Question
A new orchid that I purchased developed a large amount of a white sand-like material around the roots and lower part of the stem. Soon, all my orchids got the same white stuff. Some of the stems also look yellow close to the medium, (as if they were dry). Please, help!

Answer
Carla, it sounds like it might be a mealy bug attack, but I can't be sure from your description.  Mealy bugs appear as a white, sort of fluffy meterial, when there are lots of them.  It would be helpful if you could attach a picture. They do travel easily from orchid to orchid.  The one puzzling thing is that you didn't mention that this appeared anywhere on the leaves. I'm assuming that you have phalaenopsis orchids (also called moth orchids).