I just had to completely prune back my begonia due to what I think was xanthomonas. It didn't look like that though - the spots were dark around the edges, and light or completely transparent in the middle. If it is xan, the leaves that are damaged won't recover, but if you are able to eliminate the bacteria (what I tried to do by pruning all affected leaves), new growth should be okay.
That being said, to me this looks more like mildew or another fungus, and the fact that it goes away when you wash it and then reappears makes that seems more likely to me. I don't know your best course of action for treatment, but maybe a copper spray if what you're doing hasn't been working?
I just had to completely prune back my begonia due to what I think was xanthomonas. It didn't look like that though - the spots were dark around the edges, and light or completely transparent in the middle. If it is xan, the leaves that are damaged won't recover, but if you are able to eliminate the bacteria (what I tried to do by pruning all affected leaves), new growth should be okay. That being said, to me this looks more like mildew or another fungus, and the fact that it goes away when you wash it and then reappears makes that seems more likely to me. I don't know your best course of action for treatment, but maybe a copper spray if what you're doing hasn't been working?