Bonsai trees are prune to pests and diseases like other trees/plants. I have to admit the topic of pests and diseases is not one I ever thought I would spend so much time studying about or even become interested in. However since like all trees, bonsai are prune to both pests and disease, with time I have got to know more about the subject.I still think the best thing we can do for our trees is to provide them with the recommended care and maintenance because a healthy tree will be better equipped to fight off these awful invaders. Observing a tree regularly when carrying out regular maintenance means any invading pests or diseases can be spotted as soon as possible.
How to tell if a tree has a disease or pest
Sometimes a tree will show signs that it is under attack. I should however mention that some of these signs are not always due to pests or diseases. For example when the leaves started falling off my very first juniper bonsai, I drove myself into a mini panic attack trying to figure out what was wrong. When I asked around on a forum, more experienced growers helped me figure out that the leaves were falling because I had re located it from indoors to outside. With this in mind, below are some indications that might mean that your tree has been invaded.
- leaves turn yellow or brown
- leaves falling off the tree(die-back is when foliage starts to die from the tips and eventually falls off the tree)
- wilted leaves
- holes and colored spots on leaves
- Tree growing slower than usual
- Larvae / whitish worms on the tree or soil
- Sticky substance(honey dew) on tree(made by aphids)
- Very thin(fine) silky web between leaves and branches(these are made by red spider mites which attack especially juniper bonsai
Treating an infected tree
First step should be to confirm a tree is actually infected. Identifying bonsai tree pests and disease can be confusing. Although for most of these invaders the tree will have tale tell signs, sometimes the infection might be in the roots which makes it harder to spot. Below are some general steps that can be taken to treat infections. More specific instructions are also included further down in this article for each ailment.
For obvious infections:
- Prune the infected leaves off as soon as possible to prevent spreading
- Pruned or dropped leaves should be disposed of carefully to prevent spread of infections
- Place the plant in a secluded spot so that infection does not spread to other plants or trees
- Leaves should be treated with fungicide
- Some insects can be washed off the tree using a water hose
- Insects/pests can be sprayed using a garden insecticide
- A mild mixture of soap and water can be used leaves in certain cases.
- Any bonsai tools used like scissors should be sterilized using household disinfectant or rubbing alcohol. When using rubbing alcohol, soak a cotton swab in the rubbing alcohol and rub it on the front and back side of the scissor blades.
- Soil from an infected tree should be destroyed off properly and not re-used on another plant as fungal infection can survive in the soil
- A pot or container that housed an infected plant should be washed thouroughly before re-use
- The tree should be relocated to an open well ventilated area.
Bonsai Tree diseases
Is a fungal infection that affects the underside of leaves. It appears on leaves as small raised sections of various color including yellow, orange, red or brown. Once leaves are infected, they will curl up and eventually fall off the tree. Infected leaves should be removed from tree as soon as possible to prevent spread and disposed of safely.
When a tree is potted using compact, heavy soil like peat, the soil can become soggy due to water remaining in the pot after the tree is watered instead of draining out. If this goes on for a long time, the tree roots will start to rot. Root rot affects the whole tree. It is not uncommon for leaves to turn brown and the whole tree dying.
- Affected roots should be pruned and tree repotted in fast draining soil.
- Soil from an infected tree should not be used to repot any other plant as the infection can remain in the soil.
Is a fungal infection that produces white spots on leaves. Although the first time they appear on the leaves they are white, with time these spots can turn to different colors including black, brown or gray. Untreated leaves will fall off the tree.
Infected areas should be pruned off the tree and the rest of tree sprayed with fungicide.
Occurs when a tree lacks the falling minerals: magnesium, manganese or Iron. It is more common with bonsai that require higher acidic level like Azaleas.
Black spots are caused by a virus which makes turns leaves yellow and fall off the tree. Chinese Elm are a common victim of spots. It can be treated by pruning infected areas and spraying a fungicide on the rest of the tree.
This is technically not a disease but a sign that something is wrong with the tree. Die back occurs when a tree is stressed and starts shedding off leaves that are not necessary for the its survival. Stress might be due to root rot, over or under watering, changing the location of the tree.
Mildew is a fungus that thrives in damp conditions. It causes a white powder like substance to appear on leaves. The treatment is the same as for mold see treatment below
Mold on the other hand is a black substance that appears on the leaves and branches of trees and can be rubbed off when touched. It is caused by insects like aphids which leave a sugary extraction (honeydew) as they feed on the sap of plants. This honeydew eventually grows into mold.
Mold is technically not a disease but a sign that the tree could be infected with aphids which are common pests on bonsai trees.
Treatment of mildew and molds
Infected sections should be pruned to prevent spreading. Further still since they can spread very easily through spores, care should be taken when watering the tree to prevent further spreading.
Leaves can be washed with a mild solution of liquid soap and water. A few applications might be needed for this to be effective.
Twig and Tip blight
Is a common disease of Juniper bonsai. It occurs during warm weather and if soil with poor draining is used. It causes the tips of leaves to turn brown and fall off a tree. Sick sections of the tree should be pruned off and household disinfectant applied to the area the cut was made to prevent it spreading.
Bonsai tree care pests/insects
Before talking about the common pests and diseases, I thought it would be a good idea to look at methods used to get rid of pests and diseases on bonsai trees.
Treatment of pests and diseases
- Change location of the plant
- For outdoor trees, lady bugs which eat most insects but do not harm bonsai trees can be introduced as an environmentally responsible solution.
- For some bugs the best way to get rid of them is to pick them off the tree.
- A mild solution of water and soap can be used to get rid of some pests. (Although liquid dish soap can be used, insect soap is another environmentally friendly alternative)
- Spraying infected tree with garden insecticide. This can either be added to the soil or sprayed on the foliage.
Are tiny insects that can prove fatal to trees of not treated. They appear as red dusty particles on the underside of leaves. They are so tiny that they can be easily missed on a tree. To check if your tree has been infected, hold a white sheet of paper under a branch. Tapping the branch gently will make mites fall on the sheet of paper. Sometimes they also weave a fine web on tree leaves
To treat red mites; apply garden insecticide or organic insect soap to the underside of leaves.
Below is a video that explains how to detect and treat spider mites
Are tiny insects which produce a sticky substance. These bugs can be yellow, white or brown on color and might have a hard protective shell. Some species of Bonsai like Fukien Tea are more prone to attack by scales. Scales are so dangerous because they hide under the bark of trees.
Treatment: The best method is to pick them off the tree by hand. Spraying garden insecticide might not be effective since they hid under the bark of the tree and have a hard protective shell which can be hard to penetrate.
Vine weevils can do lots of damage to trees. Adult insects which can be easily seen on the tree lay eggs in the soil. The larvae is the most dangerous because they feed on the roots of trees and can go undetected while they damage the tree. Sometimes when you see larvae/worms in the soil they could be the larvae of vine weevils. The leaves of an infected tree will start to wilt.
Treatment: Adult insects can be sprayed to prevent them from laying more eggs. As a preventive method, insecticide can be added to soil when repotting.
Examples include black and green fly. These insects that attach themselves to the plant are notorious because they can multiply into vast numbers within a very short time period of time. They damage trees by sucking nutrients as they feed on plant sap. Signs of infection include a tree having weak leaves and branches. Leaves can also start to curl. Aphids also leave honeydew (sugary substance) on leaves which can lead to mold.
Aphids should be treated as soon as they are noticed on a plant by using insecticide sprays, a solution of soapy water sprayed on the plant can also be quite effective.
Damage a tree by feeding on large quantities of bonsai tree leaves. A sign that they have infested a tree are leaves with holes in them.
Treatment; can be removed by hand, either garden or organic insect spray is also an effective way of getting rid of them.
Are tiny insects that hide under white cotton balls on leaves and branches of trees. They group together in either clusters or clamps. They can also infect roots of trees (root mealy)
Treatment; use of systematic spray or contact insecticide
Snails and slugs
These pests which leave a slimy secretion become active only when the temperature reaches 10 C in the spring time.
Treatment: my preferred method is to pick them off the tree. Slug bait can also be used.
I sincerely hope that if your tree has any of the above infections,it makes a brilliant recovery.My advise for any infections is to isolate the tree if it is one of many and try to treat the problem as soon as possible.
Photo credit: Shipher Wu