Foliage: Tree diseases

Updated Feb 2, 2018

As soon as you notice any abnormality in a tree’s appearance, you should begin a careful examination according to guidelines set by the International Society of Arboriculture. By identifying specific symptoms and understanding their causes, you may be able to diagnose the problem and select an appropriate treatment.

Early disease symptoms usually appear in the spring. Dying branches in the upper part of the canopy (called dieback), and yellowing and flagging can be symptoms of diseases and environmental stress, while leaf drop or leaf spots are common symptoms of foliar disease.

John Lloyd, assistant professor of arboriculture at the University of Idaho, says the best first course of action is to tend to the obvious. Trees in overly dry areas should be watered. Compacted soil should be nourished with organic mulch to reduce the soil’s bulk density and to provide slow-release nutrients to surrounding trees. If a tree has a history of foliar disease, preventive sprays can be applied to reduce the chances of recurrence.”

Holding the line on DED
Dutch elm disease (DED) is certainly not new, but it’s distinction as the most devastating shade tree disease in North America is well known. It is caused by a fungus that generates rapidly reproducing spores. These spores spread toxins throughout the tree, eventually killing the tree.

One of the first symptoms of DED is the sudden wilting of leaves in the top of the tree, followed by a color change in leaves that goes from green to yellow to brown. Eventually, the leaves shrivel and die. If the tree is infected very late in the season, you could mistake the dying, falling leaves as normal. However, once leaves reemerge in spring, they are smaller and, by summer, the tree often dies.

Prevention: While elm bark beetles do spread Dutch elm disease, only a small percentage of them are carriers, so it’s not always essential to treat for them. Instead, focus on a program that strengthens the overall health of the tree. Use mycorrhizal fungi and fertilizer to improve the soil and make trees stronger.

Treatment: Fungicide injections are currently the best option for this disease.

Oak tatters targets Midwestern trees
Oak tatters is a relatively new condition that affects emerging oak leaves, causing them to appear lacy or tattered. It has been observed throughout the Midwestern United States. It affects primarily the white oak group, including white, bur and swamp white. The red oak group (such as red, black and shingle oaks) is only occasionally affected.

Damage from oak tatters appears at the time of leaf emergence, generally in middle to late May. Within two or three weeks, heavily affected trees will produce a new flush of leaves that may not have tatters. These leaves may be smaller and lighter in color than normal leaves. The damage is often evenly distributed throughout the entire crown, but sometimes may be greater in the lower crown.

Prevention: Mulch newly planted or established trees to reduce grass competition. To improve or maintain tree vigor, water during extended dry periods and fertilize trees that have a known mineral deficiency.

Anthracnose attacks shade trees
Anthracnose is a group of diseases caused by several closely related fungi that attack many shade trees. It occurs most commonly and severely on sycamore, white oak, elm, dogwood and maple. Other host plants that are usually only slightly affected include linden (basswood), tulip tree, hickory, birch and walnut.

Symptoms include defoliation of trees, especially following unusually cool, wet weather during bud break. Single attacks are not usually harmful to trees, but repeated infections will result in reduced growth.

Prevention: Depending on the fungal species involved, disease control measures can vary. Rake and remove infected leaves in the fall. Prune dead twigs and small branches, either burning or burying them to prevent spread of infection. Fertilize, if needed, to increase tree vigor.

Treatment: Spray with a fungicide containing mancozeb at budswell and twice again during leaf expansion. Follow label rates. TLC

Sources: International Society of Arboriculture, USDA, Ohio State University Extension, Virginia Cooperative Extension and

Disease diagnosis
Correct diagnosis of plant health problems requires careful examination of the situation. The ISA offers these tips to aid in your assessment:

  • Accurately identify the plant. Diseases are plant-specific, limiting the number of suspected pests.
  • Look for a pattern of abnormality. Compare the affected plant with others on the site, especially those of the same species.
  • Carefully examine the landscape. The history of the property and the adjacent land may reveal many problems.
  • Examine the roots. Note their color. Brown or black roots may signal problems.
  • Check the trunk and branches. Wounds in the trunk can provide entrances for pathogens and wood-rotting organisms.
  • Note the position and appearance of affected leaves. Dead leaves at the top of the tree are usually the result of environmental or mechanical root stress. Twisted or curled leaves may indicate viral infection, insect feeding, or exposure to herbicides.

Source: International Society of Arboriculture

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