How do I know if it is root rot, and can the plant be saved?

Root rot is not a single disease. It is the general collapse of a root system in soil that has stayed saturated long enough for oxygen to run out, usually followed by opportunistic Pythium or Phytophthora once the roots are already weakened. That is why drainage fixes it and fungicide alone rarely does.

To confirm, take the plant out of the pot — you cannot diagnose it from above. Healthy roots are firm and pale, white or tan depending on species, and hold together when you tug gently. Rotted roots are brown to black, slimy, smell stale or sulphurous, and the outer sheath slips off between your fingers leaving a thin thread of vascular tissue behind.

If more than roughly two thirds of the root mass is gone, the odds are poor and you are usually better off taking healthy cuttings from the top growth. If less than half is affected, the plant will often recover.

The procedure is straightforward. Wash all the old medium off under running water so you can see what you are cutting. Remove every soft root with clean scissors, cutting back into firm tissue. Cut back a proportional amount of foliage — a plant with half its roots cannot support all its leaves, and leaving them on drives further collapse. Repot into fresh, fast-draining medium in a pot only slightly larger than the remaining root ball, in a container with drainage holes.

Then water sparingly and put it somewhere bright but out of direct sun. Do not fertilise for a month; damaged roots cannot take up salts and feeding at this stage makes things worse. Recovery shows as new growth, and it usually takes six to ten weeks.