Poison oak gall
Fasciated stems on Poison Oak at Pillar Point Bluff County Park.
Fasciation of Poison Oak.
Fasciation on poison oak
Antler-shaped fasciations gall on terminal buds of Poison Oak (Toxicodendron diversilobum).
Glen Canyon Park
San Francisco, CA
Reference: Field Guide to PLANT GALLS of California and Other Western States by Ron Russo- ISBN 978-0-520-24886-1
Fasciation Gall. Described by Russo (2021), but cause apparently not known to science yet.