Japanese maple trees are a lovely addition to any yard or garden. Occasionally, homeowners may notice a white lacy fungus developing on the leaves of the tree. This white fungal coating is a fungal disease called powdery mildew. Japanese maples with this disease do not usually suffer serious injury, but it can become unsightly as the disease continues to cover the tree.
Powdery mildew is caused by host-specific fungi, which affect many different plant species. According to the Missouri Botanical Garden, there are over 11,000 species of powdery mildew fungi. Although the fungi that cause powdery mildew are specific to the host, the symptoms are almost identical in all plant species. Powdery mildew is most common during the spring and fall months. Warm days followed by cool nighttime temperatures are favourable for powdery mildew development.
Japanese maple trees with powdery mildew develop a delicate white fungal coating on the leaves, stems and branches of the tree. Powdery mildew fungi prefer young, succulent shoots over older leaves. Black peppery fungal spores are often seen mixed in with the white fungal coating on the host tree. Japanese maple trees with severe powdery mildew often suffer from leaf curling, yellowing of leaves and stunted growth. Infected leaves often drop from the maple prematurely.
Prevention of powdery mildew is accomplished with cultural controls. Purchase powdery-mildew-resistant plants, if possible. While powdery mildew can infect any plant, there are some varieties that are more resistant than others to the disease. Keep your Japanese maple tree in good health with proper watering early in the day. Avoid planting Japanese maples in areas of heavy shade, as this promotes high humidity and powdery mildew development. Avoid overhead watering and do not wet the leaves of your tree during watering.
If your Japanese maple tree shows symptoms of powdery mildew, there are several ways to attack the problem. First, prune out any diseased tree limbs and destroy them. Powdery mildew fungi overwinters in fallen leaf debris, leaving it on the ground promotes disease development. Fungicides are often effective in preventing powdery mildew and treating it if symptoms develop. Using a sulphur spray or copper-containing fungicide, spray both sides of your Japanese maple leaves and flowers. Repeat fungicide applications if necessary for continued control.