Powdery Mildew: a Perennial Problem that I Don’t Usually Mind

I have to admit I get somewhat excited when I see the first fuzzy powdery mildew spots of the season appear. It’s almost like playing the plant pathology lotto, betting when the environmental conditions (warm, dry days followed by cool, humid nights) are just right for the fungi to cause disease. This year, I saw the first spots on roses in mid-June. I had just received a photo from a client with a strange white growth on her rosemary transplant, and I initially thought it was too early for a powdery mildew diagnosis. But alas, I was wrong.

Powdery mildew on rose. Photo by A. Beissinger.

Powdery mildew is a disease caused by several different species and genera of fungi. Though you may see powdery mildew on herbaceous perennials, vegetables, and woody ornamentals, each species of powdery mildew fungi is usually host specific. The powdery mildew on your cucumber plant is not causing powdery mildew on your maple tree. Instead, you hit the powdery mildew jackpot and happen to have more than one species in your yard. In the lab, we identify the fungus to genus based on characteristics of their chasmothecia, or overwintering structures.

Black chasmothecia, overwintering structures on English oak. Photo by A. Beissinger

 

Powdery mildews are identified based on the morphology of chasmothecia. Pictured here is Microsphaera sp. Photo by A. Beissinger.

One of the most common questions we get in the Home & Garden Education Center is about chemical treatments for powdery mildew. Due to the biology of powdery mildew fungi, we don’t usually recommend spraying anything for woody and herbaceous perennials and here is why: powdery mildew only causes aesthetic damage and will not jeopardize the health of your plants. The fungi are obligate parasites, meaning they require a living host plant to grow, obtain nutrients, and thrive. As such these fungi have a biological incentive to keep their host plant alive; if they kill their host plant, they would not survive. It’d be more useful for you to save money and not spray a product  into the environment that will have very little success at controlling the disease.

The answer about chemical controls is a bit different for fruit and vegetable crops such as apple, grape, and cucurbits. While powdery mildew doesn’t necessary kill the host plants, the disease can present challenges for fruit quality, consistency, yield, and taste. Fruit can be deformed, have blemishes, or other markings that render them unmarketable, and produce far less than normal. In these cases, we may recommend a sulfur, neem oil, triforine, or potassium bicarbonate product. Always read the pesticide label before applying any product, and please note that chemical controls are usually only effective when appropriate cultural controls are taken as well.

Apple powdery mildew. Photo by J. Pscheidt

So, what are these cultural controls?

• Start off with resistant cultivars. Selecting plant varieties that have resistance to powdery mildew is one of the most important strategies to help prevent infection. There are many options to choose from, and require you to plan ahead before you begin planting. Garden centers and seed catalogues can be very helpful.
• Space plants adequately. Dense plantings can increase humidity, which can in turn increase disease development. Remove plants to improve airflow.
• Avoid overhead watering. Using a soaker hose, drip irrigation, or watering plants only at the base can help decrease humidity in the planting.
• Thoroughly clean up all infected plant parts at the end of the season. Many herbaceous perennials are left by gardeners to maintain fall habitat for pollinators. However, removing all infected plant parts at the end of the season will decrease the inoculum able to overwinter and infect plants the following year. Do not compost infected plants as at-home compost systems do not reach temperatures high enough to kill the fungus.

One other note about diseases in the garden: powdery mildew mycellium (a mat of fungal growth; the “fuzzy” growth you see) typically grow on the upper leaf surfaces of plants, and unlike other fungi, will not grow when a film of water is present on the leaves. Occasionally mycelium will grow on the lower leaf surface, but that is less common. If you’re seeing powdery white-grey spots only on the lower leaf surface, more than likely you’re seeing downy mildew, which is a far more serious disease. These diseases are often confused for each other because of their name and appearance. Downy mildew is caused by an oomycete rather than a fungus, and spreads when water is present. Early action is required to save your plants.

Downy mildew on grape. Note the spores are present only on the lower leaf surface. Photo by A. Beissinger.

For more information on powdery and downy mildew, visit our website for fact sheets.

-Abby Beissinger