I was happily harvesting my ranunculus flowers about a week ago and noticed that the stems had white patches on them. This is my first year growing these flowers and I was trying to convince myself that the white fuzz on the stems was catching the sun in a specific way and that it wasn't actually powdery mildew. Powdery mildew treatment can be intense and I didn't want to have to deal quite yet. But...
We'd had a hot week here in Southern California with the temps going into the high 80s and the overnight temps were staying in the low 50s which is the perfect growing condition for powdery mildew. On top of that, I noticed that my neighbors invasive ivy had managed to find any corner and crevice in the wooden fence that abuts my garden and it had white patches all over it. Apparently, they hadn't cared to do any powdery mildew treatment on the ivy plant.
That's when I knew I had gotten my first powdery mildew of the season. The weather conditions were perfect for it to grow and my ranunculus were stressed from the hot weather (they do NOT like temps in the 80s). Plus the fungus spores are airborne and I was sure the neighbor's ivy had spread them to my garden.
I spent an entire afternoon hacking away all the invasive ivy and getting rid of all the diseased leaves and I started spraying. Sigh. It's too early for this! And my poor ranunc's.
What is Powdery Mildew?
Powdery mildew is an air-borne fungus. The white powder is microscopic spores that are chained together.
6 WAYS OF PREVENTING POWDERY MILDEW
- Provide good air circulation for your plants
- Never water from above, getting leaves wet
- Be on a consistent watering schedule to not stress your plants
- Mulch well to help moisture stay in the soil.
- Sterilize all tools before moving to next plant with a hydrogen peroxide or bleach solution.
- Get rid of diseased leaves immediately
WHAT ARE THE BEST ORGANIC POWDERY MILDEW TREATMENTS?
There are four organic powdery mildew treatments that have been consistent with verifiable results.
- Use an organic fungicide such as Bacillus subtilis (safe for bees) : CEASE Biological Fungicide
- Baking soda solution sprayed on leaves. (.5 percent solution = 1 teaspoon baking soda in 1 quart water)
- Spray with Neem Oil spray. (Spray only in very early morning or evening hours. It takes about an hour to be safe for pollinators after spraying. Follow directions carefully.
- Spray with Monterey Horticulture Oil. This is a insecticide, fungicide and miticide. It helps kill the insects that can bring the powdery mildew onto the plants. Read instructions carefully.
I ended up spraying with the baking soda solution since my ranunculus are heat stressed currently and I didn't want to take any chances with the oils because they don't allow the leaves to take up any moisture while they are drying. The baking soda solution has proven effective because it changes the pH on the surface of the leaf and inhibits the growth of the fungal spores.
