Brown damp mark

You can try some H2O2 as a soil drench if you think it's root rot ... mix a 1:1 ratio of 3% H2O2 with water and soak the soil... it will help aerate the soil and kill off any fungus that may be lurking around the roots. Not entirely certain it will help if you've got stem/bud rot starting up in the stem but it certainly won't hurt your plant. Next grow, add a lot more perlite to aerate the soil and get better drainage.... Good luck!