I have a large mound of Brown-Eyed Susans. One plant has dried up. When I looked at it, the stems next to the ground were mushy and there was white stuff around them that resembled sawdust.
Stem Rot on Brown-Eyed Susan
Certified GKH Gardening Expert
The sawdust-like substance could indicate that a critter was gnawing on the stems. This could have also led to the rot you see.
Just in case it is a disease, you should remove the affected plant, including the root and any fallen leaves, and dispose of it in a tied garbage bag.