Q. My Maple tree has white stuff on its leaves, what is it?
Your beautiful maple seems to be suffering from powdery mildew.
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. You were very smart to give your tree a more sheltered spot in your new location but it is very possible that the air current in the new location has been too reduced and the spot has too much shade. Moisture is not drying from the leaves quickly enough to keep fungus from growing. The disease flourishes when some warm humidity is available.
You are certainly not alone to be experiencing fungal problems this growing season. The unusually wet spring and early summer created fungal issues for gardeners and commercial growers throughout the area. So while you may want to adjust the siting of your plant, some of the blame can be laid on the wet weather of 2017.
Not surprisingly, good maintenance practices and smart gardening are the keys to keeping your maple healthy in ensuing seasons. 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.
-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. I am sending you this link to a Missouri Botanical Garden fact sheet about Japanese maple care so you can give your tree the best possible care and position.
I hope that’s helpful.