The appearance of blight in Europe is but one of the grim accidents inflicted on history by Cristobal Colón.
His mistaken landing on San Salvador Island in 1492 triggered a union of taxonomies that had drifted apart on the continents 15 million years before.
Honeybees aren't native to North America, nor are earthworms; likewise, potatoes aren't native to Europe, nor is phytophthora infestans.
Ireland is one of the only nations in the world whose population--one still-unfurling consequence--was lower in 2000 than in 1840.
Biologists often view the location where a species displays its greatest genetic variety to be that species' place of origin.
Until recently this was assumed of P. infestans to be central Mexico, where it comprises two types that can sexually reproduce (after a fashion),
generating an egg-adjacent object called an oospore.
P. infestans originating outside Mexico lacks this facility for sexual reproduction.
That potatoes weren't even observed in Mexico until 1803 mattered not: Alexander von Humboldt, the scientist identifying the first unequivocally
Mexican potatoes, concluded that they had been imported by Spaniards and P. infestans had encounered its first S. Tuberosum thousands of years into its history.
This view was punctured in 2007, when plant geneticists at the University of North Carolina revealed that Andean potato blight comprised more haplogroups than its Mexican counterpart.
Haplogroups--groups of organisms with similar DNA patterns--are useful markers of genetic diversity, especially among beings that reproduce asexually
(i.e. all blight originating outside Mexico).
Theories for central Mexico and Peru as P. infestans' spawn point contine to compete today.
Much of the Andean genetic material analyzed at the University came from American and Irish potatoes harvested during the Great Hunger, stored in museums and herbariums.
The view generally defensible by molecular science is that P. infestans traveled first to the US, then to Europe:
In 1843 the provincial council in West Flanders voted to import potatoes from North and South America, their own potatoes having experienced a bad year.
As the guano craze was imminent in the Old World and the Spaniards had, by now, mostly exhaused South America's silver reserves,
it's believed the oomycete-bearing potatoes crossed the Atlantic on a guano ship.
By this time, the population of Europe—and especially the British Isles—had grown mightily on guano-fertilized potatoes and milk and not much else.
Tell a subsistence farmer sending 2000 extra calories per
diem that potatoes are too starchy to be suitable human food.
By November 1843 a third of the Irish crop was lost, a few months later three-quarters. The next year was worse; the next worse still.
Phytophthora infestans can be paraphrased as "vexing plant destroyer."
It attacks species of the nightshade family, which includes tomatoes, peppers, aubergine, potatoes, and weeds like bittersweet nightshade.
It is a type of oomycete, or water mold. Oomycetes occupy a place somewhere between plants, bacteria, and fungi:
they possess the branched filaments and spores of fungi, but are made of cellulose, like spinach.
Numerous types of water mold colonize water and damp surfaces. Some plague fish. All have complex reproductive cycles involving spores equipped with bacterial flagella.
P. infestans bears a mechanism for piercing the undersides of plants' dermal cells to better deliver infection. This blooms as mycelium inside the infected tissue.
Brown spots appear and spread on leaves; fruit decomposes before ripening.
Because the border between infected and healthy tissue is indistinct in fruit, blighted units must be eaten or carefully discarded.
P. infestans grows and germinates only in moisture and its method of travel is rain splash. Blighted potatoes take on a crumbly, yellow character.
Mercifully, blighted food of all varieties can be safely if miserably consumed by humans.
Potato blight continues to be one of the world's chief agricultural pathogens and one of the bigger bites out of the industrial farmer's often shockingly-thin margins.
If you've grown nightshades in natural dirt using water on any of the continents upon which farming is conducted you have hosted P. infestans.
Because of Colón it is unwise to wet the leaves of your tomato plants with the hose.
The oldest and still most effective weapon against blight is fungicide, whose unintended consequences include more robust strains of blight.
This is of particular concern because P. infestans' genome is, at 240MB, tremendous compared to that of other oomycetes, and organized to evolve quickly.
A number of plant species in South America bear genetic resistance to P. infestans.
Attempts have been made to breed and engineer food plants with the same resistances, but blight's rapid evolution has undercut them all.
Charles C. Mann, 1493: Uncovering the New World Columbus Created. Vintage, 2011.
Water Molds (cliffsnotes.com)
The Irish potato famine pathogen Phytophthora infestans originated in central Mexico rather than the Andes | PNAS
Phytophthora infestans (Phytophthora blight) (cabi.org)
Phytophthora infestans | IDphy (idtools.org)
Phytophthora infestans: An Overview of Methods and Attempts to Combat Late Blight (nih.gov)