If there is a drain hole, then water is not accumulating in the bottom. As long as you are letting the top inch or so of soil dry out, then root rot is unlikely. I suspect it was some damage done inadvertently when you repotted. It should recover.
Root rot is not caused by a fungus, so a fungicide would not be appropriate even if you did suspect root rot. Washing the roots is a drastic solution that usually does far more damage than good. It is not something I would ever do.
Monitor your watering carefully and I think you will see gradual improvement.
