The word disease in connection with plants is considered to refer to any disturbance in the normal life processes which results in such things as
(a) abnormal growth
(b) temporary or permanent check to the development or
(c) premature death of part or all of the plant.
Plant diseases can be divided into two sorts (i) Parasitic, where the trouble is due to attacks of parasites such as fungi, bacteria or viruses and (ii) Non-parasitic in which the trouble is a result of faults in the environment (soil, temperature, moisture etc). The biggest group of diseases is that caused by fungus parasites (including bacteria) which are spread about by spores produced in the fruiting bodies (equivalent to seeds of higher plants but, of course, microscopic in size). Viruses are incredibly small in size and in nature are carried from infected to healthy plants by insects, mainly aphids, and also by trimming knives and hands (in glasshouses), by knives etc, (in propagating houses) and in a very few instances by seed transmission. It will be noticed that the great difference between parasitic and non-parasitic disease is that the former is infectious while the latter is not.
Despite this, where a non-parasitic trouble begins, there is likely to be great loss unless the fault in environment is quickly corrected. The symptoms in either kind of trouble can be very similar and even almost identical so that to judge the cause it is often necessary to consult an expert to get microscopic examination and sound advice.
The control of plant disease depends on an accurate estimate of the symptoms shown by the affected plant so as to arrive at the exact cause of the trouble. Even with parasitic diseases the cause is usually microscopic, requiring a careful examination and often laboratory tests on the diseased tissues. Similarly with non-parasitic troubles the environment must be studied as well as the plants and the details of cultivation carefully considered. There may have to be a careful soil analysis as well as a study of the drainage and soil texture and there may also be an analytical test of some of the foliage or fruits which could reveal a shortage of some essential plant food.
Precautionary measures Before considering measures which have to be taken to check diseases in plant crops we may look at some of the things which can be done to guard against disease ever appearing. These can best be termed precautionary measures. They aim at building up the vigor of the plants to help them to resist any possible attack by a parasitic disease and they also include various precautions which can be taken to eliminate the possibility of disease being in the neighborhood of the crop, more especially in the soil.
It is of the utmost importance to study the special requirements of any particular plant so that the soil can be prepared in order to ensure good drainage and also that it contains sufficient organic matter (humus) and the necessary plant foods. Where some plants are concerned, for instance camellias and rhododendrons it is important to ensure that there is no lime present in the soil. Everything should be done to get vigorous plants with robust stems and foliage. In glasshouses, proper light and ventilation must be arranged and in fruit trees skillful pruning helps to build up strong, healthy shoots and buds with good circulation of air among the branches.
Rotation of crops is intended to avoid growing the same kind of plant on the same spot year after year. If crops are grown on the same site year after year any disease of that crop is encouraged to build up and the soil can become heavily infected, addition to which the same plant foods are taken out of the soil. A different crop takes different amounts of the various chemical elements so that a balance can be easily kept. In glasshouses the same crop is very often grown each year but the disease build-up is checked by suitable methods of soil sterilization. Other precautionary measures are weed eradication (eg wild celery harbours celery leaf spot disease and the common plantain can carry the virus of spotted wilt). Careful spacing is also helpful so that diseases are not provided with the humid and moist conditions between plants which they need to germinate their spores and infect the leaves.
The protection of large wounds is another obvious precaution and one which is very important where large specimen trees or even expensive fruit trees and shrubs are concerned. It is not suggested that small pruning cuts need to be treated but where a large branch is broken down by wind or snow or cracked during severe frosts it is wise to try to protect the broken or cut places. After any branch is removed the cut should be painted over with a suitable protective paint such as Stockholm tar or Arbrex to prevent the entry of fungus parasites. Not only are plum trees likely to be infected by the silver leaf fungus but many fine ornamental trees and shrubs can be lost by neglecting this simple precaution. Even after cutting out a canker from apple trees the wound should be painted.
Resistant plants An important method of avoiding plant disease is to use resistant plants. A plant immune to a particular disease is highly valued by the grower if its quality is as good as those which are susceptible to the disease. Growing immune varieties is the simplest way of avoiding disease and it is a pity that such varieties are so limited in number.
Soil sterilization One of the most important precautions taken to avoid disease in horticultural crops is the practice of soil sterilization. In this the soil in a greenhouse intended for tomatoes, cucumbers, lettuces etc, is treated by passing hot steam through it (a commercial practice) or by watering with chemicals (eg formaldehyde or cresylic acid) before the crop is planted so that any dormant spores of disease are killed. However, at the temperature used, the spores of beneficial bacteria such as the nitrogen fixers are not killed, so that after the process is completed these can begin to enrich the soil without any immediate competition from other organisms. For use in small pots and seed boxes sterilized soil made after the John Innes formulae can be bought for use by gardeners. Similarly, sterile soilless composts are now available. Small glasshouses can be washed down inside with formalin or other disinfectants and the same sort of treatment can be given to garden frames, pots, boxes, seed trays and tools. Cresylic acid is one substance used for this purpose and in some instances also for sterilizing the surface of the oil, but there are several other good disinfectants available for the gardener.
Preventing the spread of disease When, despite the precautions referred to, a disease makes its appearance it is necessary to act quickly and to take direct action measures. The chief of these is to cover the plant with a protective film of a chemical which will kill the fungus or at least prevent the germination of its spores. Diseased parts can be removed before treatment, but it must be remembered that in most diseases an affected plant is often doomed (except where the disease is superficial or where affected parts can be cut away). The object of the treatment is to protect still healthy tissues, so that it is wise to spray or dust early.
Various chemicals, known as Fungicides, which will deal with all types of disease, are widely obtainable on the market. These are applied as a fine misty spray or as dust (most people hold that spraying is more efficient than dusting). Many types of spraying and dusting machines are in use. There are also smokes which are lit to fumigate glasshouses, which have previously been cleared of all plant life. Seeds, bulbs and corms are also treated by dusting or by immersion in a liquid fungicide to give them protection from soil-borne diseases, etc., after planting.
Fungus diseases These are diseases which are caused through the attack of various parasitic fungi. Some fungi obtain food by living on the decaying organic matter which we call humus in the soil, but some obtain their food by attacking living plants and injure or kill them in the process—these are called parasites. In general, the harmless saprophytes are large and easily seen but the parasites are very small and need a microscope for their proper identification, although their presence may be detected because of some whitish or greyish mould or furry growth (eg rose mildew). These parasitic fungi grow microscopically inside plants but some have also a smothering effect (mildews) and grow on the outside of leaves and stems etc. They reproduce and spread themselves by means of innumerable spores.
Fungicides Fungicides are chemical substances which are used in the control of those diseases of plants which are caused by fungus parasites (see above). The ideal fungicide is a substance which will kill a fungus or prevent its spores from germinating, without doing any harm to the host plant. These substances are used in various forms and in various ways and many chemicals have been tried in the search for the most effective safe fungicide. Sulphur and copper are two of the oldest elements used for this purpose and in various forms are still used against some diseases. For instance copper sulphate is used to make the well-known Bordeaux mixture which
has been in use for a century, while sulphur probably dates from Biblical times.
In recent years many of these fungicides have been replaced by more modern ones as the result of much research and there is now a much greater choice. In general the modern fungicide is more specific, that is to say it will prove very effective against a certain disease but is not so generally useful against many others, whereas the older kinds had an all-round fungicidal effect and exercised a check on many diseases likely to attack the plant.
Fungicides are applied either as sprays or dusts on the foliage or in empty greenhouses as sprays or in the process of fumigation. They may also be applied as a ‘smoke’ from a special generator or tablet which is lit.
But although fungus diseases are those we most frequently come in contact with in our gardens, the viruses are also important.
Virus diseases A virus is a minute particle, visible only under the electron microscope, which causes disorders or diseases in living cells. The presence of viruses may result in leaves developing yellow or brown spots, streaks or ring patterns. Other symptoms affecting the leaves may include dark green areas along the veins (vein banding), a loss of color (vein clearing), complete yellowing, distortion and small outgrowths. Streaks and stripes may appear on stems. Other symptoms include witches’ brooms (large numbers of side shoots), distortion, color breaks or white flecks on flowers. A condition where flowers become green and leaf-like is called `phyllody’. Fruits may be small, misshapen or bumpy. Less distinct symptoms of virus infection are a general stunting and reduction of cropping. Viruses can also change the internal structure and the metabolism of plants; for example, virus-infected tomatoes are said to have a better flavor.
A plant may contain a ‘latent’ virus without any visible symptoms. There may also be interactions due to the hidden presence of viruses. Indicator plants, which give a quick and characteristic reaction when inoculated with the sap of a virus-infected plant, are widely used in the identification of viruses. Good indicator plants include Nicotiana species, beans and Chenopodium species.
Non-persistent viruses adhere to the mouth parts of insects feeding on infected plants and are carried to healthy plants. The particle remains infective for up to about two hours.
Persistent viruses are absorbed into the digestive system of the insect, and from there pass into the salivary glands where the virus multiplies. When feeding the insect injects saliva containing virus particles into plants and remains a disease carrier all its life.
Virus diseases may be mechanically transferred from one plant to another by man. Plants propagated from infected material will contain virus particles. This is important in all vegetatively propagated crops such as potatoes, fruit and bulbs.
Control Heat therapy kills or inactivates the virus, and apical meristem culture is another method of approach. As a general rule avoid the spread of virus diseases by regular spraying against the insect vectors. Clean cultivation prevents weeds acting as carriers of infection. Remains of the previous year’s crop are another source of infection. Plants susceptible to the same virus should not be grown close together. Regular inspection and rogueing of obviously diseased plants is often effective. Seed should not be sown near an old infected crop. Barrier crops are sometimes grown by planting immune plants around those liable to a particular virus infection. Early sowings may enable plants to become established before the activity of disease-carrying insects. Physiological disorders In the study of plant diseases it is usually found that most diseases are due to the attack of some small organism such as a fungus, a bacterium, or one of the microscopic viruses. The work of the plant pathologist when confronted with a disease is to try to identify the parasite which is causing the trouble and then to take steps to combat it. Long ago it was realized that plants sometimes become sick and show symptoms of ill-health although they are not infected by any parasite. This type of trouble in plants is referred to as non-parasitic disease or physiological disorder. It can arise from a multitude of factors such as unsuitable soil, lack of lime, excessive moisture, drought, lack of some essential food element, spray, fume or fumigation damage, and so forth. There is no parasite present so there is no infection to spread but it is obvious that unless something is done all the plants will become affected and weakened.
The identification of a physiological disorder is not easy and, in general, must depend on finding the symptoms as it does with the parasitic diseases which are microscopic. In both groups symptoms from different troubles are so similar that it requires an expert to recognize them.
Attempts have been made to form a rough classification of symptoms and the most study has so far been given to the effects of shortages of essential food elements which are often referred to as mineral deficiencies. These include shortages of the major elements such as nitrogen, potash, phosphate, magnesium and lime, which plants absorb in fairly large amounts and the lesser known minor or trace elements, such as boron, copper, zinc, manganese and iron, which are needed only in very small amounts. Although some shortages, for example of potash, show as a reddish-brown scorching of the leaf margin, it is usually necessary to obtain an analysis of the soil or of the leaf tissues to identify such deficiencies.
The absence of iron shows as a lack of green color until the leaves are very pale yellow or even white and this condition often results from excess of lime and is called lime-induced chlorosis’. This, however, is easily tested by one of the soil-testing field kits which will give the pH measurement of the soil showing its acidity or alkalinity. A quick remedy for iron shortage is now available by watering with the chelated compound called Sequestrene 138 Fe, which should be applied in January. A modern treatment for deficiencies is to use the spray called Foliar Feed. This is sprayed on to a crop and is said to supply all the necessary food elements through the leaves.