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Deciduous tree galls – What is happening to my tree?

It can be very alarming to see galls forming on a tree on your property, but in most instances they appear to be more formidable than they really are. Galls can be formed by insects, fungi, mites and bacteria. Some may only form on leaves, while others form on branches and even the main leading branches of trees. Here we will touch upon the few most common galls seen throughout Northern Wisconsin.

Oak Galls

These blister-like gall formations can often girdle stems and can cause branch dieback.  In spring, adult female wasps emerge from branch galls to lay eggs in oak leaves.  The eggs hatch and larvae begin feeding along the leaf veins.  The tree then produces blister-like galls due to the blocking of nutrients by feeding larvae.  By mid-summer, the adults fly from leaf galls to lay eggs in twigs.  Eggs hatch inside the twigs and larvae start feeding and developing.  Galls begin to form on twigs soon thereafter.  Larvae often remain in twigs for two to three years until adults emerge.

How can we mitigate this problem? Examine oak trees annually for galls. If found, prune branches holding the gall. Branches should then be burned or buried. Consider harvesting your oak tree if overall health is dwindling.

Maple Bladder Galls

Blister-like growths on the leaves of soft maple trees are caused by mites.  These mites overwinter in the furrows of the soft maple tree’s bark. As buds flower in the spring, the mites feed on the tree’s new growth. The leaves respond to the mites’ feeding by developing the galls. The mites then live, feed, and reproduce within the maple bladder galls.  At first, these growths are red and green, but they eventually turn black.  Galls can cause the leaves to become deformed and to drop earlier than normal.  Once formed, galls cannot be removed from the leaves.  In the fall, mites move back to the bark furrows where they overwinter.  Landowners are often alarmed when they notice maple bladder gall infestations, fearing that their trees will eventually die off.  However, trees are very unlikely to experience any injury from these galls, and the trees’ overall health and vigor will not be affected.  However, the galls can impact the aesthetic quality of the tree. 

How can we mitigate this problem? No mitigation is necessary. If galls present an aesthetic issue, remove infected branches. Infected branches can then be burned or buried.

Hickory Galls

Hickory gall

Hickory gall is also known as phomopsis gall. This is the most common gall seen throughout Wisconsin. The galls are caused by a fungus that lives on the trunk or branches of hickory trees. The fungal spores are either wind- blown or carried by squirrels and birds to nearby trees. The fungus then enters the tree through wounds and fissures in the bark. Small cankers are often lighter in color, while larger galls on the trunk are dark colored and appear to be split open. The hickory’s sap oozes from the galls as they split open. Branch wilting and branch die back may occur if the gall girdles the branch. Trunks and entire trees are usually not girdled, and tree mortality is not common. 

How can we mitigate this problem? Examine hickory trees annually for galls. If found, prune branches holding the gall. Branches should then be burned or buried. Consider harvesting your hickory tree if overall health is dwindling.

Landowners with questions related to tree galls or any general forest management related questions can call Kretz Lumber Company’s forestry services team at 800-352-1438 or email us at

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