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Spots on Your Tree’s Leaves

University of Minnesota Extension, Stearns County News
June 1, 2016        
Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns & Benton Counties


Spots on Your Tree’s Leaves
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (06/01/16) — The landscape is filled with lush green color, but some may have noticed some spots on their leaves. Cool, wet temperatures offer an ideal climate for fungal spores to multiply and leave unsightly spots on many of our trees leaves.

Anthracnose is the term used to describe diseases that impact our deciduous trees caused by several closely related fungi. Symptoms will vary, but it often starts with yellow, brown or 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.  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 cycle begins when temperatures are between 50-68 degrees Fahrenheit.  The spores travel by splashing water or by wind. 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.  Certain species such as walnut and hornbeam may continue to suffer from anthracnose throughout the summer months.

To manage most cases of anthracnose, simply wait it out; once the weather dries and becomes warmer, the fungi growth will end and the tree will possibly replace any leaves lost with new growth.  Homeowners should simply clean-up and dispose of any fallen leaves caused by anthracnose, and attempt to remove or minimize 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

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