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Bumps on Maple Leaves

University of Minnesota Extension, Stearns County News
June 7, 2017        
Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns, Benton, & Morrison Counties

Bumps on Maple Leaves
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (06/07/17) — Have you looked at your maple tree leaves lately and they have pimples?  Your tree doesn’t have acne; instead it is maple bladder galls, Vasates quadripedes.  This is nothing to be alarmed by and vary rarely causes concern.

Maple bladder galls are small bumps or pimples that are either red, pink, or green in color and approximately one-eighth inch in size. They are actually caused by an eriophyid mite when the leaves first emerge in the spring.  The eriophyid mite has likely overwinter on your maple tree. The gall is formed as the tiny mites feed on the newly emerging leaves.

The galls are growing plant parts and will require nutrients like the other parts of the leaf. In extreme cases the galls can cause set-back to the tree but is most likely a problem when galls cover young trees. There are also cases where galls return year after year on the same branch in high number and can cause damage to that particular branch. However, this is very rare and in nearly all cases the galls do not cause damage to the overall health of the tree.

Since most galls caused by the eriophyid mite do not impact the health of the tree no treatment is necessary. Chemical treatments can be done on young trees or extreme cases however timing of the application is critically important and therefore very challenging to apply at the correct time. The sprays must be applied as the mites are actively feeding in early spring. Once the galls begin to form, the damage is already done, and treatment would not be effective. Horticultural oils can be applied to the overwintering mites before their activity begins in spring. However they are microscopic in size and again make it challenging.

If you’ve noticed your maple tree has a case of acne this summer, just let it be, the time to treat has already passed and unfortunately you will just have to look at a pimpled “teen” tree. For more information on leaf galls visit and search for “leaf galls” in the search bar.

PHOTO: Maple Bladder Galls. Credit: University of Minnesota Extension.


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