By Edmayo
United States 
Our colored begonias have developed mold on all the leaves and are dying. The white begonias in the same hanging basket or pot are not affected. This is the 2nd year we have had this issue. We have been growing begonias for many years and have never ran into this problem. It starts with one plant and spreads to all the plants around the entire house with in a week or two. HELP!
16 Sep, 2014
It might be powdery mildew, to which begonias are very susceptible - if left untreated, it will kill a begonia. Remove the worst affected leaves and stems (provided that means no more than 4 stems per plant), then mix 2 parts non fat milk to 4 parts water in a sprayer, and spray till run off with that, covering leaf undersides and stems as well as the top of the leaves - this 'recipe' is from the University of Washington Botanical Gardens, and they say it also works as a preventative, so you could spray the unaffected begonias as well. Don't substitute with fat milk or milk with cream - it doesn't contribute to efficacy and may make your plants smell as it goes sour.
The other treatment option (via University of California Agriculture & Resources) is to use 1.5 tablespoons of neem oil, 1 to l.5 tablespoons of mild or natural dishwashing soap in one gallon of water, and spray with that.
16 Sep, 2014