The name Phytophthora infestans means, more or less, “vexing plant destroyer.” P. infestans is an oomycete, one of 700 or so species sometimes known as water molds. It sends out tiny bags of 6 to 12 spores that are carried on the wind, usually for no more than 20 feet, occasionally for half a mile or more. When the bag lands on a susceptible plant, it breaks open, releasing what are technically known as zoospores. If the day is warm and wet enough, the zoospores germinate, sending threadlike filaments into the leaf. The first obvious symptoms—purple-black or purple-brown spots on the leaves—are visible in about five days. By then it is often too late for the plant to survive.
P. infestans preys on species in the nightshade family, especially potatoes and tomatoes. Scientists believe that it originated in Peru. Large-scale traffic between Peru and northern Europe began with the guano rush. Proof will never be found, but it is widely believed that the guano ships carried P. infestans. Probably taken to Antwerp, P. infestans first broke out in early summer 1845, in the West Flanders town of Kortrijk, six miles from the French border.
The blight hopscotched to Paris by that August. Weeks later, it was destroying potatoes in the Netherlands, Germany, Denmark and England. Governments panicked. It was reported in Ireland on September 13, 1845. Cormac O Grada, an economist and blight historian at University College, Dublin, has estimated that Irish farmers planted about 2.1 million acres of potatoes that year. In two months P. infestans wiped out the equivalent of one-half to three-quarters of a million acres. The next year was worse, as was the year after that. The attack did not wind down until 1852. A million or more Irish people died—one of the deadliest famines in history, in the percentage of population lost. A similar famine in the United States today would kill almost 40 million people.
Within a decade, two million more had fled Ireland, almost three-quarters of them to the United States. Many more would follow. As late as the 1960s, Ireland’s population was half what it had been in 1840. Today the nation has the melancholy distinction of being the only country in Europe, and perhaps the world, to have fewer people within the same boundaries than it did more than 150 years ago.
Despite its ghastly outcome, P. infestans may be less important in the long run than another imported species: Leptinotarsa decemlineata, the Colorado potato beetle. Its name notwithstanding, this orange-and-black creature is not from Colorado. Nor did it have much interest in potatoes in its original habitat, in south-central Mexico; its diet centered on buffalo bur, a weedy, spiny, knee-high potato relative. Biologists believe that buffalo bur was confined to Mexico until Spaniards, agents of the Columbian Exchange, carried horses and cows to the Americas. Quickly realizing the usefulness of these animals, Indians stole as many as they could, sending them north for their families to ride and eat. Buffalo bur apparently came along, tangled in horse manes, cow tails and native saddlebags. The beetle followed. In the early 1860s it encountered the cultivated potato around the Missouri River and liked what it tasted.
For millennia the potato beetle had made do with the buffalo bur scattered through the Mexican hills. By comparison, an Iowa farm, its fields solid with potatoes, was an ocean of breakfast. Because growers planted just a few varieties of a single species, pests like the beetle and the blight had a narrower range of natural defenses to overcome. If they could adapt to potatoes in one place, they could jump from one identical food pool to the next—a task made easier than ever thanks to inventions like railroads, steamships and refrigeration. Beetles spread in such numbers that by the time they reached the Atlantic Coast, their glittering orange bodies carpeted beaches and made railway tracks so slippery as to be impassable.
Desperate farmers tried everything they could to rid themselves of the invaders. Eventually one man apparently threw some leftover green paint on his infested plants. It worked. The emerald pigment in the paint was Paris green, made largely from arsenic and copper. Developed in the late 18th century, it was common in paints, fabrics and wallpaper. Farmers diluted it with flour and dusted it on their potatoes or mixed it with water and sprayed.
To potato farmers, Paris green was a godsend. To chemists, it was something that could be tinkered with. If arsenic killed potato beetles, why not try it on other pests? If Paris green worked, why not try other chemicals for other agricultural problems? In the mid-1880s a French researcher discovered that spraying a solution of copper sulfate and lime would kill P. infestans. Spraying potatoes with Paris green, then copper sulfate would take care of both the beetle and the blight. The modern pesticide industry had begun.