Galls are swollen masses of abnormal tissue. They can be caused by fungi and bacteria as well as certain insects. If you cut open a gall and there is no sign of an insect, suspect disease.
Crown gall is a serious bacterial disease that infects and kills grapes, roses, fruit trees, brambles, shade trees, flowers, and vegetables. Galls are rounded with rough surfaces and are made up of corky tissue. They often occur on the stem near the soil line or graft union but can also form on roots or branches. To control it, buy healthy plants, and reject any suspicious ones. Don’t replant in an area where you have had crown gall. Avoid wounding stems, and disinfect tools between plants when pruning. Remove and destroy infected plants, or cut out galls.