Always spray before you see any mildew or rot. Once you see it, it's too late. The sprays are preventative........they stop mildew from forming, but once it's there, they don't stop or kill it. Also, spray before a rain, not after, for the same reason. You want the plants to be coated with spray (protected) before the rain comes.

Thanks, Cupshaped, for the pictures. Beginners should notice the leaves showing their undersides-- this lets you spot PM from a distance before it even produces the white stuff-- although older leaves that have stiffened up usually won't show that. Fertilome Systemic is propiconazole, same as Banner and Bonide Infuse. It seems to be as effective as anything. I read a disturbing study the other day that found even the best fungicides ineffective when rose plants were already heavily infected and conditions for disease were optimum. So picking off infected leaves and stems makes sense in that context. My primitive sulfur fungicide has done nearly a perfect job of preventing PM this year, except on the no-spray roses that don't get blackspot, and which now are getting sprayed. Sulfur is widely used in agriculture for PM, so it is effective, but only as a preventative. Drought doesn't cause PM, but rain twice a week goes a long way toward preventing it. I washed down my roses this morning before spraying. PM germinates only on dry leaves at very high humidity, with temps mainly in the upper 50s and 60s F. These conditions occur mainly at night when the temperature holds for a few hours just above the dew point. Fog and heavy dews suggest PM conditions. PM is unlikely to germinate if the dew point is below 55, or if night temps are above 71, because dew points above 70 are pretty rare. Daytime temps in the 70s-low 80s are ideal for mycelial growth and release of spores. As Ann says, it doesn't like hot weather much. (from Horst's Compendium of Rose Diseases)

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)!

Wow, there are a lot of powdery mildew spray recipes around the web and they vary quite a bit in the quantity of each ingredient. Several sites mention 2% baking soda being the maximum before you start seeing leaf burn. 16 cups to a gallon, so 1/3 cup = 2% of a gallon. I'd prefer it not be so close to the max. I'm going to try a weaker mix, at least for the regular prevention spray. Also, many recipes use less liquid soap and more milk, so I'll tinker with those also...tinker, tinker, bang, hammer, tinker, saw, scratch head, tinker, tinker and drumroll... new experimental recipes Prevention spray- 1 tablespoon liquid dish soap 2 tablespoons baking soda 1 cup milk 1 gallon water The war is on spray- 2 tablespoons liquid dish soap 4 tablespoons baking soda 2 cups milk 1 gallon water These will likely get adjusted some too. Luckily I have some spotty 30ft birdhouse gourd vines I can test on.