An old-timer that worked well and still does (if it can be found) is called ferbam.

After they are dry (which can take quite a few days) dust them with a fungus-inhibiting product containing sulfur or ferbam.

Fungicides recommended include bordeaux mixture or ferbam.

They are particularly sensitive to a rose-spraying ritual that requires use of chemicals like the fungicides benomyl, ferbam and other remedies.