What plant diseases are caused by viruses. Viral plant diseases: types of diseases, characteristics of pathogens. Varieties resistant to the virus

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Viruses You've probably heard about influenza, rabies, herpes, and AIDS viruses. These viruses cause diseases in humans and animals. There are viral plant diseases, such as tobacco mosaic, in which tobacco leaves become covered with whitish spots. Even

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Viruses and viroids are constantly present in plants, and their harmfulness usually manifests itself in stressful situations, acquiring economic importance only when infected with aggressive strains. Plants can independently defend themselves against many viruses, but the result of this fight manifests itself in the form of point or extensive necrosis, mosaics, and deformations. As a result, product quality deteriorates and yields decrease.
Chemical methods of combating viruses are not yet well developed, since the reproduction of viruses is so closely related to the metabolism of the host plant that the direct selective effect of any drugs on the pathogen itself has a negative effect on the plant cell. Therefore, protection against viruses comes down to preventing diseases, vaccination with weakly pathogenic strains of viruses, or reducing the rate of development of viral epiphytoties using various agricultural practices.
In practice, the following methods of combating viral and viroid diseases are used:
1. During vegetative propagation, periodic cleaning of the plantings of mother plants is carried out. This method is effective against pathogens with well-defined symptoms.
2. Thorough examination of plants and removal of diseased parts (phytosanitary cleaning) during the period of germination, the beginning of flowering and the beginning of fruiting.
3. Thermotherapy can dramatically reduce infestation, and sometimes completely rid plants of a number of heat-labile viruses. This method can be used both to disinfect vegetative organs and to fight infection inside seeds. Temperature conditions are strictly specific and are discussed below in the relevant sections.
4. Using the method of cultivating apical meristems allows you to get rid of most pathogens of viral infections. The method is ineffective against viroids. The best healing effect from viral infections is obtained by combining the method of culture of apical meristems with preliminary thermotherapy or chemotherapy, in which antiviral additives (glycoproteins, polysaccharides, ribonucleases, analogs and derivatives of nitrogenous bases, antibiotics) are introduced into the nutrient medium for cultivating meristems or treated with them. original donor plants of meristems.
5. Fight against virus reservoir plants and infection vectors.
6. Reducing the supply of viruses in environmental objects (in seeds and in the plants themselves).
7. Stimulation of nonspecific immunity in plants: with the help of resistance inducers (elicitors), growth regulators, etc.
8. Preimmunization, or vaccination. It is known that virulent strains do not cause disease symptoms if the plant was previously infected with a weakly pathogenic or avirulent strain of a related virus. A similar vaccination was used in greenhouses to protect tomato varieties and hybrids that are not resistant to TMV. But the preimmunization method has not been widely used in practice due to the possibility of the pathogen mutating, increasing its harmfulness when co-infected with other pathogens, and due to a number of other reasons. However, in recent years, good vaccines have been obtained not only for TMV, but also for cucumber green mottled mosaic virus (Andreeva et al., 2000).
9. Selection for virus resistance followed by the use of immune varieties and hybrids. In this case, breeding work should be carried out not only on the basis of resistance to the virus, but, preferably, also to its vector. Of no less importance is the production of tolerant (hardy) varieties in which the systemic spread of viruses is limited and their concentration is reduced. Tolerance often leads to an asymptomatic course of the disease, while plant productivity is practically not reduced.
10. Creation of transgenic plants. Changing the plant genome due to the inclusion of new resistance genes obtained from donors. When the gene responsible for the synthesis of the envelope protein of the tobacco mosaic virus is introduced into tobacco cells, resistance to this disease appears. Thus, transgenic zucchini carrying the genes for the zucchini yellow mosaic and watermelon mosaic virus shells had no symptoms of virus damage, while control plants and transgenic plants with one gene had obvious damage (Avetisov, 1999). Field tests of virus-resistant tomato, potato and many other crops obtained using this approach have shown its effectiveness and the promise of further research in this area.
11. State (external) and on-farm (internal) quarantine. When importing plants, the quarantine certificate must confirm that the material does not contain quarantine objects. Accordingly, internal quarantine involves the localization and destruction of outbreaks of diseases registered as quarantine. The effectiveness of external and internal quarantine measures largely depends on the reliability and speed of virus identification methods.
12. Organizational and economic measures include disinfection of cutting instruments and tools in disinfecting solutions (formalin, potassium permanganate, alcohol) or their heat treatment, since many economically significant viruses are transmitted by contact; work in changeable shoes and clothes; placement of disinfection mats in front of the entrance to the greenhouse; regular visual inspection of plants.
13. Relieving the symptoms of the disease by maintaining an optimal regime for growing crops, including mineral nutrition. During the development of epiphytoties, plants are sprayed with solutions of microelements, phosphorus and potassium fertilizers, which stimulate the plant to accelerate its passage through the phases of ontogenesis and, as a result, the onset of age-related resistance.
The last three methods together form the basis of preventive measures.

Structure and basic properties of phytopathogenic viruses.

Reproduction and spread of viruses.

The nature of viruses.

Structure and basic properties of phytopathogenic viruses.

The existence of viruses was first discovered in 1892 by the Russian scientist D.I. Ivanovsky while studying tobacco mosaic disease. He found that viruses are smaller in size than bacteria, they have a corpuscular structure, are infectious and reproduce only in a living plant, and they cannot be grown on an artificial nutrient medium. Soon after this, similar diseases were discovered in other plants - potatoes, cucumbers, beans, cereals, fruit and berry crops.

Currently, we can say that viruses characterized by the following features:

1. They reproduce only in the body of the host or vector; do not grow on artificial nutrient media. They have a unique mechanism of reproduction.

2. They do not have a cellular structure: they consist of RNA - ribonucleic acid (single or double-stranded) or DNA - deoxyribonucleic acid, usually surrounded by a protein shell.

3. The genome of viruses is represented only by nucleic acid, which reproduces due to the host’s enzymatic system.

4. Nucleic acid is responsible for infectivity, and protein primarily protects RNA.

Currently, most scientists believe that viruses are the simplest forms of life that do not have a cellular structure and are activated when they enter the cells of susceptible organisms.

Viruses are rod-shaped (TMV), filamentous (potato virus X, citrus tristeza), spherical (tobacco necrosis) and bacilli-shaped (wheat streak mosaic). The size of viruses ranges from 25 nanometers (nm) for tobacco necrosis virus, and up to 2500 nm for citrus tristeza; 1 nm (nanometer) is equal to 10 -9 = 0.001 µm).

Reproduction and spread of viruses.

By the nature of the impact on the affected organism, viruses are divided into two large groups - mosaic-type viruses (mosaic) and icteric-type viruses (jaundice).

As a result of infection with mosaic viruses, the color of the leaves changes, alternation of light and dark green, yellow, green areas of the leaves, the appearance of necrotic spots, streaks, rings, etc. are observed.

Such symptoms result from the destruction of chlorophyll in the affected areas of the leaf, the use of plant nitrogen and phosphorus to build virus particles, and the increased activity of plant respiratory enzymes. Sometimes, with mosaic diseases, a disturbance in the shape of the affected leaves is observed - wrinkled, threadlike and ferny-like.

Mosaic viruses are transmitted non-persistently by insects (the main carrier is aphids), but they can also spread by contact-mechanical means during inter-row treatments, when the tops are damaged and diseased and healthy plants come into contact; when caring for plants (when pinching, chasing, pruning, pinching, breaking out shoots, etc.).

Sources of infection of mosaic viruses can be dry plant debris, seeds, tubers, weeds, soil, etc.

When plants are infected with jaundice-type viruses, more severe damage is observed than when infected with mosaic-type viruses.

Viruses settle in the phloem, which disrupts the transport of carbohydrates from leaves to other plant organs. A lot of starch accumulates in the leaves, they become thick and brittle. Jaundice viruses also disrupt growth processes, which leads to dwarfism and excessive bushiness of plants, deformation of vegetative and generative organs (leaf curling, dwarfism, excessive bushiness, overgrowth). A common symptom is yellowing and chlorosis of the leaves.

Sources of infection of jaundice-type viruses can be wheatgrass rhizomes, wormwood roots, dark leafhopper larvae (pupation of oats); tubers, planting material (cuttings, layering, young trees, etc.), seeds, nematodes.

The nature of viruses. To date, there is no generally accepted unified classification of viruses, so phytovirologists prefer to use the concept of a group of viruses, and sometimes cryptograms. In each group, a typical representative is described in detail, and related viruses are indicated. All phytopathogenic viruses are grouped into 20 groups.

For example, the group of tobamoviruses includes the tobacco mosaic virus as a typical representative, which is characterized by RNA content (5%), molecular weight - RNA 2.06 10 6, virion length is about 30 nm, inactivation temperature is over 90 ° C. The virus infects a wide range of plant hosts with varying symptoms. This group includes the tomato green mosaic virus and the cucumber mottled mosaic virus.

Control questions

1 . The structure of phytopathogenic viruses.

2. Reproduction of viruses.

3. Measures to combat viral diseases.

4 . Methods for diagnosing viral plant diseases.

Literature

1. Phytopathology: Textbook / M.I. Dementieva. - M.: Kolos, 1977. - 366 p. - (Textbook and teaching aids for higher agricultural educational institutions).

2. Yakovleva N.P. Phytopathology programmed training. 2nd edition, additional: Textbook for students of higher educational institutions., M.: Kolos, 1992. – 382s.

3. Popkova K.V. General phytopathology. - M.: 2005.

Basic properties of viruses

In nature, viruses exist in two forms: extracellular and intracellular.

Extracellular form of the virus called virion - it is an inert infectious particle that consists of a nucleic acid and a protein shell - capsid . The nucleic acid in the virion - the genetic apparatus or genome - can be of only one type - either DNA or RNA. The genome can be represented by one chain (single-component or whole genome) or there are several of them (fragmented genome). Most plant viruses are RNA viruses.

Capsid consists of protein subunits - capsomers. Capsids come in various shapes:

1). Isometric: spherical or polyhedral (“polyhedron” means polyhedron) with a cubic type of symmetry.

2). Anisometric with a spiral type of symmetry - rod-shaped, thread-like. There are viruses with a combined type of symmetry, for example, in the shape of a tadpole or bacilliform.

Complex capsids are those built from more than 60 structural units containing 5 or 6 capsomeres.

The sizes of various viruses most often range from 20 to 300 nm, but filamentous viruses of greater length are found - up to 2000 nm.

Due to the presence of a protein shell in plant viruses, which encloses nucleic acid, viruses have antigenic activity, or immunogenicity, that is, they are capable of causing the formation of antibodies when introduced into the body of animals.

MANIFESTATIONS OF VIRUS ACTIVITIES

Many viruses are capable of infecting any one host. Others, such as tobacco mosaic virus (TMV), have a wide host range. Some plant viruses are capable of reproducing in the bodies of insect vectors.

The intracellular activity of plant viruses probably consists of a number of the following stages:

1. The virus enters the cell entirely - the entire NC in the capsid shell - through damage in the membrane.

2. Capsid shedding . When infected with TMV, the first symptoms appear several hours later than when infected with free RNA of this virus. This is a fact in favor of the statement that a virus that has entered a cell “undresses” - sheds its capsid.

3. Reproduction of viruses . Viral RNA more often penetrates into the nucleus of a plant cell, where the complementary RNA (¾) chain is synthesized and formed double-stranded RNA – replicative form (RF). Then, probably, multiple replication of viral RNA occurs in the nucleoli.

4. Biosynthesis of the structural protein of the virus . After viral RNA replication increases in the cell, the amount of capsid protein increases. The synthesis of these proteins takes place on the ribosomes of the host cell.

5. Aggregation of viral RNA and capsid . Appearance of mature viral particles.

6. Exit of viruses from the cell in plants occurs along plasmodesmata.

3. BACTERIOPHAGES

Bacteriophages – bacterial viruses. They consist of a head with cubic symmetry and a process or “tail” with spiral symmetry. At the end of the process there is basal plate with spines and tentacles designed to attach to the bacterial cell wall. Nucleic acid (most often DNA) is located in a polyhedral head. There are two types of life activity of bacteriophages: moderate and virulent.

Life cycle virulent bacteriophage consists of the following steps:

1. Absorption – attachment to the wall of the bacterium with the help of spines and tentacles of the basal plate.

2. Injection – injection of phage DNA into the bacterial cell. The process sheath contracts, the pressure inside the phage particle increases and DNA is injected into the bacterial cell, while the capsid remains outside the cell.

3. Incorporation into the cell nucleoid -owner.

4. Repeated self-copying of phage DNA.

5. Capsid regeneration.

6. Maturation (assembly) of phage particles can occur spontaneously, without the participation of special factors, as a result of aggregation of NK and capsid proteins.

7. Cell lysis and release of bacteriophages occurs when the concentration of phages in a cell reaches a critical level, for example, when it accumulates

10,000 viral particles per cell .

This cycle is also called lytic .

Temperate bacteriophages in the life cycle they undergo the first three stages (absorption, injection, integration into the nucleoid), and then replicate synchronously with the bacterial chromosome. This phenomenon is called lysogeny . After several generations, under the influence of environmental conditions (UV, X-ray radiation), the viral genome can pass from a moderate form to a virulent one and cause the lysis of all infected cells. In another case, the bacteriophage DNA can leave the nucleoid and leave the cell, “taking with it” part of the host DNA. This genetic information is transferred by the phage to another bacterial cell. This phenomenon is called transduction.

4.VIROIDS

Viroids – these are the smallest known pathogens; they are much smaller than the smallest viral genomes and lack a protein shell. Only plant viroids are known; they consist of a single-stranded RNA molecule that replicates autonomously in infected cells. Viroids have been identified as causative agents of dangerous diseases. One of them has caused the death of millions of coconut trees in the Philippines over the past fifty years, the other damaged the commercial cultivation of chrysanthemums in the United States in the early 1950s.

First viroid– Potato spindle tuber morphology, or PSTV, was identified in 1971. It is the largest known viroid; its RNA consists of 359 nucleotides and has either a closed ring or a hairpin structure. Complementary base pairs are joined by hydrogen bonds to form double-stranded RNA. Viroids are found only in the nuclei of infected cells. They replicate like viruses, that is, they synthesize a complementary chain that functions as a template. In this case, viroids use the enzyme systems of the host cell.

Other molecular pathogens that are not related to viroids also occur in living organisms. Some protein fragments can control their reproduction in animal cells without the participation of nucleic acids; such particles are called prions.

5. Principles of virus classification

The classification of plant viruses is based on the study of the properties of specific viral isolates . A virus isolate is a homogeneous population of a virus isolated from any source and obtained by cloning through a series of sequential passages through a suitable indicator plant. Viral isolates that do not differ in properties are classified as one strain virus. Isolates that differ slightly are classified as one mind. Plant viruses that have many similar properties constitute group viruses.

When classifying viruses, the following genetically stable characteristics are used: type of nucleic acid; number of genome strands and fragments; the nature of the distribution of genome fragments in virions; features of the terminal structure of nucleic acid molecules; number and molecular weight of capsid polypeptides; morphological properties of virions: size, shape, type of symmetry; hydrodynamic properties (character of sedimentation during high-speed centrifugation); antigenicity and features of serological tests with antisera of a certain type; modes of transmission and attitude towards vectors; range of host plants and the nature of symptoms on them.

Based on a comparative analysis of the properties of plant viruses, they are combined into 26 groups according to the classification of viruses adopted by the International Committee on Taxonomy of Viruses of the International Union of Microbiological Societies. Groups of plant viruses are heterogeneous in composition, some of them are represented by only one member. The taxonomy of plant viruses in its current state is not natural.

Nomenclature. The names of plant viruses, despite numerous attempts to latinize them, remain largely trivial, that is, formed during the initial isolation and description of the virus, mainly in accordance with the host plant and the external symptoms of the disease, e.g. tobacco mosaic virus, barley yellow dwarf virus etc. Moreover, the name of the host from which the pathogen was first isolated in biocenoses is fixed. English names of viruses are used as international names: tobacco mosaic virus, barley yellow draft virus respectively.

6. Methods for studying viruses

1).Electron microscopy. The resolution of the electron microscope is up to 1A. An image of an object is obtained as a result of scattering of a flow of electrons by the sample under study. A beam of electrons moving in a vacuum is focused by an electric or magnetic field (electron lenses). This method can be determined shape and size of virions, localization in the cell, in the plant, cytological changes in cells.

2).Ultracentrifugation. Centrifuges that reach speeds of more than 40,000 revolutions per minute are called ultracentrifuges. As a result, additional gravity develops, which promotes the sedimentation of small particles, which are viruses.

3).Electrophoresis. All viruses in the capsid contain ionized groups, which determine their mobility in an electric field. The speed of movement of viral particles depends on the molecular weight and total charge. Thanks to this method, it became possible to separate a mixture of viruses using electrophoresis - movement in an electric field.

4).Immunological (serological) methods. Any virus, be it a plant, animal or bacterial virus, when injected into rabbits or other small mammals behaves as an effective antigen. As a result, specific antibodies, which react with antigens (viruses) and are used to detect them.

For getting diagnostic antiserum Purified virus is used as an antigen. This antigen is administered intravenously or intraperitoneally to the rabbit. The number of injections can be different (4-8) with an interval of 1-2 days. 7-11 days after the last injection, blood is drawn, it is left for 1.5-2 hours at a temperature of 370C, then centrifuged. The serum is stored in ampoules of 5-10 ml at a temperature of - 40C.

To the main methods for detecting reactions antigen-antibody reactions include precipitation and agglutination.

Precipitation(from Latin praecipitacio - falling down, precipitation) - a reaction that allows the precipitation of viruses (antigens) with the help of antibodies, has high sensitivity and specificity.

Agglutination(from Latin agglutinatio - gluing) - gluing microorganisms, including viruses, into clumps (lumps) and their precipitation. Used to diagnose viral diseases.

5).Method of indicator plants. The method is based on visual inspection. When a plant is infected with viruses, symptoms of damage appear, which depend on the host plant, the strain of the virus and environmental conditions. Indicator plants – these are plants that react with characteristic symptoms to infection with a virus. For infection, young plants in the phase of 2-3 true leaves are used. Infection ( inoculation ) is carried out with an extract of diseased tissues. To do this, they are ground in a mortar with a buffer solution, the homogenate is filtered through gauze or nylon. Most often, inoculation is carried out mechanically: the extract is rubbed in with fingers, a swab, a spatula, a brush or a brush. After 5-10 minutes, excess viral material is washed off with distilled water. Plants are marked by attaching labels to infected leaves. Plants are placed in a dark place for a day, then transferred to a phytotron for 12-14 days until viral symptoms clearly appear. Viral lesions are identified using tables and photographs.

Highlight four main types of reactions plant indicators for damage by the virus:

Immunity – when plants are not affected by this virus;

Hypersensitivity – when plants are affected with the formation of local necrosis, which occurs due to the death of cells near the point of infection;

Tolerance – when the virus is transported through plant tissues, but the symptoms of the disease are mild;

Systemic damage – when the virus is transported throughout all plant tissues with a clear manifestation of disease symptoms.

7. Types of Viral Symptoms

Plants that easily show symptoms characteristic of infection with this virus are called indicator plants. In addition to external symptoms, a viral infection causes various types of histological and cytological changes in a diseased plant. They manifest themselves in anomalies of the vascular system and various types changes in cell structure– from changes in the structure of individual organelles to the formation in the cell specific viral inclusions. Inclusions can be formed by viral particles, which are localized in the cell in a manner characteristic of a given virus or in combination with products of viral influence. The type of intracellular inclusion is characteristic of a given virus and is used to identify viruses.

The following are distinguished: main types of viral symptoms :

1).Mosaic– uneven green color of the leaf blade or the presence of yellowish or light green spots.

2).Chlorosis– general or symmetrical yellowing of leaf tissues.

3).Necrosis– death of plant tissues, often a consequence of mosaic or chlorosis with their strong development, but often develops independently. Highlight local necrosis– develops in places where infection enters the plant and systemic (scattered) necrosis- can appear on any part of the plant.

4).Deformation Plant organs are diverse and can be caused by physiological disturbances that lead to changes in the morphology of organs or the entire plant. As a result of impaired growth coordination, wrinkling, curling, swelling, and curvature of shoots develop.

5).Growth inhibition can be expressed in general dwarfism of plants, shortening of internodes at the top of the shoot.

6).Withering observed with severe damage to the vascular system.

7).Growth (proliferation). The immediate causes of growth may be disturbance of the dormancy of axillary and wintering buds or degeneration and vegetative growth of generative organs. An accompanying sign of growth is threadlike stems and sprouts.

8).Abortion– falling of flowers and ovaries, drying out of set fruits or individual seeds in a fruit, seedlessness of fruits.

9).New growths – tumors on various parts of the plant (for example, proliferation of leaf veins), leaf-shaped outgrowths of enation, etc. .

10).Anthocyanosis – purple, red-violet or blue-violet coloring of leaves or their edges, veins, stems.

11). Variegation – uneven color or partial discoloration of petals, for example, in a tulip.

In most cases, a diseased plant exhibits several symptoms in combination.

8. Infection and movement of viruses in plants.

Phytopathogenic viruses infect a wide range of plants from different families. Moreover, the same plant species can be the host of many viruses. Each virus has a specific circle host plants i.e., plants susceptible to a given virus, in whose cells it can multiply, giving new generations of viral particles. For example, some viruses that infect strawberries (genus Fragaria) do not cause disease in plants of other genera. And the tomato bronze virus is characterized by a wide range of hosts: in this list of plants there are 166 species from 34 families, belonging to both the class of monocotyledons and the class of dicotyledons.

Viral infections of plants differ significantly from viral infections of animals and microorganisms.

First, phytopathogenic viruses penetrate plant cells through damage in the cell membrane when it is mechanically injured or as a result of piercing by the mouthparts of arthropod carriers.

Secondly, a plant infected with a virus becomes its permanent carrier. In this case, the virus penetrates almost all organs and tissues of the infected plant (with the exception of viruses that have tissue specificity).

Virus infection More often it is systemic, less often local. At systemic damage viral particles are able to move from cell to cell along plasmodesmata, through intercellular fluid, through conductive elements to other plant organs. When plant cells are systemically infected, they can accumulate the virus in significant quantities while remaining viable.

Local lesion can be explained by two reasons: tissue specificity and local tissue necrosis, resulting in localization of the viral infection at the site of penetration of the virus into the plant tissue.

Often plants respond to infection with both types of symptoms, and local tissue necrosis at the sites of virus penetration is combined with the development of systemic plant infection, which can also lead to local or systemic necrosis of tissues of various organs.

Movement of viruses in a plant occurs through plasmodesmata, through the intercellular fluid, through the phloem and xylem. The speed of propagation depends on temperature: the higher the temperature, the higher the speed. Viruses can spread through the phloem at a speed several centimeters per hour. Most viruses move with the flow of carbohydrates through the phloem, and less often through the xylem.

Viruses accumulate only in those cells where they reproduce. They are practically unable to reproduce in vessels. The maximum number of virus particles that accumulate in a cell depends on the host plant species. For example, 10 times more tobacco mosaic viruses accumulate in the cells of tobacco leaves than in tomato leaves.

9. Spread of viruses in biocenoses

Plant viruses have the ability to quickly spread in biocenoses. Distribution methods are different:

1. Transmission by contact-mechanical means during mutually damaging contact of a healthy and diseased plant (during pruning, pinching, harvesting, as well as in thickened plantings).

2. Distribution by seeds and pollen.

3. For vegetatively propagated crops, the main way of spreading viruses is through planting material.

4. Invertebrates with piercing sucking or gnawing apparatus (aphids, leafhoppers, thrips, scale insects, scale insects, mites).

5. Nematodes.

6. Using dodder.

7. Spores and zoospores of phytopathogenic fungi.

The intensity of epiphytoties depends on different vectors. Currently, about 400 different vectors have been identified. Most of them are insects. The period from the beginning of the acceptance of the virus from a donor until the insect becomes able to carry the virus is called incubation period. The time during which the carrier with the virus remains infectious is referred to as the definition persistence. Based on the transmission characteristics of plant viruses, three groups are distinguished: persistent, semi-persistent, non-persistent.

Non-persistent viruses transmitted by vectors directly after a short (several seconds) feeding on a diseased or healthy plant. Vectors quickly (within a few minutes) lose their ability to infect if they stop feeding on a diseased plant. Non-persistent viruses include potato U-virus, bean yellow mosaic virus, etc.

Persistent viruses are transmitted by a vector not immediately after their acquisition on a diseased plant, but after a latent period of a certain duration (from several hours to several days). The carrier retains the ability to transmit the virus for a long time, sometimes throughout his life. Among them are pathogens that do not reproduce in the vector (for example, barley yellow dwarf virus) and viruses that can reproduce in the body of the vector (thistle yellowing virus).

Semi-persistent viruses represent an intermediate group. They can be transmitted by a vector immediately after it feeds on a diseased plant. After stopping feeding, the ability to become infected remains for 3-4 days. There is no latent period. A representative of this group is the sugar beet jaundice virus.

Individual vectors can transmit many different viruses; for example, the aphid species Myzus persicae can transmit up to 70 viruses. The spread of viruses is facilitated by the cosmopolitanism of insects. Thus, some thrips (Thrips tabaci) feed on plants of 140 species from 40 families.

10. PRESERVATION OF VIRUSES THROUGH AN ANNUAL CYCLE

Preservation of viruses during the winter period can be accomplished in various ways. Perennials are characterized by preservation in stems, roots, tubers, and cuttings. Some viruses overwinter in seeds. Viruses with a wide range of hosts are well adapted to persist in nature if there are perennials among susceptible plants. Some viruses can overwinter in the eggs of insect vectors. Tobacco mosaic virus can survive in soil in plant debris. The lettuce virus can survive in the spores of the soil fungus Olpidium. If spring and winter wheat are grown in the same area, the stripe mosaic virus is transmitted from spring plants to carrion seedlings, and then to winter plants.

11. ROLE OF VIRUSES IN NATURE AND ECONOMIC IMPORTANCE OF Viral diseases

Viruses are constantly present in plants. Many viruses are capable of causing serious diseases, leading to significant yield loss or deterioration in its quality, in particular, seed germination and reproduction rate, plant resistance to infections of any etiology, etc. are reduced. Diseases caused by viruses are called viruses . Viruses directly or indirectly affect the physiological processes of the infected plant, with altered metabolism reminiscent of the normal state of an aging organism. When symptoms of virosis appear in plants, respiration increases, this is due to uncoupling of respiration and oxidative phosphorylation.

Viruses in nature probably play the role of population regulators of living organisms.

It is often difficult to estimate crop losses due to virus damage. Their sizes vary by year and region, so the average values ​​are essentially not indicative. But in the United States they believe that wheat harvest losses, according to average long-term data, are about 2%. At times these losses reach 20%. Viral diseases of perennial crops are dangerous because they cause the death or weakening of plants, which take several years to regenerate. For example, in West Africa cocoa tree shoot deformation virus periodically destroys entire plantations. Phytopathogenic viruses pose a particular danger to vegetable plants propagated vegetatively, for example, potatoes and many ornamental plants. Often all plants are sick, and at the same time the reduction in yield will be small (about 10%). However, in unfavorable weather conditions, a viral infection can develop rapidly and lead to complete degeneration of plants and planting material.

12. METHODS FOR DIAGNOSIS OF VIRAL DISEASES

Visual diagnostics. Possible only if the virus causes certain pathological changes in the body - symptoms. Diagnosis can be difficult due to the asymptomatic (latent) nature of the disease. In addition, similar symptoms can be caused by mineral nutrition disorders, damage by phytoplasmas, bacteria, and hormonal herbicides. Thus, accurate identification of viral damage only by external signs is impossible.

Indicator plant method is based on the use of test (indicator) plants that give clear symptoms.

Serological diagnosis . If you inject a rabbit with a purified plant virus preparation (antigen), the animal’s body produces specific antibodies that bind the antigen. As a result, a precipitate (precipitate or serum) is formed, visible visually or using a microscope. A modification called drip method : On a glass slide, a drop of antiserum is mixed with a drop of plant sap. After a few minutes, the reaction is assessed under a microscope at low magnification in a dark field or even visually without a microscope.

Electron microscopy method allows you to determine the shape, structure and even size of viruses.

Gel electrophoresis method. This method is based on the electrophoretic separation of pre-purified nucleic acids of a virus or viroid in a gel at a current of 3 and 6 mA with staining of the zones. When comparing the resulting colored lines with the height of standard marker zones, the mass and size of the viral structures are determined.

DNA probe method based on the principle of nucleic acid complementarity. Synthesize probes who will find out specific nucleotide sequences of the RNA virus. Depending on the choice of probes, groups, types and even strains of viruses can be differentiated.

Inclusion method. The development of some viruses in plant cells is accompanied by the formation in it of an accumulation of viral particles (inclusions, Ivanovsky crystals), which are detected even with the help of a light microscope. Each type of virus has its own form of viral inclusions, usually formed in the cells of the hairs or epidermis of leaves. For example, the tobacco mosaic virus is characterized by needle-shaped and hexagonal crystals; For potato virus X, the formation of spherical amorphous bodies is typical.

13. SYSTEM OF PROTECTIVE MEASURES AGAINST PHYTOPATHOGENIC VIRUSES

There are no direct methods for protecting plants from viruses. Applicable integrated protection system against viruses is aimed at reducing sources of infection inside and outside crops or plantings. The main areas of protection are:

1).Immunization plants with weakly pathogenic strains of the virus. Vaccine strains are used both outdoors and indoors. Seedlings (sprouts) are sprayed. The vaccine causes an asymptomatic pathological process, which is soon suppressed by the body's immune systems.

2).Plant selection aimed at strengthening immunity and tolerance. The widespread introduction into production of hybrids carrying resistance genes to certain viruses significantly curbs the spread of these viruses.

3). Elimination of sources of infection . This includes culling plants with viral symptoms from plantings, weeding, and removing plant debris. Weeds are often reservoirs of viruses. Cleaning the queen cells is of particular importance. This method is effective for plants that have clearly visible symptoms.

4).Use of virus-free seeds . Seed plants must be grown in sufficient isolation from external reservoirs of infection. It is advisable to store seeds in foil or any airtight packaging.

5).Thermotherapy allows you to dramatically reduce infestation, and sometimes completely rid plants of viruses. Warming of seeds or vegetative organs is strictly specific to each crop.

6).Chemotherapy – treatment with chemicals that inhibit the replication of viruses or reduce their infectious properties. These substances include analogues of nitrogenous bases (purines or pyrimidines).

7).Use of virus-free planting material , obtained by the method apical meristems. The best healing effect from viral infections is obtained by combining the apical meristem method with preliminary thermotherapy and chemotherapy. Special antiviral additives (glycoproteins, polysaccharides, nucleic acids, antibiotics) are introduced into the medium for cultivating meristems or the original donor plants are treated with them.

8).State or on-farm quarantine. When importing plants, the certificate must contain confirmation that the material does not contain quarantine objects.

9).Organizational and economic measures include disinfection of cutting instruments and tools in a solution of formaldehyde, potassium permanganate, alcohol or their heat treatment, work in overalls and shoes, use of disinfection mats and platforms.

10).Easing the symptoms of the disease by maintaining optimal regime for growing crops, i.e. activating the immune system. To do this, plants are sprayed with solutions of microelements, phosphorus and potassium fertilizers, which stimulate early ripening of plants and, as a result, the onset of age-related resistance.

14. VIRAL-BASED BIOLOGICAL PRODUCTS

Currently widely used vaccination (pre-inoculation) plants with weakly pathogenic strains of viruses. Vaccine strain obtained in Russia "VTM- V-69 » for tomatoes, used both in open and closed ground. Seedlings (sprouts) are sprayed. The drug is characterized by genetic stability and has a long-term vaccinating effect while being completely asymptomatic. The vaccine inhibits the development of various spots in tomatoes. The yield increase in vaccinated plantings is about 23%.

"VIROG- 43 » - vaccine preparation against green mottled mosaic of cucumber; use of the drug leads to the development of nonspecific induced immunity. 8-10 day old seedlings in the phase of expanded cotyledon leaves are vaccinated. After 10-12 days, a weak mosaic appears on the vaccinated plants, which later disappears completely. The concentration of pathogenic viruses is reduced several times. The resulting nonspecific induced effect also reduces susceptibility to certain fungal diseases.

LITERATURE

1. Boyko A. L. Ecology of plant viruses. – K.: Vishcha School, 1990.-166 p.

2. Gibbs A., Harisson B. Fundamentals of plant virology. - M.: Mir, 1978. - 430 p.

3. Gnutova R. V. Serology and immunology of plant viruses. – M.: Nauka, 1993. -301s.

4.Protection of plants from diseases in greenhouses (Handbook) /Ed. A.K. Akhatova. Moscow: Partnership of Scientific Publications KMK, 2002. – 464 p.

6. Matthews R. Plant viruses. – M.:Mir, 1973. -600 p.

7.Frenkel-Konrath H. Chemistry and biology of viruses. – M.: Moscow, 1972. -336s.