Question
I have an orange tree with a white powder on the underside of the leaf.  What is it and what is a natural remedy to get rid of it?

Answer
Sounds like 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; thus, powdery mildew may be found on leaves, green stems, and flower parts.

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.

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. These are sprayed on the foliage in the spring at bud break and a seconed application should be made aboput 10-14 days later. Check with your local nursery for one of these fungicides.

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