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Early Leaf Drop

University of Minnesota Extension, Stearns County News
May 27, 2015

Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns, Benton & Morrison Counties

Early Leaf Drop
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (05/27/15) — Are your trees losing their leaves already?  Or maybe the leaves are misshapen or have block spots on them?  The wet conditions we have recently had created the ideal conditions for fungus that may impact 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 to black spots or blotches on the leaves. Leaf drop or defoliation is possible when infection is more severe. Defoliation is most prevalent when there is cool, wet weather during bud break out, which appears to be the case this year, especially with ash trees.  Some infections occur on green twigs where small orange-brown blisters to brown bands encircle 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, virus, 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 during 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.

In most situations, to manage anthracnose simply wait for weather to become dry and the leaves to mature.  The fungi growth will end and the tree will possibly 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, avoiding injury due to mowers, or compaction from vehicle driving or parking.  To help prevent conditions favorable for disease avoid sprinklers that wet the lower or any part of the canopy, instead do base watering with a garden hose or drip line.  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


PHOTO: Ash tree leaflets with anthracnose, photo by Beth Berlin, University of MN Extension


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