Though most galls are caused by insects, this one is caused by an eriophyid mite, Eriophyes fraxinivorus .
These tiny mites (about 0.5 mm long) feed on the male flower clusters of ash early in the season, transforming the male flowers into irregular, fringed masses. These masses persist for up to two years and become more noticeable when the leaves drop in the fall. The masses will be green early in the season as they form but will turn black as they dry.
As with most galls, the Ash Flower Gall is unsightly but does not harm the health of the tree. The mites are also difficult to control because they are able to enter the flower bud before it is visibly open.
The most effective preventative method to avoid ash flower gall mites is to plant only female trees. Both female and male flowers are rarely found on the same tree. These mites attack only male flowers so planting a female tree will avoid infestations.