White Oak Galls (IMAGE) Emory Health Sciences Caption White oak leaves and marble-sized galls. Galls are growths on plants caused by invasions of parasites, viruses, fungi and bacteria -- making galls centers of activity for both attacks on a plant's system and a plant's defense of those attacks. Credit Photo by Stephen Nowland, Emory University Usage Restrictions Must include credit. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.