Haven't tried it myself, but since no one else has answered--I've heard a little baking soda in water sprayed on is effective, but I don't know the concentration. Also think Neem oil would work, and it's about as safe as anything could be; I understand people use it for stomach disorders in its native India. I think you could find the correct formula for baking soda if you do a search on this forum, and of course Neem oil would tell you on its label. Let us know how it works! Haven't tried eating tuberous begonia flowers--can't grow them here--but they'd sure add a lot of eye appeal. There is one species begonia, B. deliciosa, which by the name must be very tasty. And I bet there are others.

Some people use watered down milk to stop mildews and fungi infections.

There was an article in the Begonian last year where I guy did a taste test of most of the common forms of begonia - most were nothing special but some had a nice lemony flavor (he was eating the leaves).