Phytopthora infestans -- "plant destroyer" -- at work. In short order, these spuds will be mush -- and unfit for a short-order cook. Courtesy Rebecca Nelson, International Potato Center.

These purple leaves have just about had it with using sunlight to power the formation of sugars. This plant shows typical symptoms of late blight
Courtesy Rebecca Nelson.

A matter of simple dampness?
We've already seen the catastrophic effects of the 1840s potato famine in Ireland -- the million or more dead, the million-plus who emigrated. If the famine was a social catastrophe, its scientific side was a bit brighter.

Research into the deadly potato disease set the stage for discovering exactly what causes disease. It also helped kick-start the science of plant pathology; the study of plant diseases, with a focus on major crops like potatoes.

Today, the role of microbes in causing disease is so accepted -- at least in the West -- that grumbling "I've got a bug" automatically translates as "I feel sick." Matters were quite different in the early 1800s, when illness -- whether in plants or people -- was blamed on bad air, vapors, spiritual deficiencies, fate, the devil, or an imbalance of the essences.

In fact, one major British botanist blamed potato blight on excess water.

That may sound strange to us, especially since there were clues to the cause of disease long before the potato famine struck. Bacteria, for example, were discovered in Holland in 1683 by Anton von Leeuwenhoek, who's commonly credited with inventing the microscope. Pus -- the accumulation of dead microbes and immune cells -- must have been known to physicians. And dying potatoes and other sick plants displayed fungal structures.

Andean farmers investigate test plots while learning how micorbes cause can disease in crops.
Courtesy Rebecca Nelson, International Potato Center.

The evidence was there, but it wasn't clear what to make of it. Was pus a clue to the causation of disease, or merely a result? In our wisdom, we know potato mold caused blight. But many eminent scientists maintained that it was merely a symptom or consequence of disease.

Big botanist = big boo-boo
It was to be expected, wrote John Lindley, a botany professor from University College in London in the first blight summer, 1845, that

"as soon as living matter lost its force, as soon as diminishing vitality took the place of the customary vigor, all sorts of parasites would acquire power and contend for its destruction. It was so with all plants, and all animals, even man himself. First came feebleness, next incipient decay, then sprang up myriads of creatures whose life could only be maintained by the decomposing bodies of their neighbors."

According to Lindley's analysis, rapid growth, followed by cold and wet, would "cause a rapid diminution of vitality" that killed the plants.

With the benefit of hindsight, we can't help smirking at the misinterpretation of observational evidence, but the idea that mold was the symptom, not the cause, did have a root in reality: Most fungi (as late blight were considered until quite recently) attack dead plants, decaying them for recycling into nutrients that future plants will depend upon. In bio-jargon, these are saprophytes -- devourers of dead tissue -- rather than parasites -- eaters of living flesh. (Carnivores, but not cannibals...) Late blight delights in live potatoes -- it's a parasite that leaves a foul, stinking brew that wouldn't interest a hungry worker ant, let alone a starving Irish farmer.

This replica of a microscope invented by Anton von Leeuwenhoek is three to four inches high, and made of two riveted brass plates. A small hole in the plates holds the tiny, high-power lens. On the back, a screw with a needle holds the object and allows crude focusing. The scope is held close to the eye. The faint image is hard on the eye, and the crude focus entails great patience.
Molecular Expressions. © 1998-2001, Michael W. Davidson, Mortimer Abramowitz, Olympus America Inc., and The Florida State University.

But we're getting slightly ahead of ourselves. What role did late blight play in the development of plant pathology and the germ theory of disease, the scientific underpinning for the development of antibiotics and vaccines? Both were major causes of improved longevity in the 20th century.

And what's happened since in the realm of microbiology?