Powdery mildew looks like powdered sugar on the leaves of many plants. Your perennials bloom, your shrubs and trees have beautiful leaves, then in late August, early September, you notice a thin white powder on some plants. This powder often spreads from the lower to the upper leaves, becoming denser and thicker. Eventually, the affected leaf begins to blacken and curl up.

Powdery mildew is a fungal disease caused by airborne spores. This fungus can include several genera such as Oidium, Erysiphe, Microsphaera, Phyllactinia, Podosphaera, Sphaerotheca and Uncinula. This disease develops well under conditions of moderate heat and humidity, especially when nights are cool and humid at the end of the season.
Powdery mildew has a predilection for stressed plants. You’ll notice that powdery mildew is less common when plants are well-watered and healthy, but it can still develop during rainy summers. Some plants, like garden phlox (Phlox paniculata), can develop powdery mildew regardless of stress. This summer, marked by prolonged drought, has seen (and will continue to see) a lot of white. The recent heavy rains can do no more: the disease had already taken root at the end of July and beginning of August, and now it’s in full swing.
Powdery mildew and downy mildew are two distinct plant diseases that differ in their causative agents, symptoms, and environmental preferences. Powdery mildew is caused by fungi and appears as white or gray powdery spots on the tops of leaves, thriving in warm, dry conditions. In contrast, downy mildew is caused by water molds (oomycetes) and manifests as yellow or brown blotches on the tops of leaves, with a downy growth underneath, favoring cool, moist environments.

A Disease That Disfigures but Doesn’t Kill
Powdery mildew is mainly a cosmetic disease on perennials and woody plants, which generally survive and return each spring. However, powdery mildew can weaken plants and reduce their growth. It can also kill annuals and vegetables, albeit at the end of the season, after the plant has set seed.
Too Late to Act?
When you see the leaves turn white, the disease is already well advanced. This whitening is the last stage of the disease, as the conidia, the spore-carrying bodies, have already emerged. Preventive treatment is preferable, but treatments can still limit the spread and reduce the impact of the disease even after symptoms have appeared.
To Prevent Mildew
Eliminating powdery mildew once its symptoms are visible is difficult, but it can be prevented. Applying fungicides from the beginning of summer until the end, whether organic or chemical, can help. Home treatments, such as baking soda or milk sprays, are also used. Keeping the soil always moist with good mulch and appropriate watering can go a long way towards preventing the disease. Using antiperspirants can also help by forming a natural wax layer on the leaves, reducing evaporation and making it difficult for spores to adhere.

Growing Solutions
Improving air circulation around plants by thinning plantings can help, although this may reduce the number of flowers. Planting susceptible plants in windy locations can also be beneficial. Some plants, such as beebalm (Monarda didyma), do best in ever-moist environments, such as at the edge of a water garden, to avoid powdery mildew.

The Laidback Gardener’s Methods
Since mildew isn’t lethal and doesn’t prevent flowering or fruiting, laidback gardeners needn’t worry too much. Placing mildew-prone plants further back in the flowerbed hides their whitening foliage while leaving their flowers visible. Planting varieties resistant to powdery mildew is another effective solution. Why grow plants prone to powdery mildew when resistant cultivars are available? In September, choose plants whose foliage is still green.
So go to the nursery and store for phlox, beebalm, lilac and others that are free of white. When you get home, pull out the diseased plants and replace them with healthy ones. Life as a gardener is so much easier when you get rid of problem plants!

Larry Hodgson published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This text was originally published in Le Soleil on September 11, 2005.
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