Touching it releases a fine black powder.

Then it is most likely a fungus. If you are in the US, contact your county
extension agent. Otherwise, you need to use a fungicide listed for lawns, and
follow he directions on the label.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)