Your Japanese 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. Multiple years of early dormancy due to powdery mildew, however, may weaken the tree and increase its vulnerability to other diseases and pests.

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.

Not surprisingly, good maintenance practices and smart gardening are the keys to keeping your maple healthy in ensuing seasons.

    -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. Improve air circulation for the plant by reducing congestion in its area. Eliminating or pruning back other leafy plants so that sun and air are able to dry leaves more quickly will reduce the risk to your maple.

    -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. 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, unfortunately,  the case for maples and makes careful placement and air circulation especially important

Powdery mildew is inhibited by extreme heat and extended periods of rain. While fungicides may be effective if they are used as soon as the first white patches are evident, they require reapplication repeatedly and are in most cases impractical. If you wish to try fungicides, you should contact your county's cooperative extension office for information about the most effective products licensed in your area.

For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service