Robert Pore's Ag Blog

MANHATTAN, Kan. – As it does most years, powdery mildew is spreading rapidly through the central Great Plains. Almost every garden, landscape and farm has plants the fungus-caused infection can affect.


Unlike other fungal diseases, powdery mildew doesn’t require that host plants be wet with rain or irrigation. To thrive, most powdery mildew fungi just need warm weather with high humidity, said Megan Kennelly, plant pathologist with Kansas State University Research and Extension.


“That’s fairly typical weather in late spring and early summer, particularly at night,” she said.


Lots of the fungi are always around, too, just waiting for those right conditions.
“Powdery mildew fungi are host-specific. Different, but closely-related species cause the disease on oaks, roses, lilacs, crabapples, cucumbers, bluegrass and the like,” Kennelly said. “Even so, all of the fungi grow as white or whitish-grey colonies on the lower and upper surface of leaves. Some powdery mildews also grow on flowers, young fruit and other succulent tissues.


“Then the fungi produce massive numbers of spores. The disease can develop so quickly because those spores disperse on the wind.”


The plant pathologist said the long-term ways to combat the fungi are cultural: If possible, plant resistant varieties. Don’t plant susceptible plants in shady areas. Promote good air circulation with proper plant spacing, pruning, weeding, and trellising.


When powdery mildew attacks, however, identifying it early is vital to effective control.


“Once the infection reaches high levels, chemicals don’t do much good,” Kennelly said.


Under favorable conditions, powdery mildew colonies can expand to cover entire leaf surfaces, sometimes causing curling, twisting or other distortion. Severely infected tissues dry, turn brown and fall.


“Usually, powdery mildew weakens plants by reducing photosynthesis, the ability to feed themselves. The infection typically isn’t fatal, but the resulting weakness can make plants vulnerable to other problems,” Kennelly said. “Food crops, in particular, can also have reductions in both quantity and quality of yields – a real risk for market gardeners.”


Powdery mildew already is active this year in northeast Kansas fields of cucurbit crops (cucumber, melon, pumpkin, squash, watermelon).


“I’m recommending cucurbit growers scout their fields about once a week, checking the undersides of older leaves – where the disease usually gets established,” the plant pathologist said. “If one in 50 leaves has symptoms, the disease is active. Controls may be warranted.”


Commercial growers have many control options, but they must rotate chemicals, alternating ones with different modes of action.


“Powdery mildew fungi are notorious for developing resistance to fungicides, particularly some of the systemic chemicals,” she said. “Fungicide resistance is the last thing you want to promote.”


For homeowners, some control options include products with myclobutanil, chlorothalonil, triforine ,or thiophanate-methyl as the active ingredient. Organic options include neem extract, potassium bicarbonate, horticultural oil and sulfur products.


“Powdery mildew can be a challenging disease. It’s important to catch it before the disease pressure builds up. And, it’s important that you read and understand the label before applying any kind of control,” Kennelly said.