Azaleas have a whitish powdery substance on roots. What steps to prevent others from getting it?
Azaleas Have a Whitish Powdery Substance on Roots
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.