Powdery mildew is a common problem of maple trees, and trees in general, and is caused by a range of different fungi. The underlying fungus is different for different species, but all these fungi cause the growth of a white mold on the tree's leaves and stems. Though this mildew can be unsightly, the damage it causes a maple tree is minimal. The mold looks like a dusting of fine, white powder, hence its name.
Life Cycle
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The white patches visible on maple tree leaves are the means by which the fungus produces its spores. The spores are then spread to other parts of the maple, or to other plants, by the wind. The spores are highly specific, so the fungus that causes powdery mildew on a maple is only able to colonize other maples or similar plant species. The fungus overwinters on infected leaves or fallen leaves, and releases its spores in the spring.
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Symptoms and Damage
The white mold patches caused by powdery mildew fungi are the most obvious sign of this fungal infection. These patches can grow to cover entire leaves or patches of leaves. The leaves may then turn yellow, drop early, or become twisted. The white patches may develop black spots as the fungus prepares to survive the winter.
Environment
Powdery mildews don't require the same wet conditions that many fungi do. While high relative humidity promotes the fungus' spore germination, the fungus does not need wet leaves to infect a maple. Air circulation through a maple's branches can keep the environment from becoming damp enough for the fungus to germinate its spores. Pruning branches to increase air flow is a good idea, but improper pruning can harm the health of the tree, and stressed, unhealthy maples are more susceptible to fungi and other problems.
Management
Because the fungus causing powdery mildew overwinters in leaf debris, one way to manage it is to gather up and destroy all fallen leaves in the autumn. Fungicides can be applied during the growing season as soon as you notice white patches on your maple. You should also avoid applying nitrogen fertilizers after you've spotted the fungi. Nitrogen increases the production of foliage and provides more material for the powdery mildew to inhabit. Avoid overhead watering to help reduce dampness and humidity.