In my experience, root rot IS curable but with difficulty.
I've had limited success with a drench of bacillus subtilis, serenade.
Also spraying the plant with wilt pruf since the roots water uptake function is really impeded and can't keep up with transpiration (as you will notice with leaves drying out).
Of course after my sweet tooth mother was showing the first signs of coming back to life, my bulldog got the munchies and ate her... so eveidence is inconclusive

Causes include overwatering in poorly drained soil... can be much worse in cold conditions and/or poor ventilation... an already-stressed plant with a weakened immune system is not going to fare well...