'Phytopathology' (plant pathology) is the scientific study of plant diseases caused by pathogens (infectious diseases) 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 insects, mites, vertebrate or other pests that affect plant health by consumption of plant tissues. Plant Pathology also involves the study of the identification, etiology, disease cycle, economic impact, epidemiology, pathosystem genetics and management of plant diseases.
The ''"Disease triangle"'' is a central concept of plant pathology for infectious diseases
[George N. Agrios (1997) Plant Pathology fourth edition, Academic Press. New York.] . It is based on the principle that disease is the result of an interaction between a host, a pathogen, and environment condition.
Infectious diseases
Fungi
The majority of phytopathogenic fungi belong to the
Ascomycetes and the
Basidiomycetes.
The fungi reproduce both
sexually and
asexually via the production of
spores. These spores may be spread long distances by air or water, or they may be soil bourne. Many soil bourne spores, normally
zoospores and capable of living
saprophytically, caring out the first part of their lifecycle in the
soil.
Fungal diseases can be controlled through the use of
fungicides in agriculture, however new
races of fungi often
evolve that are resistant to various fungicides.
Significant fungal plant pathogens
★ Ascomycetes
★
★ ''
Fusarium'' spp.
★
★ ''
Thielaviopsis'' spp. (Causal agents of: canker rot, black root rot, ''Thielaviopsis'' root rot)
★
★ ''
Verticillium'' spp.
★
★ ''
Magnaporthe grisea'' (T.T. Hebert) M.E. Barr; causes blast of rice and gray leaf spot in turfgrasses
★ Basidiomycetes
★
★ ''
Rhizoctonia'' spp.
★
★ ''
Phakospora pachyrhizi'' Sydow; causes Soybean rust
★
★ ''Puccinia'' spp.; causal agents of severe rusts of virtually all cereal grains and cultivated grasses
Oomycetes
Leaf death caused by the
oomycete P. ramorum
The
oomycetes are fungal-like organisms that until recently used to be mistaken for
fungi. They include some of the most destructive plant pathogens including the
genus ''Phytophthora'' which includes the casual agents of
potato blight and
sudden oak death.
Despite not being closely related to the fungi, the oomycetes have developed very similar infection strategies and so many plant pathologists group them with fungal pathogens.
Significant oomycete plant pathogens
★ ''
Pythium'' spp.
★ ''
Phytophthora'' spp.; including the causal agent of the
Great Irish Famine (1845-1849)
Rice blast is hemibiotrofic
Bacteria
Most
bacteria that are associated with plants are actually
saprophytic, and do no harm to the plant itself. However, a small number, around 100 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 colonise the plant they have specific pathogenicity factors. There are 4 main bacterial pathogenicity factors:
1. 'Cell wall degrading enzymes' - used to break down the plant
cell wall in order to release the nutrients inside. Used by pathogens such as
Erwinia to cause
soft rot.
2. '
Toxins' These can be non-host specific, and damage all plants, or host specific and only cause damage on a host plant.
3. '
Phytohormones' - for example
Agrobacterium changes the level of
Auxin to cause tumours.
4. '
Exopolysaccharides' - these 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.
Significant bacterial plant pathogens
★
Proteobacteria
★
★ ''
Xanthomonas'' spp.
★
★ ''
Pseudomonas'' spp.
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 true 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. Normally plant viruses only cause a loss of
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 only encode 3 or 4
proteins: a
replicase, a coat protein, a movement protein to allow cell to cell movement and sometimes a protein that allows transmission by a vector.
Plant viruses must be transmitted from plant to plant by a
vector. This is normally an insect, but some
fungi,
nematodes and
protozoa have been shown to be viral vectors.
Nematodes
Nematodes are small, multicelluar wormlike creatures. Many live freely in the soil, but there are some species which parasitize plant
roots. They are mostly a problem in
tropical and
subtropical regions of the world, where they may infect
crops. Root knot nematodes have quite a large host range, whereas cyst nematodes tend to only be able to infect a few species. Nematodes are able to cause radical changes in root cells in order to facilitate their lifestyle.
Protozoa
There are a few examples of plant diseases caused by
protozoa. They are transmitted as
zoospores which 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.
When the motile zoospores come into contact with a
root hair they produce a plasmodium and invade the
roots.
Parasitic plants
Parasitic plants such as
mistletoe and
dodder are included in the study of phytopathology.
Physiological plant disorders
Significant abiotic disorders can be caused by:
:'''Natural'''
::
Drought
::
Frost damage, and breakage by
snow and
hail
::
Flooding and poor drainage
::
Nutrient deficiency
::
Salt deposition and other soluble mineral excesses (e.g.
gypsum)
::
Wind (windburn, and breakage by
hurricanes and
tornadoes)
::
Lightning and
wildfire (also often man-made)
:'''Man-made''' (arguably not abiotic, but usually regarded as such)
::
Soil compaction
::
Pollution of air and/or soil
::
Salt from winter road salt application
::
Herbicide over-application
::Poor education and training of people working with plants (e.g. lawnmower damage to trees)
::
Vandalism
Epidemiology
An
epidemic is a disease increase in a
population that is normally severe and widespread. Disease epidemics in plants can cause huge losses in yield of crops as well threatening to wipe out entire species of tree such as was the case with
Dutch Elm Disease and could occur with
Sudden Oak Death. An epidemic of
late potato blight led to the
Great Irish Famine (1845-1849) and the loss of many lives.
Factors Effecting Epidemics
The Plant Disease Triangle
There are three major factors in the development of plant disease
epidemics: the presence of large numbers of susceptible
host plants (often provided by
agriculture), a
virulent pathogen and favourable environmental conditions.
[George N. Agrios (1997) Plant Pathology fourth edition, Academic Press. New York.] Together these three factors are referred to as the plant disease trangle, as pictured right. But this is a somewhat basic representation of the factors effecting the progression of an epidemic.
Sometimes a fourth factor of
time is added as the time at which a particular infection occurs, and the length of time conditions remain viable for that infection, can also play an important role in epidemics.
[George N. Agrios (1997) Plant Pathology fourth edition, Academic Press. New York.] The age of the plant species can also play a role, as cerytain species change in their levels of
disease resistance as they mature a process known as ontogenic resistance.
[George N. Agrios (1997) Plant Pathology fourth edition, Academic Press. New York.]
When a pathogen requires a vector to be spread then for an epidemic to occur the vector must be plentiful and active.
Types of Epidemics
Monocyclic epidemics are caused by pathogens with a low
birth rate and
death rate meaning they only have one infection cycle per season. They are typical of soil born diseases such as
Fusarium wilt of
flax. Polycyclic epidemics are caused by pathogens capable of several infection cycles a season. These are most often caused by airborne diseases such as
powdery mildew.
Bimodal polycyclic epidemics can also occur. For example in
brown rot of
stone fruits the
blossoms and the
fruits are infected at different times.
For some diseases it is important to consider the disease occurrence over several growing seasons especially if growing the crops in
monoculture year after year or growing
perennial plants. Such conditions can mean that the
innoculum produced in one season can be carried over to the next leading to a build of an innoculum over the years. In the
tropics there are no clear cut breaks between growing seasons as there are in
temperate regions and this can lead to accumulation of innoculum.
Epidemics that occur under these conditions are referred to as polyetic epidemics and can be caused by both monocylcic and polycyclic pathogens.
Apple powdery mildew is an example of a polyetic epidemic caused by a polycyclic pathogen and Dutch Elm disease monocyclic pathogen that can give rise to a polyetic epidemic.
Management
★ Quarantine
★ Cultural
★ Plant resistance
★ Chemical
★ Biological
★ Integrated
See also
★
American Phytopathological Society
★
Biological Control
★
British Society for Plant Pathology
★
burl
★
Common names of plant diseases
★
Fungicides
★
Herbivory
★
List of phytopathology journals
★
Mycology
★
Pesticide
★
QoI
★
Phytoplasma
★
Plant virus
★
Strobilurins
★
Stunt
References
Further reading
External links
★
American Phytopathological Society
★
British Society for Plant Pathology
★
Plant Health Progress, Online journal of applied plant pathology
★
Pacific Northwest Fungi, online mycology journal with papers on fungal plant pathogens
★
Rothamsted Plant Pathogen Interactions Department
العربية
中国
Français
Deutsch
Ελληνική
हिन्दी
Italiano
日本語
Português
Русский
Español
