Forsythia stem gall—uncertain cause
Brown, irregular to spherical growths sometimes develop on the stems of forsythia. The cause is uncertain and has been attributed to crown gall (Agrobacterium tumefaciens), a genetic disorder, a Phomopsis species of fungus, and the Pseudomonas savastanoi bacterium that galls oleander and olive. But none of these have been proven as the cause of forsythia stem gall and there could more than one cause of this disorder.
Damage
Brown, warty swellings (galls) form on stems and twigs of forsythia, often high on the plant. Galled stems or twigs may die back.
Solutions
Protect plants from injury. Provide plants with a good growing environment and appropriate cultural care. Prune off the galls, making cuts in healthy portions of the stem. Prune forsythia only when conditions and plants are dry.
Because the galls may be caused by a pathogen that can be mechanically spread, after working on an infected plant clean tools and dip or spray them with a 10 to 20% bleach solution, 70% isopropyl rubbing alcohol, or a commercial disinfectant as directed on the product label.
Adapted from Forsythia Galls (PDF), Cornell University; Forsythia-Stem Gall, Pacific Northwest Plant Disease Management Handbook; and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
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Galls on forsythia stems.
Gall close-up on a forsythia stem.
Galls on forsythia stems during winter.
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