Fungi , a biotrophic
Ascomycete fungus 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. (Fusarium wilt disease) •
Magnaporthe grisea (rice blast) •
Rhizosphaera needle cast (e.g. in
spruce) •
Sclerotinia sclerotiorum (cottony rot) •
Thielaviopsis spp. (canker rot, black root rot,
Thielaviopsis root rot) •
Verticillium spp. (
Verticillium wilt of dicots)
Basidiomycetes '' •
Ustilago spp. (smuts) •
Rhizoctonia spp. •
Phakospora pachyrhizi (
soybean rust) •
Puccinia spp. (severe rusts of
cereals and
grasses)(fungus)|rusts. •
Armillaria spp. (honey fungus species, virulent pathogens of trees)
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 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 with "
Ca. Phytoplasma vitis" infection
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 (roughly between 0.5 and 2 Mb). They are normally transmitted by
leafhoppers (cicadellids) 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 galls Some
nematodes parasitize plant
roots. They are a problem in
tropical and
subtropical regions. Potato cyst nematodes (
Globodera pallida and
G. rostochiensis) 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 ==