A Basic Guide to Pests and Diseases That Affect Bonsai

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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.

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