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
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