Plant Diseases Caused By Viruses.
In ancient times diseases were attributed to supernatural causes. Following the discovery of the microscope, however, it was found that tiny organisms such as bacteria or fungi were associated with many diseases and in the eighteenth century the germ theory of disease was established. It was then believed that for each disease there would be isolated some organism which could be cultured and examined microscopically. This theory had to be abandoned when in 1892 Iwanowski showed that a bacteriologically sterile filtrate of sap from mosaic-infected tobacco plants caused infection in healthy plants. Eventually his work led to the recognition of the group of diseases which have become known as viruses. In 1899 Beijerinck propounded his virus theory of a “living contagious fluid.” From then on many plant-virus diseases were recorded and described, but it was not until the early 1920's that fundamental studies were undertaken. Most of our knowledge of viruses has, therefore, been accumulated during the past twenty-five years. Viruses have three distinctive features.
They are infectious.
Under a microscope using ordinary light they are invisible.
They can multiply only in living tissues.
Present workers define a virus simply as “an infectious disease, the casual agent of which is invisible.”
Regarding the nature of viruses there have been many theories but until recently it was generally accepted that they were minute organisms somewhat of the nature of very small bacteria. Studies during the past eleven years, however, have altered this conception. About twenty viruses, from a total of over two hundred known, have been isolated in a pure crystalline form. Chemical studies have shown that these are nucleo-proteins composed of nucleic acid and varying numbers of amino-acid groups. Each virus particle is a single molecule of nucleo-protein. This chemical simplicity distinguishes them from even the simplest organisms and might appear to indicate that they are inanimate chemical compounds. On the other hand they possess such indisputable properties of life as the power to multiply and mutate. Hence it must be concluded that they occupy a borderline position between the living organism and the non-living chemical.
Although each virus particle is a single molecule, the molecules them selves are relatively very large, varying in molecular weight from about 1,800,000 to 7,500,000. In actual dimensions, however, they are small, the diameter of the larger spherical particles being approximately one-fortieth of a micron compared with one micron for the smaller bacteria. The electron microscope which gives magnifications over thirty times greater than the ordinary microscope has shown that the particles vary from spherical to needle-shaped.
Virus symptoms are many, the commonest being a change in colour of foliage. Thus leaves may become chlorotic, show light and dark green mosaic mottling or a pale pattern of concentric rings. Other signs of their presence include an alteration in leaf shape or the development of necrotic areas on leaves or stems. In most cases stunting of plants accompanies foliage symptoms. Flowers are often affected, frequently showing a spectacular break in colour. In the case of fruit some viruses induce mottling and others necrotic markings. From the economic point of view the most significant symptom is reduction in cropping power. It has been shown that “carriers,” which are diseased plants showing no external symptoms, sometimes have a reduced yield. Actual death of plants rarely follows virus infection.
In general viruses are systemic. Therefore they are transmitted by all vegetative methods of propagation. They are carried in tubers of potatoes, runners of strawberries and in all cuttings, bulbs, etc., from diseased plants. Transmission may take place through the use of buds or scion wood from infected trees.
Some viruses are transmitted when leaves of healthy plants are rubbed with infective sap. Experimentally this technique is commonly practised while in the field, when the more highly infectious diseases are present, it is used unwittingly by workmen whose fingers become contaminated when handling infected plants during cultural operations.
Sucking insects are responsible for the natural spread of most viruses. After feeding on infected plants they are able to transmit the disease to healthy plants. As a rule only a few species of such insects are involved in the spread of any one virus.
Seed transmission may occur, but fortunately with only a few viruses.
Owing to their inability to produce anti-bodies, plants, unlike animals, never recover from virus infection.
The ability to mutate, upon which evolution mainly depends, may result in one virus having several strains. Studies of tobacco-mosaic mutants have shown slight differences in their chemical compositions. Symptoms produced by different strains vary considerably, some having only a slight effect on plant growth. When infected with a mild strain plants are immune to a more virulent strain. The mechanism of this immunity is different from that which occurs with animal viruses as no anti-bodies are formed. It has been suggested that the tissues can accommodate only a limited quantity of any one virus so that the presence of one strain prevents the development of a second.
Serology is assuming importance in the study of plant viruses. If the sap from an infected plant is injected into the blood-stream of an animal the virus acts as an antigen, stimulating the production on anti-bodies. When antiserum obtained from the animal is mixed with further sap from the diseased plant the anti-bodies react with the virus. Formation of a precipitate is the reaction most used. Besides being useful for identification the technique is of value in studying the relationship of different strains, these having the same serological reactions.
Host-range may be restricted or wide. Some viruses are known to infect only one species some attack a limited number of species within one family, while others may infect many species in unrelated families.
Many factors are taken into consideration in the identification of viruses, including symptoms, host-range, methods of transmission serological reactions and physical properties. Physical properties employed are thermal death-points, longevity in vitro and dilution end-point. All factors may not be applicable in the identification of any one virus.
Virus-infected plants do not recover naturally nor can recovery be brought about by treatment. Therefore, control measures must be directed towards preventing the introduction of disease into crops and spread within the crops once a disease has appeared.
The following control measures have been suggested:–
Rogueing of infected plants.
Destruction of alternative hosts.
Isolation of susceptible crops from likely sources of infection.
Sanitation in case of highly infectious viruses.
Use of disease-free seed in case of seed-carried viruses.
Use of healthy stocks in vegetatively propagated plants.
Use of resistant varieties.
Heat treatments (applicable in only a few instances where viruses have very low thermal death-point).
Control of insect vectors.
Possible use of acquired immunity.
I have made a number of definite generalisations, but one of the interesting features of viruses is that there are exceptions to most statements. For instance, a few viruses are not truly systemic, instances of recovery are known and in certain cases recovery can be brought about by treatment.
A study of virus diseases in New Zealand was begun in 1930. Since then some thirty-three viruses attacking sixty-nine hosts have been recorded and the list is being added to every year. Included in these are some of the most serious plant diseases present in the country. Though difficult to arrive at a reliable figure for losses caused by diseases it is, in a few instances, possible to make estimates. Losses through tobacco-mosaic amount to not less than £50,000 a year, while virus diseases of potatoes cause an annual loss of at least £20,000. Even greater losses must be brought about by tomato spotted-wilt, which attacks tomatoes, lettuces, peas and many garden ornamentals grown for the cut-flower trade.