bergonia.

Hello Brianyoung, welcome to GOY.
Your Begonia is in a place which is too cold and damp, causing downy mildew. Dust with green sulphur, cover at night with newspaper.

Hi Brianyoung - where is the plant; house, conservatory, heated greenhouse, ... ? My begonias are currently flowering on a south facing windowsill in my bedroom office with part-time central heating. They seem to quite like that environment.
Oops - Welcome to GOY, by the way!