Originally Posted by aunty mor

Galls shouldn't have any adverse affect on your Forsythia apart from maybe some disfigurement of the leaves so you don't need to treat it. In fact it is quite difficult to actually get at the pests that cause them and timing of any pesticide is quite crucial. How covered is it? Could you prune out most of the infected parts? (not that that is any guarantee they won't come back) Forsythias are as tough as old boots once mature so you can be quite ruthless with it.