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What are these spots on my leaves?

University of Minnesota Extension, Stearns County News
August 24, 2016           

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

What are these spots on my leaves?
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (08/24/16) — Sometimes it takes a closer look and other times it is pretty obvious that your tree’s leaves have spots or bumps on them.  A lot of things can be the cause, but two that repeatedly bring concern to people are leaf galls and leaf spot, specifically tar spot.  None of these issues cause great concern to the overall health of the tree, but can certainly be unsightly. 

Leaf galls are small ‘pimples’ on the leaves, sometimes in mass numbers. The small bumps were caused by insects or mites that were either feeding on the leaf or caused during the egg-laying activity typically during late spring. The insect or mite either damaged the plant material or produced a salivary secretion that increased the production of normal plant growth hormones. 

There are a variety of insects or mites that cause galls to form including: Eriophyid mites, Psyllids or jumping plant lice, aphids, adelgids, gall wasps, and gall midges. Some of these pests target specific plant species; others are broader and impact numerous plants. 

Most galls do not impact the overall health of the plant and should simply be ignored. Especially for homeowners, chemical treatment is not typically recommended. Treatment must be done at a precise time when the insect/mite activity first starts before gall formation begins or the treatment will be ineffective. For overwintering host plants, horticulture oils can be applied before the insect/mite activity begins in the spring.

Sometimes the spots on your leaves are not bumps but rather look like someone splattered some black paint or tar on them. This is tar spot, a disease caused by fungi in the genus Rhytisma. In general, tar spot alone will not harm the long term health of the tree, it is just unattractive and in severe infections can cause premature leaf drop.

The fungal spores overwintered on infected leaves that fell to the ground. In the 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 mid to late 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 and rain.  Applying fungicides is only recommended if trees are severely infected for several years, but is very difficult because timing of application is critical.

For more information about galls and other leaf spots visit and enter the “leaf galls” or “leaf spot” in the search bar.

PHOTO: Tar spot on a Silver Maple. Photo Credit: Beth Berlin, University of Minnesota Extension.


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