Azaleas Have a Whitish Powdery Substance on roots
Azaleas have a whitish powdery substance on roots. What steps to prevent others from getting it?
This sounds like Armillaria Root Rot, but without seeing it, one can't be sure. This article might help: http://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2057.html
For positive identification and recommendations, take a sample to your local agricultural extension office.