Powdery mildew is a fungus that loves warmth, moisture, and lower light. It appears as a greyish dusting on leaves and saps nutrients from its plant host. Whenever I overwinter my rosemary plants inside, they get powdery mildew. Last season at Garden Dreams, we tried out several home remedies and preventions for powdery mildew, and they worked quite well. There are many versions out there, but below are the formulas that worked well for us. Mix in a spray bottle and shake well before each use.
I used a cycle of baking soda spray one week, neem oil spray the next week, and milk spray the next week to control the mildew. Baking soda spray did seem to be a good preventative on suceptable plants such as the cucumber family, rosemary, and tarragon.
1. Baking Soda Preventative Spray: 1 heaping T baking soda, 1 T veg oil, 1 T unscented dishwashing soap (optional), 1 gallon warm water. Shake well and spray. Leaves a white residue.
2. Neem Oil Spray (not a home remedy but it works well)
3. Vinegar Fungicide: 3 T apple cider vinegar, 1 T molasses, 1 gallon water. Shake well and spray when temperatures are cool.
4. Milk Spray: 1 part milk to 9 parts water - works best when plants are in sun - spray when sun is shining.