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Powdery mildew looks like a light dusting of whitish-gray mold, usually on the leaves of an outdoor plant. In most cases, rubbing the leaves will remove most or all of this fungal growth, which sort of looks like dirt or dust.
The mildew can occur on roses and herb and vegetable plants, leaves, although it is known to crop up on other plants. There are commercial fertilizers and fungicides available to eliminate it, or, for a natural approach, try this spray. (Be sure to test the spray on a few leaves first to make sure that they are not too sensitive for it).
1 teaspoon baking soda
1 quart water
A few drops of liquid soap
Combine in a large container and transfer some to a spray bottle. Spray the leaves of plants where powdery mildew can be seen, then repeat a week later. For best results, treat plants as soon as you notice the powdery mildew.