If you're talking about downy mildew, the strain of it that wrecks Impatiens does not affect other plants, from what I've read about it. It does stay in the soil though, so that's why it's such a devastating problem, once it's in your soil, Impatiens can't live. There are other strains of downy mildew. There may be one that attacks Begonias, I couldn't find any info on ABS, and trying to search such just brings tons of info about Impatiens.

Have you been watering that area?

Here's some info about powdery mildew on Begonias. ABS doesn't seem to have an article about Botrytis, but that's another possibility to investigate.

The wax Begonias I've purchased this year are struggling also. All of the existing foliage has died off (I gradually picked off each leaf as it became ugly from some kind of pathogen,) although there is vigorous-looking basal growth on all. New plants put in the ground are coping better, naturally, and overwintered plants seem unaffected, or any of the other Begonias around the yard and pots.

Doesn't sound like the same sudden death you described, but interesting we're both having trouble with what should be such a trouble-free plant. Is anyone else having troubles with waxies this year? These plants, now much smaller (at about 2 inches for the biggest,) than when I bought them are NOT the star of any show and look ridiculous in hanging pots. Luckily I did mixed pots and other plants are trying to make up for it.

That was before the sun went away. If it doesn't come back soon, and never stops raining, all of the plants will be dead anyway. Might be a moot point. Splash, splash...!

I haven't had any trouble with my wax begonia, but my canes and rhizomatous are doing awful! They were absolutely beautiful last year, and around February they started to decline. They were putting out lots of new growth, but the leaves would drop practically as soon as they opened. Larger leaves would get some spots, and drop. I treated for mildew, which seemed to help, but the rhizomes are now thin and just barely growing. I cut everything back severely, and some are starting to grow back, but not very quickly. I'm about ready to throw everything out! I did buy a new Gryphon this Spring, and it's the only one that's doing great. And I'm keeping it away from the others, until I find out what's going on....

Oh, so sorry to hear. Hope things perk up, Grey!

...supposed to rain here all the way through the forecastable future... at least the next week.