About 2 months ago I transplanted coneflower sprouts from my back garden to a new front bed. They all looked fine for about 1 month, then I noticed one starting to wilt and quickly progressed to death w/i 3 days. It seemed to rot from the roots with a whitish powerdery "fungus" on them when I pulled it up. Now another one has done the same. The bed has excellent dirt (soil, compost, humus) and every other plant in it has done great. I am disappointed because I thought coneflowers were so indestructible---and I have not had this problem at all with any plant in the back garden. My one theory is that I may have over-fertilized them (excess Nitrogen), although I tried to exclude them when I scattered the stuff over other plants. What's the probable diagnosis & remedy folks??