The Intricate Story of Late Potato Blight

© Else C .Vellinga
Original publication: Mycena News, January 2009

Potatoes: staple food for many! I grew up with them in the Netherlands, and when we talk about a Dutch dinner, it is: potatoes, meat and veggies – in that sequence. My home country is big in potato growing and is the world’s leading exporter of seed potatoes. Apples of the earth is the Dutch word for potatoes, and like apples, they come in many, tasteful varieties.

Potato Disease

But there is a darker side. Potatoes are target for the late potato blight, a disastrous disease, with deep socio-economic consequences. This was the disease responsible for the infamous 19th century Irish famine and for the birth of plant pathology. The history of how this disease was understood reads like a layered detective story.

Potatoes are the tubers of Solanum tuberosum and related species. They grow naturally in the Andean countries of South America – Bolivia, Ecuador and Peru – and were an important part of the food of the native people before the arrival of the Europeans. Ancient potato-shaped vessels still show the reverence in which they were held.

Potatoes were brought to Europe in the second half of the 16th century, but at first were looked at with suspicion. It did not help that eating the berries ended in disaster, as all above-ground and green parts of the plant are highly toxic. But, at some point, potatoes really took off, and became the main carbon and food source for many people, especially small poor farmers. All the potatoes cultivated in Europe at that time came from the same small selections brought from South America, and there was great genetic uniformity among them. Some years the crops did well, but some times there were minor disasters, and the potato harvest disappointed. However, what happened in the 1840s changed this dramatically; the potato plants, and most importantly the tubers, rotted until all that was left, over a wide area, was a putrid-smelling mess. The rot appeared first in north eastern North America in 1843, spread west, east and north, and reached Belgium in the summer of 1845. In Europe that summer stood out because of its wetness and the relatively low temperatures. Within a few months, potato plants were affected from Ireland in the west to Germany in the east. At least a quarter of the potato harvest was ruined. The next year was even worse, and 90% of the potato harvest in Ireland failed. A catastrophic famine ensued as a third of the Irish peasantry was entirely dependent on potatoes.

What was the cause of the rot? That was at the core of great philosophical discussions at the time. A white fungus was found on the potatoes, but was it the result of the wetness and the decay, or the cause of the rot? The ideas of ‘generatio spontanea’ were still widely held and in this mood of credulity the disaster in the fields was variously ascribed to bad weather, bad luck, God’s will, or the devil’s. Plant pathology was an unknown branch of science.

The English clergyman M.J. Berkeley, who described so many mushroom species from all over the world, gave a detailed description of the ‘potato murrain’, and realized that the mold was the same species described by Montagne as Botrytis infestans. But it took some more time before it was generally accepted as the cause for the disease. The credit for swinging opinion in favour of the parasite goes to De Bary, a German scientist, who established the whole cycle of the organism on plants. De Bary also changed the name into Phytophthora infestans. He infected healthy plants which consequently showed the symptoms, while healthy non-infected plants stayed healthy, despite being exposed to the same wet weather disease, whether of animals, humans or crops. Phytopathology was born.

The culprit is not a fungus, but a water mold in the Straminopila, to which the giant kelps and the tiny diatoms also belong. Water molds differ from the real fungi because of the cellulose in the cell wall (fungal cell walls are made of chitin), and the fact that they store starch. The organism now ravaging the Coast live oaks and tanbark oaks in California is a close relative of the potato blight mold.

Potato Disease

The potato blight was so successful because of its rather simple life cycle. A spore which lands on a leaf surface, grows into the plant and emerges on the under-side of the leaf. There it forms a branched sporangiophore, a small tree-like structure with sporangia at the tips. Each sporangium is a sack containing up to 8 zoospores. In wet cool weather a sporangium disperses as a unit, which opens up and lets the zoospores out. In warm dry weather, the sporangium itself will germinate and the zoospore phase will be skipped. The zoospores have swimming devices in the form of 2 flagella and reinfect the plant. They can only survive on a wet leaf surface. The sporangia from the leaves get into the soil to infect the tubers, and they can spend the winters in stored tubers, to spread through the growing plants in spring. A potato plant can be turned to mush in less than three weeks.

The use of copper fungicides protected the potatoes from the blight, but in World War I, Germany needed copper to make bullets, not to protect the potatoes, and hundreds of thousands people died, because of the failing potato harvest.

The blight proved to be particularly devastating in the cool and wet countries of Europe. In the Andes many genetically different cultivars were grown, some more, some less susceptible to the disease, but the weather there is not particularly cool and wet.

New potato cultivars were made with the help of a resistant Mexican close relative, but the arms race with the pathogen kept going. In the 1970s, a new aggressive strain of the blight appeared with devastating effects. Till then the European version of Ph. infestans, had lacked the capacity for sexual recombination, but this new strain, which originated in the central highlands in Mexico, was of a different mating type. The blight could now fulfill the complete life cycle with a sexual part, and through genetic recombination soon was resistant against the applied fungicides.

Debate on the origin of the original 1840s strain has kept researchers busy. Did it co-evolve with Solanum tuberosum in the Andes, as the original researchers suggested? Did it originate in Mexico where wide genetic variation in Ph. infestans existed (this was the prevailing theory in the latter part of the 20th century)? Or did it migrate from Mexico to the Andes and, from there, to the rest of the world? Where do we have to go to find resistant potatoes or less virulent Phytophthora?

The problem of origin was solved only in 2007, by careful comparison of the genetic make up of Ph. infestans in the Andes, in Mexico and in other parts of the Americas. These strains were then compared with ones found in Ireland and elsewhere. Fortunately herbarium material of some infected Irish plants had been preserved, and DNA could still be isolated from this one and a half century old material. Historic material from South America, not as old as that from Ireland, but from before the second wave of pathogenic immigrants, was also instrumental in clinching the problem.

After the arrival of the new Mexican strain in Europe, the genetic diversity of Ph. infestans in the Netherlands became as great as that in Mexico – one reason to discard the out-of-Mexico theory. It is significant, too, that potatoes were not grown in Mexico in the first half of the 19th century. It is more plausible that the parasite came from South America in the 1840s, with the many new fast boats, than that it came with early potatoes. Even if it was on 16th century potatoes, the potatoes were likely to have been so affected by the long voyage that the diseased tubers were definitely not used for new plantings.

However, the most convincing evidence comes from comparison of nuclear and mitochondrial DNA of strains collected over a wide area. This shows that an ancestral population of Phytophthora diverged into different lineages in the Andes, growing on wild Solanum species; two of these developed into the present day types of Ph. infestans, capable of infecting potatoes, tomatoes and some other closely related Solanum species. Others evolved into several distinct species, one named Ph. andinum. The South American strains were transported to areas where potatoes were cultivated and wreaked havoc on a grand scale.

Late potato blight serves as the prototypical plant disease. Its story illustrates many of the hazardous aspects of agriculture, such as the role of the unwelcome companions who arrive with introduced species, the fragility of genetically uniform crops, and the social costs from the impacts plant diseases have on the lives of ordinary people, but also the strength of scientific evidence throughout the years. Alas, these stories can be told for many other equally devastating plant pathogens.

Some further reading: