The dreaded basal rot.
That's really hard to get rid of. You're going to have to unpot the plant, pull off all those affected leaves (even if it means there are only two, or even one left) and soak the plant, roots and all, in some kind of anti-bacterial solution. People are recommending straight hydrogen peroxide for their plants--you might give that a shot. Otherwise, physan, rd-20, that kind of thing, dilluted to manufacturer's instructions. Let it sit a good 15 minutes.
That happens notoriously in summer if you don't have enough air movement and water has collected in the leaf axils. Have you tried turning the plant upside down to see if any water runs out from where the leaves form the stem? Might as well do that test on all your phrags....