Answer: Just as you observed, spraying your rosemary plants with water actually can help to control powdery mildew. Unlike most other fungal diseases, powdery mildew can be held in check by strong sprays of water that simulate a downpour.
Another effective treatment is regular sprays with compost tea solution. To make the spray, mix one part mature compost that contains some manure with five parts water. Let the mixture sit in the shade for two weeks, then filter through cheesecloth and spray on your plants. You can dilute it by half if you like. Repeat every two weeks.
I hope you have some success with these methods. Good luck.
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