Plant pathology is the science that studies diseases affecting agricultural plants, and it is primarily concerned with minimizing the damage caused by pathogens to these plants. Plant pathologists study diseases resulting from fungi, bacteria, phytoplasma, parasitic plants, viruses, virions, nematodes, and monocytes.
And they study disturbances in plants resulting from an increase or imbalance, or a deficiency in some natural or chemical factors such as humidity, temperature, foodstuffs … etc. In addition, plant diseases caused by insects, humans, or animals are not included in the study of plant diseases.
The concept of disease in plants
The concept of disease in plants is a functional disorder in the host’s cells and tissues and their deviation from the normal state as a result of being infected with pathogens or environmental factors, leading to the emergence of symptoms, and weakening the plant completely or partially or to its death. This reduces the economic value of the affected plant in quantity or quality , or both.
And pathogens can cause disease in plants in several ways, including:
- Weakening the host by absorbing food from its cells and the pathogen consuming that food.
- Killing the host cells or hindering the process of metabolism by secreting toxic substances (toxins) or growth regulators.
- Obstructing the transport of nutrients or water through the transport vessels.
- Consumption of host cell contents when contacted.
As for diseases caused by environmental factors, they result from an increase or decrease in the basic factors of life such as heat, light, humidity … etc.
Classification of plant diseases
Plant diseases are classified according to the type of organism causing the disease and on this basis they are classified as follows:
Firstly, diseases that are infectious or caused by living organisms, including:
- Diseases caused by mycosis.
- Diseases caused by primary nuclei.
- Diseases caused by viruses and viruses.
- Diseases caused by intrusive flowering plants.
- Diseases caused by nematodes.
- Diseases caused by monocytes.
Second, non-infectious or physiological diseases:
- Low or severe fever.
- Decrease or increase in soil moisture .
- Lack or excess of light.
- Lack of oxygen.
- Air Pollution.
- Lack of nutrients.
- Metal poisoning.
- Radioactive pollution.
- Soil acidity or alkalinity ph.
- Pesticide poisoning.
- Inappropriate agricultural operations.
Damage caused by plant diseases
Plant diseases are considered to be of great harm to humans, because they cause losses to plants and plant products, as they have a great impact on people who depend in their lives on their intended products. Where famines could have caused them, as happened in the Irish famine in 1845 AD, and the Bengal famine in 1943 AD, and in general these losses can be divided into direct and indirect.
- Reducing the amount of production by damaging part or all of the crop.
- Decrease in the market value of the crop due to diseases such as scab and straw diseases on apples.
- Determine the types of plants that can grow in specific geographical areas by destroying certain types that are susceptible to infection with a specific disease.
- Determine the types of agricultural industries and the amount of labor used in a particular area, due to the effect of diseases on the quantity and quality of agricultural products.
- Some diseases make products unfit for human or animal consumption due to their contamination with toxic fruit formulations, such as Irgot disease in rye.
- Disease control costs for insuring chemicals, machines, etc.
- The farmer is forced to grow a variety that is resistant to disease but is less productive or more expensive.
- The farmer was forced to sell his crop within a short period, and the crop could not be stored for fear of spreading diseases and damaging it.
Some examples of plant disease losses worldwide
- Grain echoes are globally widespread on barley and wheat and cause significant damage annually.
- Carbonization of cereals is widespread worldwide and causes constant losses.
- Irgot rye and wheat are worldwide spread, and are toxic to humans and animals.
- Late blight on potatoes caused famine in Ireland in 1845 and 1846.
- Brown spot in rice is the cause of the famous Senegal famine in 1943.
- Southern corn leaf blight caused losses estimated at one billion dollars in America in 1970.
- Powdery mildew on the grapevine was spread worldwide and caused an epidemic in Europe in 1840-1859AD.
Among the fungal diseases also:
- The downy mildew on the vine is widespread in America and Europe and is the cause of an epidemic infection in Europe.
- The fluffy whiteness on tobacco caused an epidemic injury in Europe in the years 1950-1960 AD.
- Chestnut blight wiped out all the chestnut trees in America during 1904-1940AD.
- Coffeeabad rusted all coffee trees in Southeast Asia in 1870-1880, and those areas turned to tea cultivation.
- The peacock eye disease on olives inflicted great losses on the crop in Syria in 1984-1985.
- Ascochita blight on legumes wiped out 8,000 hectares planted with winter chickpeas in Syria in the Hasakah region in 1994.
- Apple scab disease causes large annual diseases.
- Olive wilt disease
- Powdery mildew on nightshade
- Citrus ulceration is widespread in Asia, Africa, America and Brazil, and millions of trees were destroyed in the US state of Florida in 1910.
- Fire blight on apples: widespread in North America and Europe, causing annual damage.
- Coronary tuberculosis disease: It is spread in Syria on fruit trees, especially in nurseries, and causes great damage.
- Sugar cane mosaic, which is one of the diseases spreading worldwide, causes a great loss of sugar cane and corn.
- Yellowing sugar beet is also common worldwide and causes annual damage.
- Rapid degradation of citrus (tristeza) is widespread in Africa and the Americas, wiping out millions of trees.
- Yellowing and dwarfness of barley is a disease that is widespread worldwide and causes annual damage.
- Tonsil pox, which is epidemic in Europe, on peach, peach and apricot trees.
- Yellowing of the peaches is widespread in the eastern United States, and affects ten million peach trees in Russia.
- Stolpor disease on tomatoes is most common in the southern regions of the Russian Federation.
- Root knot, which is universal and causes constant losses in vegetables and most other plants.
- Diabetic vesicular beet nematode causes severe infections in northern Europe and western United States.
Symptoms of plant diseases and how to diagnose them
Symptoms are all the visible and invisible changes in form and function that occur on the affected plant. The symptoms of the disease can be divided according to the way they appear into several main groups, including:
First, change the color:
Symptoms of discoloration include:
These symptoms arise as a result of the degradation of chloroplasts or the appearance of yellow pigments, especially carotenoids.
It is a pale green coloration of the plant tissue, as a result of a decrease in the concentration of chlorophyll, where the light is insufficient for chlorophyll.
It is the redness of the tissue in the affected plants due to the appearance of red pigments such as anthocyanins.
It is the transformation of the color of plants to white as the appearance of white seedlings in corn, cotton and squash, as a result of genetic mutations.
Mosaic and Topping:
It is an irregular change of color, where a pale green or yellow color interferes with the normal green color in the affected part.
In some cases the leaves can turn silvery, as in silver leaf disease in peaches.
The leaves are bronze in color, as is the case with tomato leaves infected with Macular wilt virus.
Second, necrosis (death of cells and tissues)
These symptoms are usually caused by localized parasitic infections caused by non-deterministic organisms that parasitize by fungi and bacteria. They include:
It is the sudden rapid death of buds, young fruits, flowers, leaves and branches, and their dark coloration, as the affected plant appears to be due to the heat of a flame due to a fire being lit near it.
Seedling fall or die (soft wilt):
It is the death of tissues or parts of the seedling near the soil surface.
It is the gradual death of branches starting at the top and moving downwards.
The presence of small dead areas on different parts of plants, especially leaves and fruits.
Tissue death and decomposition with unlimited areas and very similar to spotting, but are usually relatively large and irregular in shape, and are found on fruits or on leaves.
It is the appearance of nearly round dead spots on the leaves, and then these dead parts fall off, leaving small specific holes as in Tonsil leaf puncture disease.
It results from an analysis or imbalance in the structure of the cell wall and the protoplasm of the plant cell by the influence of enzymes secreted by the mold agent and affects all parts of the plant, but it is often seen on the succulent organs such as fruits, tubers and bulbs.
It appears on fruits and tubers when they lose their water and their texture becomes stiff and wrinkled, and is a subsequent symptom of rot .
It appears as a result of the death of areas in the peel of branches, stems and roots of plants. The ulcers vary in shape, size and depth, and may sometimes reach the cambium layer.
Specific raised, rough, rumpy spots appear on leaves, fruits, tubers, or stems. These spots often crack, giving them a rough texture.
It is the emergence of gum secretions in an abnormal way on the stem and branches, and it may appear on fruits or roots, as in citrus gum disease and tonsil gum disease.
It is the flaking of the skin cells in fruits and tubers as a result of being infected with superficial parasites as in the powdery mildew on grape fruits, or due to some external influences such as pesticides and unfavorable weather conditions, as in the phenomenon of apple peels.
It is the most common symptom of plant diseases in nature, and is caused by an imbalance in the water balance inside the plant, and it has many causes, including: Environmental physiological causes such as drought of the soil or increased water loss in the leaves due to high heat and dry winds.
Including parasitic causes such as infection of the root system of plants with fungi, bacteria, nematodes or viruses, which leads to rotting of the roots and a decrease in their ability to absorb water from the soil.
Symptoms that indicate the appearance of wilt:
The relaxation of the young, straightened plant organs, such as the leaves and the young stems. As for the veined plant members, their wilting does not necessarily mean loosening, but rather it is a gradual yellowing or sudden drying of the leaves, flowers and fruits that they bear because the sap does not reach them completely or partially.
A deformation occurs in the growth of the plant or parts of it, such as hypertrophy, stunting, etc., and plant hormones may have a role in that. The following are the main symptoms of this disease:
It is the failure of the plant or some of its members to reach the normal size. This is due to infection with viral and phytoplasmic diseases or due to some genetic factors, such as corn stunting disease.
It is an abnormal increase in plant height, which is caused by the pathogen excreting large quantities of gibberellin.
Tumors or tubercles:
They are localized swelling that results from either an increase in the size of infected cells due to the pathogen secretion of substances of an oxinizing nature that stimulate excessive growth, as in olive tuberculosis disease.
It is the shortening of the internodes of the stem or branches and the convergence of the nodes, and thus the convergence of the leaves that appear to be emerging from one place and take the shape of a rose.
It occurs as a result of increasing the speed of cell division and increasing its size on one side without the other, i.e. as a result of uneven growth on both sides, and wrinkling occurs.
It is deformed on the branches, where its section turns from a circular or polygonal shape to an elongated or oval shape.
The flowers become lush, i.e. instead of bright petals, green leaves are formed.
Deformation of fruits:
Most of the time the fruits become small in size as a result of disease, and other times the fruits are deformed.
And in it, the formation of large quantities of small, thin roots coming out of the stem or root, as in the rhizomania on sugar beet.
It is the abnormal emergence of a plant organ on another similar organ, such as the formation of lateral cranes on the original sprouts of maize.
Plant disease diagnosis
The diagnosis of plant diseases is an essential and essential step in order to control the pathogen and reduce its damage. After knowing this, we can move on and follow up to confirm the pathogen conclusively.
First, the diagnosis of infectious diseases:
It refers to diseases caused by pathogenic organisms (fungi, bacteria, viruses … etc.). Which is characterized by the presence of these pathogens on the surface or inside the infected plants.
Diagnosis of fungal diseases:
To determine whether the fungus is a pathogen or a throwing organism, the microscopic study of the external form of the mycelium, fruiting organisms and spores must be performed, and after the fungus is definitively identified, this can be compared in the scientific references to find out whether the organism is pathogenic or has no effect on plants.
Diagnosing bacterial diseases:
Bacterial diseases are difficult to diagnose compared to fungal diseases, due to the small dimensions of these organisms and their lack of distinct morphological characteristics.
Diagnosis of phytoplasmic diseases:
These diseases have distinct symptoms, the most important of which are: plant stunting, yellowing and redness of leaves, rapid growth of young organs and roots, abnormal flowers, and finally, deterioration and death of the affected plant.
Diagnosis of viral and viral diseases:
It is diagnosed through apparent symptoms, as some of these organisms are characterized by the production of distinct symptoms on their hosts, and the diagnosis is made in the following ways:
- Tests for artificial infection of reagent plants.
- Serological tests.
- Electron microscope method.
- Method of examining infected cells with light microscopy.
- Electrophoresis method.
- Polymer chain reaction method.
Second, diagnosis of non-communicable diseases:
If there is no pathogen, it means that the disease is caused by non-living environmental factors. These many factors affect plants by interfering with normal physiological processes.
In the end..
Plant diseases is an important and profound science, and to study it a good knowledge of the basics of the sciences that are related to it must be studied, to study all diseases that affect plants continuously.