Your beautiful maple may be suffering from powdery mildew. Powdery mildew is usually more unsightly than dangerous to the plant, unless it takes hold to the extent of destroying all its leaves or new growth far too early in the season. It is more typical for the disease to appear late in the growing season when leaves are expected to drop soon and have largely finished their function of providing sustenance for the tree. In that case, no treatment is necessary but precautions can reduce the risk of recurrence next year.

Powdery mildew is a fungal disease that leaves a superficial, white, dusty coating on leaves 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 middle to late summer, reducing the strength and damaging the appearance of infected plants, but rarely killing them. Phyllactinia guttata is the fungus that afflicts maples, and it can pass the infection to other trees including birch, horse chestnut, hornbeam and dogwood.

The fungal spores themselves are unusually rich in moisture so, unlike many fungal problems that need moist conditions to take hold, powdery mildew thrives with warmth (temperatures around 70 to 80 F), some humidity and shade.  Under those conditions, moisture does not dry from the leaves quickly enough to keep fungus from growing. The disease flourishes when some warm humidity is available.

Fungicides and other treatments will not rid plants of powdery mildew once it has appeared. Because the disease is rarely fatal, often it is simply tolerated and preparation is made to reduce the likelihood of reinfection in later years. You can remove infected parts of the tree to reduce spreading; do not compost the infected debris. Treatments like fungicides, horticultural oil and neem oil can be used to protect a tree before infection but imperfectly, even with frequent reapplication. Management involves preventing infection from occurring through cultural techniques.

Not surprisingly, good maintenance practices and smart gardening are the keys to keeping your maple healthy in ensuing seasons. Multiple years of early dormancy due to powdery mildew will weaken the tree and increase its vulnerability to other diseases and pests.

    -Do not overcrowd your plants or keep them in too protected a location; this will lead to poor air circulation and will create problems with the humidity level around susceptible plants.

    -During the winter the fungus survives on plant debris, so it is important to clean up around your maple, including in the tree's container.

Powdery mildew is inhibited by extreme heat and extended periods of rain. Some powdery mildews can assume a form that allows them to live on the bark or buds of their victim through the winter and that is the case for maples, so good care and location is essential.

Courtesy of NYBG Plant Information Service