Common Tree Leaf Galls

Each year in late spring and early summer, homeowners begin noticing strange growths on the leaves or stems and are concerned for the health of their trees. Several different types of galls are common in Nebraska and are caused by the feeding or egg laying activities of an insect or mite. Either the physical damage done by the insect or its salivary secretions as it feeds causes the plant to increase its production of plant hormones. These plant hormones cause abnormally increased plant growth in small, localized areas and result in the formation of a gall.

One of the most common galls found in landscape trees is Maple Bladder Gall, which is caused by a mite that overwinters on the bark of maple trees, particularly Silver Maple. The wart-like galls are formed by the plant in response to mite feeding on the leaves as they emerge from the bud in early spring. Initially the galls are green, but change to red and finally to black as they mature.  Leaves frequently become so covered with galls that they completely twist out of shape and may even drop early. Although the galls are ugly, maple leaf galls seldom, if every, cause permanent damage to trees.  Therefore chemical control is not recommended.

Maple spindle gall is commonly found on Sugar Maple. This gall appears as a thin, almost worm-like structure that grows up from the upper surface of the leaf. The galls rarely distort the leaf but considerable number of galls can make the leaves unsightly.

Ash flower gall appears as clusters of black structures attached to the tips of branches and is caused by several generations of tiny eriophyid mites that feed in the male flower clusters.  The black clusters persist until the year following their development then fall from the tree.  They are unsightly, but do not affect tree health.

hackberry nipple gall
Hackberry Nipple Gall
Hackberry Nipple Gall, which resembles a round green pouch, is very commonly found on hackberry. An insect called a psyllid causes this gall, which forms on the underside of leaves.

Oaks are susceptible to many different types of galls most of which are not serious. Translucent oak gall, oak flake gall and oak blister gall are just a few of the leaf galls that can occur in oaks.  However, oaks can also develop stem galls, such as oak bullet gall, which in high numbers may reduce plant growth.  Oak bullet gall, which affects primarily bur and swamp white oaks, develops as circular woody growths that often occur in masses along the stems.  There appears to be some genetic differences between trees within a species, so that about 20% of bur and swamp white oak trees can have very high levels of infestation.  The growth rate of heavily infected trees may be reduced.

Homeowners often become quite alarmed when they discover that their tree is infected with a gall. They fear that the tree is going to die unless something is done quickly. This is not the case. Leaf galls seldom, if ever, cause permanent injury to a tree, although they do detract from the trees' beauty.