Powdery mildew is a fungal disease that leaves a superficial, white, dusty coating on leaves, stems and sometimes the flowers and fruit of a wide variety of plants. The cause is a group of related fungi that each attack a limited number of closely related plants. Powdery mildew tends to create a problem for gardeners in humid conditions of  middle to late summer, reducing the strength and damaging the appearance of infected plants, but rarely killing them.

Powdery mildew can be differentiated from downy mildew, another fungal disease that presents a fuzzy, white growth, by the position of the white coating and the growing environment. Downy mildew grows primarily on the undersides of leaves and in more classic wet, cool, fungal disease conditions.

Fungicides and other treatments will not rid plants of powdery mildew once it has appeared. Management involves preventing infection from occurring. Not surprisingly, good maintenance practices and smart gardening is the key to healthy plants.

• Do not overcrowd your plants; this will lead to poor air circulation and will create problems with the humidity level around susceptible plants. Keep overhead canopies from reducing air circulation.
• During the winter the fungus survives on plant debris, so it is important to clean up around infected plants.
• Using high nitrogen fertilizers (the first number of three on fertilizer labels) promotes weak, leafy growth; try a balanced fertilizer with low numbers and apply only once in the spring to give perennials a boost for the season.
• Choose disease-resistant cultivars whenever possible.
• Follow the maxim “the right plant for the right place.” A healthy plant is always more resistant to disease problems.

Powdery mildew is often more unsightly than dangerous to the plant, unless it takes hold to the extent of destroying all its leaves or new growth. If your favorite perennials do get attacked, you have several options. You can tolerate some unsightly appearance or just cut back once the leaves in the late season. You can remove the worst stems if you like and apply an environmentally friendly product to the unaffected parts of the plant as soon as you notice the powdery mildew, but these work imperfectly on this disease. Some options include products that contain neem oil, sulfur or potassium bicarbonate.

Powdery mildew is inhibited by extreme heat and extended periods of rain. It will not overwinter in the garden without a plant host, so good garden hygiene is particularly important if you want the best chance of avoiding a repeat of the fungal infection in succeeding seasons. Unfortunately, some powdery mildews can assume a form that allows them to live on the bark or buds of their victim through the winter (this is the case for roses).

For tips on a variety of gardening topics, see our Plant Information Guides.

- Courtesy of NYBG Plant Information Service