I've read a lot of different suggestions for dealing with various fungal diseases in begonias--haven't tried many myself so I don't know how effective they might be. A great begonia grower who used to post regularly here said she used skim milk and baking soda--not sure if she mixed them or used them separately. Just was reading about using Physan and I've used that--it's also good for algae control, and says it's useful for virus. I had good results for algae and fungus but didn't try it for virus. There's a couple related ones whose names I can't remember, but I can look them up if you're interested. I see the spray form of Tinactin recommended fairly often--yes. it's the stuff for Athlete's Foot. May try that one myself. Easier to find than Physan! And there are, of course, general purpose fungicides at your garden store.