamental plants. Do not send suspect injury cases to us unless you wish to have us rule out other possibilities. Work with the person who has applied the chemical, and work with your Extension office to come up with a means of avoiding recurrence of the problem in the future.