Dealing with powdery mildew and other humid-summer plant diseases

Lauren Bethke

We’ve had a lot of hot, sticky weather lately, and it’s probably affecting your plants! When the air stays warm and humid for days at a time, you might start noticing a white, powdery coating on leaves, especially on squash, lilacs, phlox, and peonies. It’s probably powdery mildew, one of the most common late summer problems in Minnesota gardens. Read on for tips and advice around identifying and preventing powdery mildew, what to do if you find it in your garden, and other similar plant diseases that are common this time of year.

Powdery mildew is fairly easy to identify: it looks like a white or gray, talcum-powder-like coating on leaves, stems, and sometimes flower buds. It usually starts on the oldest, lowest leaves and works its way up the plant. Leaves may yellow, curl, or drop early. The good news is that powdery mildew rarely kills an established plant outright, but it puts stress on the plant and can reduce flowering and vegetable yields.

Other common fungal diseases that tend to pop up in late summer include Septoria Leaf Spot and Black Spot. Septoria Leaf Spot is especially common on tomatoes. It looks like small, dark spots with tan centers on lower leaves, which then yellow and drop. Black Spot is a fungal disease that often shows up on roses as round black or purple-black blotches, usually with yellowing around them.

The good news is that a few simple practices can help reduce all fungal diseases in your garden and improve overall plant health.

Improve air flow. This is the single most effective thing you can do to reduce the impact of fungal plant diseases because air movement helps leaves dry out and makes it harder for spores to develop. I like to aggressively prune the bottom leaves and small branches on tomato plants, and remove about one-third of the leaves from large zucchini and squash plants. Other tips include dividing crowded perennials, thinning out dense trees and bushes, and giving vegetables the spacing recommended on the seed packet.

Avoid wet leaves. Overhead watering, especially in the evening, keeps foliage wet overnight – perfect conditions for fungal diseases to thrive. Switch to a soaker hose or water right at the soil line, and do it early enough in the day that any splashed leaves dry before nightfall. Mulching around plants can also help avoid splashing soil (which may contain fungal spores) onto lower leaves.

Remove infected material. Even if you follow best practices perfectly, it’s hard to entirely avoid plant diseases in hot, humid weather. Once you see mildew or fungal spots, prune or pick off the worst leaves and get them out of the garden. For perennials, clean up infected leaves in the fall. This can help reduce next year's outbreak, since many of these fungi overwinter in leaf litter.

Choose resistant varieties. Certain types of plants, like tomatoes, squash, phlox, and peonies, are particularly susceptible to powdery mildew. However, many newer perennial varieties have been bred specifically for powdery mildew resistance. Disease-resistant tomato varieties in particular are widely available and usually clearly labeled.

Use fungicides as a last resort, and proactively. For high-value plants or a bad recurring problem, a fungicide can help. However, fungicides work best as prevention, applied before symptoms appear. There are many options, ranging from natural substances to plant oils to synthetic mixes. Personally, I have had success with copper fungicide. Always read and follow label directions carefully.

Overall, it's worth remembering that a mildew-dusted zucchini leaf or a few black-spotted rose leaves aren't a crisis. These diseases are cosmetic and stress-inducing more often than they're fatal, and good gardening practices can reduce the issues naturally. Think of midsummer disease pressure as useful information, helping you learn where your garden could use more airflow, less crowding, or a change in watering habits.
For more information, check out the University of Minnesota Extension Yard and Garden website. Extension resources are written by experts and contain the latest and most reliable research-based information. Happy gardening!

For more information, check out the University of Minnesota Extension Yard and Garden website. Extension resources are written by experts and contain the latest and most reliable research-based information. Happy gardening!