Do your leaves look like baby powder has spilled on them? Or that they might be a tad bit dusty? Then, my friend, you've got powdery mildew. Sounds gross, right?!?
Well, that's because it is! It's a fungus. Think "athlete's foot" but for plants. Totally rank.
Here's a more mature account from GardenGuides.com:
Powdery mildew is a common disease on many types of plants. In fact, it is one of the oldest plant diseases on record - Theophrastis wrote of powdery mildew on roses in 300 B.C. Although different species of fungi cause the disease on different plants (Erysiphe infects vegetable crops and flowers; Podosphaera species infects apples and stone fruits; Sphaerotheca species infects berries, roses, some vegetable crops, and stone fruits; and Uncinula necator infects grapes), the infections are all characterized by a powdery white to gray fungal growth on leaves, stems and heads.
Contrary to popular belief, powdery mildew generally does not require free water to establish and grow. Infection can actually occur on dry leaves. Warm temperatures and shady conditions encourage the fungus to grow and spread. However, the spores and mycelium are sensitive to extreme heat and direct sunlight.My beloved bergamot/bee balm/wild monarda have been plagued with the powdery mildew. It's not a complete surprise b/c they are susceptible, but I had tried to keep them in an area with good air circulation. Clearly, that was a garden FAIL.
But, powdery mildew is very treatable. Here are your options:
- Commercially available fungicide. Read the label & it should list powdery mildew as something it treats
- Home remedy of baking soda: Mix 1 tsp baking soda with 1 qt water & a few drops of liquid soap. Spray undersides & tops of leaves. The baking soda increases the pH of the leave surface & makes it an unsuitable growing environment for the fungus.
- Home remedy of milk: Mix 1-part milk to 9-parts water (DO NOT go above a 1-to-3 concentration. You'll then get a fungus of a different kind) & spray tops & bottoms of leaves. This works like option #2. Skim milk is preferred b/c it doesn't have the fat that could go rancid or attract critters.
- Remove fallen, infected leaves.
- Plant resistant varieties (too bad for me...I don't know of any resistant monarda varieties.)
- Plant in well-draining, sunny area.
- Don't crowd plants. Allow for good air circulation (this I am guilty of...the dahlias grew bigger than I had expected...)
- Disinfect tools used on infected plant to prevent spread to other plants.
- Water in the morning.
- Don't live in a humid climate! :)