Kay Easton wrote:

It's a very long time since I've done anything like this, but IIRC it
involved waiting till the top growth had died (or removing it if it was
badly rusted) and dunking the roots briefly in very hot water to kill
the over wintering rust spores.

This is a very old memory, so don't try this if it matters to you losing
the mint.

That's about all I can remember as well but Google should produce the
times and temperatures - which are fairly critical, a nice balance
between killing the fungus or cooking your mint. It was/is also widely
used as a control for nematodes in Chrysanths and certain bulbs.

Rod