Michael J. McGroarty
Perry, Ohio Copyright 2011
Japanese maples are actually pretty tough trees and quite disease resistant. Usually, if a Japanese maple is failing, or doesn’t look good, it’s not from disease but other issues that are pretty easy to correct. First let’s look at the things that you can control.
Japanese Maple Failure not Caused by Disease
As soon as somebody has a Japanese maple, or any plant for that matter, that doesn’t look good they immediately think it’s being attacked by some dreaded disease. In most cases that’s not the case at all. Contrary to what most people think, Japanese maples are pretty easy to raise and care for. For the most part they are usually care free plants that happily exist in just about any landscape. But there are a few things you should know about them.
1. They don’t like wet feet! In other words only plant them in good rich soil that drains well. If you have heavy clay soil that does not drain well you have to make some adjustments to how your tree is planted, but be careful not to make the wrong adjustments. Many people do it wrong and their tree dies.
In heavy clay soil you should only dig the hole half as deep as the root ball on your tree. Then set the tree in the hole and fill around and over the root ball with good, rich topsoil. Keep in mind that the root ball is made up of very loose, porous soil that water can easily drain into. Make sure you do not dig a hole that will become a bathtub that your plant can drown in. That’s why I suggest planting only half the root ball in the ground, then building a raised bed around the part that is sticking out of the ground. When planted high like this your tree will need watered about twice a week, but check the soil near the roots and make sure it’s not soggy before you add more water.
2. They don’t like wet hair! Japanese maples don’t like to have their leaves sprayed with water when the sun is out. The water droplets act as mini magnifying glasses and can leave burnt spots on the leaves. This isn’t a serious problem and not one that I worry a lot about. But given a choice water the roots and not the tops.
3. They don’t like to over eat! Japanese Maples Do Not Like a Lot of Fertilizer! In other words, it’s best not to fertilize them at all. Better to plant them in good rich soil that has a great deal of organic matter, such as composted cow manure worked into the soil before planting. After planting, if you really feel that you need to fertilize use something organic.
Insects and Diseases that can Attack Japanese Maples
Pseudomonas syringae is a common bacteria that affects many woody plants, including Japanese Maples. This bacteria is considered opportunistic because it usually attacks plants that have already been damaged by frost or by other means. Japanese maple leaves can be spotted and the veins within the leaves can be blackened. This pathogen can cause die back of small branches. As a collector of Japanese maples for many years I’ve seen little to no evidence of this on any of my plants. Or at least I haven’t noticed.
Verticillium Wilt is a disease that can attack Japanese maples. Symptoms are pretty obvious. In some cases the leaves on a single branch will discolor and die, but do not fall from the tree. This often happens in late summer or early fall. In many cases the branch dies completely and should be completely removed from the tree at first sign of the disease.
How do you know for sure the branch is dead? This is how you test to see if a plant, or a branch on a plant has died. Just scratch the bark of the plant with your finger nail. If the tissue below the bark is green and firm your plants are fine. If the tissue is brown and mushy that part of the plant is dead.
Verticillium Wilt is caused by a soil borne fungi but usually attacks plants that have been stressed by other things. Drought, frost, or wet soil. There’s really nothing you can do to treat your Japanese maples to prevent this disease and there’s nothing you can apply once they have it. It’s something that just happens.
Twice I’ve had fairly large branches on my Acer palmatum dissectum maples die back all the way to the trunk of the tree and I have to assume it was caused by Verticillium Wilt. But what’s really important to note is that even though these two trees lost large branches that had to be cut out, leaving a large hole in the tree, within a couple of years the trees filled back in and are once again nicely shaped and beautiful. And they’ve stayed healthy since.
Anthracnose is a fungal type disease that attacks a wide variety of tree and shrub species. Affected trees will often have spots or scorch like spots on the leaves. This fungal disease is prevalent during rainy seasons and conditions of high humidity. This disease remains active on the leaves and twigs that have fallen to the ground and eventually spores are released that can re-attach themselves to the tree or new leaves. The easiest method of control is to keep dead twigs and leaves raked up from under your trees. Rainy spring weather tends to perpetuate this disease where hot dry periods can halt the disease.
Powdery Mildew is a fungal disease than can attack Japanese maples. I’ve never seen it on any of my Japanese maples, yet I’ve had bad cases of it on some of my dogwood trees. Powdery mildew is easy to detect because it covers the upper side of the leaf with a white powdery film. It thrives in hot, or hot and humid conditions, especially where a lot of plants are grouped together and air circulation is poor.
Soft succulent tissue is more susceptible to this disease so avoid summer applications of nitrogen fertilizers. I’ve seen heavy infestation of powdery mildew one season and not the next, so it’s not something that I get all worked up about. Just rake up and dispose of any affected leaves. It’s best not to put the affected leaves in your compost bin.
Phytophthora Root Rot
Phytophthora is a condition caused by root systems that are too wet. Plants like Japanese maples and rhododendron are the most susceptible because they are the least likely to tolerate wet heavy soils that do not drain well. This is one of the biggest problems that I see with Japanese maples in the home landscape. Soil that does not drain well, or plants that are planted too deeply in the ground. Even in well drained soil one inch of the root ball should be raised above the existing grade of the bed. In poorly drained soil I suggest at least half of the root ball be raised above the existing grade, then covered with good rich topsoil.
Aphids can and have been known to feed on Japanese maples, but in most cases it’s not a big concern. Aphids feed by attaching themselves to the leaves of a plant and sucking nutrients out of the leaves. If there are a number of aphids or if they are there long enough, they can damage the leaves to the point that the leaves curl up and could drop from the plant. However, aphids have a number of natural predators including lady bugs so they usually don’t last long once they appear. You can treat for them with an insecticidal soap or rinse them off with a blast of water.
Japanese Maple Scale
Japanese maple scale seems to be a growing problem, mostly on the east coast. This type of scale insect is known as an armored scale because the insects protect themselves underneath an armored cover that is usually white in color. They are easy to spot on the stems of trees with dark bark. Scale insects are a sucking insect that extract plant sap from the host plant. In plants with heavy infestations premature leaf drop, branch die back, or death of the plant can occur.
Scale insects are somewhat predatory and attack unhealthy plants. The healthier your plants, the less likely they are to be attacked by scale insects. If the infestation is not heavy, you can try scrubbing the tree with soapy water and a scrub brush. On Japanese maples, scale insects usually only attach themselves to the stems of the tree and not the leaves, so scrubbing might actually work. Since these insects are under this protective armor they are difficult to control. A systemic insecticide that is applied to the root zone of the plant might work. Check with your local garden professional.
Japanese maples can be attacked by borers. These small insects drill into the stem of a tree and if the infestation is severe, serious damage can be done to the tree. Inspect the stem of your tree looking for tiny holes and saw dust. If you find borers you can treat the tree with a systemic insecticide, or there are some borer pastes on the market as well. Some of the old school gardeners heat up a wire and stick it into the hole while the wire is still quite hot. Does this work? I have no idea.