by Diana Alfuth, horticulture educator for Pierce and St. Croix counties UW-Extension
Around midsummer, we start to see a whitish coating on leaves of many plants, caused by powdery mildew, a fungal disease. In the vegetable gardens, we see it on vine crops, including squash, pumpkins (Cucurbita) and cucumbers (Cucumis sativus). Landscape plants affected include bee balm (Monarda spp.), perennial Phlox and lilacs (Syringa spp.). [Editor’s note: this year I also had in on my peonies.] Although the exact fungal disease organism that affects each plant is distinct, the fungi are closely related and appear in response to similar environmental conditions.
Most fungi like rainy, wet conditions, but powdery mildew prefers dry, humid conditions, exactly what we see in mid-summer! Luckily, on most landscape plants, powdery mildew is mostly a cosmetic issue. On vining vegetable plants, however, it can result in significant leaf loss and possibly plant death.
Vine crops should be treated as soon as symptoms appear to reduce spread. If you have a landscape plant that gets powdery mildew every year, you should preempt it in future years by using a fungicide before symptoms appear. Many commercial fungicides are labeled for use against powdery mildew. Caution is required when using fungicides because of the damage they can do to bees. Always read and follow label directions of the product you use.
You can also make a solution using baking soda. Spray the plants every seven to 14 days, beginning when they start leafing out. As always, its a good idea to pretest a small area to be sure your solution does not damage the plant.
- 1/2 tablespoon baking soda
- 3 tablespoons lightweight horticultural oil
- 1 gallon water
To reduce problems with powdery mildew, choose resistant varieties of plants and space them far enough apart to encourage air movement, which results in lower humidity. Also, since spores can overwinter on plant debris, be sure to remove the destroy any material that falls to the ground at the end of the season.