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, poor growing conditions (including poor, compacted soil), 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! Whilst healthy, vigorous trees are unlikely to be attacked, they will also be better able to survive 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 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 and 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 on trees that are known to suffer problems at certain times of the year.
FIRST AID :
Primarily, try to identify what has happened to your tree. Has it lost foliage? Do any of the leaves have discoloration 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 though 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.
· Under-watering 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 : 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 Miracid, which contains easily absorbed sequestered iron on a routine basis.
Die-back : 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 die back of the foliage.
Die-back of large areas of the tree can occur when a tree is traumatized 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 root system 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 under-watering.)
Natural wastage: Natural wastage some trees such as Pyracantha/ Firethorn and Ulmus/Elms will develop new growth from leaf axil 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.
VISIBLE PESTS ON LEAVES, BARK OR ROOTS:
If there are visible pests on the leaves, identify them and take the appropriate action;
Bonsai Pests: Aphids
Bonsai Symptoms: Buds and shoots appear deformed and curled. A sticky sugary substance known as 'honeydew' appears, attracting ants and causing unsightly sooty mould.
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 dealt with 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 used if sprayed onto infested areas.
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, which 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!
Bonsai Chemical Solutions: Most systemic insecticides are suitable. Alternatively, try 'plant pins', which when pushed into the soil are absorbed into the tree's system, preventing the need for spraying. Root aphids should also be sprayed with systemic insecticides if apparent when repotting.
Bonsai Organic Solutions: Wash off with water spray (aphids do not climb back) and control with special soft soap sprays. Sprinkle lacewing larvae over the tree from mid-spring onwards (average temperature 10ºC). 1 larva eats over 300 aphids. Also encourage ladybirds by not using insecticides in the garden.
Bonsai Pests: Caterpillars
Bonsai Symptoms: Foliage is eaten and caterpillars are usually visible. Act quickly if several caterpillars are present! Caterpillars are very destructive to leaves and young growth leaving holes in leaves and in some cases completely stripping them altogether. Often very difficult to spot through 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 affective though repeated problems can be reduced by using systemic insecticides.
Bonsai Chemical Solutions: Contact or systemic insecticides.
Bonsai Organic Solutions: Use organic spray 'Di-P Fungus disease: el' which leaves no harmful residues - or simply remove by hand.
Bonsai Pests: Slugs and snails
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 leaf from its stem. The most common telltale sign of slugs or snails is the silvery trail that they leave behind them.
Slugs and snails are only active when temperatures reach 10 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.
Bonsai Pests : Cuckoo spit
Cuckoo spit is evident by globules of white froth on the surface of leaves and stems. Inside the froth are larvae known as frog hoppers, these feed on the sap of plants in the same way as aphids do, causing die back and distortion of growth. Cuckoo spit can be removed by hand and by insecticide.
Bonsai Pests: Vine Weevil
Bonsai Symptoms: Larvae eat and destroy root system. When tree starts wilting the problem is usually very serious. Adults often leave u-shaped holes in leaves.
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 center 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 approx. 10mm in length, white with a red 慼ead? They feed on the roots of plants over winter and by early Spring pupate into adult vine weevils which 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 fertilizer 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 re-potting 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.
Bonsai Chemical Solutions: Prevention is the best way to control this 'evil weevil'. Mix specific soil insecticides into the soil when re-potting or water soil with vine weevil insecticide containing lmidachlopid. Both treatments last for six months.
Bonsai Organic Solutions: Prevent infestation by applying barrier glue to bonsai bench legs to protect plants from climbing weevils. Apply heterohabditis nematodes in late spring and in early autumn (use soil thermometer to check that soil temperature is at least 12ºC).
Bonsai Pests: Mealy Bugs
Bonsai Symptoms: Mealy bugs start as free-moving crawlers with the females becoming less mobile as they mature. While unlike many female scale insects, female mealy bugs often retain leads and can move. Yet, nevertheless, the female mealy bugs attach to a single spot and become less mobile.
Bonsai Chemical Solutions: The fastest and easiest way is to use pesticide, especially if there is a heavy infestation on our bonsai tree. Of course, this stuff is toxic and we must be careful during mixing and application. We will have to apply the pesticide again in 7-10 days as the mealy bugs may have laid eggs on our bonsai tree. A few escaped mealy bugs can repopulate a colony in just a few days.
Bonsai Organic Solutions: While female mealybugs feed on plant sap and secrete a powdery wax layer to protect themselves (that’s how they begin to look more like some kind of growth than an insect), the male mealybugs do not feed at all as adults and only live to fertilize the females (hence, the male mealybugs are short-lived).
Bonsai Pests: Scale Insect
Bonsai Symptoms: Yellowing and wilting of the leaves. Look for small limpet-like shells, often with white fluffy undersides, may be visible on the trunk, branches and leaves.Scale insects are sap sucking insects that attach themselves to the bark of bonsai and cover themselves in a protective shell brown shell. These are best removed by handpicking, as contact insecticides are unable to bypass the protective covering.
Bonsai Chemical Solutions: Systemic insecticides. For small outbreak, paint shells directly with alcohol, such as mentholated spirits.
Bonsai Organic Solutions: Pick off by hand. When temperature is above 14ºC, 'steinernema' nematodes can be applied.
Bonsai Pests: Red Spider Mite
Bonsai Symptoms: Mottling of the foliage, which turns from yellow to brown and then falls. Conifers are most often attacked.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.
Bonsai Chemical Solutions: Systemic insecticides.
Bonsai Organic Solutions: Control with a solution of organic soft soap, spraying under the leaves. When temperature is above 16ºC, 'phytoseiulus', a tiny harmless mite, can be tapped over infected plants. Increase levels of humidity around tree.
Bonsai Pests :Fungus Gnat
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.
Bonsai Chemical Solutions: By Systemic and Non-systemic way.
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 : Mildew is a fungi that thrives in damp, poorly ventilated conditions causing the presence of a white mold to form on foliage. The fungi extract sap from the host plant causing loss of vigour, distorted growth and die back.
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, 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 to prevent further infection.
Powdery mildew : Powdery mildew is a disease that will affect your plant’s health. When you see an infection caused by fungus on top of your plant’s leaves, this is a consequence from spraying water on your plant in the late evening when the cooler conditions did not allow the water to evaporate, or you are currently displaying your tree in an area where there is little air circulation. Plant fertilizers, which have high nitrogen content, can also cause this mildew. For treatment, you can use anti-mildew fungicide.
Downy mildew : Downy mildew another is downy mildew, which results from high humidity or too much water content in the soil of your bonsai plant. Oftentimes, this is a gray layer on the leaves’ underside. Chlorosis, on the other hand, occurs when the leaves start to turn yellow and their veins still remains green-colored. This is considered as an iron deficiency disease. Chlorosis can be treated through adding an iron supplement to the water.
Rust: 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.
Virus disease :
Plants are not only infected by fungus; they can also be attacked by viruses. Although these are seldom encountered, your plants have to have the best care to avoid this entirely. Otherwise, you may end up destroying your whole bonsai to stop the virus from spreading throughout the plant. Of course, no bonsai plant owner wants that!
Bacteria disease :
Black spot :Sooty mold, root rot, rust and black spots are also some of the deficiency diseases that your plant may experience. These diseases can be treated differently using insecticide or other supplements. Otherwise you can avoid fertilizers for about 8 weeks to make sure your plant is well-protected from these commercial products.
Black Spot on Chinese Elms 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. 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.
Protect: By Systemic and Non-systemic way.