Powdery mildew is one of the most commonly found fungal problems affecting plants. This fungus typically appears as a white or grayish powder on the leaves and stems of affected plants. It affects a wide variety of deciduous plants. Beebalm and phlox have always been common to infection, but in recent years we’re seeing more and more cases of powdery mildew on peonies.
Powdery mildew. It happens.
Powdery mildew on peonies is generally seen in late June through September when temperatures are hot and the humidity is high. In summers with a lot of rainfall, powdery mildew can run rampant. The mildew thrives in shaded areas and areas with poor air circulation. Even still, it can still be found in very open, sunny areas. Sometimes there’s just no way around it. Spores are spread by insects (such as aphids) and through the air. Even tiny vibrations from raindrops are to blame for spreading spores from leaf to leaf…and plant to plant. High nighttime humidity creates a perfect environment to grow spores, and low daytime humidity makes for the perfect environment to disperse them.
There are multiple genres of fungi that cause powdery mildew to grow on peonies and other plants. Each one generally only affects one or two different plants – They will spread peony to peony or rose to rose. But….(there’s always a but!) there are few species of fungi out there that will cause powdery mildew on many different types of plants.
U-G-L-Y …You ain’t got no alibi!
Powdery mildew usually begins on the lower leaves. You may notice dry or discolored leaves long before you begin to see a flour-like coating on the topside of the leaf. The leaves may curl, dry out, or fall off.
The good news is that powdery mildew on peonies and other plants is more of an eye sore than a health problem. Really severe cases of this chalky fungus can weaken the immune system in plants, but in most cases it does very little damage and does not require treatment.
Let nature do its thing.
Most professionals would agree that there is not much that needs to be done to combat powdery mildew. There are fungicides available, but they work as preventives. You need to spray before you find an infected leaf. Spraying after will help keep the mildew from spreading to other parts of the plant but will not kill what has already grown.
The easiest defense is to be sure that your peonies are properly spaced to increase air circulation. Its also helpful to know that fungi survives the winter by clinging to leaves and other parts of infected plants. Cleaning up the fallen leaves can help lessen the chance of future occurrences.
In the video below Mike explains how he handles cases of powdery mildew on peonies and other plants in his landscape.
But, wait! What about dogwood?
Here is a video that talks about powdery mildew on dogwood: