Well, I'm just guessing :-) and I don't know of a chemical remedy.
I think that many plants respond to vascular fungal attack by "sealing
off" the vascuar tissue leading to the infected region. This'll kill
that branch or region, but if successful, will prevent the spread of
the fungus throughout the entire vascular system.
I'd say prevention is the best cure. A periodic drying of the pots may
make life a little harder for the fungi (they may enter through roots
which have already died of rot?)
Michael