Phytophthora infestans
Phytophthora infestans is an oomycete that causes the serious potato
The potato is a starchy, tuberous crop from the perennial Solanum tuberosum of the Solanaceae family . The word potato may refer to the plant itself as well as the edible tuber. In the region of the Andes, there are some other closely related cultivated potato species...

 disease known as late blight or potato blight. (Early blight, caused by Alternaria solani
Alternaria solani
Alternaria solani is a fungal pathogen, producing a disease in tomato and potato plants called early blight. The pathogen produces distinctive "bullseye" patterned leaf spots and can also cause stem lesions and fruit rot on tomato and tuber blight on potato. Despite the name "early," foliar...

, is also often called "potato blight"). Late blight was a major culprit in the 1840s European
European Potato Famine
The European Potato Failure was a food crisis caused by potato blight that struck Northern Europe in the mid-1840s. The time is also known as the Hungry Forties. While the crisis produced excess mortality and suffering across the affected areas, particularly harshly affected were the Scottish...

, the 1845 Irish and 1846 Highland potato famine
Potato famine
Potato famine may refer to:* Great Famine , the famine in Ireland between 1845 and 1852* Highland Potato Famine, a major agrarian crisis in the Scottish Highlands from 1846 to 1857...

s. The organism can also infect tomato
The word "tomato" may refer to the plant or the edible, typically red, fruit which it bears. Originating in South America, the tomato was spread around the world following the Spanish colonization of the Americas, and its many varieties are now widely grown, often in greenhouses in cooler...

es and some other members of the Solanaceae
Solanaceae are a family of flowering plants that include a number of important agricultural crops as well as many toxic plants. The name of the family comes from the Latin Solanum "the nightshade plant", but the further etymology of that word is unclear...



The spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...

s of this oomycete over-winter on infected tubers, particularly those that are left in the ground after the previous year's harvest; in cull piles, soil or infected volunteer plants and are spread rapidly in warm and wet conditions. This can have devastating effects by destroying entire crops.

Spores develop on the leaves, spreading through the crop when temperatures are above 10 °C (50 °F) and humidity is over 75%-80% for 2 days or more. Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...

 can wash spores into the soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 where they infect young tubers. Spores can also travel long distances on the wind.

The early stages of blight are easily missed, and not all plants are affected at once. Symptoms include the appearance of dark blotches on leaf tips and plant stems. White mould will appear under the leaves in humid conditions and the whole plant may quickly collapse. Infected tubers develop grey or dark patches that are reddish brown beneath the skin, and quickly decay to a foul-smelling mush caused by the infestation of secondary soft bacterial rots. Seemingly healthy tubers may rot later when in store.


Until the 1970s, there was only one type of blight (A1) in the UK, and this was unable to produce resistant spores that could survive the winter. There are now two types (A1 and A2) which can mate and after that produce resistant spores, although the indications so far are that this rarely, if ever, happens in the UK. Mating can occur only between molds of different mating-types and is required for the production of resistant spores.


In 2009 scientists completed the sequencing of the genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 of P. infestans. It was found that the genome is considerably larger (240 Mb) compared to other Phytophthora species whose genomes have been sequenced; Phytophthora sojae
Phytophthora sojae
Phytophthora sojae is an oomycete and a soil-borne plant pathogen that causes stem and root rot of soybean. This is a prevalent disease in most soybean growing regions, and a major cause of crop loss . In wet conditions the pathogen produces zoospores that move in water and are attracted to soybean...

has a 95 Mb genome and Phytophthora ramorum had a 65 Mb genome. It also contained a diverse variety of transposon
Transposable elements are sequences of DNA that can move or transpose themselves to new positions within the genome of a single cell. The mechanism of transposition can be either "copy and paste" or "cut and paste". Transposition can create phenotypically significant mutations and alter the cell's...

s and many gene families encoding for effector proteins that are involved in causing pathogenicity. These proteins are split into two main groups depending on whether they are produced by the water mould in the symplast
The symplast of a plant is the inner side of the plasma membrane in which water can freely diffuse.The plasmodesmata allow the direct flow of small molecules such as sugars, amino acids, and ions between cells...

 (inside plant cells) or in the apoplast
Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant....

 (between plant cells). Proteins produced in the symplast included RXLR proteins, which contain an arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...

Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...

-arginine (where X can be any amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

) sequence at the amino terminus of the protein. RXLR proteins are avirulence proteins, meaning that they can be detected by the plant and lead to a hypersensitive response
Hypersensitive response
The hypersensitive response is a mechanism, used by plants, to prevent the spread of infection by microbial pathogens. The HR is characterized by the rapid death of cells in the local region surrounding an infection. The HR serves to restrict the growth and spread of pathogens to other parts of...

, killing the oomycete. P. infestans was found to contain around 60% more of these proteins than other Phytophthora species and this may allow it to overcome host defences more quickly. Those found in the apoplast include hydrolytic enzymes such as protease
A protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....

s, lipase
A lipase is an enzyme that catalyzes the formation or cleavage of fats . Lipases are a subclass of the esterases.Lipases perform essential roles in the digestion, transport and processing of dietary lipids in most, if not all, living organisms...

s and glycosylases that act to degrade plant tissue, enzyme inhibitor
Enzyme inhibitor
An enzyme inhibitor is a molecule that binds to enzymes and decreases their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. They are also used as herbicides and pesticides...

s to protect against host defence enzymes and necrotizing
Necrosis is the premature death of cells in living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death...

 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.


P. infestans is still a difficult disease to control today by ordinary methods. There are many options in agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

 for the control of both damage to the foliage and infections of the tuber
Tubers are various types of modified plant structures that are enlarged to store nutrients. They are used by plants to survive the winter or dry months and provide energy and nutrients for regrowth during the next growing season and they are a means of asexual reproduction...

. Potatoes grow throughout the season, but it is estimated the tubers stop growing when 75% of the canopy has been destroyed. Around the world the disease causes around $6 billion of damage to crops each year.

Genetic engineering

In recent years, a resistance gene effective against all known strains of blight has been identified and successfully copied from a wild relative of the potato, Solanum bulbocastanum
Solanum bulbocastanum
Solanum bulbocastanum is a plant in the Solanaceae family, native to Mexico and parts of the U.S. Southwest. It is closely related to the potato and, as it has evolved strong resistance to all known varieties of potato blight, has been used to genetically engineer resistance into the cultivated...

, and introduced into the genome of cultivated varieties of the potato. This is an example of cisgenic
Cisgenesis, sometimes also called Intragenesis, is a product designation for a category of genetically engineered plants. A variety of classification schemes have been proposed , that order genetically modified organisms based on the nature of introduced genotypical changes rather than the process...

 genetic engineering.

Sources of inoculum

Blight can be controlled by limiting the source of inoculum. Only good quality seed potatoes obtained from certified suppliers should be planted. Often discarded potatoes from the previous season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

 and self-sown tubers can act as sources of inoculum.

Environmental conditions

There are several environmental conditions that are conducive to P. infestans. During the 2009 growing season, which was colder than average, and with greater than average rainfall, there was a major infestation of tomato plants in the eastern United States. By using weather forecasting
Weather forecasting
Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Human beings have attempted to predict the weather informally for millennia, and formally since the nineteenth century...

 systems, such as BLITECAST, if the following conditions occur as the canopy of the crop closes, then the use of fungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...

s is recommended to prevent an epidemic
In epidemiology, an epidemic , occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience...

  • A Beaumont Period is a period of 48 consecutive hours, in at least 46 of which the hourly readings of temperature
    Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

     and relative humidity
    Relative humidity
    Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...

     at a given place have not been less than 20 °C (68 °F) and 75%, respectively.
  • A Smith Period is at least two consecutive days where min temperature is 10 °C (50 °F) or above and on each day at least 11 hours when the relative humidity is greater than 90%.

Potato varieties

Potato varieties vary in their susceptibility to blight. Most early varieties are very vulnerable; they should be planted early so that the crop matures before blight starts (usually in July). Many old crop varieties, such as King Edward potato
King Edward potato
King Edward potatoes, like the majority of European and North American potato varieties, are derivatives of the 'Rough Purple Chili' which was used as breeding stock after the 1840s Irish potato famine. The King Edward potato is one of the oldest of these varieties.-Appearance:The King Edward...

 are also very susceptible but are grown because they are wanted commercially. Maincrop varieties which are very slow to develop blight include Cara, Stirling, Teena, Torridon, Remarka and Romano. Some so-called resistant varieties can resist some strains of blight and not others, so their performance may vary depending on which are around. These crops have had polygenic resistance bred into them, and are known as "field resistant". New varieties such as Sarpo Mira and Sarpo Axona show great resistance to blight even in areas of heavy infestation. Defender is an American cultivar whose parentage includes Ranger Russet and Polish potatoes resistant to late blight. It is a long white-skinned cultivar with both foliar and tuber resistance to late blight. Defender was released in 2004. These varieties are likely to gain great popularity as consumers increasingly embrace organically produced crops and reject food items that have been grown using fungicides and other chemicals. Researchers are currently working to develop a variety of potatoes that will be resistant both late and early blight. Researchers have found the wild potato species Solanum verrucosum to resist the late blight disease. They aim to make cultivated potatoes resistant to late blight by crossing the wild, resistant strain with the vulnerable, cultivated strain. In addition to this, researchers are crossing Solanum verrucosum with another wild potato species that is resistant to early blight, making a hybrid that is resistant to both late and early blight. They plan to cross the hybrid with cultivated potatoes to pass both resistant genes onto the cultivated species.

Use of fungicides

Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...

s for the control of potato blight are normally only used in a preventative manner, perhaps in conjunction
Conjunction can refer to:* Conjunction , an astronomical phenomenon* Astrological aspect, an aspect in horoscopic astrology* Conjunction , a part of speech** Conjunctive mood , same as subjunctive mood...

 with disease forecasting
Forecasting is the process of making statements about events whose actual outcomes have not yet been observed. A commonplace example might be estimation for some variable of interest at some specified future date. Prediction is a similar, but more general term...

. In susceptible varieties, sometimes fungicide applications may be needed weekly. An early spray is most effective. Metalaxyl
Metalaxyl is a phenylamide fungicide with systemic function. Its chemical name is methyl N--N--DL-alaninate. It can be used to control Pythium in a number of vegetable crops, and Phytophthora in peas....

 is a fungicide that was marketed for use against P. infestans, but suffered serious resistance
Pesticide resistance
Pesticide resistance is the adaptation of pest population targeted by a pesticide resulting in decreased susceptibility to that chemical. In other words, pests develop a resistance to a chemical through natural selection: the most resistant organisms are the ones to survive and pass on their...

 issues when used on its own. It is strongly advised to use metalaxyl along with carbamate
Carbamates are organic compounds derived from carbamic acid . A carbamate group, carbamate ester, and carbamic acids are functional groups that are inter-related structurally and often are interconverted chemically. Carbamate esters are also called urethanes.-Synthesis:Carbamic acids are derived...

 compounds, or the especially effective synergistic Cymoxanil and Mancozeb combination as it is effective at managing metalaxyl resistant strains.

In the past, copper sulfate solution (called 'bluestone') was used to combat potato blight. Copper pesticide
Copper pesticide
A copper pesticide is a copper compound used as a pesticide or fungicide. In the UK the Soil Association permits farmers to use some copper fungicides on organic land used for the production of certified organic crops only if there is a major threat to crops...

s remain in use in rare instances on organic crops.

Control of tuber blight

Ridging is often used to reduce tuber contamination by blight. This normally involves piling soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 or mulch
In agriculture and gardening, is a protective cover placed over the soil to retain moisture, reduce erosion, provide nutrients, and suppress weed growth and seed germination. Mulching in gardens and landscaping mimics the leaf cover that is found on forest floors....

 around the stem
Plant stem
A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence , conifer cones, roots, other stems etc. The internodes distance one node from another...

s of the potato blight meaning the pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

 has farther to travel to get to the tuber.

The canopy can also be destroyed around 5 weeks before harvest
Harvest is the process of gathering mature crops from the fields. Reaping is the cutting of grain or pulse for harvest, typically using a scythe, sickle, or reaper...

. This can be done via a contact herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...

 or using sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 to burn off the foliage.

Historical impact

The effects of Phytophthora infestans in Ireland in 1845–57 were one of the factors which caused over one million to starve to death and forced another two million to emigrate from affected countries. Most commonly referenced is the Great Irish Famine, during the late 1840s.

One theory suggests that the origin of Phytophthora infestans can be traced to a valley in the highlands of central Mexico. This theory has been challenged by many authors and recent work suggests an Andean origin for the disease. The first recorded instances of the disease were in the United States, in Philadelphia and New York City in early 1843. Winds then spread the spores, and in 1845 it was found from Illinois to Nova Scotia, and from Virginia to Ontario. It crossed the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

 with a shipment of seed potatoes for Belgian farmers in 1845.

Use as a biological weapon

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

, Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, USA and the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 researched P. infestans as a biological weapon in the 1940s and 50s. Potato blight was one of more than 17 agents that the United States researched as potential biological weapons before the nation suspended its biological weapons program.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.