I bought 2 Sunset Echinaceas this year about 2 months ago. I planted them about 12" from each other and they have been growing fine. About 3 days ago I noticed the leaves on one of them started wilting/sagging and lost their luster. Yesterday some of them turned black and were easily pulled off from the base of the plant. The other one is right next to it, in the same soil, getting the same sun/water and is doing fine. I dug up the plant and the soil seems to have a nice moisture level and I didn't see any tunnels of voles or other insects. The roots did look disintrigated to some extent. What is this and how do I treat the plant/soil?