Small shoots may be defoliated and killed. Galls also form at trunk or limb wounds. Bacteria survive in the knots and are readily washed off at all times of the year.
Infection occurs at low temperatures, usually in fall or spring.
Openings are necessary for penetration of bacteria and these are provided by leaf scars, pruning wounds, or bark cracks made by freezing. Limited chemical eradication of larger galls is feasible. Two applications each year are needed for control. Because leaf scars are susceptible when fresh, treatment should be timed to protect as many leaf scars as possible. Prune trees in July to August to prevent pruning wound infection.