Most species of shrubs or trees commonly used for bonsai cultivation rarely succumb to disease if looked after carefully and given the correct environment to grow in.
It is my experience that 95% or more trees that are affected by disease or bugs are also in poor general health. Under or over watering, under or over feeding, inappropriate growing conditions (including airless, compacted soil) and poor positioning of the bonsai, all cause stress to a tree, leaving it more susceptible to infection from disease and bugs.
Bugs can attack trees randomly though you quickly learn which are likely to become infested at a moments’ notice! Healthy and vigorous trees are unlikely to be attacked, they will also be better equiped to survive if there are attacks from bugs and diseases. Trees in poor health or trees that are under stressful growing conditions will be more affected by any external attack on its weakened defences.
Precautions, such as regular spraying with systemic insecticides and fungicides, can be useful though should not be relied upon. Systemic remedies work by being sprayed onto the foliage, which digests the treatment into the sap stream of the plant where it is distributed throughout the entire plant. Attacks of fungi or bugs are quelled when they attack the plant and are exposed to the treated sap.
However, systemic treatments are not 100% effective, regular spraying is expensive, environmentally unsound; repeated use can also reduce the effectiveness of treatments when they are actually needed. In my opinion, it is far better to use systemic insecticides or fungicides only on trees that are known to suffer problems at certain times of the year.
Primarily, try to identify what has happened to your tree. Has it lost foliage? Do any of the leaves have discolouration or holes? Closely examine the tree and the foliage, is there any evidence of pests either on the tree itself, on the surface of the compost or around the surface on which the pot itself is standing. Secondly, once (hopefully) the pest or disease is identified and dealt with, it is important to identify if there is any way that you could prevent re-occurrence in the future. Some problems such as caterpillars and aphids are difficult to guard against, although you should be able to anticipate which trees in your collection are more likely to be attacked.
Yellowing Leaves/Dropping Leaves
There are only 3 ways that a healthy tree with healthy foliage will suddenly lose leaves or have leaves that suddenly turn dry and crispy (over just 2 or 3 days):
Frost, a tropical and subtropical species being exposed to frost.
Poison, the bonsai is exposed to a poisonous chemical either in the soil or the air (directly onto the foliage). Though very rare, it isn’t unknown for a tree to be badly affected when accidentally exposed to drifting spray from weedkiller use.
Underwatering is by far the most common reason for the sudden drying up and death of healthy foliage. Once there is no moisture left in the soil of the bonsai, the leaves will die within hours. Was the soil allowed to dry out completely? Was the soil watered thoroughly enough the last time the tree required water? Was the soil dry but looked wet because you misted the tree and the surface of the soil? Less severe, under watering can also lead to yellowing of the leaves; see below.
Yellowing leaves and/or dropping leaves can occur for a number of different reasons;
Chlorosis is caused by a mineral deficiency and is due to a lack of magnesium, manganese or iron. It usually only affects acid-loving species such as Azaleas. Administer a liquid fertiliser that contains trace mineral elements easily available at all garden centres. Acid-loving species such as Azaleas can (and should) be fed ericaceous-fertilizers that contain easily-absorbed sequestered iron on a routine basis.
Die-back and yellowing leaves nearly always end up dying and falling off the tree unless the cause is Chlorosis, this is likely to be dieback of the foliage. Die-back of large areas of the tree can occur when a tree is traumatised for some reason and the tree responds by dropping any foliage that is not required for its survival. The cause is often due to damage to the rootsystem by root rot through over-watering or lack of watering which has allowed the root system to dry out. Some species (particularly tropical indoor varieties) can also become stressed by moving a tree to a new position, and they will loose their foliage. (See sections on root rot and underwatering.)
Natural wastage Some trees such as Pyracantha/Firethorn and Ulmus/Elms will develop new growth from leaf axils and will then naturally discard the now redundant leaf. Check to see if new growth is appearing from the point of leaf loss. Evergreen trees will have periods each year where they drop old foliage as it is replaced by new. If leaves are yellowing and dropping from old inner areas, this is likely to be the case. However to ensure that this growth is replaced, make sure that light and energy are given to old, inner areas of the tree by pruning the apical growth. Similarly, deciduous varieties that are left un-pruned will shed inner growth at the expense of new growth at the ends of the branches.
If there are visible pests on the leaves, identify them and take the appropriate action;
Black Fly and Greenfly are both common forms of aphids. They suck sap from the tree and in large numbers can cause dieback of new or unripe growth. On trees in poor health, this can eventually lead to death if not dealt with. Aphids can also carry virus diseases from one plant to another.
Trees are normally attacked by a few aphids, which within a few days can multiply to very large numbers. Fortunately, once detected, aphids are easily dealt with. Small numbers of aphids and their eggs can be eliminated by rubbing them off with fingers. Larger infestations can be quickly killed off by using one of any number of insecticide sprays. Soapy water can also be sprayed onto infested areas, the increased surface-tension of soapy water engulfs a pest and causes it to suffocate.
Some trees such as Acers are particularly susceptible to aphid infestation and systemic insecticide use might be worth considering during periods of repeat attack. It should be noted that the presence of ants should be looked for on trees that are repeatedly infested by aphids. Ants will commonly carry aphid eggs into trees, protect them from predators and milk them of their sticky, sweet excretion called honeydew. If ants are spotted, they should also be dealt with!
Caterpillars are very destructive to leaves and young growth, leaving holes in leaves and in some cases completely stripping them altogether. They are often very difficult to spot due to excellent camouflage, close inspection of leaves, stems and in particular, the underside of foliage is required to find and remove them by hand. Immature caterpillars will often be spotted in leaves that are folded over to protect them from predators. Contact insecticides are rarely effective though repeated problems can be reduced by using systemic insecticides.
Slugs and snails are also very destructive and quickly cause large areas of defoliation. During periods of warm, damp evenings they are particularly prevalent causing holes around the edges of leaves, this can be so extensive as to completely strip all of the leaf from its stem. The most common tell-tale sign of slugs or snails is the silvery trail that they leave behind them. Slugs and snails are only active when temperatures reach 10°C in the Spring and can be picked off by hand at night time, or killed by using proprietary slug bait in the form of pellets or liquid solution.
Cuckoo spit is evident by globules of white froth on the surface of leaves and stems. Inside the froth are larvae known as froghoppers, these feed on the sap of plants in the same way as aphids do, causing dieback and distortion of growth. Cuckoo spit can be removed by hand and killed by insecticide.
Vine weevils are probably the worst enemy of bonsai! Unlikely to be actually seen on the plant, their presence can be determined by irregular notches taken out around the edge and centre of leaves. Far more destructive to bonsai are the larvae of vine weevil, which feed on the root system commonly causing the eventual death to the plant. Adult vine weevil are 8-10mm in length, black with white/yellow markings running the length of their bodies.
Vine weevils are unable to fly but are excellent climbers and can occasionally be seen on the underside of infected plants. Adult vine weevils are easiest removed from affected plants by shaking or brushing the foliage from which grazing adults will be dislodged. Vine weevil grubs are approximately 10mm in length, white with a red ‘head’. They feed on the roots of plants over winter and by early Spring pupate into adult vine weevils, that are all female and can go on to lay up to 1,000 eggs over the course of the year. Vine weevil eggs are spherical, brown and less than 1mm in diameter – they should not be confused with slow-release fertiliser pellets used by nurseries which are larger in size.
Larvae that hatch in the warm summer months can become adults by autumn. The presence of larvae is most frequently discovered when repotting in spring or when trees suddenly die from a lack of roots! Vine weevil grubs can only be dealt with at present by removal by hand or by a small number of proprietary chemicals on the market. Most effective is “Bio Provado Vine Weevil Killer” which is used as a soil drench protecting the foliage against adult vine weevil attack for a month and vine weevil larvae attack for 6 months.
Scale insects are sap sucking insects that attach themselves to the bark of bonsai and cover themselves in a protective brown shell. These are best removed by hand-picking, as contact insecticides are unable to bypass the protective covering.
Red spider mites are very tiny sap-sucking insects that attack trees (especially coniferous plants) in hot, dry periods. The mites are hard to see with the naked eye, but their presence can be detected by fine webbing around the foliage. Contact insecticides are effective against affected trees and regular misting of foliage in hot, dry weather will deter infestation.
Fungus Gnat/Scarid Fly are tiny flies that can be seen flying around trees that are kept indoors. The flies themselves are no more than an irritation, however their grubs feed on the root system of the bonsai. Fungus Gnats are drawn to overly wet soils, particularly if they contain moss. Though the Gnats are simple to kill with the use of insecticides or household fly sprays, it is also important to improve the condition and drainage of the soil as well as ensuring that the soil is not kept permanently wet.
Viruses and Fungi
Viruses are most commonly detected by the presence of leaves or flowers that are distorted or discoloured, growth can be stunted abnormally and the plant can dieback. Treatment should include removal of all infected growth. Plants suffering with a virus should not be grouped together with other plants of the same Genus as to avoid cross-contamination, and tools should be sterilised after use. Mildew, rust and black spot are all common to weak and stressed trees. If a tree is affected by these diseases it is important to try and discover the cause of the underlying weakness in the bonsai itself.
Mildew is a fungi that thrives in damp, poorly ventilated conditions causing the presence of a white mould to form on foliage. The fungi extract sap from the host plant causing loss of vigour, distorted growth and dieback. The fungus overwinters in buds, so that young foliage emerges in Spring already infected. Spores are produced that can be spread to healthy foliage via water; hence mildews can spread quickly during warm, rainy periods. Confusingly though, whilst water droplets can aid the dispersal of the mildew spores, the water stress brought on by lack of water to the root system in hot weather reduces the natural resistance of the tree to infection. Once affected, it is not possible to rid a leaf of mildew. Infected shoots and leaves should be removed as soon as possible and healthy foliage should be sprayed with fungicide. Again to prevent further infection.
Rusts are fungal diseases that cause raised, brown or orange areas to develop on the underside of leaves (which can sometimes be seen from above the leaf) particularly on Beech and Birch species. Rust is not only unsightly but causes loss of vigour to the plant. As with Mildew, Rust is dealt with by removing affected leaves and applying fungicide, again, good air circulation will help trees avoid infection.
Black Spot on Chinese Elms can be prone to developing clusters of black spots less than 1mm in diameter on the surface of their leaves. The foliage then goes on to yellow and drop. This is caused by a virus known as Black Spot. As with mildews and rusts, once a leaf is found to be infected, it must be removed to halt the spread of the disease. As already said, care should be taken not to spray the foliage as water helps the spores travel around the plant. Avoid standing the tree in persistent rain. The remaining, healthy foliage should be sprayed with fungicide.