Mildew can be the bane of a lot of begonias including some canes (I have noticed the ones with big polka dots are more prone to mildew than plain or silver splashed leafed varieties), Mallet types, rexes, and even some rhizomatous. It sounds like you only have grown this cane indoors so I wonder where it picked up mildew (not that it really matters at this point).

I've never tried milk spray myself but have read accounts on its effectiveness more from the colonial days when the cooks/servants would throw out leftover milk and water on the nearest vegetables/herbs at the back door. They observed that the plants receiving the diluted milk often were healthier than the plants next to them. Hence the milk solution theory. Basically you are changing the pH of the surfaces which makes it more hostile for mildew spores to flourish.

Warm air, good light, and supposedly good air circulation is the natural way to control mildew but indoors in winter may be a tad difficult to achieve. In this case good habits and maybe a friendly spray may be the next best thing. Remove the worst leaves and discard, spray any leftover leaves and the stems (canes). Leaves will regrow but if the canes die then it may be hard to keep it alive. Fertilizer may help the plant in fighting the mildew attack. I've noticed that mildew often attacks younger greener stems than the older woody stems so either treat your green stems or cut them out.

The pot seems a little large for the canes in this photo so be careful on watering so it does not get waterlogged for days.

Once upon a time, the thick stem of B. 'Lucerna' was showing mildew. On someone's suggestion, I forgot who, I rubbed off the mildew, sprayed the stem lightly with hair spray. It changed the color of the stem but mildew never repeated there. I agree, lack of fresh air in winter conditions, contributes to the problem.

Thank you for the great advice!

Yes, the container size was too large to begin with. I justified it to myself because it has great drainage & I usually use a water meter prior to watering. My objective last year was to give it lots of room to grow & to prevent it from becoming top heavy as it did.

That being said, I am guilty of over-watering about three weeks ago. The leaves were looking dry and brown & beginning to fall, so I didn't use the meter. I now realize that it was exhibiting symptoms of the PM rather than being thirsty! I most likely added to the existing problem when I made that mistake. So I'm holding off with water until I see what happens. The meter is reading a consistent 6/10 in all areas now.

As far as the origin of the PM, I have no clue. I started misting the leaves between watering the plant this summer. Also, I rearranged furniture this summer and the begonia was moved a couple feet, which resulted in reduced sunshine exposure. Maybe the combo??? I'm a fresh air kinda person, so as it has been getting cooler I still leave windows open. From here through the winter, the room it's in will stay within from 55-72 degrees. Humidity will vary, so I'll try to be vigilant at times that it is extremely high or low.

So far the milk hasn't had a bad effect. I'm 24 hours in since the first treatment. There is noticeably less PM, but this could be from being sprayed in general. The sprayer creates a generous amount of fine mist. Who knew that the milk was controlling pH?! What a great concept & such an interesting story about the colonial discovery. :) Thanks for sharing!

One of the larger leaves fell off today. I then removed the last large leaf once I read your replies. None of the remaining leaves look like they're affected right now. One new leaf has started to open today, which gives me a glimmer of hope!! Lost 2 leaves, but gained 1...for a grand total of 13 lucky leaves on a nearly 4 foot tall begonia. What abuse I've caused. :(

I sprayed again today, about 3 ounces total at a 10% solution. Made sure I coated every square inch of canes and leaves....and also sprayed the top of the soil.

My plan now is to monitor it daily. I'll use a low dose of a balanced 10-10-10 next time I'm able to water. If things start going downhill, I'll up my game. Hairspray?!! Who knew? I literally LOL with your story. That'll be my second line of defense if it comes to it.

I'm left wondering what else I need to be doing????..... Do I need to plan to repot this begonia if it makes a come back??... Any other thoughts or suggestions are highly welcome!!! I have grown to love this plant over most of my others (and I have soooo many plants around my home)!

Hmmm, that is a hard call to make on potting it down at this point. It certainly may be a good thing if it continues in a downward spiral. I think if you can give it as much light as possible then it may pick up.

I often remove every leaf if a plant has a bad mildew problem or just ugly torn leaves. This often encourages the plant to put out lots of new growth. Not only on begonias but other plants as well (hydrangeas come to mind).

My own experience in over potting happened in 2005. I bought gallon sized begonias at PHOE (Palm Hammock Orchid Estate) in May at their annual begonia show and sale. I was elated at how fast Caribbean King and C. Queen grew in one summer. They were each in their own 18 inch pot and filled it beyond expectations. The problem was I left them out in early November and they got a killing frost. That nipped almost all the leaves but the rhizomes were unfazed. What I did next was stupid when I look back on it. I moved them in these extra large pots into our cool, dim garage for winter. The next year they did not attain the same giant size and I repeated the move back into the garage. The following year they got even smaller - about the size when I bought them. I down sized them that fall and moved them to the basement under lights. It has taken years to bring them back to a respectable size and they are still no where near where they were the first year. Lessons learned: use a big pot for a big plant and move them back to a proper pot indoors?

Here is that extra large C. Queen & C. King. Queen is larger than the King (on other side of sidewalk and darker green). These are 18 inch containers.