With the warm weather and recent rain, many homeowners are seeing full, healthy gardens. But the persistently high humidity and lingering rain showers have also made conditions perfect for the onset of many fungal diseases.

The most prevalent of these is powdery mildew. Powdery mildew is caused by several genera of fungi; each species within those genera attack different groups of garden plants. But each fungus causes the same disease, and each requires the same treatment. However, the different sources of disease make it likely that while one plant is infected, others nearby are not. This doesn’t mean gardeners should not be watchful for other symptomatic plants, however: if powdery mildew appears on one plant, conditions favor the development of the disease on all plants.

After stretches of wet, muggy weather, gardeners should be on the watch for powdery mildew symptoms, especially on phlox, lilac, roses, zinnia, birch, oak, cucumbers, and squash, plants highly susceptible to the disease. Plants infected with powdery mildew appear to be dusted with fuzzy white, light-gray, or slightly brown fungus. This “powdered sugar” mildew most often is visible on the leaves. In advanced stages, buds fail to open and diseased leaves turn brown and dry.

Powdery mildew spores overwinter in infected plant parts, plant debris, and dormant buds. Each spring they blow to new sites and re-infect their host plants in both the same and new locations. Careful control can prevent severe infections, especially with timely diagnosis. Gardeners should not grow mildew-prone plants like phlox or roses in shady areas where circulation is poor, and should be sure to prune and thin plant material to promote healthy air flow.

In greenhouses, where powdery mildew is perhaps the most common disease, gardeners can ventilate well in the late afternoon to drive out moisture before nightfall, when humidity is high, and infected plants in any area should be treated by removing infected debris. This can be composted, but requires a high temperature to be thoroughly eliminated. Homeowners can dispose of infested debris in normal garbage.

Infected plants can be treated with fungicides; they are usually more effective if applied in the earlier stages of the infection. Fungicides for ornamentals should be used for plants on their label; triforine (Funginex, Triflorine), sulfur (Sulfur, wettable sulfur), copper sulfate, myclobutanil (Systhane), thiophanate-methyl (Cleary 3336, Domain, Fungo, and Benefit), and dinocap (Karathane) fungicides are among the most effective. Baking soda (11/2 tablespoons) and horticultural oil (3 tablespoons) in water (1 gallon) is also effective for powdery mildew control, and most of these treatments will require repetition every 7-14 days until conditions change.

Gardeners with more questions on powdery mildew should consult the UW-Extension fact sheets on the disease (hort.uwex.edu/articles/powdery-mildew-trees-shrubs or learningstore.uwex.edu/assets/pdfs/A2404.pdf), email Marissa Hatlen at [email protected], or call the Extension office with questions at (715) 365-2750.