Sounds like it could be bacterial leaf spot. Not much you can do except remove what damaged leaves you can, avoid getting water on the leaves and keep the begonia well fertilized. Here is a link to a publication about bacterial leaf spot on begonias by the Illinois extension: http://web.aces.uiuc.edu/vista/pdf_pubs/659.pdf
Good luck and let us know what happens!
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Welcome! I’m Margaret Roach, a leading garden writer for 25 years—at ‘Martha Stewart Living,’ ‘Newsday,’ and in three books. I host a public-radio podcast; I also lecture, plus hold tours at my 2.3-acre Hudson Valley (NY) Zone 5B garden, and always say no to chemicals and yes to great plants.