Q: The leaves on my oak tree have these hard fuzzy things on them and they are all over the tree, are they eating my tree and what do I spray on them to get rid of them?
A: What you have is a form of what is know as a plant “gall”. They come in many forms from the fuzzy thing you have noticed to round marble sized things to large hard ones the size of a golf ball to hollow ones the size of a tennis ball. Most form of the leaf but a few form on twigs, They are cause by an insect laying an egg in or on the affected part of the plant in spring or early summer. After the egg hatches the larvae secrete hormones that cause the plant to grow these structures around it as it feeds. Most of these are totally harmless, especially the ones that form of the leaves. The worst is that it deforms the leaf and makes the plant look bad, the ones that form in or on the twigs can do some damage to the plants though. The only treatment that I can recommend is to spray with a systemic insecticide early in the season or use a systemic drench around the affected trees.