Phytophthora infestans is an oomycete or Oomycete, a fungus-like microorganism that causes the serious potato and tomato disease known as late blight or potato blight. Early blight, caused by Alternaria solani, is also often called "potato blight". Late blight was a major culprit in the 1840s European, the 1845–1852 Irish, and the 1846 Highland potato . The organism can also infect some other members of the Solanaceae.
Etymology
The genus name
Phytophthora comes from the Greek φυτόν (), meaning 'plant' – plus the Greek φθορά (), meaning 'decay, ruin, perish'. The species name
infestans is the present participle of the Latin verb infestare, meaning 'attacking, destroying', from which the word "to infest" is derived. The name
Phytophthora infestans was coined in 1876 by the German mycologist Heinrich Anton de Bary (1831–1888).
Life cycle, signs and symptoms
The asexual life cycle of
Phytophthora infestans is characterized by alternating phases of
growth, sporulation,
sporangia germination (either through
zoospore release or direct germination, i.e. germ tube emergence from the
sporangium), and the re-establishment of hyphal growth.
There is also a sexual cycle, which occurs when isolates of opposite mating type (A1 and A2, see below) meet. Hormonal communication triggers the formation of the sexual
, called
.
[Judelson HS, Blanco FA (2005) The spores of Phytophthora: weapons of the plant destroyer. Nature Microbiology Reviews 3: 47–58.] The different types of spores play major roles in the dissemination and survival of
P. infestans. Sporangia are spread by wind or water and enable the movement of
P. infestans between different host plants. The zoospores released from sporangia are
and
chemotactic, allowing further movement of
P. infestans on water films found on leaves or soils. Both sporangia and zoospores are short-lived, in contrast to oospores which can persist in a viable form for many years.
People can observe P. infestans produce dark green, then brown then black spots on the surface of potato leaves and stems, often near the tips or edges, where water or dew collects.
Under ideal conditions, P. infestans completes its life cycle on potato or tomato foliage in about five days.
P. infestans survives poorly in nature apart from on its plant hosts. Under most conditions, the hyphae and asexual sporangia can survive for only brief periods in plant debris or soil, and are generally killed off during frosts or very warm weather. The exceptions involve oospores, and hyphae present within tubers. The persistence of viable pathogen within tubers, such as those that are left in the ground after the previous year's harvest or left in cull piles is a major problem in disease management. In particular, sprouting from infected tubers are thought to be a major source of inoculum (or ) at the start of a growing season.
Mating types
The
are broadly divided into A1 and A2.
Until the 1980s populations could only be distinguished by
and mating types, but since then more detailed analysis has shown that mating type and
genotype are substantially decoupled.
These types each produce a
mating hormone of their own.
Pathogen populations are grouped into clonal lineages of these mating types and includes:
A1
A1 produces a mating hormone, a
diterpene α1.
Clonal lineages of A1 include:
-
CN-1, -2, -4, -5, -6, -7, -8 – mtDNA haplotype Ia, China in 1996–97
-
– Ia, China, 1996–97
-
– Ia, China, 2004
-
– IIb, China, 2000 & 2002
-
– IIa, China, 2004–09
-
– Ia/IIb, China, 2004–09
-
– (only presumed to be A1), mtDNA haplo Ia subtype , Japan, Philippines, India, China, Malaysia, Nepal, present some time before 1950
-
– Ia, India, Nepal, 1993
-
– Ia, India, 1993
-
JP-2/SIB-1/RF006 – mtDNA haplo IIa, distinguishable by RG57, intermediate level of metalaxyl resistance, Japan, China, Korea, Thailand, 1996–present
-
– IIa, distinguishable by RG57, intermediate level of metalaxyl resistance, Japan, 1996–present
-
– IIa, distinguishable by RG57, intermediate level of metalaxyl resistance, Japan, 1996–present
-
sensu Zhang (not to be confused with #KR-1 sensu Gotoh below) – IIa, Korea, 2002–04
-
KR_1_A1 – mtDNA haplo unknown, Korea, 2009–16
-
– Ia, China, 2004
-
– Ia, India, Nepal, 1993, 1996–97
-
– Ia, Nepal, 1997
-
– Ia, Nepal, 1999–2000
-
– (Also A2, see #the A2 type of NP2 below) Ia, Nepal, 1999–2000
-
(not to be confused with #US-1 below) – Ib, Nepal, 1999–2000
-
(not to be confused with #NP3/US-1 above) – Ib,
-
– Ia, Nepal, 1999–2000
-
– mtDNA haplo unknown, Nepal, 1999–2000
-
– IIb, Taiwan, Korea, Vietnam, 1998–2016
-
– IIb, China, 2002 & 2004
-
-
-
-
– Ia, Indonesia, 2016–19
-
A2
Discovered by John Niederhauser in the 1950s, in the
Toluca Valley in Central Mexico, while working for the Rockefeller Foundation's Mexican Agriculture Program. Published in Niederhauser 1956.
A2 produces a mating hormone α2.
Clonal lineages of A2 include:
-
CN02 – See #13_A2/CN02 below
-
– with mtDNA haplotype H-20
-
– IIa, Japan, Korea, Indonesia, late 1980s–present
-
sensu Gotoh (not to be confused with #KR-1 sensu Zhang above) – IIa, differs from JP-1 by one RG57 band, Korea, 1992
-
– mtDNA haplo unknown, Korea, 2009–16
-
– Ia, China, 2001
-
– (Also A1, see #the A1 type of NP2 above) Ia, Nepal, 1999–2000
-
– Ib, Nepal, 1999–2000
-
– Ia, Nepal, 1999–2000
-
– Ia, Thailand, China, Nepal, 1994 & 1997
-
Unknown – Ib, India, 1996–2003
-
– Brazil
-
-
-
– IIa, Korea, 2002–03
-
Self-fertile
A self-fertile type was present in China between 2009 and 2013.
Physiology
is the in ''P. infestans''. Hosts respond with [[autophagy]] upon detection of this [[elicitor]], Liu et al. 2005 finding this to be the only alternative to mass hypersensitivity leading to mass programmed cell death.
Genetics
P. infestans is
diploid, with about 8–10
, and in 2009 scientists completed the sequencing of its
genome. The genome was found to be considerably larger (240
megabase) than that of most other
Phytophthora species whose genomes have been sequenced;
P. sojae has a 95 Mbp genome and
P. ramorum had a 65 Mbp genome. About 18,000
were detected within the
P. infestans genome. It also contained a diverse variety of
and many
gene families encoding for
that are involved in causing
. These proteins are split into two main groups depending on whether they are produced by the water mold in the
symplast (inside plant cells) or in the
apoplast (between plant cells). Proteins produced in the symplast included
RXLR proteins, which contain an
arginine-X-
leucine-arginine (where X can be any
amino acid) sequence at the
amino terminus of the protein. Some RXLR proteins are
avirulence proteins, meaning that they can be detected by the plant and lead to a hypersensitive response which restricts the growth of the pathogen.
P. infestans was found to encode around 60% more of these proteins than most other
Phytophthora species. Those found in the apoplast include hydrolytic enzymes such as
,
and
that act to degrade plant tissue,
to protect against host defence enzymes and
necrosis toxins. Overall the genome was found to have an extremely high repeat content (around 74%) and to have an unusual gene distribution in that some areas contain many genes whereas others contain very few.
The pathogen shows high allelic diversity in many isolates collected in Europe.[
]
Research
Mycology of
P. infestans presents sampling difficulties in the United States.
It occurs only sporadically and usually has significant
due to each epidemic starting from introduction of a single
genotype.
Origin and diversity
The highlands of central
Mexico were considered to be the center of origin of
P. infestans, although others have proposed its origin to be in the
Andes, which is also the origin of potatoes.
A study published in 2014 evaluated these two alternate hypotheses and found conclusive support for central Mexico being the center of origin.
However, their study did not include either an extensive global sampling of
P. infestans or historic genomes. Support for a Mexican origin specifically the
Toluca Valley came from multiple observations including the fact that populations are genetically most diverse in Mexico, late blight is observed in native tuber-bearing
Solanum species, populations of the pathogen are in Hardy–Weinberg equilibrium, the two mating (see § Mating types above) types occur in a 1:1 ratio, and detailed
phylogeographic and evolutionary studies.
For instance, while sexual recombination is regarded as evidence for a Mexican origin,
P. infestans is mostly asexual and does not widely engage in sexual reproduction, despite the migration of the A2 mating type into Europe. Furthermore, the sister lineages of
P. infestans, namely
P. mirabilis and
P. ipomoeae are endemic to central Mexico.
Others have proposed an Andean origin for Phytophthora infestans. In 2002, Ristaino assessed the evidence for both the Mexican and South American origin hypotheses 24. She pointed to the absence of potato exports during the 1840s, which posed a challenge to the notion of a Mexican origin for the blight's migration to the US and Europe 24. Furthermore, historical accounts of a similar disease in the Andean region and the presence of the cosmopolitan US-1 lineage in South America since at least the 1980s (yet absent in Mexico) were invoked by Ristaino, potentially supporting the idea of a South American origin 24. In 2016, the Ristaino lab with collaborators Mike Martin and Tom Gilbert, at the University of Copenhagen, conducted the largest whole genome sequencing project to date with historic and modern day lineages of P infestans (25). Analysis of these more extensive genomic dataset that included both P. infestans and P. andina isolates documented an Andean origin of the species 25. Lineages of Andean origin were found to be more closely related to historical P. infestans lineages from the famine era, implying an Andean origin with later subsequent migration and diversification occurring in Mexican lineages 25. Significant admixture between the historic P infestans and P andina was also documented 25. Several close relatives of P infestans have been found in the Andes in South America, including P. andina, P urerae and P betacei.
Coomber et al., examined the evolutionary history of Phytophthora infestans and its close relatives in the 1c clade using whole genome sequence data from 69 isolates of Phytophthora species in the 1c clade and conducted a range of genomic analyses including nucleotide diversity evaluation, maximum likelihood trees, network assessment, time to most recent common ancestor and migration analysis 26}..
Migrations from Mexico to North America or Europe have occurred several times throughout history, probably linked to the movement of tubers.
Disease management
P. infestans is still a difficult disease to control.
There are many chemical options in
agriculture for the control of damage to the
foliage as well as the fruit (for tomatoes) and the
tuber (for potatoes). A few of the most common foliar-applied fungicides are
Ridomil, a Gavel/
SuperTin tank mix, and
Previcur Flex. All of the aforementioned fungicides need to be tank mixed with a broad-spectrum fungicide, such as
mancozeb or
chlorothalonil, not just for resistance management but also because the potato plants will be attacked by other pathogens at the same time.
If adequate field scouting occurs and late blight is found soon after disease development, localized patches of potato plants can be killed with a desiccant (e.g. paraquat) through the use of a backpack sprayer. This management technique can be thought of as a field-scale hypersensitive response similar to what occurs in some plant-viral interactions whereby cells surrounding the initial point of infection are killed in order to prevent proliferation of the pathogen.
If infected tubers make it into a storage bin, there is a very high risk to the storage life of the entire bin. Once in storage, there is not much that can be done besides emptying the parts of the bin that contain tubers infected with Phytophthora infestans. To increase the probability of successfully storing potatoes from a field where late blight was known to occur during the growing season, some products can be applied just prior to entering storage (e.g., Phostrol).
Around the world the disease causes around $6 billion of damage to crops each year.
Resistant plants
Breeding for resistance, particularly in potato plants, has had limited success in part due to difficulties in crossing cultivated potato with its wild relatives,
which are the source of potential resistance genes.
In addition, most resistance genes work only against a subset of
P. infestans isolates, since effective plant disease resistance results only when the pathogen expresses a RXLR effector gene that matches the corresponding plant resistance (R) gene; effector-R gene interactions trigger a range of plant defenses, such as the production of compounds toxic to the pathogen.
Potato and tomato varieties vary in their susceptibility to blight.
Genetic engineering may also provide options for generating resistance cultivars. A resistance gene effective against most known strains of blight has been identified from a wild relative of the potato, Solanum bulbocastanum, and introduced by genetic engineering into cultivated varieties of potato.[ Free version ]
Melatonin in the plant/ P. infestans co-environment reduces the stress tolerance of the parasite.
Reducing inoculum
Blight can be controlled by limiting the source of .
Only good-quality
and tomatoes obtained from
certified suppliers should be planted. Often discarded potatoes from the previous
season and self-sown
can act as sources of inoculum.
Compost, soil or potting medium can be heat-treated to kill oomycetes such as Phytophthora infestans. The recommended sterilisation temperature for oomycetes is for 30 minutes.
Environmental conditions
There are several environmental conditions that are conducive to
P. infestans. An example of such took place in the United States during the 2009 growing season. As colder than average for the season and with greater than average rainfall, there was a major infestation of tomato plants, specifically in the eastern states.
By using weather forecasting systems, such as
BLITECAST, if the following conditions occur as the canopy of the crop closes, then the use of
is recommended to prevent an
epidemic.
-
A is a period of 48 consecutive hours, in at least 46 of which the hourly readings of temperature and relative humidity at a given place have not been less than and 75%, respectively.
-
A is at least two consecutive days where min temperature is or above and on each day at least 11 hours when the relative humidity is greater than 90%.
The Beaumont and Smith periods have traditionally been used by growers in the
United Kingdom, with different criteria developed by growers in other regions.
[ The Smith period has been the preferred system used in the UK since its introduction in the 1970s.]
Based on these conditions and other factors, several tools have been developed to help growers manage the disease and plan fungicide applications. Often these are deployed as part of decision support systems accessible through web sites or smart phones.
Several studies have attempted to develop systems for real-time detection via flow cytometry or microscopy of airborne sporangia collected in air samplers.
Use of fungicides
for the control of potato blight are normally used only in a preventative manner, optionally in with disease forecasting. In susceptible varieties, sometimes fungicide applications may be needed weekly. An early spray is most effective. The choice of fungicide can depend on the nature of local strains of P. infestans. Metalaxyl is a fungicide that was marketed for use against P. infestans, but suffered serious resistance issues when used on its own. In some regions of the world during the 1980s and 1990s, most strains of P. infestans became resistant to metalaxyl, but in subsequent years many populations shifted back to sensitivity. To reduce the occurrence of resistance, it is strongly advised to use single-target fungicides such as metalaxyl along with carbamate compounds. A combination of other compounds are recommended for managing metalaxyl-resistant strains. These include mandipropamid, chlorothalonil, fluazinam, triphenyltin, mancozeb, and others. In the United States, the Environmental Protection Agency has approved oxathiapiprolin for use against late blight.
In organic production
In the past, copper(II) sulfate solution (called 'bluestone') was used to combat potato blight. remain in use on organic crops, both in the form of copper hydroxide and copper sulfate. Given the dangers of copper toxicity, other organic control options that have been shown to be effective include horticultural oils, , and rhamnolipid , while sprays containing "beneficial" microbes such as Bacillus subtilis or compounds that encourage the plant to produce defensive chemicals (such as knotweed extract) have not performed as well.[Gevens, Amanda, University of Wisconsin Madison Extension. Managing Late Blight in Organic Tomato & Potato Crops .]
During the crop year 2008, many of the certified organic potatoes produced in the United Kingdom and certified by the Soil Association as organic were sprayed with a copper pesticide
Control of tuber blight
Ridging is often used to reduce tuber contamination by blight. This normally involves piling soil or mulch around the Plant stem of the potato blight, meaning the pathogen has farther to travel to get to the tuber.
Historical impact
The first recorded instances of the disease were in the United States, in Philadelphia and New York City in early 1843.
The effect of Phytophthora infestans in Ireland in 1845–52 was one of the factors which caused more than one million to starve to death
During the First World War, all of the copper in Germany was used for and electric wire and therefore none was available for making copper sulfate to spray potatoes. A major late blight outbreak on potato in Germany therefore went untreated, and the turnip winter contributed to the deaths from the blockade.[ ]
Since 1941, Eastern Africa has been suffering potato production losses because of strains of P. infestans from Europe.
France, Canada, the United States, and the Soviet Union researched P. infestans as a biological weapon in the 1940s and 1950s.[" Chemical and Biological Weapons: Possession and Programs Past and Present", James Martin Center for Nonproliferation Studies, Middlebury College, April 9, 2002, accessed November 14, 2008.] Dr. Mannon Gallegley, deceased faculty from WVA worked in the late blight bioweapons program in the 1940s. It is unclear whether the pathogen was ever deployed. Whether a weapon based on the pathogen would be effective is questionable, due to the difficulties in delivering viable pathogen to an enemy's fields, and the role of uncontrollable environmental factors in spreading the disease.
Late blight (A2 type) has not yet been detected in Australia and strict biosecurity measures are in place. The disease has been seen in China, India and south-east Asian countries.
A large outbreak of P. infestans occurred on tomato plants in the Northeast United States in 2009.
In light of the periodic epidemics of P. infestans ever since its first emergence, it may be regarded as a periodically emerging pathogen – or a periodically "re-emerging pathogen".[()]
Further reading
External links
