Let’s know about Plant Pathology. Plant pathology (also phytopathology ) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors) .  Organisms that cause infectious disease include fungi , oomycetes , bacteria , viruses , viroids , virus -like organisms, phytoplasmas , protozoa , nematodes and parasitic plants . Insects such as ectoparasites are not included, Mites , vertebrates or other insects that affect plant health by feeding on plant tissue . Plant pathology also includes the study of pathogen identification, disease etiology, disease cycle, economic impact, plant disease epidemiology , plant disease resistance , how plant diseases affect humans and animals, pathosystem genetics, and management of plant diseases. Is.
Control of plant diseases is critical to the reliable production of food, and it poses significant problems in agricultural use of land, water, fuel and other inputs. Plants in both natural and cultivated populations have inherent disease resistance, but there are many examples of devastating plant disease such as the Great Famine of Ireland and chestnut blight , as well as recurring serious plant diseases such as rice blast , soybean cyst nematode , and citrus . Canker _ (Plant Pathology)
However, disease control is largely successful for most crops. Disease control is achieved by the use of plants that have been bred to have good resistance to many diseases, and plant cultivation methods such as crop rotation , use of pathogen-free seed, suitable planting date and plant density , field Moisture control and pesticide use. Continuing advances in the science of plant pathology are needed to improve disease control, and with changes in disease pressure due to the ongoing evolution and movement of plant pathogens and changes in agricultural practices.
Plant diseases cause huge economic losses to farmers around the world. In large areas and many crop species, it has been estimated that diseases typically reduce plant yields by 10% each year in more developed settings, but yield losses from diseases often exceed 20% in less developed settings. She goes. The Food and Agriculture Organization estimates that pests and diseases are responsible for 25% of crop losses. To solve this, new methods for early detection of diseases and pests are needed, such as novel sensors that detect plant odor and spectroscopy and biophotonics that are capable of diagnosing plant health and metabolism .
In most pathosystems, the virus is dependent on hydrolases—and a broad class of cell wall degrading proteins—that degrade the cell wall. The vast majority of CWDPs are pathogen-producing and pectin-targeted (for example, for pectinesterase, pectate lyase, and pectinases). Cell wall polysaccharides are themselves a food source for microbes, but most often only one obstacle has to be overcome.
Many pathogens also develop opportunistically when the host breaks down its own cell walls, most often during fruit ripening. 
Most of the phytopathogenic fungi belong to the Ascomycetes and Basidiomycetes. Fungi reproduce both sexually and asexually through the production of spores and other structures. Spores can be spread over long distances by wind or water, or they can be soil-borne. Many soil-dwelling fungi are capable of living karyotypically, completing part of their life cycle in the soil. These are alternative saprotrophs. Fungal diseases can be controlled through the use of fungicides and other agricultural practices. Although, New races of fungi often develop that are resistant to various fungicides. Biotrophic fungal pathogens colonize living plant tissues and obtain nutrients from living host cells. Necrotrophic fungal pathogens infect and kill host tissue and extract nutrients from dead host cells. Important fungal plant pathogens include:
- Fusarium spp. (Fusarium Wilt Disease)
- Thilaviopsis spp. (Canker Rot, Black Root Rot, Thilaviopsis Root Rot)
- Verticillium spp.
- Magnaporthe grisea (rice blast)
- Sclerotinia sclerotiorum (cotton rot)
- Ustilago spp. (smuts) barley smut
- Rhizoctonia spp.
- Phacospora pachyrizzi (soybean rust)
- Puccinia spp. (severe rust of grain and grass)
- Armillaria spp. (Honey fungus species, viral pathogens of trees)
Oomycetes are fungi-like organisms.  These include some of the most destructive plant pathogens, including the genus Phytophthora , which is the causative agent of potato late blight  and sudden oak death.   Particular species of oomycetes are responsible for root rot.
Despite not being closely related to fungi, oomycetes have evolved similar infection strategies. Oomycetes are able to use effector proteins to turn off plant defenses in their infection process.  Plant pathologists usually group them with fungal pathogens. (Plant Pathology)
Important oomycete plant pathogens include:
- Pythium spp.
- Phytophthora spp., with potato blight of the Great Irish Famine (1845-1849)
Some slime molds in Phytomyxea cause significant diseases, including club root in cabbage and its relatives, and powdery scab in potatoes. These are caused by the species of Plasmodiophora and Spongospora respectively.
Most of the bacteria associated with plants are in fact saprophytic and do not cause any harm to the plant itself. However, a small number, about 100 known species, are capable of causing disease.  Bacterial diseases are more prevalent in subtropical and tropical regions of the world.
Most plant pathogenic bacteria are rod-shaped (bacilli). They have specific pathogenic factors to be able to colonize the plant. Five main types of bacterial pathogenicity factors are known: cell wall degrading enzymes, uses of toxins, effector proteins, phytohormones and exopolysaccharides.
Pathogens such as Erwinia species use cell wall-degrading enzymes to cause soft rot. Agrobacterium species cause tumors by altering auxin levels with phytohormones. Exopolysaccharides are produced by bacteria and block the xylem vessels, often leading to plant death.
Bacteria control the production of pathogenicity factors through quorum sensing.
Important Bacterial Plant Pathogens:
- Burkholderia 
- Xanthomonas spp.
- Pseudomonas spp.
- Pseudomonas syringae pv The tomato makes tomato plants bear less fruit, and it “continues to adapt to the tomato by reducing its recognition by the tomato’s immune system.” 
Phytoplasma and Spiroplasma
Phytoplasma and Spiroplasma are genera of bacteria that lack a cell wall and are related to mycoplasma, which are human pathogens . Together they are called Mollicutes. They also have smaller genomes than most other bacteria. They are usually transmitted by sap-sucking insects, transferred to the phloem of the plant where it reproduces.
Viruses, viroids and virus-like organisms
There are many types of plant viruses, and some are asymptomatic. Under normal circumstances, plant viruses only damage crop yields. Therefore, trying to control them is not economically viable, with the exception being when they infect perennial species, such as fruit trees.
Most plant viruses have small, single-stranded RNA genomes. However some plant viruses also have double stranded RNA or single or double stranded DNA genomes. These genomes can encode only three or four proteins: a replicon, a coat protein, a movement protein to allow cell to cell movement through plasmodesmata, and sometimes a protein that acts as a vector. allows transmission. Plant viruses can contain many more proteins and employ many different molecular translation methods.
Plant viruses are usually transmitted from plant to plant by a vector, but mechanical and seed transmission also occur. Vector transmission is most often by an insect (for example, aphids), but some fungi, nematodes and protozoa have been shown to be viral vectors. In many cases, insects and viruses are specialized for virus transmission such as the beet leafhopper which transmits the disease-causing curly top virus in many crop plants.  An example is the mosaic disease of tobacco, where leaves become stunted and the leaves lose chlorophyll. Another example is the bunchy top of a banana, where the plant is dwarf, and the upper leaves form a tight rosette.
Nematodes are small, multicellular worm-like animals. Many live freely in the soil, but there are some species that parasitize plant roots. They are a problem in tropical and subtropical regions of the world, where they can infect crops. Potato cyst nematodes ( Globodera pallida and G. rostochiensis ) are widely distributed in Europe and North and South America and cause $300 million worth of damage in Europe each year. Root knot nematodes have a fairly large host range, they parasitize the root system of the plant and thus directly affect the water and nutrients required for normal plant growth and reproduction, Whereas cyst nematodes are capable of infecting only a few species. Nematodes are capable of radically changing root cells to facilitate their lifestyle.
Protozoa and Algae
There are some examples of plant diseases caused by protozoa (eg, Phytomonas , a kinetoplastid).  They are transmitted as durable zoospores that may be able to survive in soil resting conditions for many years. In addition, they can transmit plant viruses. When motile zoospores come into contact with root hairs, they produce a plasmodium that invades the roots.
Some colorless parasitic algae (eg, cephaluros ) also cause plant diseases. [ citation needed ]
Parasitic plants such as broom, mistletoe and dodder have been included in the study of phytopathology. For example, dodder can be a conduit for transmission of viruses or virus-like agents from a host plant to a plant that is not usually a host, or to an agent that is not graft-infectious.
Common Pathogenic Infection Methods
- Cell wall-degrading enzymes : These are used to break down the cell wall of plants so that nutrients can be released inside.
- Toxins : These can be non-host-specific, which harm all plants, or host-specific, which harm only one host plant.
- Effector proteins : These can be secreted into the extracellular environment or directly into the host cell, often through the type 3 secretion system. Some effectors are known to suppress host defense processes. These may include: reduced signaling mechanisms intrinsic to plants or decreased production of phytochemicals.  Bacteria, fungi and oomycetes are known to have this function.  
- spore: Spores of phytopathogenic fungi can be a source of infection on host plants. The spores first adhere to the dermal layer on the leaves and stems of the host plant. For this to happen infectious spores must be carried from the pathogen source, be it through air, water, and vectors such as insects and humans. When favorable conditions are present, the spore will produce a modified hyphae called a germ tube. This germ tube later forms a bulge called apressorium, Which builds up melanized cell walls to create turgor pressure. Once sufficient turgor pressure has accumulated, the appresorium exerts pressure against the cuticle layer in the form of a rigid entry peg. This process is also facilitated by the secretion of enzymes that destroy the cell wall from the appressorium. Once the entry peg enters the host tissue it develops a specialized hyphae called a haustorium. Depending on the pathogen’s life cycle, this haustorium may invade and feed on neighboring cells intracellularly or may be present intracellularly within a host.
physiological plant disorder
Some abiotic disorders can be confused with pathogen-induced disorders. Abiotic causes include natural processes such as drought, frost, snow and hailstorm; flooding and poor drainage; nutritional deficiencies ; deposition of mineral salts such as sodium chloride and gypsum; broken by typhoons and storms; and forest fire. Similar disorders (usually classified as abiotic), can be caused by human intervention, resulting in soil compaction, pollution of air and soil, salinization caused by irrigation and road sanding, over-application of herbicides, clumsiness handling (eg damage to lawnmower trees), and vandalism.
Epidemiology: The study of factors affecting the outbreak and spread of infectious diseases.
A disease tetrahedron (disease pyramid) best captures the elements associated with plant diseases. This pyramid uses the disease triangle as a foundation, which includes elements such as: host, pathogen and environment. In addition to these three elements, man and time combine the remaining elements to form a pathological tetrahedron.
History: Plant disease epidemics that are historically known to cause tremendous damage:
– Irish Potato Late Blight 
– Dutch elm disease 
– Chestnut blight in North America 
Factors affecting the epidemic:
Host: Resistance or susceptibility level, age and genetics.
Pathogen: Inoculum quantity, genetics and reproduction types
Plant immunity is the plant’s ability to prevent and eliminate infection by pathogens.
Structures that help plants prevent disease are: the dermal layer, the cell wall, and the stomatal guard cells. These act as a barrier to prevent pathogens from entering the plant host.
Once diseases overcome these barriers, plant receptors initiate signaling pathways to make molecules to compete against foreign molecules. These pathways are influenced and triggered by genes within the host plant and are susceptible to be manipulated by genetic breeding to create plant varieties resistant to destructive pathogens. 
A diseased patch of vegetation or individual plants can be isolated from other healthy growth. Specimens may be destroyed or transferred to greenhouses for treatment or study.
Port and Border Inspection and Quarantine
Another option is to avoid the introduction of harmful non-native organisms by controlling all human traffic and activity (eg, the Australian Quarantine and Inspection Service), although legislation and enforcement are critical to ensure sustainable effectiveness. Today’s global trade volume is and will continue to provide unprecedented opportunities for the introduction of plant pests. [MCC 1] In the United States, to get even a better estimate of the number of such introductions required, and thus, would require a substantial increase in port and border quarantine and inspection imposing inspections. [MCC 2]In Australia a similar lack of understanding has a different origin: port inspections are very useful because inspectors are not aware of very little classification. There are often pests that the Australian government has prioritized as harmful to keep out of the country, but which have taxonomic relatives that confuse the issue. And the inspectors do the opposite – harmless natives, or undiscovered natives, or recently discovered natives that run into them need not bother, but which are easy to confuse with their illegal alien family members. Is. [bh 1]
Food X-rays and electron-beam/e-beam radiation have been tested as quarantine treatments for fruit objects originating from Hawaii. The US FDA (Food and Drug Administration), USDA APHIS (Animal and Plant Health Inspection Service), manufacturers and consumers were all acknowledging the results—more thorough insect elimination and less flavor degradation—than heat treatment. 
In some societies farming is kept on a small scale, reared by people whose culture includes farming traditions dating back to ancient times. (An example of such traditions would be lifelong training in the techniques of plot terracing, weather anticipation and response, fertilizing, grafting, seed care, and dedicated gardening.) Plants that are intensively supervised often not only benefit from active outdoor protection. but greater overall power. While being primitive is by far the most laborious solution in the sense, where practical or necessary it is more than enough.
Sophisticated agricultural developments now allow growers to systematically choose from cross-bred species to ensure the greatest hardiness in their crops, as appropriate to the disease profile of a particular region. Breeding practices have been perfected for centuries, but with the advent of genetic manipulation even better control of a crop’s immune traits is possible. The engineering of edible plants may be less profitable, however, the higher production is often offset by popular skepticism and negative opinion about this “tampering” with nature.
Several natural and synthetic compounds can be used to combat the above hazards. This method works by directly destroying disease-causing organisms or by stopping their spread; However, this much broader effect has been shown, generally, to be good for the local ecosystem. From an economic standpoint, all-simple natural additives can disqualify a product from “organic” status, potentially reducing the value of the produce.
Crop rotation can be an effective means to prevent a parasitic population from becoming well established, as the organism affecting the leaves will starve when the leafy crop is replaced with a tuberous type, etc. There may be other means of weakening the parasite without attacking it directly.
The use of two or more of these methods in combination provides a high probability of effectiveness.
Plant pathology has evolved since ancient times, starting with Theophrastus, but scientific study began with the invention of the microscope in the early modern period, and developed in the 19th century.
Who is the father of Plant Pathology?
Heinrich Anton de Berry is known as the father of mycology. He is also considered the father of plant pathology.