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Spots on Your Maple Leaves

Tar spots on leaves

University of Minnesota Extension, Stearns County News
June 15, 2016        
           
Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns & Benton Counties


Spots on Your Maple Leaves
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (06/15/16) — As we look at the beautiful lush green landscape, when you take a closer look at some of your tree’s leaves, you might be alarmed by some spots.  Spring and early summer weather can sometimes be ideal for various fungi growth, one commonly referred to as tar spot or more generically leaf spot.

Tar spot is a disease caused by fungi in the genus Rhytisma and gets its name because it gives the looks like droplets of tar on leaf surfaces. In general, tar spot alone will not harm the long term health of the tree, but is simply aesthetically unappealing and in severe infections can cause premature leaf drop.

The fungal spores overwintered on infected leaves that fell to the ground. This spring those spores matured and traveled up to new growth by wind or water, which starts the disease cycle on the new growth. Initial infection appears as small, approximately 1/8-inch yellow spots. On several maples species, the black “tar” spot develops quickly within that yellow spot. By later in the summer, these fungi spores continue to grow in size including thickness and appear like tar spots splattered on the leaf surface. 

To manage tar spot and other leaf spot diseases, the first step is sanitation.  Simply rake and destroy leaves as they fall or in autumn. Ultimately removing these infected leaves reduces the number of spores and helps minimize repeat infection next spring.  Keep in mind if your neighbors’ trees are also infected, they too will need to do a leaf clean up or the spores will readily return to your trees in the spring winds.  Applying fungicides is only recommended if trees are severely infected for a few years, but is very difficult because timing of application is critical. 

For more information about all leaf spots, visit http://www.extension.umn.edu/garden/ and type in “tar spot” in the search bar.

 

 


Photo Credit: M. Grabowski, University of Minnesota

Contacts

Beth Berlin
Extension Educator, Horticulture
(320) 255-6169
adam0062@umn.edu
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