Plant disease

Plant diseases are diseases in plants caused by pathogens and environmental conditions. 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, vertebrates, or other pests that affect plant health by eating plant tissues and causing injury that may admit plant pathogens. The study of plant disease is called plant pathology.

Plant pathogens

Fungi

Most phytopathogenic fungi are Ascomycetes or Basidiomycetes. They 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 role of their life cycle in the soil. These are facultative saprotrophs.
Fungal diseases may be controlled through the use of fungicides and other agricultural 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:

Ascomycetes

Fusarium spp.
Magnaporthe grisea
Rhizosphaera needle cast
Sclerotinia sclerotiorum
Thielaviopsis spp.
Verticillium spp.
Basidiomycetes
Ustilago spp.
Rhizoctonia spp.
Phakospora pachyrhizi
Puccinia spp. |rusts.
Armillaria spp.
Fungus-like organisms

Oomycetes

The oomycetes are fungus-like organisms among the Stramenopiles. They include some of the most destructive plant pathogens, such as the causal agents of potato late blight root rot, and sudden oak death.
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.

Phytomyxea

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

Bacteria

Pathogenic bacteria

Most bacteria associated with plants are saprotrophic and do no harm to the plant itself. However, a small number, around 100 known species, cause disease, especially in subtropical and tropical regions of the world.
Most plant pathogenic bacteria are bacilli. Erwinia uses cell wall–degrading enzymes to cause soft rot. Agrobacterium changes the level of auxins to cause tumours with phytohormones.
Bacterial plant pathogens include:

Burkholderia
Pseudomonadota
* 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."
Mollicutes

Phytoplasma and Spiroplasma are obligate intracellular parasites, bacteria that lack cell walls and, like the mycoplasmas, which are human pathogens, they belong to the class Mollicutes. Their cells are extremely small, 1 to 2 micrometres across. They tend to have small genomes. They are normally transmitted by leafhoppers and psyllids, both sap-sucking insect vectors. These inject the bacteria into the plant's phloem, where it reproduces.

Viruses

Many plant viruses cause only a loss of crop yield. Therefore, it is not economically viable to try to control them, except when they infect perennial species, such as fruit trees.
Most plant viruses have small, single-stranded RNA genomes. Some also have double stranded RNA or single or double stranded DNA. These may encode only three or four proteins: a replicase, a coat protein, a movement protein to facilitate cell to cell movement through plasmodesmata, and sometimes a protein that allows transmission by a vector.
Plant viruses are generally transmitted by a vector, but mechanical and seed transmission also occur. Vectors are often insects such as aphids; others are fungi, nematodes, and protozoa. 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

Some nematodes parasitize plant roots. They are a problem in tropical and subtropical regions. Potato cyst nematodes are widely distributed in Europe and the Americas, causing 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, 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

A few plant diseases are caused by protozoa such as Phytomonas, 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. When the motile zoospores come into contact with a root hair they produce a plasmodium which invades the roots.

Physiological plant disorders

Some abiotic disorders can be confused with pathogen-induced disorders. Abiotic causes include 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.

Economic impact

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. The Food 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 and spectroscopy and biophotonics that are able to diagnose plant health and metabolism.
the most costly diseases of the most produced crops worldwide are:

Crop	Disease Latin name	Disease common name
Banana and plantain	banana bunchy top virus	banana bunchy top
Banana and plantain	Mycosphaerella fijiensis	black sigatoka
Banana and plantain	Fusarium oxysporum f.sp. cubense	Panama disease
Barley	Fusarium graminearum	Fusarium head blight
Barley	Blumeria hordei	powdery mildew
Barley	Puccinia hordei	barley stem rust
Cassava	African cassava mosaic virus	African cassava mosaic disease
Cassava	Xanthomonas axonopodis pv. manihotis	bacterial blight
Cassava	cassava brown streak virus	cassava brown streak disease
Cotton	Xanthomonas citri pv. malvacearum	bacterial blight
Cotton	Fusarium oxysporum f. sp. vasinfectum	Fusarium wilt
Cotton	Verticillium dahliae	Verticillium wilt
Maize/corn	Aspergillus flavus	Aspergillus ear rot
Maize/corn	Fusarium graminearum	Giberella stalk and ear rot
Maize/corn	Cercospora zeae-maydis	grey leaf spot
Palm fruit	Ganoderma orbiforme/Ganoderma boninense	Basal stem rot
Palm fruit	Phytophthora palmivora	bud rot
Peanut	groundnut rosette virus	Groundnut rosette disease
Peanut	GNV satellite RNA	Groundnut rosette disease
Peanut	groundnut rosette assistor virus	Groundnut rosette disease
Potato	Ralstonia solanacearum	Potato brown rot
Potato	Phytophthora infestans	late blight
Rapeseed and mustard	Leptosphaeria maculans	Phoma stem canker
Rapeseed and mustard	Sclerotinia sclerotiorum	Sclerotinia stem rot
Rice	Magnaporthe oryzae	rice blast
Rice	Xanthomonas oryzae pv. oryzae	rice bacterial blight
Rice	Rhizoctonia solani	sheath blight
Sorghum and millet	Colletotrichum sublineolum	Anthracnose
Sorghum and millet	Exserohilum turcicum	Turcicum leaf blight
Soybean	Heterodera glycines	soybean cyst nematode disease
Soybean	Phakopsora pachyrhizi	Asian soybean rust
Sugar beet	Cercospora beticola	Cercospora leaf spot
Sugar beet	beet necrotic yellow vein virus	rhizomania
Sugarcane	Leifsonia xyli subsp. xyli	Ratoon stunting
Sugarcane	Colletotrichum falcatum	red rot
Sweet potato	sweet potato feathery mottle virus	sweet potato virus disease
Sweet potato	sweet potato chlorotic stunt virus	sweet potato virus disease
Tomato	Phytophthora infestans	late blight
Tomato	tomato yellow leaf curl virus	tomato yellow leaf curl
Wheat	Fusarium graminearum	Fusarium head blight
Wheat	Puccinia graminis	wheat stem rust
Wheat	Puccinia striiformis	wheat yellow rust
Yam	Colletotrichum gloeosporioides	anthracnose
Yam	yam mosaic virus	yam mosaic disease