Galls are abnormal growths that form when plant tissue is damaged. They can be triggered by insects, mites, bacteria, fungi, or physical injury. Galls typically appear on leaves, twigs, branches, and sometimes the main trunk of a tree. In this case, the galls are specifically affecting the male flowers of ash trees.
Although the mites are too small to see without a magnifying lens, their impact has been noticeable this year, especially on white ash trees. During the winter, the mites hide under flower buds, and in spring, they begin feeding and laying eggs in the developing flowers. The ash tree responds by growing new, deformed tissue, known as galls, around the mites. These galls provide some protection for the mites against weather, predators, and parasites.
The ash staminate flower galls can remain on the tree for up to two years. Initially green, they gradually turn brown or black during the growing season. Fortunately, research shows that these galls do not harm the tree's overall health or growth rate. In rare cases, the weight of the galls can cause small branches to strain, particularly in young, newly planted ash trees.
This year’s increase in gall formation could be due to last years’ mild winter, which has allowed the mite population to grow. Additionally, many homeowners have planted seedless ash trees, which are all male and therefore more prone to this problem.
In most cases, the galls are a cosmetic issue, and chemical treatments are unnecessary and often ineffective. The best solution is to plant a variety of tree species. Ash trees are one of the most planted trees in Minnesota, but they are prone to several decline issues. Diversifying the types of trees in your yard can help protect against these and other problems.
Author: Robin Trott, Extension educator, Douglas County