Question
We have just moved to a home in Northern California with a large, mature oak tree that has nodules or round pod-like growths on its branches. The growths appear in clusters and are green when smaller, turning brown as they grow to be about the size of a tennis ball, eventually falling off of the tree. The tree is beginning to flower and appears to be healthy otherwise. Any help you can provide is greatly appreciated.

Answer
Suzanne:
These may be one of many types of insect galls which are caused by small wasps.  If this is, they usually pose no significant impact on the tree health.  If you like, you can send me a photo and I can try to ID for sure.  Review you image on your computer before sending to be sure it is in focus.  Also tell me if it is soft and thin walled or a hard and solid.

Steve