Bonsai Tree Diseases

This heading covers a number of problems which can affect the life of a plant. Causing it to wither and eventually die. We generally distinguish between fungal diseases, deficiency diseases caused by an imbalance in plant nutrients, and bacterial and viral diseases (the latter usually incurable and fatal to the infected plant).

Although fungal growth is looked on with misgiving, it should be remembered that some fungus is not only harmless but beneficial. For instance, fine white mycelium growing around the roots should be encouraged as it is in certain cases a sign of good health.

Powdery mildew

Powdery mildewThis is certainly the most widespread fungus disease. It shows itself in the form of a white, floury layer which gradually spreads over the shoots and leaves.

The mycelium (equivalent to a root system) of the fungus draws sap directly from the plant’s cell tissues which subsequently die. This fungus generally thrives in a hot, humid, poorly ventilated atmosphere and is encouraged by an excess of nitrogen.

Some specific treatments are available to deal with this fungus, most of them sulphur based. Use them to prevent the disease, especially where conditions favour the development of the fungus. Be careful not to wet the foliage over much. As this is a frequent cause of mildew.


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 at-tacked your plant, use a sulphur or mancozeb based product.


This is one of the commonest deficiency diseases, caused exclusively by a shortage of iron in the plant nutrients. The leaf blade or needles turn yellow, while the veins usually remain green. Chlorosis generally occurs in chalky or limy soil, which locks up the iron. The best remedy then is to change the potting compost when repotting. Iron can also be given in liquid form, by adding sulphate of iron to the water when watering.

Other deficiency diseases

Potassium deficiency can kill off the leaves, which turn yellow at the edges, shrivel and finally drop.

Shortage of nitrogen hinders photo-synthesis 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.


bonsai powder disease prevention dispenserThis fungal disease shows itself as a greyish mould on the undersides of the leaves and on the sepals of the flowers. Marking them with a brown halo.

Good ventilation and careful watering usually suffice to put an end to this trouble. If this is not enough, use a suitable fungicide, preferably in powder form, to destroy the fungus.

With most fungal control, prevention is better than cure. For instance, although powdery mildew is treated by simple methods, it can be very persistent. Therefore start treating potential mildew victims before the mildew appears.

Root rot

This serious fungal disease affects only the roots. It is most often caused by contamination from poorly decomposed organic fertilizer used straight on the roots (fresh manure, for example). The fungus is encouraged by over generous watering, poor drainage and by standing the container on a tray or saucer.

As root rot affects the underground part of the plant, it is difficult to treat. If you are worried, you should examine the roots carefully when lifting or repotting the bonsai. It is hard to combat root rot. The best way is to remove all infected roots, which could itself have grave consequences for the bonsai, as the imbalance between roots and top growth could kill it. Lift the plant and brush away all contaminated soil as quickly as possible. After transplanting in fresh compost, spray the foliage well to compensate for the water the plant roots are unable to provide.


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 plants 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.

Control materials

The chemical products used to control these diseases are offered either as dilutable liquids (or powders to be dissolved in water), or as powders to be used in that form. So you need a sprayer and a powder blower as well.

The sprayer is also valuable for damping over the plants’ foliage as necessary, so it is an indispensable item of equipment. A powder blower, on the other hand, is used less frequently, and only in special situations. What is more, some products which are offered in powder form can only be used when conditions are right for dusting.

24. February 2012 by Dave Pinkney
Categories: Bonsai, Bonsai Care | Tags: , , , , , , | Comments Off on Bonsai Tree Diseases


Get every new post delivered to your Inbox

Join other followers: