Monday, July 30, 2018

Summer Mystery: Powdery Mildews by Olivia Tracy

Photo courtesy of M. Grabowski, UMN Extension
If you’ve gone out to your peonies and found that they look like someone dusted them with talcum powder, you likely have a case of powdery mildew. Varieties of powdery mildew can affect almost every type of plant (although particular infections are host-specific), and the leafy portions of the plant are typically most affected. The original whitish-gray, powder-like growth will eventually turn brown, and then black, and can ultimately cause leaves or buds to drop off the plant.

Powdery mildew is caused when humidity in the air is high; spores germinate in the humidity and grow on dry leaves. Some cultivars of plants, including roses and peonies, are resistant to powdery mildew; when shopping for new plants, consider buying these mildew-resistant cultivars.

Controlling and Treating Powdery Mildew

Many think that powdery mildew grows because leaves get wet; in fact, the mildew can't take hold on wet leaves. However, reducing general overhead watering can also reduce the relative humidity in the air. It's probably best to water your plants from the roots to prevent humid conditions from forming around your plants. 

Once you have signs of powdery mildew, remove and destroy all infected leaves, buds, etc. Don’t place these in the compost. Powdery mildew can also thrive in overcrowded plant environments. If your plants are overcrowded, consider pruning them back to increase air circulation around the leaves. 

If these strategies don’t control the problem, you can also use chemical-based fungicides, including sulfur, neem oil, triforine, and potassium bicarbonate. Alternatively, you can mix baking soda with a small amount of horticultural oil. For detailed information about these chemical controls, see the linked CSU Extension page below. 

This post was adapted from the CSU Extension Factsheet Powdery Mildews: 2.902.