This is a mildew closely related to fungus. The powder is the mildew's reproductive spores. It thrives in an acid environment in a temperature range of 60-70?F (15-21?C) with a humidity above 50%. The spores are floating in the air and there is no practical way to screen them out. Instead, try to change the environment so that conditions don't match the mildew's needs. This may require raising the temperature or lowering humidity. Use Neem Oil: Neem oil is pressed from the nut of the Indian Neem Tree. It protects against and kills mildew by interfering with respiration and collapsing the cell wall. Some growers claim that plants grow more vigorously when sprayed with dilute neem oil twice a month. There are many brands of neem oil available. Many of them are listed as organic
