Powdery Mildew on Tuberous Begonias

Just noticed there is the first signs of Powdery Mildew attacking the leaves of some of my Tuberous Begonias. It's normally an autumn thing but I assume this unusually cold wet weather has brought it on early. Everything I look at in the Garden Centres specifically says not to be used on Begonias, or it does when you search the manufacturers web site.
These are some expensive show begonias and some I inherited from my mother in 1993 so I don't want to lose any of them.
Anyone know what I can use to cure or prevent it spreading?

-- Regards
Bob Hobden
Posting to this Newsgroup
from the W.of London. UK