I have over 80 varieties of Japanese maples growing in my landscape and have been growing them since the mid 1980's. Over the years I have lost only one tree in my landscape due to disease, or any other cause for that matter. The tree I lost was one that was planted by the previous homeowner in an area of the landscape that had poor soil drainage. It was too large for me to dig and replant or relocate. Eventually, the tree developed root rot and died. Instead of replacing it with another Japanese maple or improving the drainage in the area I replaced it with a bald cypress tree, which loves the soggy soil. Anyway, one dead tree out of 80 over a 30 year period isn;t too bad!
In general, Japanese maples are very hardy and long-lived trees that have very few if any problems. However, like almost any other plant or tree, Japanese maples are susceptible to diseases...some which are inconsequential, others which can be deadly if not caught and treated early. That being said, most all diseases can be prevented with proper cultural care and sanitary practices. Selecting varieties that have demonstrated the best overall performance in your area is important too.
The various diseases that can infect threadleaf Japanese maples include anthracnose, leaf spot, powdery mildew, leaf blight, verticillium wilt and rot. Here are some brief descriptions of these diseases.
Root rot is probably the most common disease in Japanese maple trees and is caused by the fungal pathogens pythium, fusarium and verticillum in the soil. These pathogens develop in soil that is overly wet and saturated for a long period of time and can survive until the excess moisture is eliminated. The first signs are browning or blackening of the tips of the leaves. Root rot does not respond to fungicides. Healthy, vigorous trees are able to recover from root rot infections once they experience adequate drainage. For established trees, the best way to determine whether root rot is the true cause for symptoms of decline in your Japanese maple is to dig up soil to expose a portion of the tree's root system. Observe whether the roots appear waterlogged or rotten, indicating that the pathogen is disintegrating the tree's root system. Fruiting bodies or reddish-brown cankers may also be present in the soil around the root system or on the root system itself. If your tree has root rot take immediate steps to improve soil drainage or reduce irrigation.
Anthracnose, which first shows up as brown spots on the leaves that spread along the veins or as irregularly shaped splotches on the leaves in late spring or early summer, can be dangerous because it can cause the tree to lose all its leaves, which opens the door to more serious problems. During late spring and early summer the Japanese maple may exhibit some of the following symptoms: a thinner canopy, leaf spots with angular edges, areas of dead leaf tissue, a die-back of twigs and cankers on small branches. Untreated anthracnose can eventually cause cankers on the tree's branches and trunk. Cankers are dead areas that 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. When left untreated, anthracnose is fatal to Japanese maple trees. Certain varieties of Japanese maple planted in wet, warm climates are prone to this fungus. Constantly soggy soil due to poor drainage or overwatering can be a cause.
To prevent the spread of anthracnose, rake and destroy or compost infected fallen leaves. Maintain the vigorous health of your Japanese maple through proper fertilization, pruning, watering and pest control to increase tolerance to anthracnose. Always plant Japanese maples in well-drained soil. The soil can and should be moist, but it must be well-drained. You can also spray the tree with a fungicide such as Daconil, making sure to follow instructions on the label for mixing and applicatication. Make sure to spray during the early morning hours to avoid leaf scorch.
Powdery mildew is a fungal infection that appears as a grey-white, powder-like mildew that forms like a blanket on foliage. It is rarely a serious disease and more prevalent in moist climates or during periods of abnormally heavy rainfall during the warm season. This being said, I live in a very hot a humid area in mid Georgia and none of my Japanese maples have ever had powdery mildew.
Maintain the vigor of your tree to prevent powdery mildew infection and remove and destroy infected fallen leaves. Neem oil can be used to control powdery mildew. If you spray, make sure to do so in the early morning hours and always follow instructions on the label for proper mixing and application.
Various leaf spot diseases caused by fungal infection (like Phyllosticta and Septoria fungi) are found on Japanese maples, causing visible spots on the surface of leaves. Often triggered by a wet spring season, this fungal infection may spread on your Japanese maple tree, generally causing cosmetic injury. Illness is generally not serious but may cause premature defoliation.
Leaf scorch is a disease on Japanese maple trees that causes leaves to appear burned; darker areas of brown and black become visible on the surface of the leaves. Leaf scorch is caused by a bacterium-like pathogen that is spread by leaf hoppers. Leaf scorch lesions will be the first symptom to appear, followed by diminished health and a loss of leaves in which foliage remains only on large branches close to the interior crown of the Japanese maple tree. There is no effective treatment for bacterial leaf scorch, though antibiotic injections can prolong the life of the tree.
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 is 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. Treating the disease is almost always unsuccessful. The only method of prevention is to have your soil tested for the presence of the fungus.
Disease PREVENTION Tips for Japanese Maples
- Light: With the exception of a few varieties that have demonstrated tolerance to all-day direct sun, most prefer part shade. Morning sun with afternoon shade or filtered sun is best.
- Soil: Plant Japanese maples in moist, well-drained, fertile soil.
- Irrigation: Water Japanese maples only enough to keep the soil in the surrounding area moist, but not constantly soggy. Water in the early to mid morning. Late evening or night watering will promote development of fungal diseases. Water on foliage during the hot afternoon hours can scorch leaves.
- Sanitation: Remove fallen leaves and other debris from underneath and around the tree.
- Selection: To find out which Japanese maple varieties have demonstrated the best overall performance in your area, consult with your local professional nurseryman, landscaper, arborist or county extension agent.
Follow the guidelines and tips provided in this article and you should have very good success growing Japanese maples in your landscape!