Tuberous Begonia Blues . . .

I'm in a sad state with my tuberous begonia plants. I have 10 or so (2 very large ones) in containers on the shady front porch (Cleveland OH Zone 6). I bought them all from local nurseries in spring, and all have been doing wonderfully until about 3-4 weeks ago. First one plant, then all the others, have gradually developed first brown (prematurely brown) flowers and then white filigreed spots on leaves. The flowers aren't just spent -- they are attached quite firmly but are dead-brown and papery.
Are the plants goners? My research on the web shows a powdery mildew fungus the likely cause, although they are on a protected porch, and I never water the leaves and am careful not to over-water.
On advice from the local nursery, I've sprayed all plants with a Daconil-type fungicide, but the result was awful. The plants now look even worse -- white residue coating all the leaves, and the fungicide has killed all the remaining flowers it touched.
Do I keep up with the weekly application of fungicide and put up with the unsightly residue/no flowers? What's the point of that, I wonder? However, I have so much $$ invested in these beautiful plants that I hate to give them up to the fungus. Any advice appreciated!
-- pelirojaroja
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