Answer: I think the problem is powdery mildew. It's one of those persistent garden companions, and quite common on begonia's. The mildew will win if you loose interest or patience with the control regime. It requires that you keep vigilant, washing top and bottoms of leaves weekly with water. If you have a compost pile, make some compost tea and spray it on leaf surfaces every couple of weeks, or use baking soda-oil spray (1 tsp. baking soda and 2 tsp. summer oil in 1 gallon of water) sprayed once per week. A commercial organic alternative is Soap Shield Fungicidal Soap from Gardens Alive (5100 Schenely Pl., Lawrenceburg IN 47025; ph# 812/537-8650).
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