Question
I have a mature live oak in my back yard that now has yellowish, fuzzy things growing on the leaves.  What are they and will they harm the tree?  I also have what looks like spider webbing on the brances of the tree.  Are these two problems associated?  Thanks for your help!

Answer
They are all 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. No need to worry about the leaf galls since they do no real harm to the health of the tree.

The webbing on the branches sounds like bark lice.

Bark Lice are more properly called Psocids (pronounced "SOSS-sids"). They are also called tree cattle because of their habit of living as a group on the bark of hardwoods, particularly oaks, and have been reported locally on citrus trees.  Large numbers of adults and nymphs are occasionally observed on tree trunks often living underneath the fine silken webbing they construct.  These insects do not attack the tree, but feed on lichens, fungi, and dead animal and plant matter.
Usually, the first noticeable sign of the presence of this insect is the fine silken webbing. The bark lice secrete this as a protective covering on the bark.  This silken webbing has a silvery sheen and may cover parts of the tree trunk and large limbs.  When the webbing is pulled off, it is not uncommon to see hundreds of these insects.  This webbing is not found on the twigs or leaves, and looks different than webbing produced by destructive caterpillars.
Webbing barklice are found throughout Florida, and along the Gulf coast from Texas and along the Atlantic coast north to South Carolina.  They are distant relatives of the booklice, which are household pests that can consume wood and paper products.  These outside Bark lice are not a threat to either the inside environment or to Florida Yards.   Populations of these tree cattle rise and fall with the seasons - cooler weather kills off many in the winter, and as springtime temperatures rise, food sources increase, and more webbing appears.  The greatest populations are found in the fall before the onset of cooler weather.
No control measures are recommended for these insects.  If the property owner objects to the webbing of these insects, a strong blast of water from a hose will dislodge them from the tree.  But as scavengers, they perform a valuable function in consuming excess accumulations of lichens, dead bark, and other materials found on the outside of the tree.  On trees that are regularly sprayed with insecticides for other pests, psocids will rarely be seen.  If the homeowner does insist on removing these insects, a Pest Control Operator may be contracted to apply a legal insecticide.
Here is a web site that gives more information and pictures of bark lice.
http://www.ag.auburn.edu/enpl/bulletins/barklice/barklice.htm  

Again they do little harm so it is not real important to treat them unless you can not stand the sight.