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Plant pathology (also phytopathology) is the scientific study of
disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
s in
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...
s caused by
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...
s (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
, oomycetes,
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
,
viruses A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's ...
, viroids,
virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...
-like organisms,
phytoplasmas Phytoplasmas are obligate intracellular parasites of plant phloem tissue and of the insect vectors that are involved in their plant-to-plant transmission. Phytoplasmas were discovered in 1967 by Japanese scientists who termed them mycoplasma-lik ...
,
protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
,
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant- parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...
s and
parasitic plant A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All parasitic plants develop a specialized organ called th ...
s. Not included are ectoparasites like insects, mites,
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
, or other pests that affect plant health by eating plant tissues. Plant pathology also involves the study of pathogen identification, disease etiology, disease cycles, economic impact,
plant disease epidemiology Plant disease epidemiology is the study of disease in plant populations. Much like diseases of humans and other animals, plant diseases occur due to pathogens such as bacteria, viruses, fungi, oomycetes, nematodes, phytoplasmas, protozoa, an ...
, plant disease resistance, how plant diseases affect humans and animals,
pathosystem A pathosystem is a subsystem of an ecosystem and is defined by the phenomenon of parasitism. A plant pathosystem is one in which the host species is a plant. The parasite is any species in which the individual spends a significant part of its lifes ...
genetics, and management of plant diseases.


Overview

Control of plant diseases is crucial to the reliable production of food, and it provides significant problems in agricultural use of land, water, fuel and other inputs. Plants in both natural and cultivated populations carry inherent disease resistance, but there are numerous examples of devastating plant disease impacts, such as the Great Famine of
Ireland Ireland ( ; ga, Éire ; Ulster-Scots: ) is an island in the North Atlantic Ocean, in north-western Europe. It is separated from Great Britain to its east by the North Channel, the Irish Sea, and St George's Channel. Ireland is the s ...
and chestnut blight, as well as recurrent severe plant diseases like rice blast, soybean cyst nematode, and citrus canker. However, disease control is reasonably successful for most crops. It is achieved by use of plants that have been bred for good resistance to many diseases, and by plant cultivation approaches such as
crop rotation Crop rotation is the practice of growing a series of different types of crops in the same area across a sequence of growing seasons. It reduces reliance on one set of nutrients, pest and weed pressure, and the probability of developing resistant ...
, use of pathogen-free seed, appropriate planting date and
plant density Plant density is the number of individual plants present per unit of ground area. It is most easily interpreted in the case of monospecific stands, where all plants belong to the same species and have germinated at the same time. However, it could ...
, control of field moisture, and application of
pesticide Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and ...
s. Continuing advances in the science of plant pathology are needed to improve disease control, to keep up with the ongoing evolution and movement of plant pathogens, and to keep pace with changes in agricultural practices. Plant diseases cause major economic losses for farmers worldwide - see
Economic impact An economic impact analysis (EIA) examines the effect of an event on the economy in a specified area, ranging from a single neighborhood to the entire globe. It usually measures changes in business revenue, business profits, personal wages, a ...
. Across large regions and many crop species, it is estimated that diseases typically reduce plant yields by 10% every year in more developed settings, but yield loss to diseases often exceeds 20% in less developed settings. The
Food and Agriculture Organization The Food and Agriculture Organization of the United Nations (FAO)french: link=no, Organisation des Nations unies pour l'alimentation et l'agriculture; it, Organizzazione delle Nazioni Unite per l'Alimentazione e l'Agricoltura is an intern ...
estimates that pests and diseases are responsible for about 25% of crop loss. To solve this, new methods are needed to detect diseases and pests early, such as novel sensors that detect plant odours and
spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter ...
and biophotonics that are able to diagnose plant health and
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...
.


Plant pathogens

In most
pathosystem A pathosystem is a subsystem of an ecosystem and is defined by the phenomenon of parasitism. A plant pathosystem is one in which the host species is a plant. The parasite is any species in which the individual spends a significant part of its lifes ...
s,
virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ...
is dependent on hydrolases - and the wider class of
cell wall degrading protein Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
s - that degrade the
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...
. The vast majority of CWDPs are pathogen-produced and
pectin Pectin ( grc, πηκτικός ': "congealed" and "curdled") is a heteropolysaccharide, a structural acid contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal, chemical component o ...
-targeted (for example,
pectinesterase Pectinesterase (EC 3.1.1.11; systematic name pectin pectylhydrolase) is a ubiquitous cell-wall-associated enzyme that presents several isoforms that facilitate plant cell wall modification and subsequent breakdown. It catalyzes the following reac ...
,
pectate lyase Pectate lyase () is an enzyme involved in the maceration and soft rotting of plant tissue. Pectate lyase is responsible for the eliminative cleavage of pectate, yielding oligosaccharides with 4-deoxy-α-D-mann-4-enuronosyl groups at their non-re ...
, and
pectinase Pectinases are a group of enzymes that breaks down pectin, a polysaccharide found in plant cell walls, through hydrolysis, transelimination and deesterification reactions. Commonly referred to as pectic enzymes, they include pectolyase, pectozyme, ...
s). For microbes the cell wall
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...
s are themselves a food source, but mostly just a barrier to be overcome. Many pathogens also grow opportunistically when the host breaks down its own cell walls, most often during fruit ripening. .


Fungi

Most phytopathogenic fungi belong to the phyla Ascomycota and Basidiomycota. The fungi reproduce both sexually and asexually via the production of spores and other structures. Spores may be spread long distances by air or water, or they may be soil borne. Many soil inhabiting fungi are capable of living saprotrophically, carrying out the part of their life cycle in the
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
. These are facultative saprotrophs. Fungal diseases may be controlled through the use of fungicides and other agriculture practices. However, new races of fungi often evolve that are resistant to various fungicides. Biotrophic fungal pathogens colonize living plant tissue and obtain nutrients from living host cells.
Necrotroph A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from t ...
ic fungal pathogens infect and kill host tissue and extract nutrients from the dead host cells. Significant fungal plant pathogens include:


Ascomycetes

* ''
Fusarium ''Fusarium'' is a large genus of filamentous fungi, part of a group often referred to as hyphomycetes, widely distributed in soil and associated with plants. Most species are harmless saprobes, and are relatively abundant members of the soil mi ...
'' spp. (Fusarium wilt disease) * ''
Thielaviopsis ''Thielaviopsis'' is a small genus of fungi in the order Microascales, and family Ceratocystidaceae. The genus includes several important agricultural based pathogens. The most widespread is ''T. basicola'', the causal agent in several root rot ...
'' spp. (canker rot, black root rot, ''Thielaviopsis'' root rot) * ''
Verticillium ''Verticillium'' is a genus of fungi in the division Ascomycota, and are an anamorphic form of the family Plectosphaerellaceae. The genus used to include diverse groups comprising saprobes and parasites of higher plants, insects, nematodes, moll ...
'' spp. * '' Magnaporthe grisea'' (rice blast) * '' Sclerotinia sclerotiorum'' (cottony rot)


Basidiomycetes

* ''
Ustilago ''Ustilago'' is a genus of approximately 200 smut fungi parasitic on grasses. Uses ''Ustilago maydis'' is eaten as a traditional Mexican food in many parts of the country, and is even available canned. Farmers have even been known to spread th ...
'' spp. (smuts) smut of
barley Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley p ...
* '' Rhizoctonia'' spp. * '' Phakospora pachyrhizi'' (
soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean, which has numerous uses. Traditional unfermented food uses of soybeans include soy milk, from which tofu ...
rust) * '' Puccinia'' spp. (severe rusts of
cereal A cereal is any grass cultivated for the edible components of its grain (botanically, a type of fruit called a caryopsis), composed of the endosperm, germ, and bran. Cereal grain crops are grown in greater quantities and provide more food ...
s and
grass Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns a ...
es) * '' Armillaria'' spp. (honey fungus species, virulent pathogens of trees)


Fungus-like organisms


Oomycetes

The oomycetes are fungus-like organisms. They include some of the most destructive plant pathogens including the
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
'' Phytophthora'', which includes the causal agents of potato late blight and sudden oak death. Particular species of oomycetes are responsible for root rot. Despite not being closely related to the fungi, the oomycetes have developed similar infection strategies. Oomycetes are capable of using effector proteins to turn off a plant's defenses in its infection process. Plant pathologists commonly group them with fungal pathogens. Significant oomycete plant pathogens include: * '' Pythium'' spp. * '' Phytophthora'' spp., including the potato blight of the Great Irish Famine (1845–1849)


Phytomyxea

Some slime molds in Phytomyxea cause important diseases, including
club root Clubroot is a common disease of cabbages, broccoli, cauliflower, Brussels sprouts, radishes, turnips, stocks, wallflowers and other plants of the family Brassicaceae (Cruciferae). It is caused by '' Plasmodiophora brassicae'', which was once co ...
in cabbage and its relatives and powdery scab in potatoes. These are caused by species of '' Plasmodiophora'' and ''
Spongospora Powdery scab is a disease of potato tubers. It is caused by the cercozoan ''Spongospora subterranea'' f. sp. ''subterranea'' and is widespread in potato growing countries. Symptoms of powdery scab include small lesions in the early stages of the ...
'', respectively.


Bacteria

Most
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
that are associated with plants are actually
saprotrophic Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi ...
and do no harm to the plant itself. However, a small number, around 100 known species, are able to cause disease. Bacterial diseases are much more prevalent in
subtropical The subtropical zones or subtropics are geographical and climate zones to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately 35° north a ...
and
tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
regions of the world. Most plant pathogenic bacteria are rod-shaped ( bacilli). In order to be able to colonize the plant they have specific pathogenicity factors. Five main types of bacterial pathogenicity factors are known: uses of cell wall–degrading enzymes, toxins, effector proteins,
phytohormones Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, ...
and exopolysaccharides. Pathogens such as ''
Erwinia ''Erwinia'' is a genus of Enterobacterales bacteria containing mostly plant pathogenic species which was named for the famous plant pathologist, Erwin Frink Smith. It contains Gram-negative bacteria related to ''Escherichia coli'', ''Shigella'', ...
'' species use cell wall–degrading enzymes to cause soft rot. ''
Agrobacterium ''Agrobacterium'' is a genus of Gram-negative bacteria established by H. J. Conn that uses horizontal gene transfer to cause tumors in plants. ''Agrobacterium tumefaciens'' is the most commonly studied species in this genus. ''Agrobacterium ...
'' species change the level of auxins to cause tumours with phytohormones. Exopolysaccharides are produced by bacteria and block xylem vessels, often leading to the death of the plant. Bacteria control the production of pathogenicity factors via quorum sensing. Significant bacterial plant pathogens: * Burkholderia *
Pseudomonadota Pseudomonadota (synonym Proteobacteria) is a major phylum of Gram-negative bacteria. The renaming of phyla in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature. Th ...
** '' Xanthomonas'' spp. ** '' Pseudomonas'' spp. * Pseudomonas syringae pv. tomato causes tomato plants to produce less fruit, and it "continues to adapt to the tomato by minimizing its recognition by the tomato immune system."


Phytoplasmas and spiroplasmas

'' Phytoplasma'' and ''
Spiroplasma ''Spiroplasma'' is a genus of Mollicutes, a group of small bacteria without cell walls. ''Spiroplasma'' shares the simple metabolism, parasitic lifestyle, fried-egg colony morphology and small genome of other ''Mollicutes'', but has a distincti ...
'' are genera of bacteria that lack cell walls and are related to the mycoplasmas, which are human pathogens. Together they are referred to as the mollicutes. They also tend to have smaller genomes than most other bacteria. They are normally transmitted by sap-sucking insects, being transferred into the plant's phloem where it reproduces.


Viruses, viroids and virus-like organisms

There are many types of plant viruses, and some are asymptomatic or latent. Under normal circumstances, plant viruses cause only a loss of
crop yield In agriculture, the yield is a measurement of the amount of a crop grown, or product such as wool, meat or milk produced, per unit area of land. The seed ratio is another way of calculating yields. Innovations, such as the use of fertilizer, the c ...
. Therefore, it is not economically viable to try to control them, the exception being when they infect
perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also widel ...
species, such as fruit trees. Most plant viruses have small, single-stranded RNA
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...
s. However some plant viruses also have double stranded RNA or single or double stranded DNA genomes. These genomes may encode only three or four
proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
: a replicase, a coat protein, a
movement protein In order for a virus to infect a plant, it must be able to move between cells so it can spread throughout the plant. Plant cell walls make this moving/spreading quite difficult and therefore, for this to occur, movement proteins must be present. A ...
, in order to allow cell to cell movement through plasmodesmata, and sometimes a protein that allows transmission by a vector. Plant viruses can have several more proteins and employ many different molecular translation methods. Plant viruses are generally transmitted from plant to plant by a vector, but mechanical and seed transmission also occur. Vector transmission is often by an
insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...
(for example, aphids), but some
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
,
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant- parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...
s, and
protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
have been shown to be viral vectors. In many cases, the insect and virus are specific for virus transmission such as the
beet leafhopper The beet leafhopper (''Circulifer tenellus''), also sometimes known as ''Neoaliturus tenellus'', is a species of leafhopper which belongs to the family Cicadellidae in the order Hemiptera. Morphology A lot of morphological diversity has been ...
that transmits the
curly top Curly Top is a 1935 American musical romantic comedy film starring Shirley Temple, John Boles and Rochelle Hudson. Plot A bachelor wants to adopt an orphan, but she refuses to leave behind her older sister, so he adopts them both. The man eventu ...
virus causing disease in several crop plants. One example is mosaic disease of tobacco where leaves are dwarfed and the chlorophyll of the leaves is destroyed. Another example is Bunchy top of banana, where the plant is dwarfed, and the upper leaves form a tight rosette.


Nematodes

Nematodes are small, multicellular, wormlike animals. Many live freely in the soil, but there are some species that parasitize plant
root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...
s. They are a problem in
tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
and
subtropical The subtropical zones or subtropics are geographical and climate zones to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately 35° north a ...
regions of the world, where they may infect crops. Potato cyst nematodes (''Globodera pallida'' and ''G. rostochiensis'') are widely distributed in Europe and North and South America and cause worth of damage in Europe every year. Root knot nematodes have quite a large host range, they parasitize plant root systems and thus directly affect the uptake of water and nutrients needed for normal plant growth and reproduction, whereas cyst nematodes tend to be able to infect only a few species. Nematodes are able to cause radical changes in root cells in order to facilitate their lifestyle. There are also beneficial nematodes that can be used as a biological control for crop-destroying pests. A promising example is the effectiveness of the nematode ''Steinernema rarium'' PAM25 against the ''Drosophila suzuki'', a type of fruit fly.


Protozoa and algae

There are a few examples of plant diseases caused by
protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
(e.g., ''
Phytomonas ''Phytomonas'' is a genus of trypanosomatids that infect plant species. Initially described using existing genera in the family Trypanosomatidae, such as ''Trypanosoma'' or ''Leishmania'', the nomenclature of ''Phytomonas'' was proposed in 1909 ...
'', a kinetoplastid). They are transmitted as durable zoospores that may be able to survive in a resting state in the soil for many years. Further, they can transmit plant
viruses A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's ...
. When the motile zoospores come into contact with a root hair they produce a
plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a blood-feeding insect host which then injects parasites into a ve ...
which invades the
root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...
s. Some colourless parasitic
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
(e.g., ''
Cephaleuros ''Cephaleuros'' is a genus of parasitic thalloid green algae comprising approximately 14 species. Its common name is red rust. Specimens can reach around 10 mm in size. Dichotomous branches are formed. The alga is parasitic on some import ...
'') also cause plant diseases.


Parasitic plants

Parasitic plants such as
broomrape ''Orobanche'', commonly known as broomrape, is a genus of over 200 species of small parasitic herbaceous plants, mostly native to the temperate Northern Hemisphere. It is the type genus of the broomrape family Orobanchaceae. Description Broom ...
,
mistletoe Mistletoe is the common name for obligate hemiparasitic plants in the order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which they extract water and nutrients from the host plant. ...
and dodder are included in the study of phytopathology. Dodder, for example, can be a conduit for the transmission of viruses or virus-like agents from a host plant to a plant that is not typically a host, or for an agent that is not graft-transmissible.


Common pathogenic infection methods

* Cell wall-degrading enzymes: These are used to break down the plant
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...
in order to release the nutrients inside. * Toxins: These can be non-host-specific, which damage all plants, or host-specific, which cause damage only on a host plant. * Effector proteins: These can be secreted into the extracellular environment or directly into the host cell, often via the
Type three secretion system Type three secretion system (often written Type III secretion system and abbreviated TTSS or T3SS, also called Injectisome) is a protein appendage found in several Gram-negative bacteria. In pathogenic bacteria, the needle-like structure is u ...
. Some effectors are known to suppress host defense processes. This can include: reducing the plants internal signaling mechanisms or reduction of phytochemicals production. Bacteria, fungus and oomycetes are known for this function. * Spores: Spores of phytopathogenic fungi can be a source of infection on host plants. Spores first adhere to the cuticular layer on leaves and stems of host plant. In order for this to happen, the infectious spore must be transported from the pathogen source. This occurs via wind, water, or vectors such as insects and humans. When favourable conditions are present, the spore will produce a modified hyphae called a germ tube. This germ tube later forms a bulge called an appressorium, which forms melanized cell walls to build up turgor pressure. Once enough turgor pressure is accumulated the appressorium asserts pressure against the cuticular layer in the form of a hardened penetration peg. This process is also aided by the secretion of cell wall degrading enzymes from the appressorium. Once the penetration peg enters the host tissue it develops a specialized hyphae called a haustorium. Based on the pathogens life cycle, this haustorium can invade and feed neighboring cells intracellularly or exist intercellularly within a host.


Physiological plant disorders

Some abiotic disorders can be confused with pathogen-induced disorders. Abiotic causes include natural processes such as
drought A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D.  Jiang, A.  Khan, W.  Pokam Mba, D.  Rosenfeld, J. Tierney, an ...
,
frost Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above-freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase change from water vapor (a g ...
,
snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughou ...
and
hail Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...
;
flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...
ing and poor drainage; nutrient deficiency; deposition of mineral salts such as
sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...
and
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywa ...
;
wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ...
burn and breakage by storms; and
wildfire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identi ...
s. Similar disorders (usually classed as abiotic) can be caused by human intervention, resulting in
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
compaction,
pollution Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, th ...
of air and soil, salinization caused by irrigation and road salting, over-application of
herbicide Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page f ...
s, clumsy handling (e.g. lawnmower damage to trees), and
vandalism Vandalism is the action involving deliberate destruction of or damage to public or private property. The term includes property damage, such as graffiti and defacement directed towards any property without permission of the owner. The ter ...
.


Epidemiology

Epidemiology: The study of factors affecting the outbreak and spread of infectious diseases. A disease tetrahedron (disease pyramid) best captures the elements involved with plant diseases. This pyramid uses the disease triangle as a foundation, consisting of elements such as: host, pathogen and environment. In addition to these three elements, humans and time add the remaining elements to create a disease tetrahedron. History: Plant disease epidemics that are historically known based on tremendous losses: - Irish potato late blight - Dutch elm disease - Chestnut blight in North America Factors affecting epidemics: Host: Resistance or susceptibility level, age, and genetics. Pathogen: Amount of inoculum, genetics, and type of reproduction


Disease resistance

Plant disease resistance is the ability of a plant to prevent and terminate infections from plant pathogens. Structures that help plants prevent disease are: cuticular layer, cell walls and stomata guard cells. These act as a barrier to prevent pathogens from entering the plant host. Once diseases have overcome these barriers, plant receptors initiate signaling pathways to create molecules to compete against the foreign molecules. These pathways are influenced and triggered by genes within the host plant and are susceptible to being manipulated by genetic breeding to create varieties of plants that are resistant to destructive pathogens. Among defense mechanisms, chemical deterrence of pest settling and feeding, like the induction of defensive compounds, may be a key strategy for reducing herbivore damage.


Management

Before control measures can be taken, the pathogen must be detected. Ancient methods of leaf examination and breaking open plant material by hand are now augmented by newer technologies. These include
molecular pathology Molecular pathology is an emerging discipline within pathology which is focused in the study and diagnosis of disease through the examination of molecules within organs, tissues or bodily fluids. Molecular pathology shares some aspects of practice ...
assays such as
polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) ...
(PCR),
RT-PCR Reverse transcription polymerase chain reaction (RT-PCR) is a laboratory technique combining reverse transcription of RNA into DNA (in this context called complementary DNA or cDNA) and amplification of specific DNA targets using polymerase chai ...
and loop-mediated isothermal amplification (LAMP). Although PCR can detect multiple molecular targets in a single solution there are limits. Bertolini et al 2001, Ito et al 2002 and Ragozzino et al 2004 developed PCR methods for multiplexing six or seven plant pathogen molecular products and Persson et al 2005 for multiplexing four with RT-PCR. More extensive molecular diagnosis requires PCR arrays.


Domestic quarantine

A diseased patch of vegetation or individual plants can be isolated from other, healthy growth. Specimens may be destroyed or relocated into a greenhouse for treatment or study.


Port and border inspection and quarantine

Another option is to avoid the introduction of harmful nonnative organisms by controlling all human traffic and activity (e.g., the
Australian Quarantine and Inspection Service The Australian Quarantine and Inspection Service was the Australian government agency responsible for enforcing Australian quarantine laws, as part of the Department of Agriculture. Following a period operating under the name DAFF Biosecurit ...
), although legislation and enforcement are crucial in order to ensure lasting effectiveness. Today's volume of global trade is providing—and will continue to provide—unprecedented opportunities for the introduction of plant pests.p.17, "It is clear, however, that continuing increases in global trade and travel will provide opportunities for nonindigenous species to be transported into the U.S. at rates that are unprecedented in world history." In the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
, even to get a better ''estimate'' of the number of such introductions, and thus the need to impose port and border quarantine and inspection, would require a substantial increase in inspections.p.17, " A more comprehensive estimate of the frequency and diversity of nonindigenous plants, particularly those introduced as contaminants in cargo, would likely require a substantial increase in inspection efforts by APHIS personnel." In
Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
a similar shortcoming of understanding has a different origin: Port inspections are not very useful because inspectors know too little about taxonomy. There are often pests that the
Australian Government The Australian Government, also known as the Commonwealth Government, is the national government of Australia, a federal parliamentary constitutional monarchy. Like other Westminster-style systems of government, the Australian Governmen ...
has prioritised as harmful to be kept out of the country, but which have near taxonomic relatives that confuse the issue. And inspectors also run into the opposite - harmless natives, or undiscovered natives, or just-discovered natives they need not bother with but which are easy to confuse with their outlawed foreign family members.p.39, Table 2
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
and electron-beam/E-beam irradiation of food has been trialed as a quarantine treatment for
fruit In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in partic ...
commodities originating from
Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
. The US FDA (
Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...
), USDA APHIS ( Animal and Plant Health Inspection Service), producers, and consumers were all accepting of the results - more thorough pest eradication and lesser taste degradation than heat treatment. The
International Plant Protection Convention The International Plant Protection Convention (IPPC) is a 1951 multilateral treaty overseen by the United Nations Food and Agriculture Organization that aims to secure coordinated, effective action to prevent and to control the introduction an ...
(IPPC) anticipates that molecular diagnostics for inspections will continue to improve. Between 2020 and 2030, IPPC expects continued technological improvement to lower costs and improve performance, albeit not for
less developed countries A developing country is a sovereign state with a lesser developed industrial base and a lower Human Development Index (HDI) relative to other countries. However, this definition is not universally agreed upon. There is also no clear agreem ...
unless funding changes.


Cultural

Farming in some societies is kept on a small scale, tended by peoples whose culture includes farming traditions going back to ancient times. (An example of such traditions would be lifelong training in techniques of plot terracing, weather anticipation and response, fertilization, grafting, seed care, and dedicated gardening.) Plants that are intently monitored often benefit from not only active external protection but also a greater overall vigor. While primitive in the sense of being the most labor-intensive solution by far, where practical or necessary it is more than adequate.


Plant resistance

Sophisticated agricultural developments now allow growers to choose from among systematically cross-bred species to ensure the greatest hardiness in their crops, as suited for a particular region's pathological profile. Breeding practices have been perfected over centuries, but with the advent of genetic manipulation even finer control of a crop's immunity traits is possible. The engineering of food plants may be less rewarding, however, as higher output is frequently offset by popular suspicion and negative opinion about this "tampering" with nature.


Chemical

Many natural and synthetic compounds can be employed to combat the above threats. This method works by directly eliminating disease-causing organisms or curbing their spread; however, it has been shown to have too broad an effect, typically, to be good for the local ecosystem. From an economic standpoint, all but the simplest natural additives may disqualify a product from "organic" status, potentially reducing the value of the yield.


Biological

Crop rotation Crop rotation is the practice of growing a series of different types of crops in the same area across a sequence of growing seasons. It reduces reliance on one set of nutrients, pest and weed pressure, and the probability of developing resistant ...
may be an effective means to prevent a parasitic population from becoming well-established. For example, protection against infection by ''
Agrobacterium tumefaciens ''Agrobacterium radiobacter'' (more commonly known as ''Agrobacterium tumefaciens'') is the causal agent of crown gall disease (the formation of tumours) in over 140 species of eudicots. It is a rod-shaped, Gram-negative soil bacterium. Symptom ...
'', which causes gall diseases in many plants, by dipping cuttings in suspensions of '' Agrobacterium radiobacter'' before inserting them in the ground to take root. Other means to undermine parasites without attacking them directly may exist.


Integrated

The use of two or more of these methods in combination offers a higher chance of effectiveness.


Economic impact

the most costly diseases of the most produced crops worldwide are:


History

Plant pathology has developed from antiquity, starting with
Theophrastus Theophrastus (; grc-gre, Θεόφραστος ; c. 371c. 287 BC), a Greek philosopher and the successor to Aristotle in the Peripatetic school. He was a native of Eresos in Lesbos.Gavin Hardy and Laurence Totelin, ''Ancient Botany'', Routle ...
in the ancient era, but scientific study began in the Early Modern period with the invention of the
microscope A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisi ...
, and developed in the 19th century.


See also

* American Phytopathological Society * Australasian Plant Pathology Society * Biological control with micro-organisms * British Society for Plant Pathology * Burl *
Common names of plant diseases This is a list of articles that are lists of plant diseases. A * List of foliage plant diseases (Acanthaceae) * List of African daisy diseases * List of African violet diseases * List of foliage plant diseases (Agavaceae) * List of alfalfa di ...
*
Disease resistance in fruit and vegetables There are a number of lines of defence against pests (that, those animals that cause damage to the plants we grow) and diseases in the orchard, principal among these being the practice of good husbandry, creating healthy soil and ensuring high st ...
* Forest pathology * Gene-for-gene relationship * Global Plant Clinic *
Glossary of phytopathology This is a glossary of some of the terms used in phytopathology. Phytopathology is the study of plant diseases. It is a multi-disciplinary science since prerequisites for disease development are the presence of a susceptible host species, a pathog ...
* Herbivory * Horsfall-Barratt scale * Inducible plant defenses against herbivory * List of phytopathology journals *
Microbial inoculant Microbial inoculants also known as soil inoculants or bioinoculants are agricultural amendments that use beneficial rhizosphericic or endophytic microbes to promote plant health. Many of the microbes involved form symbiotic relationships with the ...
*
Phytopharmacology Phytopharmacology is the study and practice of eradicating plant pathology originated from the ''Verbandes Deutscher Pflanzenärzte'' (1928–1939), (German Plant Physicians Society), headed by Otto Appel, known as the ''Organiser of German Plant ...
* Plant defense against herbivory * Plant disease forecasting * Stunting


References

* *


External links


International Society for Plant Pathology

Australasian Plant Pathology Society

American Phytopathological Society

British Society for Plant Pathology

Contributions toward a bibliography of peach yellows, 1887–1888
Digital copy of scientist Erwin Frink Smith's manuscript on peach yellows disease.
Erwin Frink Smith Papers
Index to papers of Smith (1854–1927) who was considered the "father of bacterial plant pathology" and worked for the United States Department of Agriculture for over 40 years.
Plant Health Progress, Online journal of applied plant pathology

Pacific Northwest Fungi, online mycology journal with papers on fungal plant pathogens

Rothamsted Plant Pathology and Microbiology Department

New Mexico State University Department of Entomology Plant Pathology and Weed Science

Pathogen Host Interactions Database (PHI-base)

Grape Virology

Opportunity in Plant Pathology

Facebook page for Asian Association of Societies for Plant Pathology

The Pest and Pathogens Glossary
{{Authority control Agronomy Pathology Plant diseases