Bonsai - Diseases and Pests

Plants are much like people; when they feel comfortable in their environment and are well cared for, they tend to get sick less often. Their natural resistance protects them against diseases and pests. But if they are compromised too much as a result of poor air circulation or temperatures that are too high, or lack of care (which sometimes cannot be avoided), your bonsai might fall prey to fungi or pests. Prevention-meaning attentive and tender care-is the best medicine for your bonsai. However, even the most expertly cared for plants can get sick. There is a list of the most common bonsai diseases and pests. With this information, you may be able to effectively intervene early. Fortunately for all of us who love nature and plants, it is now easier than ever to combat diseases with organic, biological methods, as natural and environmentally "friendly" products have to a large degree replaced chemical preparations.

Powdery Mildew

A fungus infection occurring on the tops of leaves. It usually is the result of too little air circulation, or may result if a plant has been sprayed too late in the evening and the water didn't have a chance to evaporate. True mildew also can be caused by a plant fertilizer too high in nitrogen. If possible, treat your plant with a natural antimildew fungicide.

Downy Mildew

A grey covering on the underside of leaves, with yellow spots on the top. It is caused when air circulation around bonsai is insufficient or the humidity in the air and the water content of the soil are too high. To treat bonsai with false mildew, relocate the affected plant to a place with better air circulation and spray it with a fungicide-the earlier, the better. A spray of 1tablespoon (15 ml) sodium bicarbonate plus 2 tablespoons horticultural oil dissolved in 1 quart of water (1000 ml) can be used as a fungicide for either type of mildew. Some plants become photosensitive, however, after being sprayed, so check with a nursery to be sure before using this recipe.

Chlorosis

Your bonsai has chlorosis when the leaves turn yellow while the veins remain green. This iron deficiency can be eliminated by adding a chelated iron supplement to the water.

Sooty Mold

A black, soot like deposit that appears primarily on older leaves and often only on one side of bonsai. This fungus growth usually follows infestation by aphids (plant lice). The fungus grows on the honeydew the aphids secrete. Treat your bonsai with a contact insecticide.

Root Rot

May result when water remains in the saucer for too long, or when the plant has been fed too heavily. It may cause the leaves to become discolored, or branches may break off. Affected roots and root fibres turn brown and mushy and must be removed. After the affected portion of the roots system has been cut off, transplant bonsai with fresh soil and water it well. For the next couple of weeks, reduce watering so that new roots have a chance to grow and are able to take up water. Do not fertilize your bonsai for the next 8 weeks, at least. Also, during this time of convalescence, do not expose your bonsai to direct sunlight.

Rust

Another common fungal disease, which occurs in the form of orange or brown patches or even blisters spreading over the underside of the leaves, which curl up and eventually drop. These patches and blisters result from infection by the Phragmidium fungus, which also thrives in a hot, humid atmosphere. Rust growth is commonly encouraged by an excess of potassium.

Black spot

Black spot is another formidable fungus, which mostly attacks the leaves (elms being particularly susceptible). The leaf becomes progressively covered in black patches, until it eventually shrivels and falls off.
Preventive treatment with a general fungicide is usually sufficient to deter black spot. If you discover it has attacked your bonsai, use a sulphur or mancozeb based product.

Other deficiency diseases

• Potassium deficiency can kill off the leaves, which turn yellow at the edges, shrivel and finally drop.
• Shortage of nitrogen hinders photosynthesis resulting in the leaves turning pale, with brownish patches appearing on the stalks of plants.
• Magnesium deficiency can depress flowering, which becomes rather sparse. This deficiency also causes a slight yellowing of the leaves.
• Phosphorus deficiency also restricts flowering and the leaves, though remaining green, fall prematurely.

All these deficiency diseases can be easily cured by a balanced feeding programme. This is why the choice of the right fertilizers is so important.
Your choice of fertilizer should take into account the precise needs of the individual tree, with special reference to the growth cycle of the tree.

Canker

This is a bacterial rather than a fungal disease, which shows as swelling of the bark and callosities. It often arises after pruning with tools that have not been sterilized after cutting infected material (which shows the wisdom of passing the blades of a grafting knife or secateurs through a flame). The only effective way of controlling canker is to cut out infected areas -cutting away a branch or removing infected tissue. Be careful to dress the wounds made in this way with mastic to help them heal and to exclude further infection. It is vital to burn all diseased wood immediately and to sterilize the tools you have used.

Viral diseases

These are, fortunately, seldom encountered, for one can only control this trouble by destroying infected bonsai to stop the disease spreading to other plants and becoming an epidemic.
The most dangerous viral disease of cultivated plants is without doubt mosaic, which appears as a mottling alternate green and yellow stripes or other shaped areas -on the leaves, and by a highlighting of the leaf veins. The leaves eventually shrivel, dry up and fall.

Pests can also be fought successfully with beneficial insects, particularly if the pests are detected early. Using natural predators natural enemies of a pest-is often very difficult for gardeners to accept at first. However, once they have observed this natural process of combating pests, most quickly become convinced of its effectiveness.

The basic principle underlying this approach has to do with competition between organisms. By using the natural enemies of a pest, the pest can be kept from multiplying. In the absence of the pests' natural enemies, as is often the case with small trees in containers, the pests can multiply and in the end consume your plant.

When using the beneficial insects, you will note that they only live as long as their food (the pests) is available. When the food is gone, so are the predators.

For this treatment to work, temperatures must be at least 64°F (18°C). You need to also be aware that this method will not work as fast as chemical pesticides. Be patient. Growing bonsai requires patience anyhow. Let's look at the specific pests.

Aphids

Usually found on the underside of leaves or around a newly forming bud. They literally suck the life out of a plant. Often it is sufficient to give bonsai a shower (in the bathtub) in order to get rid of them. If this doesn't work, spray bonsai with a garden spray containing pyrethrum, like Metasystox. If you want to use the natural predators, you will have success with lacewing larvae, known as "aphid lions." Also, yellow stickers (cards with a sticky surface) are very effective against flying aphids. The yellow color attracts them, and they adhere to the sticky substance.

Scales

Appear as small, brownish mound-shaped insects-looking not unlike pockmarks-on either the underside of leaves or the stems. Frequently they can be scratched off with a toothpick or removed with your fingernail. Or you could paint on and around the scales with a cotton swab dipped in methylated spirit. If that doesn't help, spray bonsai with a garden spray containing dormant oils or a borax and kerosene combination.

Spider Mites

Present when you see a fine web spreading across yellow leaves. You will see the mites if you shake a branch from the affected bonsai over a white piece of paper. They usually look like a red powder, like paprika, but sometimes they are brown or yellow. You can see the mites with a magnifying glass. In order to quickly rid your bonsai of this pest, you can use Metasystox-R (oxydemeton-methyl) or Pentac (dienochlor), an acaricide.
If you detect spider mites early, or if you want to follow up after treatment with a preventative, the use of predatory mites, the natural enemies of spider mites, is recommended.

Springtails

Live in, or on top of, the soil. They move about in a jumpy fashion. The presence of an individual insect is considered rather beneficial; however, when they appear in large numbers, they become pests, because then they attack the roots. Springtails only multiply in wet soil. To get rid of them, treat bonsai with a household spray.

Whiteflies

Often found on Sageretia, pomegranate, and hibiscus. They thrive best in stale air and a dry environment. Their larvae and eggs usually hide on the underside of leaves. The tops of the leaves sometimes show a yellow sprinkling. You can combat this pest with its natural enemy: the ichneumon wasp. In addition, a garden spray with an oil emulsion or a contact insecticide is very helpful, as are organic preparations of pyrethrum and the yellow sticker cards.

Mealy bugs

Look like little cotton balls sitting on the axil of branches, twigs, and leaves. The small insect hides in the center of the cotton ball and is protected from predators by a wax like covering. Treat the infested bonsai with a pyrethrum spray or other contact insecticides. Mealy bugs have no natural enemy.

Root mealy bugs

Can cause yellowing leaves and often kill bonsai. If you take bonsai out of its container and examine the roots and find whitish-grey clumps that look like cotton balls, your bonsai is infested with this insect. You can use diazinon to combat root mealy bugs. When the infestation is severe, water the soil with a solution of Metasystox.

Caterpillars

There are many different kinds of caterpillars, all with a voracious appetite for foliage, which attack bonsai leaves, to the point of total destruction.
Some of them develop in cocoons on the undersides of leaves before appearing in the daylight. They twist themselves into silky webs, hence their name, tortrix caterpillars. Sometimes they develop in the leaf and flower buds.
Caterpillars are not always easy to eliminate, as they are sometimes resistant to insecticides. Remove them by hand where you see them and destroy them. An insecticide powder, which stays on the leaves longer, can give good results, as the caterpillars absorb it as they eat their way through the leaves. It may be possible to catch some species in a saucer of glue placed at the base of the tree. This will prevent moths from climbing the tree at night in autumn and laying their eggs on the leaves.

Insect larvae

Some insects may lay eggs in the bonsai compost, that is, in the root ball. The larvae which hatch are especially fond of young roots, which they devour, depriving the plant of nourishment. So keep a watchful eye open during repotting, when you can destroy larvae with your fingers. You could also use a suitable insecticide like HCH.

Ants

Ants are formidable pests in several ways. Due to their fondness for aphids, on whose excretions they feed, they can almost be said to 'farm' them, since they convey them from one plant to another plant and protect them from predators such as ladybirds. In order to be near their food, ants also tend to congregate in nests in the soil around the roots, causing havoc in the root system and cutting through roots as they carve out tunnels. The sole remedy is to set a trap for them on the surface of the root ball. If an ant's nest is setup in the root ball, the only solution is to lift bonsai from its tray. Brush off the soil and eliminate as many larvae as possible. When disturbed, ants are easily dislodged.

In general, when combating pests that have invaded your bonsai plant, keep in mind that, wherever there is soil, there is life, and not everything that moves is dangerous. Over time you will develop a keen eye and learn to detect what is dangerous and what is not. Some species react badly to some pesticides; check with a nursery before you use one, if possible.

Whenever you make a decision to use a particular preparation, don't stop treatment too soon-even when you think you have won the battle. Continue treatment two or three more times. Then you can be sure that the eggs that some pests might have left behind are also killed.

Be sure to keep all pesticides and other chemicals out of the reach of children and pets, in clearly marked containers. Read and follow the instructions carefully, and remove people and pets from the area when you apply the chemicals. Spray in a well-ventilated area. Protect yourself with gloves and a face mask.