The Japanese maple tree (Acer palmatum) is a slow-growing species that reaches a maximum height of only about 20 feet, though many cultivars are smaller. Japanese maples create wonderful focal points in gardens or in groves, mixed in with other types of trees. They are generally hardy, but are susceptible to diseases that range from inconsequential to deadly.
Two diseases that affect the appearance of Japanese maple trees without endangering their lives are tar spot and sooty mold. Tar spot appears on the leaves, usually in mid-June, as yellow spots that eventually develop dark centers. The Plant Disease Diagnostic Clinic at Cornell reports that tar spot is a fungal infestation that rarely causes serious damage, but extreme infestations can cause massive leaf drop.
Sooty mold is a secondary infestation that attacks trees weakened by sucking insects like aphids. The mold is dark grey and velvety, and usually appears on the leaves. The Rhode Island University Landscape Horticulture Program (RIULHP) notes that this disease almost never kills the trees outright, but it is extremely unsightly and indicative of a potentially more serious problem.
Treating Cosmetic Diseases
To control tar spot, you need to rake up the affected leaves in the fall and destroy them. Some fungicides can also help, but because you have to apply them carefully and evenly, Cornell doesn't recommend them as a first line of defense.
RIULHP recommends treating sooty mold in two steps: Control the insect infestation by applying an insecticidal soap or a stronger chemical remedy. Then blast the leaves of the maple tree with jets of water, to wash away the mold.
Several different funguses can cause anthracnose, which shows up as brown spots on the leaves that spread along the veins or as irregularly shaped splotches on the leaves. The Alabama Cooperative Extension Service (ACES) says anthracnose is especially dangerous to Japanese maples because it can cause the tree to lose all its leaves, which opens the door for other more serious infestations. Trees planted in wet, warm climates are particularly prone to this fungus.
Untreated anthracnose can eventually cause cankers on the tree's branches and trunk. Cankers are dead areas can grow until they encircle the trunk or branch and kill it. On Japanese maples, cankers look tan or grey compared to the usual red color of healthy branches.
Treating Serious Diseases
Control anthracnose by raking up and destroying all the blighted leaves, preferably before the tree starts to put out new leaves in the spring. Fertilize and water the tree generously to help it recover.
Apply a fungicide to kill off the underlying culprit and protect your tree from further defoliation. Apply the fungicide just before the tree leafs out, and apply it once every 10 to 14 days until the leaves are fully mature. Fungicides that ACES recommends retail under the trade names of Daconil 2787 4F, Daconil Ultrex, Dithane T/O, Fore, Protect T/O, 3336 50W, 3336 4.5F and Halt. Follow the package directions carefully when you apply the fungicide to your tree.
To control canker on twigs or branches, prune out any limbs that have the lesions and discard the trimmings.
Potentially Fatal Diseases
Another deadly disease that attacks maples is verticillium wilt. Also called maple wilt, this disease originates with Verticillium dahliae, a fungal infestation in the soil that the Oregon State University Extension (OSUE) describes as "almost impossible to eradicate." Once the tree absorbs the fungus through its roots, the fungus blocks the flow of water into the tree while producing toxins that make the leaves of the tree suddenly wilt and drop. Some trees die almost immediately, while some manage to live for several years.
Coping with Maple Wilt
Treating the disease is rarely successful, so the best way to avoid it is to have your garden soil tested for the presence of Verticillium dahliae before you plant a maple. You can buy a soil testing kit at a local nursery or get it tested through your state agriculture department or county extension service. Microsclerotia in the soil should be interpreted as a potential disease risk.
To prevent the fungus from spreading, remove and burn any affected limbs. OSUE reports that gardeners have sometimes managed to eliminate the fungus by mixing Italian ryegrass clippings into the soil and covering it with plastic sheeting for three months late in the summer.