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Problems with Shade Trees

University of Minnesota Extension, Stearns County News
June 11, 2014       
Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns & Benton Counties


Problems with Shade Trees
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (06/11/14) — Finally the leaves have emerged and our deciduous trees are looking lush and green; or are they? The wet conditions we have had this spring are optimal for a wide variety of diseases impacting our ash, maples, oak, and many other deciduous tree species.  In general these diseases are called anthracnose.

Anthracnose is the term used to describe diseases that impact our deciduous trees caused by several closely related fungi. Each group of anthracnose fungi species affects only a limited number of tree species. Symptoms will vary on severity, but commonly it starts with brown spots or blotches on the leaves. Defoliation is possible when infection is more severe; defoliation is most prevalent when there is cool, wet weather during bud break out. Some infections occur on green twigs where small orange-brown blisters to brown bands encircling the young twig, causing shoot death.  Although unattractive, single attacks of anthracnose seldom cause harm to the tree.  However, if the tree sustains anthracnose damage for several continuous years it can cause reduced growth, vigor, and may weaken the tree, making it more susceptible to other more harmful diseases, viruses, or other dangers.

Anthracnose fungi can overwinter in the buds, twigs, fruit, and fallen leaves, depending on the host plant and which specific fungi.  The disease is most common in cool, wet springs when the disease cycle begins and temperatures are between 50-68 degrees Fahrenheit.  The spores travel by splashing water or greater distances by air. The impact at this point is to the newly forming leaves; certain species such as the ash, maple, and oak are largely impacted on the young leaves and shoots, but tend to be resistant to anthracnose fungi once mature, expanded leaves have been reached.  Anthracnose can also occur later in the summer when cool, wet conditions are in conjunction with the succulent leaf growth.  Certain species such as walnut and hornbeam may continue to suffer from anthracnose throughout the summer months.

To manage anthracnose, in most situations, is to simply wait until the weather becomes dry and the leaves mature.  The fungi growth will end and the tree will replace any leaves lost with new growth.  The leaf spots and distortion should have very little impact on the overall health of the tree.  However if the situation or other stresses occur year after year it may weaken the tree.  In those cases, while the tree is vulnerable, other pests may optimize the tree’s weakness and cause more significant damage. Homeowners should simply clean-up and dispose of any fallen leaves caused by anthracnose, and attempt to remove or alleviate any other stresses the tree may have throughout the rest of the growing season.  This may include watering during dry periods.  To help prevent conditions prone for disease be sure to avoid sprinklers that wet the lower or any part of the canopy.  In addition, proper pruning will help increase air circulation and help prevent disease. 

Anthracnose can leave our trees looking less attractive and even sick, but by removing other stresses, your trees should be just fine. For more information on anthracnose visit

(top) Picture of a Maple tree affected by Anthracnose, Credit: M. Grabowski, University of Minnesota


Beth Berlin
Extension Educator, Horticulture
(320) 255-6169
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