Plant diseases are diseases inplants caused bypathogens (infectious organisms) and environmental conditions (physiological factors).[1] Organisms that cause infectious disease includefungi,oomycetes,bacteria,viruses,viroids,virus-like organisms,phytoplasmas,protozoa,nematodes andparasitic plants.[2] Not included areectoparasites likeinsects,mites,vertebrates, or otherpests that affect plant health by eatingplant tissues and causing injury that may admit plant pathogens. The study of plant disease is calledplant pathology.
Most phytopathogenic fungi areAscomycetes orBasidiomycetes. They reproduce bothsexually andasexually via the production ofspores 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 livingsaprotrophically, carrying out the role of their life cycle in thesoil. These are facultative saprotrophs.
Fungal diseases may be controlled through the use offungicides and other agricultural practices. However, newraces of fungi oftenevolve 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.[3]
Significant fungal plant pathogens include:
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Theoomycetes are fungus-like organisms among theStramenopiles.[9] They include some of the most destructive plant pathogens, such as the causal agents ofpotato late blight[9]root rot,[10] andsudden oak death.[11][12]
Despite not being closely related to the fungi, the oomycetes have developed similar infection strategies, using effector proteins to turn off a plant's defenses.[13]
Someslime molds inPhytomyxea cause important diseases, includingclubroot in cabbage and its relatives andpowdery scab in potatoes. These are caused by species ofPlasmodiophora andSpongospora, respectively.[14]
Mostbacteria associated with plants aresaprotrophic and do no harm to the plant itself. However, a small number, around 100 known species, cause disease, especially insubtropical andtropical regions of the world.[15][page needed]
Most plant pathogenic bacteria arebacilli.Erwinia uses cell wall–degrading enzymes to causesoft rot.Agrobacterium changes the level ofauxins to cause tumours with phytohormones.
Significant bacterial plant pathogens include:
Phytoplasma andSpiroplasma are obligateintracellular parasites, bacteria that lack cell walls and, like themycoplasmas, which are human pathogens, they belong to the classMollicutes. Their cells are extremely small, 1 to 2 micrometres across. They tend to have smallgenomes (roughly between 0.5 and 2 Mb). They are normally transmitted byleafhoppers (cicadellids) andpsyllids, both sap-sucking insect vectors. These inject the bacteria into the plant'sphloem, where it reproduces.[19]
Many plant viruses cause only a loss ofcrop yield. Therefore, it is not economically viable to try to control them, except when they infectperennial species, such as fruit trees.[citation needed]
Most plant viruses have small, single-strandedRNAgenomes. Some also have double strandedRNA or single or double strandedDNA. These may encode only three or fourproteins: areplicase, a coat protein, amovement protein to facilitate cell to cell movement throughplasmodesmata, and sometimes a protein that allows transmission by a vector.[citation needed]
Plant viruses are generally transmitted by avector, but mechanical and seed transmission also occur. Vectors are ofteninsects such asaphids; others arefungi,nematodes, andprotozoa. In many cases, the insect and virus are specific for virus transmission such as thebeet leafhopper that transmits thecurly top virus causing disease in several crop plants.[20]
Somenematodes parasitize plantroots. They are a problem intropical andsubtropical regions. Potato cyst nematodes (Globodera pallida andG. rostochiensis) are widely distributed in Europe and the Americas, causing$300 million worth of damage in Europe annually. 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,[21] 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.[22]
A few plant diseases are caused byprotozoa such asPhytomonas, akinetoplastid.[23] They are transmitted as durablezoospores that may be able to survive in a resting state in the soil for many years. Further, they can transmit plantviruses. When the motile zoospores come into contact with aroot hair they produce aplasmodium which invades theroots.[citation needed]
Some abiotic disorders can be confused with pathogen-induced disorders. Abiotic causes include natural processes such asdrought,frost,snow andhail;flooding and poor drainage;nutrient deficiency; deposition of mineral salts such assodium chloride andgypsum;windburn and breakage by storms; andwildfires.[24]
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Plants are subject to disease epidemics.
The introduction of harmful non native organisms into a country can be reduced by controlling human traffic (e.g., theAustralian Quarantine and Inspection Service). Global trade provides unprecedented opportunities for the introduction of plant pests.[McC 1] In theUnited States, even to get a better estimate of the number of such introductions would require a substantial increase in inspections.[McC 2] InAustralia 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 theAustralian Government has prioritised as harmful to be kept out of the country, but which have near taxonomic relatives that confuse the issue.[BH 1]
X-ray andelectron-beam/E-beamirradiation of food has been trialed as a quarantine treatment forfruitcommodities originating fromHawaii. The US FDA (Food and Drug Administration), 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.[25]
TheInternational Plant Protection Convention (IPPC) anticipates thatmolecular diagnostics for inspections will continue to improve.[26] Between 2020 and 2030, IPPC expects continued technological improvement to lower costs and improve performance, albeit not forless developed countries unless funding changes.[26]
Many natural and synthetic compounds can be employed to combat plant diseases. 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 is a traditional and sometimes effective means of preventing pests and diseases from becoming well-established, alongside other benefits.[27]
Other biological methods include inoculation. Protection against infection byAgrobacterium tumefaciens, which causes gall diseases in many plants, can be provided by dipping cuttings in suspensions ofAgrobacterium radiobacter before inserting them in the ground to take root.[28]
Plant diseases cause major economic losses for farmers worldwide. 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. TheFood and Agriculture Organization 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 andspectroscopy andbiophotonics that are able to diagnose plant health andmetabolism.[29]
As of 2018[update] the most costly diseases of the most produced crops worldwide are:[30]
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