Question
QUESTION: What are the small yellowish fuzz balls on the underside of most of the leaves on my oak trees.   Thank you, Terry

ANSWER: They sound like galls caused by an insect (small wasp) The wasp lays its eggs on the surface of the leaf or twig and secretes a chemical that causes the tissue of the leaf or twig to swell and cover the egg. the egg hatches and moves on. The galls themselves will not harm the overall health of the tree although they can look bad. Leaf galls may cause the leaves to drop prematurely. There are many types of these wasps some attack the twigs and others the leaves. Control is not usually needed and has to be timed with the wasp presence and not the galls themselves. Here are a web links to a couple of sites on gall wasp that discuss the wasps and control measures.
http://entomology.unl.edu/ornamentals/pestprofiles/gallwasps.htm
http://www.ces.ncsu.edu/depts/ent/notes/O&T/trees/note05/note05.html

Not a problem that will cause any health problems to the tree. I would not worry about them.


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QUESTION: It's hard to believe that ever leaf on the tree and all my neighbors tress has something caused by wasps. Especially since we have very few wasp around that I've noticed.  Do you still think it's caused by wasp if it's every leaf and tree in the area?

Answer
It sure sounds like it. If the population is high then there would be more signs. http://www.bio.brandeis.edu/fieldbio/Plant_Galls/callirytislanata.html
These are tiny wasps and unless you watch closely you more than likely will not see them. Here is a link to these tiny wasps.http://en.wikipedia.org/wiki/Gall_wasp