Potato blight makes history

Leibniz Association

In the context of the simultaneous exhibition “8 Objects, 8 Museums“ by the Leibniz research museums, the Senckenberg Naturmuseum in Frankfurt/Main presents how it tracked down the agent responsible for a historic famine.

Devastating – and still feared today
The potato blight – also known as leaf or tuber blight – is a dreaded plant disease. Each year, it destroys approximately 20% of the worldwide harvest. During the mid-19th century, Ireland experienced the “Great Famine”, caused by the potato blight, which destroyed a large part of the harvest. Only recently has it been possible to more closely identify the pathogen that causes this disease.

The potato blight is caused by the oomycete Phytophthora infestans. Oomycetes (sometimes referred to as pseudo-fungi) are more closely related to brown algae and diatoms than to the true fungi. However, similar to the true fungi, they consist of a mesh of cellular threads, the mycelium. If tubers are infected in the soil, the oomycete begins to grow inside the tuber. The interior of the afflicted tuber shows a brownish discolouration and the tubers become inedible – both unappetising and harmful to human health, due to the presence of toxins.

Once the potato germinates, P. infestans grows into the sprouting plant. Brown areas and dark spots appear along the edges of the leaves and soon spread across the entire leaf surface.

The underside of the leaves on the infected plant is covered with a white layer of fungal hyphae, the so-called “potato mildew”. Here, the sporangia develop, which release spores during wet conditions. This can lead to a rapid spread of the oomycete, particularly during moist weather. While wet conditions damage the potato plants, they favour P. infestans, which requires moisture and moderate temperatures for an effective propagation. During dry weather, there is no risk of infection.

P. infestans made history when the organism that causes the potato blight destroyed large parts of the European potato harvest between 1845 and 1852. Ireland was hit the hardest. The potato constituted the country’s primary food source and was usually planted in monocultures. Therefore, the potato blight that wiped out almost the entire harvest in 1846 and 1847 had catastrophic consequences. The number of people who died of starvation is estimated at around 1 million.

An additional million Irish left their country and emigrated to Canada, Australia and the USA. As a result, the population of Ireland shrank by about 3 million people between 1841 and 1871.

Nature’s databases
In order to study the historical distribution of the potato blight, the scientists compared the currently active lineages of the pathogen as well as two closely related specimens with material from eleven historical herbarium sheets containing infected plant material from the past 150 years. The herbarium voucher species came from Ireland, Great Britain, Continental Europe and North America and are kept in the herbaria of the State Botanical Collection Munich and the Royal Botanical Gardens Kew in London.

A herbarium is a collection of dried and pressed plants. The Herbarium Senckenbergianum in Frankfurt/Main comprises approx.1.2 million of such herbarium voucher specimens – the oldest dating back to the 17th century.

The collected plants are stored on so-called herbarium sheets. Each plant is glued to the herbarium sheet as intact as possibly, including leaves, the shoot, roots, flowers and fruits.

In addition, a herbarium label is attached that states the location and date of collection, the collector’s name and the plant’s scientific name as well as a collection number, as shown here using the mallow plant as an example.

One of the eleven historical herbarium sheets used in the study came from the collection at Kew Gardens, dated 1846, and contains the leaf of a potato plant infected with P. infestans.

The historical and current samples were compared using modern decoding methods for the genetic material and DNA analysis. The comparison of certain sections of the DNA makes it possible to determine relationships between different organisms. In this process, mathematical algorithms are used to estimate at what time the individual Phytophthora lineages became genetically separated from each other – even beyond the 150-year period in which the samples originated.

Who was the “perpetrator”?
At the beginning of the 16th century, during the time of the first contact between Americans and Europeans in Mexico, the genetic diversity of the pathogen causing the potato blight began to increase. It is likely that at that time, the pathogen underwent a sudden and extensive spread from its place of origin in Mexico. New lineages were able to develop in the new habitats. After the introduction of new potato varieties in the 20th century, the so-called US-1 lineage became the most successful P. infestans lineage. For a long time, it was believed that this lineage, which is still widespread today, was also the cause of the Great Famine around 1845. However, this assumption has now been disproved by the genetic examination of the eleven historical herbarium sheets.

The US-1 lineage of P. infestans (blue lines in the figure) can hardly be considered the “perpetrator”, since it could not be found in the historical vouchers from the 19th century and likely did not become established in Europe until around 1900.

The Irish famine was caused by a hitherto unknown P. infestans lineage, referred to as HERB-1 (red line in the figure), which had already spread to Europe around 1845, shortly after becoming genetically separated from the US-1 lineage.

For more than fifty years, HERB-1 went on a worldwide rampage; it presumably became extinct, when the first resistant potato plants were cultivated at the beginning of the 20th century. Thus, more than 160 years later, it has finally been revealed which pathogen was responsible for the death of millions of people – and that this pathogen itself became extinct in Europe and the US.

The Senckenberg Gesellschaft für Naturforschung 
The self-proclaimed task of the Senckenberg Gesellschaft für Naturforschung (Senckenberg Society for Nature Research) is to “conduct nature research and to make the results of the studies accessible to the public through publications, teaching and its nature museums.” This is currently realised at six research institutes and three museums.

The topics of the exhibitions at the Senckenberg Naturmuseum in Frankfurt/Main (left), which is part of the Senckenberg Gesellschaft, range from the fauna and flora of long bygone times across geology and the development of the Earth to the diversity of modern organisms. In the process, the transformation of habitats over millions of years is clearly illustrated. With more than 38 million objects, the Senckenberg Gesellschaft houses one of the world’s most extensive natural history collections. Most of the objects are not on display; they are used solely for research purposes.

The dinosaur skeletons are the highlights of the exhibition: Iguanodon (left), original skeleton of a Diplodocus longus (right). This is the only skeleton of its kind on display outside the USA.

The varied exhibition areas of the Senckenberg Naturmuseum in Frankfurt display countless exhibits. Many of these represent extremely rare or even unique specimens, including a mounted Anaconda, which is among the world’s largest snakes. Shown here is the enormous prey (in this case a Capybara) this snake is able to devour in one piece.

In the next few years, the museum’s exhibition space will be expanded from 6,000 to 10,000 square metres. A new, globally unique concept provides for four major exhibition areas: Human, Earth, Cosmos and Future.

Credits: Story

“8 Objects, 8 Museums” is a collaboration project between the Leibniz research museums and the Leibniz-Institut für Wissensmedien in Tübingen in the Leibniz Year 2016.

Research project by the Senckenberg Gesellschaft für Naturforschung regarding »Tubers infected with the potato blight«

All documents and photos:
Senckenberg Gesellschaft für Naturforschung, Photos: Marco Thines, Sven Tränkner

Julius Kühn-Institut, Photo: Thilo Hammann
LIFE Photo Collection
K. Yoshida et al. (2013). The rise and fall of the Phytophthora infestans lineage that triggered the Irish potato famine.
Hertie-Stiftung 2016, Dr. Alexander Grychtolik, Photo: U. Dettmar

Text and object selection: Torsten Collet, Bernd Herkner, Sören Dürr (Senckenberg Gesellschaft für Naturforschung)
Collaboration: Alexandra Donecker, Hildegard Enting, Christina Höfling, Sven Tränkner (Senckenberg Gesellschaft für Naturforschung), Stephan Speicher
Translation: Hendrik Herlyn

Literature: K. Yoshida et al. (2013). The rise and fall of the Phytophthora infestans lineage that triggered the Irish potato famine.

Credits: All media
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