Here is my take on growing begonias indoors - the air is too dry for a lot of begonias to flourish. You can attempt to acclimate them to their new permanent setting but it may take months to do so if at all. The easiest thing to do is enclose them in some kind of humid environment - baggie, plastic cup or larger container, or a glass housing such as a fish tank or a glass jar and slowly wean them to a drier atmosphere. Another option is to put them in a tray that can hold some water in the bottom.

If a leaf is rotting, remove at least the rotting part if you intend to use the healthy part for propagation. Older leaves can be removed when you are ready to deal with them. Often diseased leaves are best removed anyway so that new healthy leaves will soon replace the discards.

For spraying purposes, you can spray them with most fungicides that are safe for houseplants. If you are unsure then only spray one leaf to see if there is any harm to that leaf before spraying the entire plant. I know Neem oil, Milstop, sulfur-pyrethrin, and insecticidal soap are safe for begonias. I typically stay away from chemicals that say to wear Hazmat suits (kidding here but there are some chemicals that have a long list of safety precautions so do read all the instructions before buying or using).

One of our club members says she never gets mildew but then she keeps all her plants in humidity controlled enclosures and rarely exposes her plants to household air that may contain mildew spores. The downside to this is any begonias that are predisposed to become large plants will not do so in this constrained environment.

Thank you so much for all your advice. I have read a lot of places suggesting the putting water trays under the plants with rocks or something to keep them directly out of it. I may try this next. It seems a lot easier to maintain constant moisture to the air around them this way. I will also try some of the sprays you suggested. I'm the same way about the chemical stuff. I've had great success with a diluted milk solution to totally wipe out powdery mildew in my garden....Don't know if it's such a good idea inside ha ha. I would love to hear success stories of others who have begonias inside. I have a couple like my "iron cross" , "muddy waters" and "black fancy" that all do well. Nothing seems to phase them but they are by windows, not under lights and without any extra humidity. I'm finding out no two varieties act the same.

If you look through the old threads on this forum you will find some references to diluted milk. I've never tried it but did find some interesting stories on how that developed over the centuries. I've heard you can also use baking soda but again I haven't tried that. The way I understand it is the pH of the leaf is changed making it a more hostile place for the spores to grow in.

I had a ton of pea gravel in most of my aluminum trays in the basement. I finally got rid of it all since it was hard to clean once soil and other debris fell in to the gravel. It also seemed to speed up the corrosion of the aluminum as well. So if I add water to the bottom of the pan, I try to keep it at a very low level so it will evaporate quicker and not cause water logging to the pots. Most of my pots do not have saucers either.

Most begonias will tolerate the low humidity in the house but there are some that absolutely will die if they don't have the high humidity that their genetic makeup requires. The problem is knowing which ones to baby and which ones to "neglect". Most will appreciate more humidity but some are so succulent that they would probably be better treated like a cactus (venosa comes to mind).