black spots help

one of my plants has some black spots on the roots what is it? i m using rockwool as my medium if it is fungus how do i fix it?

ROOT ROT. FLUSH REZ WITH CLEAN PURIFIED H20 run no nutes for a week then run 1/4tab of AG nutes and keep an eye on it Check out SICC and see what he says. dont rape his thread though. he hates that