Garden Plant Diseases and Disorders
Garden Plant Diseases and Disorders
In addition to the numerous harmful insects and other small animals that damage plants, many plant diseases and disorders are also destructive in the garden and greenhouse. Some are familiar, others more obscure, though no less damaging on that account. The effect of some can be devastating, as in the case of the Dutch elm disease, which has killed so many fine elms. The spores of this fungus are carried from tree to tree by small beetles, with dire results.
Most of the diseases in our gardens are caused by fungi, although viruses are also responsible for heavy crop losses. Both viral and bacterial diseases are difficult to control, and usually the only solution is to burn infected material. The array of effective fungicides, however, makes the control of fungus diseases a less drastic affair.
Most fungus diseases we are concerned with in the garden in no way resemble the toadstool-shaped kinds with conspicuous fruiting bodies. The majority are invisible to the naked eye, and it is the symptoms that we recognize.
Powdery mildew is an unsightly and weakening disease frequently seen on roses. If allowed to progress, the pale, powdery coating will soon cover twigs, foliage and even flowers during the summer. There are other species of powdery mildew, and these can be seen on a wide range of trees and other plants. The leaves of apple and oak are common victims, as are gooseberries and blackcurrants. The twigs of the last two may become distorted, and should be removed if this happens, otherwise the disease may persist the following year.
Black-spot is another unsightly disease found on roses, although a form also occurs on sycamore. The characteristic black spots, which are the first sign of the disease, are very disfiguring. It is more prevalent in the clean atmosphere of country areas than in towns, where the pollution seems to act as a natural fungicide.
Coral-spot is another disease with a descriptive name. It produces numerous reddish or deep orange spots on dead twigs. Unfortunately it often spreads from these to living twigs, and can be very destructive to old red currant bushes, although gooseberries and apples are among the other plants attacked.
Honey fungus is even less welcome. This large honey-coloured fungus grows on old free stumps, and spreads from these by means of black, underground root-like rhizomorphs, known as ‘bootlaces’. The problem lies in the fact that it is likely to spread from dead stumps to living roots of trees and even to any growing nearby.
Rust fungus is a disfiguring disease, and there are species that affect plants such as hollyhocks, antirrhinums, mint and . The characteristic sign is the spread of numerous orange and brown pustules on the under surface of the leaves. The disease may eventually kill the plants, and should always be treated quickly. Fortunately there are rust-resistant antirrhinums that can be planted on ground known to contain the spores.
Peach leaf curl is common on peach trees and some related species such as nectarines, almonds and other Prunus species. The symptoms are twisted and contorted leaves with red swollen areas; these are conspicuous and unsightly.
Club-root is not visible until the plants are uprooted, although the problem is manifest in the unhealthy and stunted growth made by the plant. Cruciferous plants are affected, and it is often worst on badly-drained acid soils. Affected plants develop large gall-like swellings on the roots, and when these rot in the ground spores are released and the ground is contaminated for more than a decade. Even crop rotation is of limited value because the spores remain viable for such a long time. With suitable treatment it may be possible to grow brassica crops, but it is best to avoid the disease if your garden is not already affected, by raising all your own brassica plants.
Apple canker is a secondary infection that usually enters through an existing wound, although it can also be spread by aphids. It is a serious disease of apples but also affects some other trees, including pear and beech. First signs are small depressions in the bark, often near a leaf scar. The area affected becomes larger and more oval, and soon the tissue in the centre dies and flakes off to leave a ragged wound surrounded by cracked bark.
Potato blight is a serious potato disease. Brown or black patches appear on the leaves, particularly on the edges, and soon the leaves turn yellow or brown. In moist weather a whitish mould may be seen on the undersides of the brown patches. The disease can also affect the tubers.
Bacterial diseases are far less common in plants than in animals, which is fortunate as they are not readily controlled by normal garden chemicals. Strict garden hygiene is the best solution, burning all diseased tissue. As the bacteria may be carried on seeds, never save seed from affected plants.
The two bacterial diseases most likely to be encountered are potato blackleg and soft rot.
Potato blackleg may reveal its presence any time from mid-June onwards. The plants turn a greenish-yellow and become stunted and unhealthy in appearance. If the stems are pulled up they will be seen to be black and rotten. If a stem is cut across, brown or black spots may be seen. Affected tubers will rot in store, and probably spread the disease to other tubers.
Soft rot may be caused by various fungi or bacteria. The bacterial kind usually affects carrots, and other root vegetables. The soft tissue usually disintegrates and becomes soft and slimy. Stored roots are often most affected. Celery heart rot is caused by the same organism.
Viruses are particularly troublesome because they are so difficult to control. Once a group of plants has become infected it is usually best to burn them and start again.
A typical symptom is a yellowing and mottling of the leaves, sometimes with distortion. The plant looks sickly, and the yield of food crops is significantly reduced. Sometimes, however, the mottling is considered attractive in ornamental subjects; the variegated abutilons are an example.
In fruit and vegetables, however, virus-affected plants must be destroyed immediately. Always buy certified virus-free stock whenever possible, and keep the plants free by controlling the aphids and other sap-sucking insects that transmit the diseases.
Tomato spotted wilt virus causes the young top leaves to brown or bronze, and sometimes concentric rings appear. The whole plant appears to stop growing, and the leaves tend to curl downwards, with thickened veins. This is an especially unwelcome disease as it also affects many other plants, such as winter cherries (Solanum capsicastrum), (Sinningia), and arum (Zantedeschia aethiopica).
Mosaic viruses cause a yellow mottling of the leaves, sometimes accompanied by distortion. Many ornamental plants may be affected, as well as soft fruit such as raspberries and vegetables such as . It may be possible to buy virus-free lettuce seed from one of the specialist suppliers.
Not all unhealthy plants are suffering from pests or diseases. Sometimes a physiological disorder is responsible. Lack of water may cause leaves to wilt and drop; cold draughts may make them turn yellow and fall; chemicals in aerosols can cause spots to appear on the leaves of houseplants; blossom end rot of tomatoes is primarily due to irregular watering.
If there is not an obvious culprit, question the cultural procedure or the environment. Remember also that a deficiency of certain nutrients can cause symptoms resembling diseases. A manganese deficiency, for example, can cause a yellowing of the leaves that could resemble a virus disease.
The chemicals mentioned in this site are safe if used in accordance with the manufacturers’ instructions. But always follow them to the letter.
Sensible spraying and dusting will help in the fight against– but apply chemicals safely and sensibly. Remember:
- Read the label carefully – all of it!
- Certain chemicals may damage particular groups of plants (see labels).
- Mixing stronger solutions will not increase the potency but may damage the plants.
- Do not spray in strong sunshine as this may damage the plants.
- Always dispose of unused spray solutions safely (the manufacturers’ instructions will usually advise or pour over the ).
- Insecticides and fungicides are usually most effective if applied as soon as possible after the problem has been diagnosed. Fungus diseases may be particularly difficult to eradicate once the infection has become established (a systemic fungicide may be the best control in this situation).
- Use dusts when the morning dew is still on the leaves powder will adhere more readily.
- Cover the undersides of the leaves as well as the topsides and shoots when applying sprays.
Consider the other creatures too. Some insecticides will also kill beneficial insects, although pirimicarb is harmless to lacewings, ladybirds and bees. It is best to spray in the morning or late afternoon when bees are least active.
Even ‘natural’ chemicals such as pyrethrum may be toxic to fish, so be particularly careful to check the label before applying a garden chemical within close proximity to a pond.