Maples (Acer spp.) are shade trees widely grown for their for brilliantly coloured fall foliage. A disease common to maple trees, however, often diminishes the impact of maples' autumn display. It conceals those bright leaves in a coating of white mould. This infection, called powdery mildew, results from fungal infestation. Norway maples are especially susceptible to powdery mildew. While the white mould certainly detracts from the maples' appearance, it seldom does more than cosmetic damage.
More than 1,100 different fungus species produce powdery mildew in thousands of plants. The two most common species attacking maple trees are Phyllactinia corylea and Uncinula circinata.
Powdery mildew's white mould attacks a maple's leaves and tender, new growth. Early spring infestations may also produce curled, deformed new leaves. Young leaves develop blisters with mouldy surfaces. Severely infected new growth may die. The maple's older foliage also develops white mould without other symptoms. As the disease progresses, the mouldy leaf tissue may become tan or grey with black specks. They develop a feltlike texture. Infected leaves are more likely to die and drop prematurely.
Dormant strands, or mycelia, of the powdery mildew fungi spend their winters on a maple's branch and twig bark or inside the tree's dormant leaf buds. When the buds break in spring, the mycelia release spores. Wind or rainwater carries the spores to the maple's healthy foliage, where they infect both leaf surfaces. The fungus is most likely to spread during spring and fall's warm days and cool nights. It flourishes in damp shade and temperatures between 15.6 and 26.7 degrees Celsius.
Saturating the maple with wettable sulphur sprayed when temperatures are less than 90 is an effective powdery mildew preventive measure. Add surfactants to the wettable sulphur to ensure that it adheres to the trees. Spraying the tree with Bacillus subtilis-based fungicide is a biological control method, but not as effective against existing white mould as the sulphur. A third alternative is to spray the maple with horticultural oil. Like wettable sulphur, this oil is safe below 90 degrees. Continue the applications on a seven- to 10-day basis throughout the spring and fall, when the fungus is most likely to spread.
Reduce your maple's exposure to white mould by planting it where it will receive at least six hours of daily sunlight. Separate it from other maples to ensure adequate air circulation. Water the soil around the tree weekly during dry spells to a depth of between 8 inches and 1 foot. Watering the soil directly keeps the leaves dry. Pruning the infected twigs and branches, raking up the dead leaves and disposing of both -- by burning, if possible -- limits your tree's chances of reinfection.