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Ash flower galls

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Common name:  Ash flower gall

Scientific name:  Aceria fraxiniflora

Hosts:  Ash trees (Fraxinus species)

Many home gardeners become alarmed when they see green or brown clusters hanging from branches on their ash trees. The clusters are ash flower galls and do not harm the tree. They are caused by a tiny eriophyid mite.

Early in spring, an eriophyid mite, Aceria fraxiniflora, feeds on male ash flowers. The mite’s feeding causes the tree to produce a proliferation of flower bud tissue. This tissue grows around the mites. As a result, the flower buds enlarge and become deformed. They can stay on the tree for up to two years, instead of dropping off the plant during the first spring. Look for round, green, ½ to 1 inch tumor-like growths. The galls are green at first, but become brown later in the growing season. They can be seen easily when the leaves fall off in autumn. The galls do not harm the tree, but they can be considered unsightly.

Ash flower gall mites overwinter as fertilized females. The mites are tiny and may be difficult to see, even with a magnifying glass. They spend the winter under flower bud scales or bark. In spring, they begin feeding and laying eggs in the developing flowers. The feeding stimulates plant tissues and forms a gall around the mites. There are several generations during the spring and summer. In the fall, fertilized females move to bark crevices and beneath bud scales to overwinter.

The galls are usually just a cosmetic problem so management is not necessary. Treatment with insecticides is not recommended, as it is difficult to time the sprays and the insecticides may be harmful to beneficial insects.

For more information, contact The Morton Arboretum Plant Clinic (630-719-2424 or