The plant disease that killed the potatoes was caused by a fungus, Phytophthora infestans (fie-tof-thor-uh in-fes-tans).

This news story tells you about a researcher who is recreating bits of the DNA of the fungus to help in answering questions about where the 1840s P. infestans came from.

Samples from Irish Potato Blight May Shed Light on Crop-Killer
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As the article describes, the scientist is gathering samples of 150-year-old preserved potato leaves to screen them for the fungus DNA. The DNA screen she has developed may be useful in detecting P. infestans in potato seeds before they are planted, which would help prevent blight disease. The fungus is still out there today in many countries and it still causes disease in potatoes; in fact, a new strain even more destructive than the 1840s Irish strain is now threatening potato fields in Russia:

Potato Late Blight Threatens Russian Crop
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But there has never been such a widespread and devastating outbreak as during the Irish Potato Famine.

There are many reasons the fungus caused so much destruction in Ireland in the mid-1800s. One is the way the potatoes were grown. If you’ve ever left a potato in the pantry for too long, you’ve probably noticed sprouts coming from the spots or "eyes" on the vegetable. You can grow a whole new potato plant by cutting out the eye of a potato and planting it. The new plant will be an exact copy or clone of the potato you started with. This is called vegetative propagation (prop-uh-gay-shun) and this is how potatoes were grown and spread across Ireland (potatoes originated in South America and were brought to Europe in the 1600s). However, the original potatoes and therefore almost all of the cloned plants had no natural ability to fight off P. infestans. So when the fungus sprang up, it spread like wildfire across the entire country.

The fungus affected potato crops in other parts of Europe, too. But no other country relied so heavily on potatoes. Ireland’s peasants had become very, very dependent on potatoes as their primary food source. Potatoes were the main dish at every meal and often the only food on the table because they yield the most amount of food per acre of land. So when the fungus hit, there was almost nothing left for the peasants to eat but grass and weeds. It would be like having bombs blow up every grocery store in your state leaving you with no place to get food.

While everyone realized that the potatoes were dying, nobody then knew why. Some blamed the rotting on "the little people." Others thought it was the work of the Devil and sprinkled holy water in the fields to try to drive the demons away. Others believed that the land had simply given out from over-farming. One widely accepted idea was that the trains that chugged across the country were discharging electricity into the air that caused the blight. It wasn’t until 1861 that Anton De Bary, who is considered the father of modern plant disease study, proved that the fungus caused the blight disease.

P. infestans is now controlled by chemical fungus killers and better management of crops.