Galls on Witch Hazel (image) Howard Hughes Medical Institute Share Print E-Mail Caption Most of the galls made by Hormaphis cornu aphids are green. But mutations in one gene trigger the development of a red gall instead. Credit David Stern Usage Restrictions Must use credit. Share Print E-Mail 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.