What is this?
Insects and mites are the most common cause of galls, though they can also be caused by fungi, bacteria, and viruses. The formation of the gall is the trees’ hormonal response to being infected or injured by a pest. Galls can come in all shapes and sizes – some are small clustered bumps on leaves, while others grow along stems or twigs, and still others can appear to hang from the tree, similar to a fruit. Many have a hard shell, but others can appear soft.
How do I fix it?
You can prune off affected parts of the tree as galls appear, but it isn’t necessary.
Can it be prevented in the future?
To help prevent gall-producing pests from returning the following year, be sure to rake and dispose of all leaf litter over the winter.