The ash flower gall is an unsightly plant growth caused by a mite that appears on ash trees. It specifically infects the clusters of male flowers ashes produce. Clusters of these tumor-like growths start to expand and become woody in the spring. They turn dark brown in the summer, can persist for more than one year, and make the tree look unsightly. Trees are not generally harmed by this gall, although heavier galls can occasionally cause limbs to break. The mites that cause these galls have one generation per year and overwinter in the buds of ash trees.

Control of this pest is difficult. Some ash trees are more susceptible to this gall so planting appropriate trees is one option to eliminate the problem. Spraying is rarely effective. The entire tree must be thoroughly covered in the spring when the flower buds first start to break to control the mites. This can be impractical for large trees. Often the best course of action is to do nothing since this gall does not threaten the health of the tree.