Things You'll Need
Tea tree oil
Powdery mildew can form just about any place with humid conditions, including on plants. Powdery mildew on plants can cause them to rot and die. Neem oil, an essential oil that has anti-fungal properties, can be quite effective in killing powdery mildew. Unfortunately, neem has a strong, bitter garlic-like scent that can be fairly unpleasant. Tea tree oil shares neem oil's anti-microbial properties, though it is not as effective, and has a pleasant scent. Studies have also shown that a combination of oregano and clove oils in water can be quite effective at wiping out powdery mildew.
Neem and Tea Tree Oils
Mix one quart of warm water with one teaspoon of neem oil, one teaspoon of tea tree oil and 1/4 teaspoon of mild, non-antibacterial dish detergent. Use a whisk when mixing the oils into the water to help fully blend the three liquids; when you are finished, the water should be cloudy and there should not be any oil floating on top of the water. Add the detergent last and stir gently to avoid creating too much foam.
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Pour the mixture into a spray bottle with a misting function and spray any surface covered in powdery mildew. Don't worry about these oils getting into the soil around your plants; just avoid saturating the soil and overwatering the plants.
Allow the mixture to take effect for a few minutes before wiping down each surface with a soft cloth or tissue. The mildew will be killed, but will leave a blackish residue. Repeat the process every day for two to four days, even if there is no mildew, to ensure that all the spores have been killed.
Clove and Oregano Oils
Mix 1 quart of warm water with three to five drops each of clove oil and oregano oil and pour into a spray bottle with a misting function. These essential oils can be found at specialty food stores or any place that supplies holistic medicines.
Shake the bottle vigorously for one to two minutes to blend the oils with the water. This is made easier by using warm water but avoid using hot water; it can diminish the effectiveness of the oils. As with the tea tree and neem oil solution, the water should be cloudy with no oil floating on top of it.
Spray the oil mixture onto any surface affected by powdery mildew. Allow the mixture to work for a few minutes and then wipe the surfaces clean with a soft cloth or paper towel. Use this spray once a week to prevent future outbreaks of powdery mildew.