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There aren’t too many acer plant problems. The acer tree (known also as the Japanese maple) is a relatively easy plant to look after. However, it can be susceptible to a few harmful conditions. The environment, insects, fungus, disease and even you can cause these acer plant issues, sometimes causing the Japanese maple to die.
Environment problems such as too much sun result in scorched leaves on the acer tree. This is where the leaves turn brown at the sides and the tips, then curl up and become dry. I have an article that explains what causes scorched leaves and what to do about them.
Problems may also occur in the wintertime if it’s a very cold winter. Protect your plant to ensure its survival to the next growing season. I have advice on how to do that in How to protect an acer tree over winter?
If your leaves in autumn aren’t very colourful, it could be because your acer plant isn’t getting enough sun. Darker colour leaves (red, purple) need quite a lot of sun to develop and maintain their colour.
Acer trees may sometimes be visited by harmful insects. The vine weevil feeds on many ornamental plants and trees, especially those growing in containers. Look for notches on the edges of the leaves.
In addition, you may see the young grubs as shown in the picture above eating away at the roots closest to the surface. The adults are active in the spring to late summer, while the grubs are busy in the summer through to the spring. The grubs have opaque white bodies with brown heads.
Control vine weevils by picking them off in the evenings. You need to use a torch to see them in the dusk. Gently shake the tree over newspaper to dislodge the insects. Alternatively, use sticky barriers around the tree to trap the weevils before they get to it. Or encourage the wildlife that eats these nuisances – birds and hedgehogs for example.
Biological control containing nematodes is also available to use. Failing all these methods, pesticides are the last option, But take care as these chemicals are not selective in what they kill.
Diseases, often caused by a fungus, can hit acer trees. The two most common of these acer plant problems are phytophthora and verticillium wilt.
Phytophthora and verticillium wilt
Phytophthora is a root rot condition that arises when the acer root system is too wet. As Japanese maples don’t like heavy and wet soil, it’s particularly prone to this disease. The leaves of the acer become pale green and wilt, and then start to easily fall off. Any new shoots die back and eventually the tree is just a collection of bare branches.
Treat phytophthora by removing the soil from around the tree right down to the top of the root ball. Exposing the root system gives it an opportunity to dry. Also, cut away any infected branches. This stops the spread and may save the tree.
Verticullium wilt shows up in late spring and early summer. The leaves of the acer tree wilt and then dieback from the edges. This is followed by the branches blackening and then this condition spreads down the trunk of the tree. Verticullium wilt is a fungus that enters the tree through the roots if they’re waterlogged.
Apart from ensuring good drainage in the soil, there’s nothing you can do to prevent this. And nothing you can do to stop it once it sets in beyond cutting off the affected branches and correcting the soil.
Another fungus to watch out for is powdery mildew. This appears as a white powder in patches on the leaves. It likes moderate temperatures and shade, and also flourishes where there’s poor air circulation. Lightly prune your acer tree if the branches are all growing together to get the ventilation going.
Powdery mildew can’t survive in water so lightly hosing down your tree can remove it. Additionally, neem oil or jojoba oil destroys the spores and prevents the spreads.
If the mould that you find is black instead of white, your tree has sooty mould. This fungus loves to grow in the sticky substance left by aphids and other insects. Aside from working in the spring to control the aphids, you can wipe off sooty mould with a damp cloth.
Because of you
Something you do may cause your Japanese tree distress. One example is sap bleeding from the branches which weakens the tree. This usually happens if you prune your tree at the wrong time. Make sure to prune your acer tree (if you must but it’s not necessary) when it’s fully dormant. Aim for between November and January to be safe.
Over- and under-watering of your tree falls into this category. You’re in a relationship with your acer tree and should monitor it regularly to see how it’s getting along. Adjust your watering schedule to its needs and the weather.