Black, Black-Eyed Susans
 

Some of my black-eyed susans have had their green undergrowth turn black &
crispy, yet the tops of the plants seem normal and healthy. I've noticed this
occuring where the plants have overgrown onto hot pavement, and figure the
problem there is too much reflected heat and sunlight from the concrete. But
it's also happening where these plants are far from any sources of extra heat
or sunlight.

These plants are all Rudbeckia fulgida. Care tag proclaims: "Grows in
virtually any location, tolerates poor soil, heat and drought, looks best with
some summer water."

Could too much water, or the compost rich soil I planted them in be *too* good
for these plants?

TIA