Tom Volk's Fungus of the Month for March 2001

This month's fungus is Phytophthora infestans, cause of late blight of potato and the Irish potato Famine

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This is the alternate version of the Fungus of the month for March 2001 that lacks the QuickTime video of swimming zoospores.

Irish mother with children, planting potatoes, London Illustrated News, 1849I can hear you asking-- why didn't you think of this before? The logical choice for the FOTM in the month we celebrate St. Patrick's Day is Phytophthora infestans, cause of the Irish potato famine from 1845 to about 1860. This month's fungus is a not really a fungus at all. It's a member of the Oomycota, another one of those phyla "banished" from the fungal kingdom. Check out my Introduction to the Kingdom Fungi for a list of characteristics of the fungal kingdom. The two main reasons that the Oomycota are excluded are the cell walls composed of cellulose and food being stored as starch. One other unusual characteristic of the Oomycota is swimming spores. Just for having these swimming spores, the Oomycota and the Chytridiomycota were thrown out of the fungal kingdom. However recently the Chytridiomycota have been reinstated, since they have chitin cell walls and store their food as glycogen, like other fungi. The first time I took Mycology (at Ohio University with Jim Cavender in 1978!) these two phyla "definitely belonged in the fungal kingdom." By the time I was teaching Mycology, they were definitely not fungi. Now, using modern methods such as DNA sequencing, the Chytridiomycota are definitely fungi and the Oomycota definitely are not. At least for *this* week....

Phytophthora infestans sporangium with kneesThe Oomycota are now placed in the kingdom Stramenopila, which you may have never heard of. This kingdom was erected to include the Oomycota, the brown algae (giant kelps), the diatoms the yellow-green algae, and the golden-brown algae. Some researchers place everything that's not a plant, animal, fungus, or bacterium in a "dumping ground" kingdom called the Protista. It's likely the Protista will be split into a few dozen kingdoms in the coming years. The slime molds are another group that's put in the Protista.

Most mycologists still include these non-fungal groups in their mycology courses, and researchers who work on these groups form a significant part of any Mycology or Plant Pathology meeting. Ecologically, many of these organisms behave like fungi, especially those that are plant pathogens. We're glad to still have them in our mycology fold.

whiplash and tinsel flagellaPhytophthora infestans (pronounced fy-TOF-thor-uh in-FEST-ans) is a rather common pathogen of potatoes wherever they are grown, but it is usually not a problem unless the weather is unusually cool and wet. The water is necessary for the spores to swim to infect the leaves of the potatoes; the tubers and roots of the potato are more resistant to the pathogen. The name, meaning "infesting plant destroyer" is especially appropriate, because under the right conditions and with the correct susceptibility genes in the host, Phytophthora can kill off a field of potatoes in just a few days!

Phytophthora infestans sporangium with kneesPhytophthora infestans is so virulent in wet weather because it produces enormous numbers of swimming spores called zoospores in zoosporangia, shown to the left. The red structures are the sporangiophores poking out of the stomata of an infected potato leaf. The oval-shaped sporangia have fallen off, although a few are still visible in the field of view. The zoosporangia crack open and release dozens of zoospores, as shown in the above video. These zoospores have two flagella; a whiplash flagellum faces the back and pushes the spore through the water and a tinsel flagellum points forward and pulls the spores through the water. The flagella are shown on the scanning electron micrograph above and to the right. I'm sorry I don't know the source of this picture; it was given to me in a set of slides by Dr. Bill Whittingham, with whom I was first a TA in Mycology in 1981.

A bit of history of the Irish Potato Famine

Late blight of potato is an example par excellence of the impact that a "fungal" disease has had on the political, economic and social atmosphere of several nations.

Although this disease is best known as being responsible for the Irish potato famine in the 1840's we have to go back 3.5 centuries to look at a some of the history of the potato.

Before 1500, the potato was unknown except in a few regions of Central and South America, where its native habitat was in mountainous regions. It was cultivated by the Incas in the Andes Mountains and served as a main food staple. There were (and still are) many varieties, none of which resemble the modern Irish potato. The other varieties are not just big bags of starch, but are smaller and very tasty.

Spanish seamen carried the potato to Europe, where it was a curiosity in private horticultural gardens for two centuries. It was not often eaten because gardeners knew it was in the same family (Solanaceae) as deadly nightshade. In fact all parts of the potato plants, except the tubers, are poisonous to people. The tubers may become poisonous on exposure to light when they begin to turn green. Sometime after 1800, Europeans found the potato tuber (really an underground stem anatomically) was edible, and it was quickly adapted as a staple crop-the climate and soil in Europe was similar to that of the Andes and thus ideal for cultivation. The potato was especially attractive to Irish peasants. Why should this be so?

Most of the land in Ireland was owned as large estates by absentee English landlords, who made long-term leases to English middlemen. They, in turn, subdivided the estates into small parcels and rented them at high rates to Irish tenants (often to the same people whose families had historically owned the land). Although the Irish peasants were poor, they could pay the high rent in the form of produce, grains, and sometimes pigs. Thus the main problem for tenants was to have a crop to sustain the family for almost a year while growing conditions were adverse. The potato provided the perfect solution. It gave a good yield and satisfied their hunger-- because of its bulk, stomachs were distended from 8-14 pounds (4- 6 kg) of potatoes per person per day!! Think about a 10 pound bag of potatoes; that's a lot of potatoes! This was virtually all the peasants had to eat during the winter months--the average family consumed up to a ton of potatoes per month.

The potato did well for the Irish, for the population of Ireland exploded from 4.5 million in 1800 to about 8 million in 1845. This is particularly amazing because most of the population was dependent on the potato for their nutrition for 10 months of the year. But then came the late blight of potato.

infected potato leafPeople noticed localized outbreaks of the disease during the early 1800's, although they didn't know the cause. There was always a background level of the disease, but it didn't become a problem until 1845. An ominous warning appeared in the Gardener's Chronicle and Agricultural Gazette: "A fatal malady has broken out amongst the potato crop. On all sides we hear of destruction. In Belgium the fields are said to have been completely desolated." The disease struck down plants like a hard frost in summer. It was said to spread faster than cholera does in humans. It reduced the foliage to a putrid mass in a few days, and the tubers were affected to various degrees in a similar way, although they did not rot as rapidly.

One amateur mycologist, the Rev. M.J. Berkeley, noticed the mycelium on the leaves and proposed that it was a fungal disease. This was not widely accepted-- scientific thinking at the time dictated that a fungus couldn't be the cause of a disease, so it must be a secondary invader. Of course Rev. Berkeley was right.

In September 1845, the journal announced that the disease had become established in Ireland. The weather was unusually cool and wet that year allowing easy distribution of the zoospores of the pathogen. With the tremendous destruction it wrought, famine was inevitable, and (according to some sources) the English realized this. To stave off starvation, they considered importing cereal grains such as wheat, barley, and corn, but at the time the English had Corn Laws (for those of you on this side of the pond, grains in England were called corn, while what we call corn here is called maize in Britain). The Corn Laws imposed a high tariff on imported grains, but not corn (maize), so they couldn't import enough wheat or barley to sell cheaply to the peasants.

The winter of 1845 was disastrous. The stored potatoes were rotting because they had been harvested too early. So desperate were they that many of the stored seed potatoes were eaten, and many rotted. Thus the following year was critical. According to some reports, the English tried importing maize to ward off starvation for the Irish. However, the Irish refused to eat the corn-- it was not as filling as the potato, and they considered it chicken feed. As the growing season of 1846 progressed, those that had salvaged seed potatoes were optimistic, since the new plants seemed very healthy. It seems their prayers had been answered. BUT---.

In July 1846 the disease struck again; there was a cool, wet period, just like the year before, allowing the Phytophthora's zoospores to multiply and spread. From that time on the disease was there to stay. In some years (warm and dry), the disease was localized; in wet years, the disease became epidemic. The corn laws were repealed, but it was too late.

St. Patrick's Day parade on Fifth Avenue in NYC 2000Between 1845 and 1860 over a million Irish people died as a consequence of the blight; another 1.5 million emigrated, mostly to the east coast of the United States, where many became New York or Boston policemen (some shown to the right in the 2000 St. Patrick's Day parade in NYC) or workers on the transcontinental railroad (remember that the Irish laid track from the east and the Chinese from the west.) Nearly 150 years later, the Irish population has still not recovered from the disaster caused by the late blight of potato.

Only in the past 25 years or so has there been increased fervent study of the history of the potato famine. Many would like to have forgotten the whole thing, but many Irish feel it is important to confront the sociological and political reasons for the famine, which is still responsible for some of the tension and animosity toward the English in Ireland today. There are many web pages on the subject, and rather than list them all, I recommend you do a web search for "late blight of potato" or "Irish potato famine" and you'll find dozens of sites, many of which deal with the controversial role of the English in the famine. There is certainly no consensus on historical details, and 150 years has really muddied the waters.

St. Patrick's Cathedral in NYCInterestingly, after many decades of its not being much of a problem, late blight of potato is a re-emerging disease, especially in the United States. The disease had been brought under control by the use of chemical pesticides (fungicides) and other agricultural control practices. Phytophthora was not able to quickly develop resistance because it was primarily reproducing only asexually, through mitosis. This was because there was only one mating type present in North America. However about 15 years ago, the second mating type of Phytophthora (which happens to be a more virulent pathogen) was first noticed in Wisconsin, and had probably by that time already spread throughout much of North America. The presence of the second mating type allows sexual reproduction (meiosis), which allows for quicker recombination of genes. This in turn leads to quicker development of resistance to pesticides. Late Blight is a much bigger problem today than anyone ever anticipated. Other members of the potato family (Solanaceae), such as tomatoes and eggplant are also affected by late blight, although usually to lesser degrees.

I hope you enjoyed hearing the story of this month's fungus. One of the lessons you should get from this is that planting of genetically identical plants throughout the fields is a very bad agricultural practice. When a pathogen gets into the fields it can destroy the crop much more easily if the plants are all genetically identical. We must conserve the genetic diversity of crop plants, much of which lies in the wild relatives of cultivated plants. Plant breeders and plant pathologists still have much work to do.

Another lesson you should learn is that diseases have had a great impact on history. This is but one example. There are LOTS of fungi, and each one has an interesting story.

For more information about fungi that produce swimming zoospores, please see Zoosporic Fungi Online from the University of Georgia.

If you have anything to add, or if you have corrections or comments, please write to me at

This page and other pages are © Copyright 2001 by Thomas J. Volk, University of Wisconsin-La Crosse.

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