Plant pathology

Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors).[1] Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. Not included are ectoparasites like insects, mites, vertebrate, or other pests that affect plant health by eating of plant tissues. Plant pathology also involves the study of pathogen identification, disease etiology, disease cycles, economic impact, plant disease epidemiology, plant disease resistance, how plant diseases affect humans and animals, pathosystem genetics, and management of plant diseases.

Black rot lifecycle
Life cycle of the black rot pathogen, Xanthomonas campestris pathovar campes

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 Irish potato famine and chestnut blight, as well as recurrent severe plant diseases like rice blast, soybean cyst nematode,[wheat rust caused by Puccinia], [downy mildew of grapes caused by Plasmopara viticola] and citrus canker. However, disease control is reasonably successful for most crops. Disease control 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, use of pathogen-free seed, appropriate planting date and plant density, control of field moisture, and pesticide use. 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. Continuing advances in the science of plant pathology are needed to improve disease control, and to keep up with changes in disease pressure caused by the ongoing evolution and movement of plant pathogens and by changes in agricultural practices. Plant diseases cause major economic losses for farmers worldwide. The Food and Agriculture Organization estimates indeed that pests and diseases are responsible for about 25% of crop loss. To solve this issue, new methods are needed to detect diseases and pests early, such as novel sensors that detect plant odours and spectroscopy and biophotonics that are able to diagnose plant health and metabolism.[2]

Plant pathogens

Powdery mildew
Powdery mildew, a biotrophic fungus

Fungi

Most phytopathogenic fungi belong to the Ascomycetes and the Basidiomycetes. 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 soilborne. Many soil inhabiting fungi are capable of living saprotrophically, carrying out the part of their life cycle in the soil. 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. Necrotrophic fungal pathogens infect and kill host tissue and extract nutrients from the dead host cells. Significant fungal plant pathogens include:

Rice blast
Rice blast, caused by a necrotrophic fungus

Ascomycetes

Basidiomycetes

Fungus-like organisms

Oomycetes

The oomycetes are fungus-like organisms.[3] They include some of the most destructive plant pathogens including the genus Phytophthora, which includes the causal agents of potato late blight[3] and sudden oak death.[4][5] 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.[6] Plant pathologists commonly group them with fungal pathogens.

Significant oomycete plant pathogens include:

Phytomyxea

Some slime molds in Phytomyxea cause important diseases, including club root in cabbage and its relatives and powdery scab in potatoes. These are caused by species of Plasmodiophora and Spongospora, respectively.

Bacteria

Agrobacteriumgall
Crown gall disease caused by Agrobacterium

Most bacteria that are associated with plants are actually saprotrophic and do no harm to the plant itself. However, a small number, around 100 known species, are able to cause disease.[7] Bacterial diseases are much more prevalent in subtropical and tropical 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 and exopolysaccharides.

Pathogens such as Erwinia species use cell wall–degrading enzymes to cause soft rot. 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.

Vitis vinifera phytoplasma
Vitis vinifera with "Ca. Phytoplasma vitis" infection

Significant bacterial plant pathogens:

Phytoplasmas and spiroplasmas

Phytoplasma and Spiroplasma 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 virus, and some are even asymptomatic. Under normal circumstances, plant viruses cause only a loss of crop yield. Therefore, it is not economically viable to try to control them, 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 may encode only three or four proteins: a replicase, a coat protein, a movement protein, 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 (for example, aphids), but some fungi, nematodes, and protozoa have been shown to be viral vectors. In many cases, the insect and virus are specific for virus transmission such as the beet leafhopper that transmits the curly top virus causing disease in several crop plants.[10]

Nematodes

Nematodes are small, multicellular wormlike 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 may 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 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,[11] 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.

Protozoa and algae

There are a few examples of plant diseases caused by protozoa (e.g., Phytomonas, a kinetoplastid).[12] 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. When the motile zoospores come into contact with a root hair they produce a plasmodium which invades the roots.

Some colourless parasitic algae (e.g., Cephaleuros) also cause plant diseases.

Parasitic plants

Parasitic plants such as mistletoe and dodder are included in the study of phytopathology. Dodder, for example, is used as a conduit either 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 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. Some effectors are known to suppress host defense processes. This can include: reducing the plants internal signaling mechanisms or reduction of phytochemicals production.[13] Bacteria, fungus and oomycetes are known for this function.[3][14]

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, and 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 tugour 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 neighbouring cells intracellularly or exist intercellulary within a host.[15]

Physiological plant disorders

Abiotic disorders can be caused by natural processes such as drought, frost, snow and hail; flooding and poor drainage; nutrient deficiency; deposition of mineral salts such as sodium chloride and gypsum; windburn and breakage by storms; and wildfires. Similar disorders (usually classed as abiotic) can be caused by human intervention, resulting in soil compaction, pollution of air and soil, salinisation caused by irrigation and road salting, over-application of herbicides, clumsy handling (e.g. lawnmower damage to trees), and vandalism.

Tobacco mosaic virus symptoms orchid
Orchid leaves with viral infections

Epidemiology

Epidemiology: The study of factors affecting the outbreak and spread of infectious diseases.[16]

Plant Disease Triangle
Plant disease triangle

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[17]

- Dutch elm disease [18]

- Chestnut blight in North America[19]

Factors affecting epidemics:

Host: Resistance or susceptibility level, age and genetics.

Pathogen: Amount of innoculum, 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 over come these barriers, plant receptors initiate signalling 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.[20]

Management

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. Another option is to avoid the introduction of harmful nonnative organisms by controlling all human traffic and activity (e.g., AQIS), although legislation and enforcement are crucial in order to ensure lasting effectiveness.
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
(See: pesticide application) 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 may be an effective means to prevent a parasitic population from becoming well-established, as an organism affecting leaves would be starved when the leafy crop is replaced by a tuberous type, etc. 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.

History

Plant pathology has developed from antiquity, starting with Theophrastus, but scientific study began in the Early Modern period with the invention of the microscope, and developed in the 19th century.[21]

See also

References

  1. ^ Agrios GN (1972). Plant Pathology (3rd ed.). Academic Press.
  2. ^ Martinelli F, Scalenghe R, Davino S, Panno S, Scuderi G, Ruisi P, Villa P, Stroppiana D, Boschetti M, Goulart LR, Davis CE (January 2015). "Advanced methods of plant disease detection. A review" (PDF). Agronomy for Sustainable Development. 35 (1): 1–25. doi:10.1007/s13593-014-0246-1.
  3. ^ a b c Davis N (September 9, 2009). "Genome of Irish potato famine pathogen decoded". Haas et al. Broad Institute of MIT and Harvard. Retrieved 24 July 2012.
  4. ^ Kamoun S, Furzer O, Jones JD, Judelson HS, Ali GS, Dalio RJ, Roy SG, Schena L, Zambounis A, Panabières F, Cahill D, Ruocco M, Figueiredo A, Chen XR, Hulvey J, Stam R, Lamour K, Gijzen M, Tyler BM, Grünwald NJ, Mukhtar MS, Tomé DF, Tör M, Van Den Ackerveken G, McDowell J, Daayf F, Fry WE, Lindqvist-Kreuze H, Meijer HJ, Petre B, Ristaino J, Yoshida K, Birch PR, Govers F (May 2015). "The Top 10 oomycete pathogens in molecular plant pathology" (PDF). Molecular Plant Pathology. 16 (4): 413–34. doi:10.1111/mpp.12190. PMID 25178392.
  5. ^ Grünwald NJ, Goss EM, Press CM (November 2008). "Phytophthora ramorum: a pathogen with a remarkably wide host range causing sudden oak death on oaks and ramorum blight on woody ornamentals". Molecular Plant Pathology. 9 (6): 729–40. doi:10.1111/J.1364-3703.2008.00500.X. PMID 19019002.
  6. ^ "Scientists discover how deadly fungal microbes enter host cells". (VBI) at Virginia Tech affiliates. Physorg. July 22, 2010. Retrieved July 31, 2012.
  7. ^ Jackson RW (editor). (2009). Plant Pathogenic Bacteria: Genomics and Molecular Biology. Caister Academic Press. ISBN 978-1-904455-37-0.
  8. ^ Burkholder WH (October 1948). "Bacteria as plant pathogens". Annual Review of Microbiology. 2 (1 vol.): 389–412. doi:10.1146/annurev.mi.02.100148.002133. PMID 18104350.
  9. ^ "Research team unravels tomato pathogen's tricks of the trade". Virginia Tech. 2011.
  10. ^ Creamer R, Hubble H, Lewis A (May 2005). "Curtovirus Infection of Chile Pepper in New Mexico". Plant Disease. 89 (5): 480–486. doi:10.1094/PD-89-0480. PMID 30795425.
  11. ^ Huynh BL, Matthews WC, Ehlers JD, Lucas MR, Santos JR, Ndeve A, Close TJ, Roberts PA (January 2016). "A major QTL corresponding to the Rk locus for resistance to root-knot nematodes in cowpea (Vigna unguiculata L. Walp.)". TAG. Theoretical and Applied Genetics. Theoretische und Angewandte Genetik. 129 (1): 87–95. doi:10.1007/s00122-015-2611-0. PMC 4703619. PMID 26450274.
  12. ^ Jankevicius JV, Itow-Jankevicius S, Maeda LA, Campaner M, Conchon I, Carmo JB, Dutra-Menezes MC, Menezes JR, Camargo EP, Roitman I, Traub-Csekö YM (1988). "Ciclo biológico de Phytomonas" [Biological cycle of Phytomonas]. Memórias do Instituto Oswaldo Cruz (in Portuguese). 83: 601–10. doi:10.1590/S0074-02761988000500073. PMID 3253512.
  13. ^ Ma, Winbo (March 28, 2011). "How do plants fight disease? Breakthrough research by UC Riverside plant pathologist offers a clue". UC Riverside.
  14. ^ "1st large-scale map of a plant's protein network addresses evolution, disease process". Dana-Farber Cancer Institute. July 29, 2011. Archived from the original on 12 May 2012. Retrieved 24 July 2012.
  15. ^ Mendgen K, Hahn M, Deising H (September 1996). "Morphogenesis and mechanisms of penetration by plant pathogenic fungi". Annual Review of Phytopathology. 34 (1): 367–86. doi:10.1146/annurev.phyto.34.1.367.
  16. ^ "American Phytopathological Society". American Phytopathological Society. Retrieved 2019-03-26.
  17. ^ "Great Famine (Ireland)", Wikipedia, 2019-03-25, retrieved 2019-03-26
  18. ^ "Dutch elm disease", Wikipedia, 2019-02-17, retrieved 2019-03-26
  19. ^ "Chestnut blight", Wikipedia, 2019-02-12, retrieved 2019-03-26
  20. ^ Andersen EJ, Ali S, Byamukama E, Yen Y, Nepal MP (July 2018). "Disease Resistance Mechanisms in Plants". Genes. 9 (7): 339. doi:10.3390/genes9070339. PMC 6071103. PMID 29973557.
  21. ^ Aisnworth GC (1981). Introduction to the History of Plant Pathology. Cambridge University Press. ISBN 978-0-521-23032-2.

External links

Allen Kerr

Allen Kerr AO, FRS, FAA (born 1926) was a Scottish-born Professor of Plant Pathology at the University of Adelaide. His most significant work was his study of crown gall - a plant cancer induced by Agrobacterium tumerfaciens.

American Phytopathological Society

The American Phytopathological Society (APS) is an international scientific organization devoted to the study of plant diseases (phytopathology). APS promotes the advancement of modern concepts in the science of plant pathology and in plant health management in agricultural, urban and forest settings.

The Society has nearly 5,000 plant pathologists and scientists worldwide. It is the oldest and largest organization of its type in the world. It is also a member of the International Society for Plant Pathology.APS provides information on the latest developments and research advances in plant health science through its journals and its publishing arm, APS Press.

APS advocates and participates in the exchange of plant health information with public policy makers and the larger scientific community, and it provides advice on education and training.

Armillaria

Armillaria, is a genus of parasitic fungi that includes the A. mellea species known as honey fungi that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as A. mellea. Armillarias are long-lived and form some of the largest living organisms in the world. The largest known organism (of the species Armillaria solidipes) covers more than 3.4 square miles (8.8 km2) in Oregon's Malheur National Forest and is more than 2,400 years old. Some species of Armillaria display bioluminescence, resulting in foxfire.

Armillaria can be a destructive forest pathogen. It causes "white rot" root disease (see Plant Pathology section) of forests, which distinguishes it from Tricholoma, a mycorrhizal (non-parasitic) genus. Because Armillaria is a facultative saprophyte, it also feeds on dead plant material, allowing it to kill its host, unlike parasites that must moderate their growth to avoid host death.In the Canadian Prairies (particularly Manitoba), Armillaria is referred to often as openky (Ukrainian: опеньки), meaning “near the stump” in Ukrainian.

Australasian Plant Pathology

Australasian Plant Pathology is a peer-reviewed international journal that publishes original research and critical reviews on phytopathology in the Australasian region. It is published by Springer Science+Business Media in cooperation with the Australasian Plant Pathology Society.

The journal began in 1972 as the APPS Newsletter. In 1978 it was renamed A.P.P. Australasian Plant Pathology, and in 1984 it adopted its current name. As of early 2007 there have been 36 volumes, at six issues per year. According to the Journal Citation Reports, its 2009 impact factor is 0.943.

Australasian Plant Pathology Society

The Australasian Plant Pathology Society (APPS) is a scientific association whose members study plant diseases. Its members are located in Australia, New Zealand and Papua New Guinea, and also the Indian, Pacific and Asian regions. The society was founded in 1969.

As part of their membership in APPS, members are also associate members of the International Society for Plant Pathology.

The 2013 APPS conference was held in Auckland, New Zealand.

British Society for Plant Pathology

The British Society for Plant Pathology, or BSPP, is a UK based organisation of British plant pathologists but accepts members from all countries. It was founded in 1981 and publishes three scientific journals: Plant Pathology, Molecular Plant Pathology and New Disease Reports.The organisation actively tries to encourage its younger members to participate. Each year an award is given out each year for the best student paper published in one of societies journals. The PH Gregory prize is awarded to the best presenter of an oral paper at the annual presidential meeting.

Like other organisations of its type it arranges conferences and also awards various scholarships and fellowships.

Clara H. Hasse

Clara Henriette Hasse (1880 – 10 October 1926) was an American botanist whose research focused on plant pathology. She is known for identifying the cause of citrus canker, which was threatening crops in the Deep South.

East Ridge Historical Area

The East Ridge Historical Area of UMass Amherst consists of several of the older lecture halls on campus, primarily those used by the entomology, plant pathology and other natural science programs. These buildings were originally located adjacent to the campus orchard where the Central Residential Area currently stands today. The district consists mainly of lecture halls that were built prior to the first World War by the Massachusetts Agricultural College; however, it is also home to the Boltwood-Stockbridge House, the oldest house in Amherst.

Edwin John Butler

Sir Edwin John Butler FRS (13 August 1874 – 4 April 1943) was an Irish mycologist and plant pathologist. He became the Imperial Mycologist in India and later the first director of the Imperial Bureau of Mycology in England. He was knighted in 1939.. During his twenty years in India, he began large scale surveys on fungi and plant pathology and published the landmark book Fungi and Disease in Plants: An Introduction to the Diseases of Field and Plantation Crops, especially those of India and the East (1918) and has been called the Father of Mycology and Plant Pathology in India.

Forest pathology

Forest pathology is the research of both biotic and abiotic maladies affecting the health of a forest ecosystem, primarily fungal pathogens and their insect vectors. It is a subfield of forestry and plant pathology.

Forest pathology is part of the broader approach of forest protection.

Glenn Simpson Pound

Glenn Simpson Pound (March 7, 1914 – July 6, 2010) was an American educator and acting Chancellor of the University of Wisconsin–Madison in 1977.

Born in Hector, Arkansas, Pound graduated from University of Arkansas in 1940 and then received his doctorate degree from University of Wisconsin in 1943. In 1946, Pound was named professor of plant pathology and was dean of the UW College of Agriculture from 1964 until 1979. In 1977, Pound was acting Chancellor of University of Wisconsin–Madison. Pound retired in 1979 and moved to La Jolla, California. He served as adjunct professor of plant pathology at University of California, Riverside. He died in La Jolla, California in 2010.

Kishan Singh (biologist)

Kishan Singh (born 1931) is an Indian plant pathologist, known for his contributions to the pathology of crops, especially sugarcane. An alumnus of the Chandra Shekhar Azad University of Agriculture and Technology, he is reported to have done seminal research on the epidemiology and control of sugarcane diseases and suggested disease management through hot air therapy. He has published his research findings by way of articles and books, which include Soil fungicides (2 volumes), Recent advances in plant pathology,The national research grid for sugarcane in India, Sugarcane diseases and prospects of their control, Diseases of sugarbeet in India, Grassy shoot disease of sugarcane : III: response of varieties to infection,Innovations in companion cropping with sugarcane and Laminar infection of sugarcane leaves by red rot (Physalospora tucumanensis) organism in nature. The Council of Scientific and Industrial Research, the apex agency of the Government of India for scientific research, awarded him the Shanti Swarup Bhatnagar Prize for Science and Technology, one of the highest Indian science awards, in 1976, for his contributions to biological sciences.

List of botany journals

The following is a list of botanical scientific journals.

Miles Joseph Berkeley

Miles Joseph Berkeley (1 April 1803 – 30 July 1889) was an English cryptogamist and clergyman, and one of the founders of the science of plant pathology. The standard author abbreviation Berk. is used to indicate this person as the author when citing a botanical name.

Molecular Plant Pathology

Molecular Plant Pathology is a bimonthly open access peer-reviewed scientific journal published by Wiley-Blackwell on behalf of the British Society for Plant Pathology. It was established in January 2000 by Gary D. Foster, University of Bristol, who acted as editor-in-chief from 2000 to 2012. The journal covers research concerning plant pathology, in particular its molecular aspects such as plant-pathogen interactions. The current editor-in-chief is Ralph A. Dean (North Carolina State University). The journal had a 2017 impact factor of 4.188, ranking it 17th out of 223 journals in the category "Plant Sciences". The journal became open access in January 2019.

Pathogenic fungus

Pathogenic fungi are fungi that cause disease in humans or other organisms. Approximately 300 fungi are known to be pathogenic to humans. The study of fungi pathogenic to humans is called "medical mycology". Although fungi are eukaryotic, many pathogenic fungi are microorganisms. The study of fungi and other organisms pathogenic to plants is called plant pathology.

Plant Pathology (journal)

Plant Pathology is a peer-reviewed scientific journal published by Wiley-Blackwell in association with the British Society for Plant Pathology. It was established in 1952 and was originally published by the Ministry of Agriculture. The journal publishes research articles and critical reviews on all aspects of plant pathology except for articles on pesticide and resistance screening. The editor-in-chief is Matt Dickinson.

Trial garden

A garden grown specifically for the purpose of testing and evaluating plants is called a trial garden. Universities, plant breeders, and garden-industry companies frequently have trial gardens, as do many private and public botanical gardens and professional garden journalists. In the classic trials model, newly developed varieties of plants are compared with the closest similar industry standard plant throughout their life cycle—from germination/propagation through maturity, from seed to harvest. By growing new varieties side-by-side with existing ones, researchers can determine whether these new varieties are indeed better, and, if so, in what respects.

The Muck Crops Research Station, near Kettleby and Ansnorveldt, in Ontario, Canada, operated by the University of Guelph, tests new species and houses a plant pathology laboratory. Its focus is on plants that grow in muck soil.

Any gardener might enjoy creating his or her own trial garden to see which plants fare best in a specific garden environment. Most trials programs are very formal, with scientific designs including random replicated plots to minimise any risk of bias due to placement of plants. The volunteer judges evaluate entries for desirable qualities such as novel flower forms, flower colors, flower show above foliage, fragrance, length of flowering season, and disease or pest tolerances or resistance. Vegetables are judged looking for such traits as speed to harvest, total yield, fruit taste, fruit quality, ease of harvest, plant habit, disease, and pest resistance.

Viroid

Viroids are the smallest infectious pathogens known. They are composed solely of a short strand of circular, single-stranded RNA that has no protein coating. All known viroids are inhabitants of higher plants, in which most cause diseases, ranging in economic importance.

Discovery of the viroid triggered the third major extension of the biosphere in history to include smaller lifelike entities—after the discovery of the "subvisible" microorganisms by Antonie van Leeuwenhoek in 1675 and the "submicroscopic" viruses by Dmitri Iosifovich Ivanovsky in 1892.

The unique properties of viroids have been recognized by the International Committee for Virus Taxonomy with the creation of a new order of subviral agents.The first recognized viroid, the pathogenic agent of the potato spindle tuber disease, was discovered, initially molecularly characterized, and named by Theodor Otto Diener, plant pathologist at the U.S Department of Agriculture's Research Center in Beltsville, Maryland, in 1971. This viroid is now called Potato spindle tuber viroid, abbreviated PSTVd.

In a year 2000 compilation of the most important Millennial Milestones in Plant Pathology, the American Phytopathological Society has ranked the 1971 discovery of the viroid as one of the Millennium's ten most important pathogen discoveries.As cogently expressed by Flores et al: Viruses (and viroids) share the most characteristic property of living beings: In an appropriate environment, they are able to generate copies of themselves, in other words, they are endowed with autonomous replication (and evolution). It is in this framework where viroids represent the frontier of life (246 to 467nt), an aspect that should attract the attention of anybody interested in biology.Although viroids are composed of nucleic acid, they do not code for any protein. The viroid's replication mechanism uses RNA polymerase II, a host cell enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes "rolling circle" synthesis of new RNA using the viroid's RNA as a template. Some viroids are ribozymes, having catalytic properties that allow self-cleavage and ligation of unit-size genomes from larger replication intermediates.With Diener's 1989 hypothesis that viroids may represent "living relics" from the widely assumed, ancient, and non-cellular RNA world—extant before the evolution of DNA or proteins—viroids have assumed significance beyond plant pathology to evolutionary science, by representing the most plausible RNAs capable of performing crucial steps in abiogenesis, the evolution of life from inanimate matter.

The human pathogen hepatitis D virus is a "defective" RNA virus similar to a viroid.

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