page_title: Phytophthora infestans
section_title: Origin and diversity of P. infestans
hierarchical_section_title: Phytophthora infestans / Origin and diversity of P. infestans
caption_attribution_description: English: Model from the collection of the Botanical Museum in Greifswald, Germany. . see also universitaetssammlungen.de/modelle for more information on the models Deutsch: Modell der Sammlung des Botanischen Museums in Greifswald OLYMPUS DIGITAL CAMERA
context_page_description: Phytophthora infestans is an oomycete or water mold, a 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 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 12–18 °C in water-saturated or nearly saturated environments, and zoospore production is favored at temperatures below 15 °C. Lesion growth rates are typically optimal at a slightly warmer temperature range of 20 to 24 °C.
context_section_description: The highlands of central Mexico are considered by many 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 recent study evaluated these two alternate hypotheses and found conclusive support for central Mexico being the center of origin. Support for Mexico 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 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. Some of the differences between strains may be related to variation in the RXLR effectors that are present.
caption: Histological model of a potato leaf with Phytophthora infestans, Botanical Museum Greifswald
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