Master Gardener Column by Jan Phipps

Planting and blooming season has started, and so has the potential for powdery mildew.  Many gardeners think powdery mildew is caused by one fungus that can spread throughout the yard. Relax, there are many powdery mildew-causing fungi, and most are specific to a particular host plant. So, just because your peonies get powdery mildew doesn’t mean your other plants will “catch it”. The other good news is that, while very unsightly, plants are rarely killed by powdery mildew.

Most fungal and bacterial diseases are caused by warm weather, high humidity, and water standing on the leaves. However, powdery mildew favors cool nights in spring and fall, and wet leaves aren’t required. Because they like cool nights, they might be active during winter and early spring.

Mint is particularly susceptible since it thrives under the same conditions as the mildew. Also, mint-related weeds like henbit and dead nettle. If you grow mint, try to eradicate the other two from your yard.

Powdery mildew is treatable. However, infections occurring late in the growing season may not be harmful enough to treat beyond good sanitation. Culturally, plant for good air movement and light penetration around the susceptible plants. Also, don’t overdo the fertilizer. Fast-growing, new growth is particularly susceptible.

Treatment options exist. They are called fungicides and should be labeled for powdery mildew. Systemic fungicides are more effective than the sprays or neem oil. Avoid home recipes or any product containing baking soda (sodium bicarbonate), which can damage plants if the toxic ions build up. Instead, look for potassium bicarbonate fungicides.

Of course, don’t allow any infected plant material to overwinter on site. Once plants begin to grow, watch them closely for the first sign of infection and treat immediately.

Call the Extension office at 217-465-8585 or email sarahep@illinois.edu with your gardening questions.