Question
I have a live oak tree which suddenly has a white grwoth all over the leaves and branches. It looks almost like lint from a dryer filter. Can you tell me what it is, and what to do about it.  

Answer
Could be a couple of things--Powdery mildew or wooly oak aphid.

Check and see if the fluff moves if touched--if so it is an insect called an aphid. IF not then it more than likely is the fungi.

wooly oak aphid
  These insects have sucking mouth parts and can secrete a waxy material that is white. One of these is the Wooly oak aphid. You should be able to feel the sticky honeydew on the foliage. And it maybe raining down under the tree.
The fluff is waste material from the aphids.
I would spray the foliage with one of the following insecticides.
Numerous contact insecticides are registered for aphid control. If you do not obtain reasonable control, consider rotation to another insecticide. Contact insecticides currently registered for aphid control include: acephate (Orthene), bifenthrin (Talstar), chlorpyrifos (Dursban),  diazinon, malathion, nicotine sulfate, pyrethrum, rotenone, resmethrin, and tetramethrin + sumithrin.

IF the tree is too large to spray the foliage then a soil applied insecticide can be used. A systemic insecticide can be used that the tree takes the insecticide into the root system and to the leaves and twigs and kills the aphid when they suck the plant juices.
Imidacloprid (Bayer Advanced tree and shrub  Insect Control) is a newly available systemic insecticide that can provide aphid control on trees for many months following application to the soil.

Here is a web link to this product http://www.bayeradvanced.com/tree-shrub-care/products/12-month-tree-shrub-insect...

Powdery mildew
   Powdery mildew fungi belong to the group of plant pathogens called obligate parasites. Plant pathogens within this group can only grow and reproduce on or in a living host plant. S. pannosa can infect any green tissue

This fungus lives largely on the outer surface of the host plant. It has a high demand for the nutrients necessary for growth and spore production. It obtains these from host plant cells by means of small, root-like organs, called haustoria, that feed within the epidermal layer of the host plant. Although powdery mildew rarely kills a plant, infection reduces host vigor and lowers aesthetic value.

Several fungicides (Table I) can be used to control powdery mildew. It is a good idea to use a different fungicide each season or alternate between different fungicides during the growing season. This is done to prevent the development of fungicide resistance in the natural powdery mildew population.

Table I. Fungicides for control of powdery mildew of roses:
  Banner MAXX? Immunex?
  Benomyl? Benlate
  Folpet? Phaltan
  Bayleton? Strike?Triadimefon
  Funginex? Triforine
  Triact? Rose Defense? Neem oil
  Pipron?
  Phyton 27? copper sulfate
  Rubigan AS?
  Cooke's?wettable, Safer?Garden Fungicide, Microthiol Special?Sulfur