Informational table showing disease name, symptoms, pathogen/cause, and management of Forsythia diseases. Disease Symptoms Pathogen/Cause Management Galls Knobby galls form along branches. Uncertain causes. Crown gall, the fungus Phomopsis, and genetic abnormality have been blamed for this disease but none has been conclusively proven to be the cause. It is possible that all three can cause galls.
Prune infected branches. Disinfest the pruning shears thoroughly after use. Twig blight The ends of lower branches, especially those near the ground, die. Black nodules of fungal tissue are found inside the stems when split open. Young nodules of fungus in the pith are white at first. Sclerotinia Avoid overhead watering in the spring. Prune infected branches. Thin the shrub to ensure good air circulation. Apply a fungicide to protect plants.

Black Sclerotinia sclerotium (overwintering structure) in the pith.

Twig blight (Sclerotinia) of branch tips.