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Species of single celled organism

Phytophthora infestans is an oomycete or water mold, 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 famines. The organism can also infect some other members of the Solanaceae. The pathogen is favored by moist, cool environments: sporulation is optimal at in water saturated or nearly saturated environments, and zoospore production is favored at temperatures below 15 °C (59 °F). Lesion growth rates are typically optimal at a slightly warmer temperature range of .

The genus name Phytophthora comes from the Greek φυτό (phyto), meaning "plant" – plus the Greek φθορά (phthora), 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).

The asexual life cycle of Phytophthora infestans is characterized by alternating phases of hyphal 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 spores, called oospores. 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 biflagellated 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. The sporangia and sporangiophores appear white on the lower surface of the foliage. As for tuber blight, the white mycelium often shows on the tubers' surface. Under ideal conditions, P. infestans completes its life cycle on potato or tomato foliage in about five days. This can have devastating effects by destroying entire crops.

P. infestans is diploid, with about 8–10 chromosomes, and in 2009 scientists completed the sequencing of its genome. The genome was found to be considerably larger (240 Mbp) 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 genes were detected within the P. infestans genome. It also contained a diverse variety of transposons 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 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 proteases, lipases and glycosylases that act to degrade plant tissue, enzyme inhibitors to protect against host defence enzymes and necrotizing 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.

Study of P. infestans presents sampling difficulties in the United States. A recent study evaluated these two alternate hypotheses and found conclusive support for central Mexico being the center of origin. Support for Mexico specifically the Toluca Valley comes 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. Furthermore, the closest relatives of P. infestans, namely P. mirabilis and P. ipomoeae are endemic to central Mexico. On the other hand, the only close relative found in South America, namely P. andina, is a hybrid that does not share a single common ancestor with P. infestans. Finally, populations of P. infestans in South America lack genetic diversity and are clonal. Migrations from Mexico to North America or Europe have occurred several times throughout history, probably linked to the movement of tubers. Until the 1970s, the A2 mating type was restricted to Mexico, but now in many regions of the world both A1 and A2 isolates can be found in the same region. The co occurrence of the two mating types is significant due to the possibility of sexual recombination and formation of oospores, which can survive the winter. Only in Mexico and Scandinavia, however, is oospore formation thought to play a role in overwintering. In other parts of Europe, increasing genetic diversity has been observed as a consequence of sexual reproduction. This is notable since different forms of P. infestans vary in their aggressiveness on potato or tomato, in sporulation rate, and sensitivity to fungicides. Variation in such traits also occurs in North America, however importation of new genotypes from Mexico appears to be the predominant cause of genetic diversity, as opposed to sexual recombination within potato or tomato fields. Many of the strains that appeared outside of Mexico since the 1980s have been more aggressive, leading to increased crop losses. (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 isn't 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. 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. This is an example of cisgenic genetic engineering. Melatonin in the plant/P. infestans co environment reduces the stress tolerance of the parasite.

Blight can be controlled by limiting the source 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.

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 fungicides 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. 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. Whilst these methods show potential to allow detection of sporangia in advance of occurrence of detectable disease symptoms on plants, and would thus be useful in enhancing existing Decision support systems, none have been commercially deployed to date.

Fungicides for the control of potato blight are normally only used in a preventative manner, optionally in conjunction 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 African smallholder production fungicide application can be necessary up to once every three days.

In the past, copper(II) sulfate solution (called 'bluestone') was used to combat potato blight. Copper pesticides 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, phosphorous acids, and rhamnolipid biosurfactants, 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. 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 to control potato blight. According to the Soil Association, the total copper that can be applied to organic land is /year.

Ridging is often used to reduce tuber contamination by blight. This normally involves piling soil or mulch around the stems of the potato blight meaning the pathogen has farther to travel to get to the tuber. Another approach is to destroy the canopy around five weeks before harvest, using a contact herbicide or sulfuric acid to burn off the foliage. Eliminating infected foliage reduces the likelihood of tuber infection.

The effect of Phytophthora infestans in Ireland in 1845–52 was 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, from which the Irish population has still not fully recouped. 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 with a shipment of seed potatoes for Belgian farmers in 1845. All of the potato growing countries in Europe were affected, but the potato blight hit Ireland the hardest. Implicated in Ireland's fate was the island's disproportionate dependency on a single variety of potato, the Irish Lumper. The lack of genetic variability created a susceptible host population for the organism after the blight strains originating in Chiloé Archipelago replaced earlier potatoes of Peruvian origin in Europe. During the First World War, all of the copper in Germany was used for shell casings 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 resulting scarcity of potatoes led to the deaths of 700,000 German civilians from starvation. 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. 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. 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".