Issue 16, September 19, 2011

Leaf Galls on Pin Oak

Abnormal leaf growths show up at various times during the year on our landscape plants. Leaves get bumps, blisters, and other warty type growths varying in size from a pin head to marble sized. These are leaf galls and they are unattractive only because they are not normal. When found in high numbers they cause the owner much concern but really do very little harm to the plant. This fall we are seeing high number of leaf galls on pin oak in the Quad Cities area.

Galls develop when plant tissue has been irritated or stimulated by some type of pest. Aphids, midges, wasps, or mites cause the majority of known types of galls while bacteria, fungi, and viruses cause the rest. Galls are abnormal plant growth or swellings made up of plant tissue. They can be thought of as benign plant tumors. There are several different types of galls that infest oaks and not only on the leaves but also on the twigs and stems. Infestations vary from year to year and this year seems to be a good year for leaf galls. Often when a pest population reaches high numbers they crash and for the next few years you see fewer. But the numbers will build again to another bumper year.

Leaf galls often are found on green leaves that are still capable of photosynthesizing, producing food for the tree. You may see premature leaf drop but this late in the season the tree is starting to shut down normally and leaf drop is soon to occur regardless of leaf galls.

Once galls begin to develop it is impossible to stop or reverse development. This leaf gall is caused by a small wasp that attacks the leaves in the spring. As with most leaf galls chemical controls are not warranted since the damage is mainly aesthetic. Also, timing of application is difficult to achieve for effective control. Treatments must be correctly timed and thorough coverage is essential. Cultural controls such as raking and destroying fallen infested leaves reduces overwintering numbers.

Leaf galls are primarily a problem in the eyes of the tree owner. Some years there are higher numbers than others. We all get a few bumps and warts over the years and we manage to survive just fine. (Martha A. Smith, Horticulture Educator)

Martha Smith

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