Oak Tree Pest?
I have approximately 5 acres on a hill side in central California covered with oak trees, the last couple of years I have found a peculiar red growth on some of my trees (see pictures) I'm not sure what it is or if I need to worry about it.
Fresno County California tree health
Thanks for your question, very interesting.
From the pictures the growth on the oak leaf is most likely a gall caused by a small wasp, a cynipid wasp probably. The majority of gall makers that attack oaks are wasps, but in some cases flies are responsible. Galls are caused by powerful plant growth regulating chemicals or other stimuli produced by the insect that react with the plant hormones. The majority of oak leaf galls do little or no harm to their host. However, twig galls may cause severe injury and bring about tree death. The galls you have in the pictures is a leaf gall.
You can Google oak leaf galls to learn more. They are very fascinating.
Hope this helps....