The grayish-white coating that is commonly found in mid-summer on foliage of peonies, lilacs, zinnias, roses, vine crops, and many other plants is a disease called Powdery Mildew.
Powdery Mildew is caused by many different species of fungi. The disease develops most severely during periods of high humidity, and is a common mid-to-late-summer disease. 
Although the disease does little long-term damage to some plants like peonies and lilacs, it’s still unsightly. Powdery mildew is more serious on plants like squash, cucumbers, melons and pumpkins because the disease can deteriorate the foliage, sometimes severely, which reduces the plant vigor and reduces fruit production and yield.
Practicing good sanitation is always a good idea with foliage diseases. Remove and discard affected vines during fall cleanup. Cut back peonies and other mildewed perennials and discard their tops in fall after frost.
But powdery mildew fungi are ever-present, and are easily carried by wind, so sanitation alone isn’t enough to prevent powdery mildew.
Once powdery mildew affects a plant, there is no reversal for the current season. Powdery mildew must be prevented, before it becomes entrenched on the plant. If a plant is mildewed, strive for prevention next year.
The three best ways to prevent powdery mildew:
- When possible, select varieties that have been bred for mildew resistance. Tags and plant descriptions usually indicate this, if the variety is resistant.
- Increase air flow around plants. Powdery mildew is worse in high humidity. Pruning or spacing plants to maximize air flow helps reduce the local humidity, which reduces incidence. Avoid wetting foliage when watering, as wet foliage increases plant humidity.
- Perhaps the best remedy – apply fungicides preventatively before the disease starts, or at the earliest symptoms. Apply a fungicide product that lists powdery mildew under ‘diseases controlled’. A common active ingredient is chlorothalonil.
