THE NATURE OF PLANT DISEASES
PLANTS, just as much as animals, may suffer from various diseases which can injure or kill them. All types of plants, from, perennials and bulbous plants to shrubs and large trees of every kind, can be attacked,so that a knowledge of the early signs of infection and the way in which a disease can be combated is a valuable asset.
New varieties of plants are being produced all the time by various methods of selection and inbreeding. Some of these are highly bred, but highly bred plants are not necessarily highly resistant to disease, and with these in particular the prevention of disease can assume great importance. Much research work is, therefore, devoted to the prevention and curing of plant ailments.
Where one particular type of plant is grown in quantity, often on the same ground year after year, an outbreak of disease can cause serious losses. Such a disease finds ample opportunity to live on its particular host plant during the summer, and will usually have some method of persisting through the winter. It may remain in the, and although the soil may be only slightly contaminated at first, the trouble may build up to serious proportions. This type of trouble is more likely to arise in greenhouses, garden frames and probably formal beds. Eradication will often necessitate treatment of the soil in the dormant season when the site is vacant.
With growing crops swift action is likely to be needed if serious losses are to be avoided. The more common troubles which can affect plants should be easily recognized and the appropriate remedy applied. Outside advice may be quickly available, but with fast-spreading diseases early treatment has great advantages.
The term ‘plant disease’ refers not only to foreign organisms (parasites) which can injure or kill a plant but also to anything — except insect damage — which may check the growth of a plant, cause abnormal growth or cause the death of part or all of the plant. Consideration must therefore be given not only to ill-health resulting from the invasion of plants by parasitic organisms but also to cases where plants fail to thrive because of unsuitable soil, incorrect temperature, injury from fumes and sprays, excessive liming or even damage from hail or frost. Fruits and vegetables — such as apples, pears,and carrots — continue to live even when stored, and can suffer from various troubles which either reduce their food value or destroy them entirely.
Plant diseases may be divided into two main groups:
A. Non-parasitic diseases, which are not infectious; and
B. Parasitic diseases which are infectious.
These latter may be subdivided into two classes:
1. Fungus and bacterial diseases; and
2. Virus diseases.
A. NON-PARASITIC DISEASES
This type of plant disorder is often called ‘functional disorder’ or ‘physiological disturbance’. The term is, however, in-tended to include all those plant troubles which are not the result of infection by parasitic fungi, bacteria or viruses. It covers, for example, lasting ill-health due to waterlogged soil; excessive lime in the soil; shortage of an essential food element; excess humidity; fume and spray injury; and even short term damage by lightning, frost, hailstones and drought.
B. PARASITIC DISEASES
1. Fungus diseases are those caused by parasitic fungi, and with them are grouped the very similar bacterial diseases.
Parasitic fungi are mostly microscopic. They invade higher plants and grow in their tissues (cells), which they kill and then absorb the contents for food. They penetrate and grow in the plant cells by means of fine fungal threads, and spread from plant to plant by means of spores (the equivalent of seeds in higher plants). These spores are formed at the ends of special threads, often inside special fruit bodies, and they are produced in enormous numbers. When released they are carried by wind currents or water (by splashing) to healthy plants, where they alight, germinate, grow into the tissue and thus spread the disease again.
Most of the fungus parasites overwinter on the plant or in the soil by forming a type of thick-walled spore, or some other structure which is resistant to adverse weather.
These fungus parasites may be roughly divided into two types. The first — which includes the powdery mildews (common on many plants such as roses,, Michaelmas daisies and marigolds) — produces an obvious and superficial whitish growth on the surface of the leaves, stems and petals. This growth is made of fungal threads and spores which cover the leaf surface and feed by sending down a kind of sucker into the surface cells (epidermis) to absorb nourishment. In the second type the parasite grows down deeply into the internal tissues, sending up threads to produce spores at the surface. The first type is easy to check but, unfortunately, most fungus diseases belong to the second.
Bacteria which attack plants are much smaller than parasitic fungi, but infect in a similar manner by living in, and killing, the tissues. They cannot, however, form resting spores. They are able to persist by remaining in plant debris or in dormant cells in the tissue of seeds, corms, bulbs, etc. — a method often used by some fungus parasites as well, despite their ability to form resting spores.
2. With virus diseases, the exact identification of the parasite is difficult. Viruses are so small that they cannot be seen through the ordinary microscopes used to detect and study fungus and bacterial parasites. They can be seen only by means of modern electronic microscopes, but even so they are something of a mystery. Undoubtedly, plants suffering from virus disease have some form of infectious agent in their sap, but in many cases its exact nature has not been identified. It is, however, known to be very small and to multiply within the plants’ cells, so that it is usually distributed throughout all the tissues, with the exception of the seed.
Results of Virus Infection
Plants, unlike animals, do not seem to produce antibodies to fight viruses, although, in some cases, they are able to resist to a certain extent. Thus, more than one virus can exist in a plant at the same time. For some plants, though not many, virus attack means sudden death, but usually infected plants become more crippled and degenerate with the passing of each season.
Seeds of infected plants are usually free from virus, so, by saving seed, clean stock can be obtained again. This method is, however, suitable only in the case of fairly short-lived plants.
Special care has to be taken, for ex-ample, to exclude the risk of virus infection from good stocks of fruit trees, which are all propagated vegetatively as increase by seed is not feasible.
Symptoms of Virus Infection in Plants
The symptoms of virus infection are very varied. Common signs are stunted growth and mottled patterns on the leaves (often referred to under the general term ‘mosaic’). Other signs are ring-like markings on the leaves (ring-spots); curling or distortion of leaves and shoots; ‘breaking’ of flowers (white streaking in the colour of the petals); abnormal production of shoots (proliferation), and many other abnormalities. Infection usually results in all the cells of the plants being invaded by the virus, although shoots already developed are not usually much altered. New shoots and leaves, however, begin to show abnormal symptoms as they grow.
The symptoms of the same virus may vary in different plants — or even in different varieties of the same species. For example, some varieties of strawberry affected by the so-called yellow edge virus may be quickly and severely crippled, while other varieties, similarly affected, show no outward signs of the disease. The latter varieties are referred to as ‘carriers’ and, therefore, should not be planted near a very susceptible variety.
Plant viruses injected into the blood stream of animals, such as rabbits, may produce antibodies, with the result that animal antisera can be prepared and used in the detection of a specific virus. If the animal antisera is mixed with a dilute suspension of the virus from the sap of a plant, a cloudiness (precipitate) will appear.
Serological reactions such as this are used extensively in the identification of some plant viruses, and can be of great value in the work of virus classification. No generally accepted method of classification has, however, been adopted. Scientists usually use the name of the host plant first and follow this with the most obvious symptoms caused by the virus (for example,black ring-spot virus and banana bunchy top virus).
The Spreading of Virus Diseases
Aphids are mostly responsible for the spread of virus diseases, although a few are spread by leaf-hoppers, thrips and whiteflies. These insects are referred to as insect ‘vectors’. When feeding they may take up the virus from the sap of virus-infected plants and transmit it to healthy plants.
Viruses can also be transmitted by the propagation techniques of budding and. This is a very common method of increase and spread of diseases, so that the use of clean stocks is essential for propagation.
Only rarely does a virus travel with the seed, and then only in a minute percentage: examples aremosaic, cucumber mosaic and bean mosaic. It is considered that there are many different strains of virus and that these, like other parasitic organisms, may vary greatly in their virulence towards different hosts.
Some viruses remain in and contaminate the soil in which virus-infected plants have been grown.
Raspberries are a case in point, and although it is not understood exactly how the soil is contaminated, it is certain that there are some fields where raspberries cannot be grown with profit because the young shoots become infected with virus from the soil.