Did this stuff start as a white powdery look if so this might be fungus Botrytis ( if so remove all of the damaged leaves and spray the plant down with some 1/2 milk with 1/2 water solution)
also could be a chemical burn so after you try that fungus solution try flushhing the plant if that didnt work
and the last thing it could be is root rot which is possible since i dont see any perlite in your medium and they are only 2 ways to stop root rot if it isnt to late 1 is to add root rot fungicide and when doing that
lower your ph below 6.0 to prevent spreading and the second solution is to try and aerate the soil by transplanting
P.S. wouldnt flushuntil you try the fungus test so try that then if you see it start to appear try flushing and if you dont think its a chemical burn then try teating it like root rot