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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. Not included are ectoparasites like insects, mites, vertebrate, or other pests that affect plant health by consumption 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.
- 1 Overview
- 2 Plant pathogens
- 3 Common pathogenic infection methods
- 4 Physiological plant disorders
- 5 Epidemiology
- 6 Disease resistance
- 7 Management
- 8 Timeline of plant pathology
- 9 See also
- 10 References
- 11 External links
Control of plant diseases is crucial to the reliable production of food, and it provides significant reductions 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 (see Irish Potato Famine, Chestnut blight), as well as recurrent severe plant diseases (see Rice blast, Soybean cyst nematode, 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.
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. These are known as facultative saprotrophs.
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. See Powdery Mildew and Rice Blast images below.
Significant fungal plant pathogens include:
- Fusarium spp. (causal agents of Fusarium wilt disease)
- Thielaviopsis spp. (causal agents of: canker rot, black root rot, Thielaviopsis root rot)
- Verticillium spp.
- Magnaporthe grisea (causal agent of blast of rice and gray leaf spot in turfgrasses)
- Sclerotinia sclerotiorum white mold
- Ustilago spp.
- Rhizoctonia spp.
- Phakospora pachyrhizi (causal agent of soybean rust)
- Puccinia spp. (causal agents of severe rusts of virtually all cereal grains and cultivated grasses)
- Armillaria spp. (the so-called honey fungus species, which are virulent pathogens of trees and produce edible mushrooms)
The oomycetes are not true fungi but are fungus-like organisms. They include some of the most destructive plant pathogens including the genus 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 very 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
Some slime molds in Phytomyxea cause important diseases, includind club root in cabbage and its relatives, and powdery scab in potatoes. These are caused by species of Plasmodiophora and Spongospora, respectively.
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. Bacterial diseases are much more prevalent in sub-tropical 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, use Cell wall-degrading enzymes to cause soft rot. Agrobacterium changes 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.
Significant bacterial plant pathogens:
- 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 ('Mycoplasma-like organisms') and spiroplasmas
Phytoplasma and Spiroplasma are a genre 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 plants 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.
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, 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). They are transmitted as zoospores that are very durable, and may be able to survive in a resting state in the soil for many years. They have also been shown to transmit plant viruses.
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. Bacteria, fungus and oomycetes are known for this function.
Physiological plant disorders
Significant abiotic disorders can be caused by:
- Man-made (arguably not abiotic, but usually regarded as such)
- Wherein a diseased patch of vegetation or individual plants are isolated from other, healthy growth. Specimens may be destroyed or relocated into a greenhouse for treatment/study. Another option is to avoid introduction of harmful non-native organisms by controlling all human traffic and activity (e.g., AQIS) although legislation and enforcement are key in order to ensure lasting effectiveness.
- 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 foodplants may be less rewarding, however, as higher output is frequently offset by popular suspicion and negative opinion about this "tampering" with nature.
- (See: pesticide application) Many natural and synthetic compounds that could be employed to combat the above threats exist. 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.
- 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.
- The use of two or more of these methods in combination offers a higher chance of effectiveness.
Timeline of plant pathology
- 300-286 BC Theophrastus, father of botany, wrote and studied diseases of trees, cereals and legumes
- 1665 Robert Hooke illustrates and plant pathogenic fungal disease, rose rust
- 1675 Anton van Leeuwenhouek invents the compound microscope, in 1683 describes bacteria seen with the microscope
- 1729 Pier Antonio Micheli, father of mycology, observes spores for the first time, conducts germination experiments
- 1755 Tillet reports on treatment of seeds
- 1802 Lime sulfur first used to control plant disease
- 1845-1849 Potato late blight epidemic in Ireland
- 1853 Heinrich Anton de Bary father of modern mycology, establishes that fungi are the cause, not the result of plant diseases, publishes "Untersuchungen uber die Brandpilze"
- 1858 Julius Kühn publishes "Die Krankheiten der Kultergewachse"
- 1865 M. Planchon discovers a new species of Phylloxera, which was named Phylloxera vastatrix.
- 1868-1882 Coffee rust epidemic in Sri Lanka
- 1876 Fusarium oxysporum f.sp. cubense ,responsible for Panama disease, discovered in bananas in Australia
- 1878-1885 Downy mildew of grape epidemic in France
- 1879 Robert Koch establishes germ theory: diseases are caused by micro-organisms
- 1882 Lehrbuch der Baumkrankheiten is authored by Robert Hartig in Berlin, the Diseases of Trees is the first textbook of forest pathology.
- 1885 Bordeaux mixture introduced by Pierre-Marie-Alexis Millardet to control downy mildew on grape
- 1885 Experimental proof that bacteria can cause plant diseases; "Erwinia amylovora" and fire blight of apple
- 1886-1898 Recognition of plant viral diseases; Tobacco mosaic virus
- 1889 Introduction of hot water treatment of seed for disease control by Jensen
- 1902 First chair of Plant Pathology established; Copenhagen
- 1904 Mendelian inheritance of cereal rust resistance demonstrated
- 1907 First academic department of Plant Pathology established; Cornell University
- 1908 American Phytopathological Society founded
- 1910 Panama disease reaches Western Hemisphere 
- 1911 The scientific journal Phytopathology founded
- 1925 Panama disease reaches every banana growing country in the Western Hemisphere
- 1951 European and Mediterranean Plant Protection Organization (EPPO) founded
- 1967 Recognition of plant pathogenic mycoplasma-like organisms
- 1971 T. O. Diener discovers viroids, organisms smaller than viruses
The historical landmarks in plant pathology are taken from unless otherwise noted.
- American Phytopathological Society
- Australasian Plant Pathology Society
- Biological control with micro-organisms
- British Society for Plant Pathology
- Common names of plant diseases
- Disease resistance in fruit and vegetables
- Forest pathology
- Gene-for-gene relationship
- Global Plant Clinic
- Glossary of phytopathology
- Inducible plant defenses against herbivory
- List of phytopathology journals
- Microbial inoculant
- Plant defense against herbivory
- Plant disease forecasting
- Agrios, George N. PLANT PATHOLOGY.3rd ed. New York: ACADEMIC PRESS INC., 1972. print.
- Nicole Davis (September 9, 2009). "Genome of Irish potato famine pathogen decoded". Haas et al. Broad Institute of MIT and Harvard. Retrieved 24 July 2012.
- "Scientists discover how deadly fungal microbes enter host cells". (VBI) at Virginia Tech affiliates. Physorg. July 22, 2010. Retrieved July 31, 2012.
- Jackson RW (editor). (2009). Plant Pathogenic Bacteria: Genomics and Molecular Biology. Caister Academic Press. ISBN 978-1-904455-37-0.
- Burkholder (October 1948). "Bacteria as Plant Pathogens". Annual Review of Microbiology (Cornell University) 2: 389–412. doi:10.1146/annurev.mi.02.100148.002133. PMID 18104350.
- "Research team unravels tomato pathogen's tricks of the trade". Virginia Tech. 2011.
- Creamer, Rebecca; H. Hubble and A. Lewis (May 2005). "Curtovirus Infection of Chile Pepper in New Mexico". Plant Diseases 89 (5): 480–486. doi:10.1094/PD-89-0480.
- Jankevicius, J.V. et al. Ciclo biológico de Phytomonas / Biological cycle of Phytomonas. Memórias do Instituto Oswaldo Cruz, Rio de Janeiro, v. 83, supl. 1, 1988.
- Ma, Winbo (March 28, 2011). "How do plants fight disease? Breakthrough research by UC Riverside plant pathologist offers a clue". UC Riverside.
- "1st large-scale map of a plant's protein network addresses evolution, disease process". Dana-Farber Cancer Institute. July 29, 2011. Retrieved 24 July 2012.
- "History of Plant Pathology". Retrieved 5 February 2015.
- "Plasmopara viticola, the Cause of Downy Mildew of Grapes". The Origin of Plant Pathology and The Potato Famine, and Other Stories of Plant Diseases. Retrieved 4 February 2015.
- "Fusarium oxysporum : The End of the Banana Industry?". The Origin of Plant Pathology and The Potato Famine, and Other Stories of Plant Diseases. Retrieved 4 February 2015.
- "Plant Diseases - History Of Plant Pathology". Retrieved 5 February 2015.
- Aisnworth GC. (1981). Introduction to the History of Plant Pathology. Cambridge University Press. ISBN 0-521-23032-2.
- International Society for Plant Pathology
- Australasian Plant Pathology Society
- American Phytopathological Society
- British Society for Plant Pathology
- Food Security Journal
- 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