What's wasting them spuds?
Late blight was new to Europe in 1845, and the speed of its march was stunning and frightening. The disease was first spotted in June on the Isle of Wight; by early August, it had spread to every European country. Whole potato fields would look verdant and healthy one week, and be essentially dead the next. To potatoes, the blight was as deadly as a midsummer frost.

To aid air dispersal, Phytophthora infestans makes sporangia on stalk-like sporangiophores. Left: sporaniosphores are seen through a microscope. Middle: sporaniosphores interspersed with leaf hairs, through a dissecting scope. Right a closeup of a germinating zoospore from a detached sporangium.
Courtesy R. Vaughan James, William E. Fry and B. Gillian Turgeon respectively.

Although potato diseases had struck Ireland seven times between 1832 and 1842, the 1845 blight was in a class by itself in terms of deadliness and sheer velocity. People who depended almost entirely on spuds for food had plenty of reason to panic, and 1846 was even worse.

Anton de Bary proved that spores could carry the potato blight.
Courtesy Peter v. Sengbusch.

Many fingers have been pointed at the British Government for failing to sustain its relief efforts, but the Irish got precious little help in confronting the blight from the scientific establishment. Scientists, in fact, did not know what to make of late blight. As historians have noted again and again, speculation and observation were the major modes of action for natural philosophers, university scientists and amateur scientists alike.

The disputes appeared in The Gardener's Chronicles, a British periodical edited by the botanist Lindley, who argued that excess water caused the disease, and the visible signs of blight were merely symptoms of the loss of vitality. A 1940 history of plant disease put it this way:

"Dr. Lindley's theory was that as a result of such changes in the weather the potato plants had become in some way overladen with water; they had been growing away fast and furiously during the good weather, then when the fogs and the rain came they absorbed moisture with avidity, and in the absence of sunshine transpiration was checked, the plants had been unable to get rid of the excess water in their usual way, and so they had contracted a kind of dropsy and wet putrefaction set in" (see "The Advance... " in the bibliography).

Under cool conditions, the lemon-shaped sporangia of Phytophthora infestans produce swimming spores called zoospores. In warm temperatures, the sporangia germinate directly (left), producing a hyphal germ (right) tube that can infect the host plant.
Courtesy William Fry.

The scientists, with Lindley in the lead, had it wrong. Ironically, in light of current evolution-versus-creationism debate between scientists and fundamentalists, it was a religious man, the Reverend M.J. Berkeley, who correctly deduced that the crusty discolorations -- the fungal structures -- were causing the blight. To quote the 1940 history again, Berkeley "was of contrary opinion. He had at once connected the potato disease with the prevalence of a kind of mould on the affected tissues." Lindley apparently thought that Berkeley, a fungus expert, was "preoccupied with toadstools and mushrooms and moulds and mildews, all the greater and the lesser fungi, [and] was attaching far too much importance to a little growth of mould on the diseased potato plants."

All wrong, but half right
Although Lindley was all wrong about the cause of the blight, in one respect he was half right. In the cool, damp summer of 1845, water did play a key role in late blight, but for reasons that only became clear once the identity and life cycle of the late blight pathogen were known. Late blight -- Phytophthora infestans -- is now considered a water mold -- water plays a key role in forming sporangia, spreading the spores and allowing them to penetrate plant tissue. The wetter the year, the worse the blight.

As blight continued to decimate the Irish crop, scientists continued to argue, and the Irish continued to die. Through the 1840s, there was no real progress in understanding the cause, or, more important, any way to control or prevent it. In hindsight, the first real blight science waited until 1855, when two German scientists shed light on the simple question that had so flummoxed Lindley. Was the blight caused by the fungussy grunge on the potato leaves and tubers, or by simple waterlogging?

Phythopthora infestans resembles a fungus, but it's really a protist. Here are the branching, threadlike hyphae of the mycelium. Courtesy Mary Powelson.

First, J. Speerschneider of Blankenburg probed the relationship between fungal spores and potato disease with a controlled experiment much like a smart 21st century college biology student might devise, but a real innovation to the speculation- and observation-heavy "natural science" of the period. Speerschneider buried healthy potatoes in moist soil taken from potato-free fields, placing infected potato leaves under some tubers. Ten days later, mycelia -- fungal threads -- were visible in the tubers that sat on the sicko leaves.

It spread. But what exactly?
That demonstrated contagion, but didn't prove that an organism was causing the disease - which could have been caused by the transfer of a chemical or something from the first sick plant. De Bary went a step further by isolating the spores so at least the plant tissue wasn't a variable. Only later did Robert Koch reach the "gold standard" of proof by isolating a bacterium by growing it in pure culture. Yes, the same organism was attacking tubers and plants.

Blight strikes quickly. Six days after infection (above),
these Yungay potatoes look great, but they're living dead.
After 18 days (below), they're kaput.
Courtesy Rebecca Nelson, International Potato Center.

In 1861, botanist Anton de Bary went further by applying blight spores to potato leaves and watching the spores germinate and send tubes into the leaf to start the infection. Even more interesting, de Bary watched some spores swell and release tiny "zoospores" in water, which then penetrated the plant. The zoospores hardly seemed vegetative -- possessed as they were with tiny propulsive tails, de Bary knew zoospores from studying algae.

Very viable variables
Eventually, de Bary repeated Speerschneider's experiment and proved that blight was caused by fungi: He took healthy plants, inoculated some with blight spores, and watched as only those plants sickened and died. In contrast to Speerschneider, de Bary isolated the spores -- which were a critical variable. By designing an experiment that focused on one entity, the spores, de Bary placed plant pathology -- indeed all of the emerging science of microbiology -- on a firm footing. The rapidity of infection -- combined with the ability of one plant to produce millions of spores that could float or blow to other plants -- refuted the common skepticism that a disease could kill as quickly as late blight did.

The burgeoning knowledge of blight's life cycle gave some major clues to managing potato blight -- removing and burning diseased plants would reduce the number of infective spores. Although it was not clear why the pathogen could survive the winter and infect the next crop, diseased seed potatoes were major suspects. Eventually, knowledge seeped out from the elite, Latin-speaking world of university botany into gardening and farming circles, helping to contain if not eliminate blight.

What's this got to do with yogurt?