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Annual Plant Reviews: Plant Pathogen Interactions
Tap cross to close filters. 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. 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. 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. 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.
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Necrotrophic fungal pathogens infect and kill host tissue and extract nutrients from the dead host cells. Significant fungal plant pathogens include: [ citation needed ].
The oomycetes are fungus-like organisms. 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. 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.
Most bacteria that are associated with plants are actually saprotrophic and do no harm to the plant itself.
However, a small number, around known species, are able to cause disease. Most plant pathogenic bacteria are rod-shaped bacilli.
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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. 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.
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.
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.
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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.
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 a few examples of plant diseases caused by protozoa e. 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. 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. 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. 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. 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. 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.