The Origin of Plant Pathology, Potato Famine, Downy Mildew of Grapes and Tree Diseases

The Origin of Plant Pathology and The Potato Famine, and Other Stories of Plant Diseases.

The problems of fungal diseases on plants have been known since the beginnings of agriculture. However, until the late 1800's, the actual causes of plant diseases were unknown. The discolorations of the plants and the fungal growth associated with many plant diseases were only looked upon as "punishment from the gods" or other such fanciful reasons to more sophisticated explanations such as "the plants are dying of some unknown malady". Although the latter reason did recognize that something was causing the plants to die, it was not thought fungi that was the cause, even when they were. Instead when fungal growth was observed on plants, it was thought to merely represent growth that occurred on the plant after it had already been stricken or died.

Probably the most significant impact that fungi have on people is that they cause diseases on our crop plants. There are many such diseases and many have fascinating stories. Some are more interesting than others and I would like to go over two that I feel are probably the most interesting and had historical impact and was of economic importance at the time that they occurred. The two diseases that we will discuss are the Late Blight of Potato and the Downy Mildew of Grapes, which are caused by Phytophthora infestans and Plasmopara viticola, respectively. Because of their significance, both diseases have been very well documented. We will discuss the Potato Blight first because this disease actually marked the beginning of Plant Pathology, the branch of botanical sciences that study the diseases of plants. In addition, we will also discuss two diseases of native trees in North America, in the United States, Cryphonectria parasitica, the cause of the Chestnut Blight and Ophiostoma ulmi, the cause of the Dutch Elm Disease.

When agriculture began, different cultures selected different staples on which their civilizations were built. For example, the Asian cultures grew rice and corn was the staple for the Mayans and Aztecs of Central America and Mexico. In Western Europe wheat and rye were the crops that were usually grown. A lesser known staple is the potato. Its place of origin was in the new world tropics, in the highlands of South America, between Peru and Bolivia. This is the crop on which the Incan civilization was based and the subject of part of today's topic on the origin of plant pathology. The potato plant was an ideal crop in many respects. The potato grows underground and originates from the swollen, underground stem of the plant where it is protected from the hazards of the above ground part of the plant. It is a complete food that is high in carbohydrates, proteins and vitamins on which the Incas were able to build their civilization. The use of the potato as the Incas' staple could be traced as far back as 400 B.C. in the Incan religious and agricultural records.

As was the case in many civilizations in the New World, the Incan empire was conquered by the Spanish Conquistadors during the 16th. Century for their gold, silver and jewelry. They did not find these treasures, but did discover the potato at this time. The potato was brought back to Europe, not as a potential new crop, but as part of the provisions for the sailors on their long voyage home. Potatoes soon became a standard on their sailing ships because it was noted that sailors who ate potatoes did not suffer from scurvy. However, because of the poor storage facilities, most of the potatoes that were not eaten did not survive the trip. It was the few surviving potatoes that gave rise to the cultivar (=cultivated variety) that we commonly refer to as the "Russet" or "Irish" Potato (Figure. 1). The latter name reflects its association with the Irish.

Fig. 1: The Russet or Irish Potato, characterized by its large, brown, oblong shape.

The potatoes were thought to have arrived in Spain around 1570 and from there was distributed through much of Europe and England. However, there was reluctance in accepting the potato as a new food crop. It would be another two centuries after its introduction before there would be any significant plantings of the potato for food. The reasons for not accepting it as a new food were varied. Many religious leaders discouraged the eating of potatoes because they felt that crops that grew in the soil, rather than on top of the soil, was not suitable for consumption by people. Also, since it was not mentioned in the Bible, there had to be something inherently evil about the potato. There was even a logical reason for not eating potatoes. Because the potato plant is a member of the Nightshade Family, which includes mostly species of plants containing alkaloid poisons, it was felt that the potato might also have these poison. This, in fact, is true! One of the very interesting characteristic of the potato is that the roots, stems, leaves, flowers and fruits of the potato plant actually are poisonous. The potato is the only part of the plant that is edible and even the potato can begin to accumulate the alkaloid poisons that are found in the rest of the plant if it begins to sprout and turn greenish with age. However, some people still experimented with eating different parts of the plant. The fruit of the potato plant was the most common part eaten. Aside from being poisonous, it was also very bitter. Based on the bitter taste of the fruit that were tried by many people, it was decided that the whole plant was worthless as a food crop. Despite these problems, the potato, nevertheless, eventually became accepted as a food crop, not just in Ireland, but throughout Europe. Farmers discovered that it was a crop that required little care, it was very nutritious and because of its subterranean habitat, it was safe from invading armies which was one of the reasons it was adopted by the Irish. Prior to the potato becoming the staple of Ireland, the poor of Ireland consumed mostly rye, which was also true of most of the peasant people in Western Europe, but grains were often diseased and unfit for consumption, and the amount of grain available, after harvest, was meager when compared to the potato crop, which had high yields, even when grown in the boggy areas of Ireland. For these reasons, the potato became an important crop in Europe sometime during the 17th Century. The potato was believed to be responsible, in part, for agrarian revolution of the 17th and 18th Century as well as for the population increase in Europe during this period. With the increase in population, the farmers went from subsistence farming to making a profitable living. Although the potato was, in part, responsible for the beginning of this prosperous time in Europe's history, that is only part of the story. Prior to this period, the European staples were rye, if you were of the peasant class and wheat, if you happened to be wealthy. The abandonment of these grains in their diet may also have played a major role in the increase of the European population. We will discuss this theory later in the semester.

The potato would not be introduced into Ireland until the mid 1700's, but by the 1800's the potato would become a major part of their diet, supplying them with 80% of their calories. The potato was also used as fodder for their farm animals that would supply them with milk, eggs and meat to supplement their diet. Thus, the Irish was totally dependent upon the potato for their food. Unfortunately, it was this dependence that led to the famine that would occur in the mid 1850's.

The Late Blight of Potato was first documented in Ireland during the late Summer of 1845. Thus, the name Late Blight, and eventually would be responsible for the death of approximately one million people and the migration of another 1½ million people to other parts of the British Empire, Canada and the United States. The fungus caused a mildew to form on all parts of the plant, which led to the eventual death of the potato plant.

The population of Ireland in 1845 was approximately eight million. So the disease impacted more than a quarter of the Ireland's population. It was all the more devastating because during 1844, the potato crop had increased its yield, dramatically, and during the early Summer of 1845, it appeared to be one of the best harvest year for potatoes. However, without warning, the weather became more overcast and rainy for weeks and it was during this period that the potato crop throughout Ireland appeared to rot overnight. It was all the more frustrating for the Irish because there was nothing that they could do since the actual cause of the Potato Blight was unknown. It was the failure of the potato crop that in part led to the death of one million Irish peasants. If you have taken high school biology or have taken an introductory course in biology at some time during your undergraduate career, you have probably heard this much of the story. However, there is much more to this story and than just the failure of the potato crop which eventually led to these deaths.

There was already a great deal of negative interactions that occurred between the Irish and English, even prior to the crop failure. The tragedies caused by the famine was merely just one more reason that there is still continuing conflicts between the English and Irish to this day. Also, it is not a simple matter of the English Protestants against the Irish Roman Catholics as the news media always seem to portray this conflict. The conflict is an old one that has been going on for centuries. In biology we tend to emphasize that the fungus, Phytophthora infestans, was responsible for the mass starvation that occurred, and it certainly was responsible for the failure of the potato crop. However, the Irish blamed the English for their dependence on a one crop staple.

Prior to the famine, one of the causes for the Irish resentment towards the English was the exorbitant rent that was being paid to people that were referred to as the absentee landlords. These landlords were English nobles who came into possession of the land, in the 1640s, when English forces under Oliver Cromwell invaded Ireland and seized Irish land. Although, the reason for the rent being so exorbitant was not illegal, even by today’s standard, it is certainly not a situation that any of us would want to be in. The landlords resided in England and charge of the land was actually under the control of agents of the landlords whose job was to lease the land. Thus, the name absentee landlords. The lease from the agent usually involved a hundred acres. The man who leased from the agent would then sub-let most or all of the land in five acre lots at an even higher rental. These plots were then further subleased into smaller portions and rented again at still higher rates. At the bottom, with a quarter acre, was the Irish peasant who had to grow enough to feed a family and pay the rent on the land. The Irish peasants were also the poorest people of all the renters and were paying the highest rate per unit area in renting the land. Although the potato was the only crop that the Irish peasants grew for their own consumptions, there were many other crops that were grown in order to pay for the rental of the land. Artist conceptions of life during and after the Potato Famine may be viewed from this link.

Because of the exorbitant rent, there were often tenants that were unable to pay their rent or became rebellious. When tenants resisted eviction or rebellions occurred, English soldiers were sent in to burn down the homes and crops of the tenants, and often killed their pigs, which were kept for an emergency food supply in the event of food shortages. However, the potato had the advantage of being "hidden" from the soldiers so that there was still a food supply even after the soldiers had taken their food and burned what remained. In addition, as mentioned above, rye, which was the staple, until the mid 1700's, when the potato was adopted, could not be grown in adequate quantities to feed the Irish and much of what was available was often found to be covered with fungal growth while the potato grew in great abundance in soil-poor areas where other crops would not grow.

Once the potato became the established, staple crop, in Ireland, by 1800, the population of Ireland doubled, going from 4.5 to 8 million people by 1845. The potato was the main course at every meal for the Irish peasant and often may have been the only food for each meal. The typical Irish peasant ate 8-14 pounds of potato each day. However, potatoes are nutritious and have substantial carbohydrates, as you might guess, and also proteins and vitamins. Although not the best tasting diet, this diet was forced upon the peasant farmers because they were poor in the extreme. Families typically lived in a one room, windowless huts with little furniture or possessions, on their quarter acre plots. Fortunately, the potato grew well in the moist, cool climate of Ireland which was similar to the South American highlands where it was native.

The cultivation of potatoes as a crop was also relatively easy and fast. Potatoes were grown by a process known as vegetative propagation. This procedure involved taking a small piece of the potato which included the "eye" which is actually a bud from which the potato plant is grown. The planted potato eye is often referred to as the seed potato, but is not a seed in the true sense of the word. This has a number of advantages. The eye has substantial food reserves to nourish the bud of the potato plant so that the the potato plant grew more rapidly than if it were started from true seeds. The other advantage to note is that the potato plants when grown in this fashion are genetically identical to the potato from which it was derived because it is a clone of that potato. So if the potato from which the seed potato was derived had the desirable quality that you wanted, then the potatoes produced from it will also have those qualities since it would be a clone of that potato. Whereas if you grew potato plants from actual seeds, which are products of sexual reproduction, each seed will give you genetically different potatoes and you could not be sure as to the quality of the potato crop. Remember that this can also be a disadvantage, and this was one of the main reasons that all the potato crop was so susceptible to the Potato Blight disease. Once the disease struck one plant, it would surely kill all of the potato plants since the potato plants were genetically uniform. It would then continue to the next field and do the same for the potato plants there. So you run a great risk when you grow genetically uniform, i.e. clones, crops of any sort. The Irish potato was already at great risk even if seed potatoes were not used. Remember that few potatoes survived the trip from South America to Spain, and upon its arrival, there were only a few individual that could be used for breeding purposes. Therefore, there was already little genetic diversity even before the potato became a major crop in Europe. By the time the Irish began growing potatoes, it must have already been very genetically uniform. In addition, the seed potato is more likely to harbor diseases than a true seed since the nutritious part of the potato from which the bud will get its food will also attract bacteria and fungi that may kill the bud. Seeds, on the other hand, have a protective layer to protect the young embryo and food supply from infection.

This then sets the background for the Potato Famine. Lets summarize the salient points. From 1800 to 1845 the population of Ireland almost doubled and became wholly dependent upon a single crop, with no alternative food source should that crop fail. You may think that it was very short sighted of the Irish peasants to be so dependent upon a single crop, but they had little choice. Other crops were grown and even farm animals may have been raised, but the Irish could not keep these for their own use. Instead, these crops and animals were used to pay the rent to the absentee landlords. If rent was not paid the farmers were evicted from their land and they would then surely starve to death. Selling the crops elsewhere was also not a possibility, because the English Corn Act specifically forbade the export of crops from Ireland, the farmers were forced to sell the crops in England and at a fraction of the cost they could have obtained if it were a free market. These other crops usually only earned enough to pay their rent.

Although I have presented here a number of reasons why the Irish were able to only have a single crop staple, this practice is not an uncommon one. There are currently over six billion people in the world today. Yet, most of the world relies on three staple crops: wheat, corn and rice.

The summer of 1845 was like most summers, on the whole hot and dry, but then a change occurred throughout Europe. In Ireland, the temperature dropped from 1.5-7ºF below the average temperatures of the last 19 years. In just a few weeks, the potato plants became a blighted mass of decaying vegetation. When the farmers dug up the potatoes they were also found to be decaying, but some "looked" healthy, but they also rotted later in storage. The blight occurred throughout Europe and was not limited to Ireland, but it was disastrous in Ireland because of their dependence on a single crop. You should also bear in mind that even though the change in climate impacted other crops as well by decreasing their yield, the Late Blight affected only the potato. Crop failure of the potato had occurred and people had starved before, but none as serious as in 1845. So the farmers knew from the beginning that there was something new and evil that was killing their crops.

That the potato was dying was obvious, but nobody knew why. There were various reasons for this. At this time few of the common people were educated. Only the wealthy had the resources for an education and the leisure time after to look at the workings of the universe. Even these educated people were not necessarily scientists. For the most part, they were men trained in medicine or religion. These people did know of the existence of microorganisms, such as bacteria and fungi, since Anton van Leeuwenhoek had observed microorganisms, under the microscope, approximately 200 years prior to the Potato Famine. However, their place in nature were not understood. At the time of the Potato Famine, spontaneous generation was still used to explain the presence of the vast populations of microorganisms that could be found on diseased or dead tissue (Does this seem reasonable? After all, it was not known where microorganisms came from at the time). So when mycelium was discovered on the the dying potato plants and potatoes it was considered to be growth that occurred after the death of the plant rather the cause. Locally, a reward was offered to the best explanation for the cause of the disease, and all sorts of explanation came forth, some of which sound rather silly today. Some thought that the "little people" started it, others thought that the land had become spent from over farming, some thought it was the Devil's work and sprinkled holy water in the fields to drive the Devil away. Still others thought it was God's punishment for the waste that occurred from previous years. Because of the ideal conditions, the previous year, there was such a large quantity of potatoes grown that it was more than could be eaten, and the excesses dumped out in the field, and for this waste God was punishing them by destroying their crops. One popular notion was that the "puffing, hooting locomotives that thundered up and down the countryside at the unholy speeds of 20 miles per hour were discharging electricity into the air" caused the blight. Popularity of this concept came about because some scientist lent credence to this theory by stating that static electricity in the air might be responsible since it was responsible for a number of ills

There were a handful of people that thought that disease was killing the dying plants, but what did they mean by disease? It was thought that the change in the weather caused the Late Blight. They reasoned that since people could become sick when the weather became cold and wet, that this was also making the potato plants sick. However, no one was able to say why only the potato plants were affected and not the grains. There were two scholars that even debated their ideas in print. Dr. John Lindley, Professor of Botany at University College, in London, and editor of the Gardener's Chronicle and Agricultural Gazette and the Reverend Dr. Miles J. Berkeley, whose hobby of collecting and studying fungi had made him an outstanding authority in this field. Both were well known in the community. Lindley theory as to the cause of the blight was very unscientific for a scientist. He argued that the potato plants had been growing at a great rate while the weather was normal and with the deluge of rain that came, the potatoes had sucked up water through their roots until they were already saturated with water and, not being able to get rid of the excess, the plant tissue became swollen and their tissues rotted away.

Berkeley, on the other hand, had a more plausible explanation, at least by today's standards. He had based his theory on his observations of leaves, from infected potato plants, which he had examined under the microscope. Berkeley observed that infected leaves were covered with a mold, similar to one he had seen on a sick onion, but was convinced that it was a new and different mold that was killing the potato plant in Ireland. Photographs of diseased potatoes and leaves, and a brief summary about the fungus can be found at this link. Shortly after his observation, Berkeley received a letter, from Dr. Jean Montagne, a physician in Napoleon's army and an old friend. With the letter were specimens of potatoes infected with the blight, and a drawing of the fungus. Berkeley was able to see immediately that Montagne's drawing of the fungus was the same as that which he observed on the blighted potatoes, from Ireland and became convinced that this new fungus was the cause of the blight. Although by today's standards, Lindley's theory would seem rather far fetched, it was Berkeley's theory that was deemed as rather bizarre when he put forth his theory in a letter to the Gardener's Chronicle. Of all the theories that I just went over, Berkeley's theory was considered to be the most bizarre and revolutionary theory. None of the readers of his letter gave much credence to his theory. This was understandable since Berkeley was just giving his opinion based only on his observations. That there was a fungus growing on the plants was not in question. Lindley pointed out to Berkeley that this new fungus was an opportunistic fungus that was invading the dead tissue of the potato plant that had already been killed by the wet, as he had already proposed. These arguments would drag on while the blight continued to take its toll.

It would not be until 1861 that Anton de Bary, who is considered the father of modern plant pathology, that the question as to the cause of the blight was finally settled. He did what would be today a rather simple experiment. He grew two groups of healthy potato plants which he subjected to the same cool and wet environmental conditions that favored the the blight fungus. To one group he applied the sporangia which he had collected from blighted plants while the other group was kept fungus free. In a matter of a few days, the group to which sporangia was applied already showed signs of the disease and eventually rotted. In the control group, disease did not occur. This convincingly demonstrated that it was indeed the fungus which caused the blight and not saturation from too much water. It was this experiment that led other scientists to critically look at, not only diseased plants, but animal and human diseases as well. This was not only the beginning of plant pathology, but a year later, in 1862, Louis Pasteur's Germ Theory would replace concept of spontaneous generation of microorganisms in dead or dying organisms. So, de Bary's work actually preceded Pasteur's Germ Theory and should probably have been credited with this theory as well.

The Potato Blight returned year after year, despite cold winters that we would assume would destroy the fungus. How then did the fungus then survive the Winter to continue its devastation the following year? De Bary determined that it was the farmers themselves that perpetuated the disease. The Late Blight fungus was able to survive the winter in the potato tuber. Tubers that were not consumed that did not appear to be diseased, but in some cases were actually infected, were placed in storage bins to be planted the following year. Under such conditions, if even a single tuber carried the Potato Blight fungus, the rest of the tubers in the storage could become infected. When planted the following year, the disease grew upward into the stem and leaves of the potato plants and finally produced sporangia and spores that would further perpetuate the disease. Another source was the rotted tubers that were discarded in the same fields that the potatoes were planted. These tubers would produce potato plants before the potato fields were planted and became a ready source of the disease. This was the reason why entire fields of potatoes could seemingly become infected overnight. It was these practices that were responsible for the the devastating famine of 1845. The previous year, the potato crop had been a particularly good one. Far more potatoes were produced than could possibly be consumed by both the Irish peasants or their livestock. The surplus potatoes were stored and many discarded. Some were infected with the Late Blight fungus which grew slowly in storage during the Winter. The Blighted tubers were discarded in the Spring of 1845 along with surplus healthy tubers. Thus, the Late Blight Fungus was in place before the planting of the potato plants during the Summer of 1845.

Why did the Late Blight fungus suddenly destroy the potato plants of Europe in 1845? It is now known that the fungus is probably native to South America where it still cause disease on potatoes. With repeated voyages to South America, and the continued transport of potato, the Late Blight fungus was inadvertently transported with the potatoes and brought to Europe. This probably happened a number of times and during the long voyage the fungus often did not survive under the harsh conditions. However, it is thought that some faster crossings allowed the fungus to survive in the tuber which then became planted in Europe. This, together with the environmental conditions which favored the growth of the Late Blight fungus and the genetic uniformity of the potatoes throughout Europe was responsible for the sudden appearance of the disease.

But what happened in England during the blight? Starvation was what killed the people of Ireland, but politics played a large role as well. When the disease struck in 1845, it was devastating, as you might imagine. When word of the disease came to the farmers, they dug under the wilted plants to find healthy tubers, but most had been, by this time, rotted. Fields were searched for healthy tubers. Even diseased tubers were saved, the rotted portions washed and the rest grated to produce potato flour. However, this was not nearly enough. Families starved and became weak and hungry. Out of desperation, the woods were combed for berries, nuts, fruits, ferns and roots for consumption. Many died as a result of poisons consumed from these plants. While the Irish peasants were starving, the landlord's agents piled high, carts with grains, and flocks of sheep, cattle and pigs, were driven to the export market. All the Irish could do was to watch them go.

Meanwhile in London, the one hundred and five Irish member of the Union of Parliament pleaded on behalf of their people for financial aide. However, the economist argued against this plan of action because this would violate economic principles and that the Irish must be left alone to solve their own problem. According to these experts, "Any tampering with the delicate balance of supply and demand would surely result in economic ruin". Despite the warning from the economists, Sir Robert Peel, who was leader of the Tories, the landowners' party, setup up an Irish Relief Commission, in November 1845, which bought in American corn. In order to do this he had to repeal the "Corn Act" which placed a high tariff on the import of foreign grains. This was done to protect the market value of domestic grain. By February 1846, food depots had been set up in Irish Seaport where the corn was sold. In order to buy the corn, the Irish laborers were given government relief work on dock repairs, roadwork and drainage. This got the Irish through the 1845-46 season and prepared them for the planting season of 1846.

The fields were once again planted during the Spring and the fields were green with potato plants in May, but by June the rain came again, and by July there would be scarcely a good potato in the field. Once again, the scramble for food was on, in the woods, streams, roads and to the sea with much the same results. In addition to starvation, disease also came about mostly in the form of cholera, dysentery, scurvy and typhus. The workhouses and soup kitchens brought the people together from near and far thereby spreading the diseases. Although many of the healthy stayed to care for the sick and hungry, by this time, even they had decided to migrate elsewhere while their families were still well and strong. Emigration from Ireland has continued to this day, but now for different reasons. In the first ten years following the famine, one million died and almost two million migrated. The drop in the population of Ireland would continue well into the 20th. Century and it was not until the 1960s that an increase in the population would occur (Figure 2.)

Fig. 2: Graph of population of Ireland, 1821-2001. Rise in population did not begin to increase until the 1960s. (Use of this graph is through the courtesy of Dr. James Hughes, Professor of Geography, Geology and the Environment, Slippery Rock University of Pennsylvania)

The failure of the potato crops also had other far reaching effects on English history. The Corn Law was passed in 1815, after the end of the Napoleonic Wars. The Corn Law imposed a heavy tariff on grains that were imported into Great Britain, thereby protecting the grains grown by the landowners, by keeping the price of imported grain high on the British market. The Whigs in Parliament had long argued that if this tax was removed the price of flour might be lowered to the level that would be affordable to the industrial workers. However, it would be Robert Peel, the head of the Tories, that would repeal the Corn Act, before the government changed from the Tory to Whig. As much as the repeal of the Corn Act displeased the landowners, this was the first step toward a policy of free trade and because of this Great Britain became, during the last half of the 19th. Century, the center of world trade market for all commercial goods.

During the time of the Potato Famine was the first effort made by the state to aid its starving people. Before this time, victims of famine had been forced to fend for themselves. Personal disasters had been treated as a man's own concern or the concern of his family, but definitely not the responsibility of the state.

The effects of the famine was also not limited to Great Britain. Its impact was felt throughout throughout much of the Western World. For example, Irish migration to the United States began. Although not well received when they first emigrated because of the fear of disease and bad feelings about admitting multitudes of people that were paupers, a great deal of bigotry met the Irish, especially from the militant Protestants who were all for deporting the Irish. Nevertheless, the Irish eventually, worked themselves into political power, often voting in bloc for the democratic candidates. The Roman Catholic religion at that time had a very minor role in spiritual affairs in this country, but today this religion is one of the major religions in this country. Also, most Irish worked themselves up from the slums, and in 1960, we had our first Irish, Roman Catholic president, John F. Kennedy.

In Europe a great deal of change also took place. The Late Blight of Potato also struck throughout Europe. However, famine did not occur because of diversified agriculture. Nevertheless, hunger did come and prices did go up because of the failure of the potato crop. This caused discontent among the working class people throughout Europe. Riots and revolts occurred throughout the capitals of Europe. This was the time in which Karl Marx wrote the Communist Manifesto and Das Kapital, and there was growing sympathy for the causes of young radicals: Kossuth, Hungarian revolutionary who fought for an independent Hungary and led the provisional government in 1849 until Russia intervened; Mazzini, an Italian patriot who fought for an independent and unified Italy; and Lamartine, French poet and minister of foreign affair in 1848. In Paris, Berlin, Vienna, etc., monarchies fell, republics were borne. New constitutions were granted

Although social changes occurred almost immediately, an end to the Potato Blight did not. The disease struck again in 1872 and again in 1879. Although John Lindley did observe, as early as 1846, that downwind from the copper works, in Swansea, Wales, the potato plants remained green and healthy, it would be many years later, in 1885, that a copper sulfate and lime solution, named the Bordeaux Mixture, would be discovered that would kill this dreaded plant disease.

However, even after the means to control this disease was discovered, there was one more major outbreak of the Potato Blight, which caused the death of many people as a result of famine. The famine took place, in 1916, in Germany, during WW I. Although it was now well known that spraying the plants with Bordeaux mixture would kill the fungus, all the copper that Germany had was being used for shell casings and electric wire. None was spared for the making of copper sulfate to spray the potato plants. This famine killed seven hundred thousand people during the winter of 1916-17. Until that time, the war was going well for Germany. They were on the verge of defeating Russia, in the east and was readying themselves against England in the west. However, because of the starvation that was occurring, morale became very low in the German army and they were unable to launch a successful attack. This setback was all that was needed to turn the tides of the war against Germany. By 1917, the United States had entered the war and joined with the British and French, and would eventually defeat Germany. If not for the Potato Blight, at this period of time, the outcome of WWI may have been quite different.

Most literature related to the potato famine is not very favorable towards the English. However, I was able to find a web page commemorating the famine in which Mr. Conrad Bladey presented a different point of view on the famine. If you wish to read his opinion of the famine, please click here. It is always interesting to read history from different perspectives. If you wish to read more about the Irish famine click here. The best web page that I found on the famine can be found here.

The fungicide that would destroy the Late Blight fungus was not developed until 1885 and not specifically for the Late Blight, but instead to a related fungus, Plasmopara viticola, the cause of the Downy Mildew of Grapes.

Although Downy Mildew of Grapes was an important crop disease, fortunately it was not as devastating, in terms of human lives as the Late Blight disease. The Downy Mildew of Grapes also has a rather interesting story which involved a number of interconnected events which impacted several countries.

In 1865, the vineyards in the Valley of the Rhone began dying and each year the disease spread further. The disease began with reddening and yellowing of the leaves, which withered and fell. The rest of the plant was stunted. The grapes only partially ripened and by the following season, the plants were almost dead from the ground level up. Digging down into the roots, a M. Planchon readily observed the problem. The roots were swollen with irregular nodule at the base of the plants. Everybody could agree that this was the cause of the grape plants dying, but there was still the question as to what caused it. When news of the disease spread throughout France, there were a number of suggestions as to what caused the disease. Unfortunately, these were the usual, unhelpful suggestions. It was due to some miasmic (=disease) in the atmosphere. Some blamed the late frost of that year. However, Planchon examined the swollen areas of the root more carefully. Using a lens, he was able to observed that there were insects, specifically a species of aphid, that had invaded the roots. He also found eggs and adult females. The aphid was later identified as a new species of Phylloxera, which was named Phylloxera vastatrix. So this would be a job for the applied entomologists to handle.

The damage was done to the roots by the adult aphids sucking the sap from the roots. It is the puncturing and sucking that led to the abnormal swelling that could be seen in diseased plants.

The Phylloxera eventually spread throughout France and later to every wine making country in Europe. By 1885, it had also reached Algeria and Australia. In France, by 1875, it was causing a loss of about fifty million pounds sterling per year to the wine industry and approximately two and a half million acres of vines were affected.

Various desperate means were tried to ease the spread of the aphids. Vineyards were flooded in hopes of drowning the aphids and poisonous vapors were injected into the soil to gas the aphids. However, what eventually stopped the destruction of the the vineyards came from America.

The answer came from Charles Riley, at that time, an entomologist for the state of Missouri. He recognized the Phylloxera to be native to America and that it was probably introduced into into Europe on an American vine. It was at first thought to be different because the American Phylloxera attacked only the leaves of American grapes plants while it attacked the roots of the European cultivated grape (Vitis vitifera). Riley believed that the long association between the native American grapes and the aphid had developed a degree of balance that allowed both to coexist. By 1872, Riley had been to France to observe the dying vines and talked to French scientist and growers as to the significance of the fact that American vines in France seemed to be spared except for some galls on the leaves. What Riley suggested was for the French vineyards to use American root stocks that have become adapted to the Phylloxera and graft them onto French grape plant. Some growers were convinced by Riley and began importing root stocks from America. However, to many growers this was not acceptable.

Many French growers could not think of replacing the superior French vine stocks with American stocks, believing that American grapes were raw and horrible. To the French wine connoisseur, it was an insult. Some growers felt if that was all that could be done, the wine industry was doomed anyway, and it would be far better to give up wine and go the way of the English and start brewing beer than to use American root stocks. This may seem like an over reaction since a solution was being offered that would save the wine industry, but you must realize that to the French, the vineyards and viticulture was more than just an industry, it was a part of their culture. So, only a few growers, at first, followed Riley's idea to remedy the problem of the aphids. This American root stock, however, did eventually solve the dilemma of the Phylloxera and appeared to save the French wine industry. However, another plague would soon replace it.

In America, there was also a fungus that grew on the native grapes, Plasmopara viticola. This fungus attacked the leaves of all species of grapes and attacked it much in the same way that the Late Blight attacked the potato. Plasmopara, in fact, it is a related fungus belonging to the same division of fungi. However, it seemed to do little harm to the American grapes. In fact, it was even looked upon as beneficial by American growers. According to William Farlow, who was then the mycologists at Harvard: "Our native vines have a luxurious growth of leaves, and the danger is that in our short summers the grapes will not be sufficiently exposed to the sun to ripen". The Plasmopara appears at just the right moment to shrivel up the leaves so that the direct rays of the sun may reach the grapes. However, Farlow also added that if this fungus were to be introduced into Europe it would be quite a different story. In the moister climate, the attack on the grape vines might prove as disastrous as the Phylloxera, and the fungus was inadvertently introduced into Europe.

The fungus was first observed in France by Millardet and Planchon in 1878. It is not certain as when, where or how it had been introduced. Only that it was, and probably during the importation of American root stocks to counter the Phylloxera. The spread of the fungus was rapid and by 1882, it was in every wine growing district in France, and progressing fast over the vines of Italy and Germany. To see pictures of diseased grape vines and grapes click here.

With this most recent of diseases, France had now endured almost 40 years of various plant diseases. However, the French would not give up because the diseased grapes were not just a crop that was grown by tenants as they were in Ireland. The vines were grown by the owners of the land and their very lively hood depended upon it. So the search to find a solution to the fungus continued.

By 1882, Pierre Millardet, Professor of Botany at Bordeaux University, and a student of Anton de Bary, had been carrying out research on the Downy Mildew fungus for several years and had clearly demonstrated that the infection of this fungus was much like that of the Late Blight. In October of that year, Professor Millardet was strolling through a vineyard. There had been much mildew in the locality that year, and he was surprised to see that the vines beside the roads were still leafy, while elsewhere they were bare. Examining these leaves more closely he found traces of a bluish-white deposit on them as though someone had treated them with some chemical. Millardet then went to see Mr. Ernest David, the manager of that vineyard. He learned from Mr. David that it was common practice for the vine growers to spray the vines beside the roads with a conspicuous poisonous looking substance to discourage passer-by from sampling the grapes. To do this, the grapes were sprayed with a solution composed of copper sulphate and lime. Mr. David had never really noticed that the sprayed plants remained healthy while those left untreated went the way of all other diseased vines.

For the next two years Millardet tested variations of the mixture used by Mr. David and found the copper sulphate best for not only controlling the Downy Mildew, but also the Late Blight as well. The copper sulphate is what we now call the Bordeaux Mixture, named for the area of France where it was discovered. However, rumors of his copper sulphate cure had already spread far by this time. Now many others were claiming credit for the discovery of the Bordeaux Mixture. Fortunately, Millardet along with Planchon were the ones to discover the Downy Mildew on grape in 1878 and since that time Millardet had continued to work on the disease. Thus, his claim to be the discoverer of this cure was the most credible. However, this did not keep many other researchers from producing variants of the Bordeaux Mixture and claiming it to be their own invention.

So now a means was found by which the Downy Mildew of grapes could be controlled and the French vineyards were thought to be saved. However, this was not the end of France's problems. During the aftermath of the disease, something else happened that almost caused financial ruin to the wine industries of France and some of the Mediterranean countries, but this had nothing to do with fungi, insects or any other types of diseases of grape. Instead this came about through greed. When it appeared that the wine industry of France would collapse, some of the Mediterranean countries thought that they would be able to fill the void that France would certainly leave with the collapse of their wineries. The supply of available wine soon started to far exceed the demand and there was a glut on the market, which nearly caused financial ruin in all these countries.

Although a control was discovered for the Late Blight of Potato and Downy Mildew of Grapes diseases, both began with the inadvertent introduction of the disease causing fungus. As devastating as these diseases were, most countries, including the United States, still did not implement preventive measures to stop introduction of potential diseases and pests. It was not until 1912 that the Plant Quarantine Act was passed and that was only after Cryphonectria parasitica (=Endothia parasitica), the cause of the Chestnut Blight, had destroyed millions of Castanea dentata or American Chestnut trees. Even after this policy was adopted, Ulmus americana, the American Elm, has almost been driven to extinction by Ophiostoma ulmi, the cause of the Dutch Elm Disease.

We will go over the story of these two diseases of trees that are native to North America. Unlike the two previous diseases that were just discussed, the American Chestnut and Elm were not crop plants, but forest trees occurring in their natural habitats. Although the chestnut was a forest tree logged for its wood and the elm was desirable as an ornamental shade tree, both were of value beyond what can be evaluated in terms of dollars and cents and were a part of the American culture.

In its natural habitat, at the turn of the century, the American Chestnut's distribution ran from New England to the Mid-Atlantic States to the Midwest and down the Appalachian highlands to the Gulf States. Trees were typically up to 100 feet fall, with a columnar growth, 3-4 feet in diameter. Large trees could be found that were 10-12 feet in diameter, with crowns that were up to 100 feet wide. The wood of the Chestnut was utilized for a variety of purposes. Its appearance was like that of oak, but not as strong or dense, making it a more workable wood for the building of furniture. Because of its resistance to decay, it was also valued for usage as railroad ties, utility poles and coffins as well as for its tannin. In these usages, no single species has been able to replace the chestnut. However, the Chestnut was more than a species that was logged for its wood. It is part of our American heritage. The fruit of the chestnut was an important food for the Native American Indians that lived within the distribution of this species and for the Europeans who later colonized North America as they moved westwards. Before the blight, it was very much part of Thanksgiving and Christmas. The nuts of the Chestnut were used for part of the turkey dressing and was food for the turkey and other animals that lived in the forests, as well. Even though this generation of children have probably never seen a Chestnut, it is still a well known tree because it has become part of our culture. This is the tree that Nat King Cole sang about in his Christmas Song (click here for excerpt) and it is the tree that had inspired Henry Wadsworth Longfellow to compose his poem, The Village Blacksmith: (an excerpt for your edification)

Cryphonectria parasitica, the cause of the Chestnut Blight, was first detected in 1904, by Mr. Herman Merkel, the head forester, at the Bronx Zoo. Merkel took immediate action when the disease was discovered. He sprayed, he cut down the dead trees and thought that he the disease under control. However, the following year, more trees became infected and the blight was now further spread. His report on this disease was sent to the Department of Agriculture, in Washington and plant pathologists were soon dispatched to New York. Although, they could see that it was a serious and virulent disease, they didn't feel that there was any cause for alarm. It was, after all, a simple matter of removing the dead and infected trees and replanting. Unfortunately, the disease did not end in New York. The following year, reports were received from outside the park, and chestnuts were dying throughout New York State. Now people were becoming alarmed about the disease, as they should be, and although pathologists now gave the disease top priority, the disease continued slowly through the eastern forests.

Although the disease was first reported from the Bronx Zoo, in 1904, it seems likely that the outbreak of the disease did not originate there. It was only because of the watchful eyes of Mr. Merkel that the disease was detected immediately. According to Dr. Sandra Anagnostakis, the introduction of the causal organism must have occurred much earlier, possibly during the 1880's and from Japan, not China, as was first believed. Also, the introduction occurred not once by many times by importers of different species of chestnuts trees. A review as to what Dr. Anagnostakis believes to have occurred can be read here. According to Anagnostakis, an earlier introduction is more likely since the blight would otherwise have been unable to spread as rapidly as it did.

An adequate means of controlling the disease, in the United States, has never been found and by 1915, the government had even given up trying to eradicate this disease. As far as the government was concerned, the American Chestnut would either survive on its own or become extinct. The government did, however, pass the Plant Quarantine Act, in 1912, in order to reduce the likelihood of such a catastrophe happening again. By 1923, the blight had spread to approximately 80% of all American Chestnuts throughout their natural distribution. By the 1950s, 80% of all had succumbed to the blight. It is now impossible to find an uninfected tree in its natural range of distribution. Yet, the species has managed to survive. The disease does not affect the roots of the tree and each time a tree dies, sprouts grow from the old stumps of the diseased trees, but growth does not occur for more than 15 feet. At that time the sprouts eventually are infected and die from the blight. Nevertheless, as long as the Chestnuts continue to sprout in this matter, there is hope that perhaps some means will be discovered that will save this species or that a mutation will arise that will be resistant to the disease.

Unfortunately, the blight did not end in the United States. Once the disease destroyed the American Chestnut forests, it was inadvertently introduced across the Atlantic, sometime during the 1930s, where it found a suitable host in the European Chestnut, which made up approximately 15% of the forests of Italy and southern European countries.

Infection of this disease occurs through openings, from wounds, in the bark of twigs. The fungus then grows into through the bark and works its way into the water and food conducting tissues. Infections normally begin on young twigs. Where growth of the fungus occurs, cankers or dead areas form around the circumference of the twigs, preventing water and minerals, from being transported to all parts of the tree, above the canker. Infections occur from twig to twig and death occurs rapidly in these terminal branches and when the infection reaches the main trunk, death of the entire tree will follow.

Unlike the crop plants which we have discussed, superficial spraying with a fungicide is useless, since the infection occurs deep within the tree, below the bark. In theory, the disease can be controlled in a single tree or perhaps a small number of trees, if all the infected twigs are pruned and destroyed while the infection is just beginning. However, with the millions of trees that that were once present, this was not practical. Although the government had given up doing anything about the Chestnut by 1915, this does not mean that nothing was being done. There have been attempts at hybridizing the American Chestnut with various species of chestnuts in hopes that resistant to the disease can be found. The Chinese Chestnut, which is resistant to the disease has been successfully hybridized with the American Chestnut. Unfortunately, as in the case of many hybrids, their progeny are sterile.

The loss of the American Chestnut, did also have an economic impact. Billions of dollars from losses in lumber and as shade trees. Wildlife, such as deer, squirrels, and wild turkeys were also decimated by this disease because of their dependence on the fruit of the Chestnut.

Sometime when you're sitting around and waiting for a miracle, one does occur. In the 1950s, some trees in Italy were found with cankers of the Chestnut Blight, but were not being killed by the disease. Studies have demonstrated that isolates from these trees were a new strain of the fungus that was much weaker than the common strains. These strains have been designated as hypovirulent. Not only are these weaker, but when they come in contact with a virulent strain, one that will kill the tree, the hyphae fuse and the factor that causes the hypovirulence is transferred to the virulent strain. This phenomenon appears to be caused by virus in the fungus that is transferred from fungus to fungus. Extensive research now has begun, in an attempt to utilize this hypovirulent strain of this fungus to control the disease, which has been successful in parts of Europe. Although in theory this method should control the blight in the United States, it has not done so. For reasons not yet understood, the hypovirulent strain of the blight seems to remain localized and does not disperse, but research continues in this area since it seems the most likely means of control of the blight and has been successful elsewhere.

The Dutch Elm Disease first appeared in Holland in 1919 where it devastated the elm trees and would later spread throughout Europe. In 1927, it crossed the English Channel and attacked the elm trees in England. In a few short years, the disease would find its way across the Atlantic where it would eventually almost drive the American Elm to extinction. Thus, the enactment of Plant Quarantine did not prevent another catastrophe as serious as the Chestnut Blight. The cause of this disease is Ophiostoma ulmi (=Ceratocystis ulmi). The disease caused by this fungus infects the elms in a similar fashion as the Chestnut Blight, but with some help.

The fungus by itself would be a formidable problem, but with its insect ally, the European Elm Bark Beetle, which was responsible for disseminating the fungal spores, the disease proved to be unstoppable. The European Elm Bark Beetle had preceded the fungus to the United States, and this had alarmed the entomologists who knew that the insect's nibbling damaged the European Elm. However, it was soon classified as just a pest because the American Elm was able to resist the nibbling and boring of this introduced beetle just as it was able to resist the nibbling and boring of the American Elm Beetle. To thrive the European Elm Bark Beetle had to find dead elms, or at least dead branches, under whose bark their larvae could develop. Because the American Elm was so healthy, the European Elm Bark Beetle was at first unsuccessful at establishing a niche in its new home. That is until the introduction of the Dutch Elm Disease.

As usual, the fungus was inadvertently introduced. In this case, the source was known. It came in on imported European Elm logs on at least three separate occasions. Its presence was first confirmed in Ohio, in 1930. The occurrence of this fungus in the United States was discovered by two Dutch plant pathologist, Christine Buisman and Bea Schwarz who simultaneously discovered the disease in Cleveland and Cincinnati. They were all too familiar with this fungus since it had already laid waste to the elms from their native Holland.

The disease, like the Chestnut Blight, must enter the host through a wound in the bark, and almost invariably the wound is caused by the two bark-boring insects, the European and American Elm Bark Beetles. As the beetles chew their way into the juicy sap that flows in the food conducting cells, they carry on their legs the spores of the Dutch Elm disease. The mycelium and spores then cause what is called a vascular wilt. The wilt clogs up the water transport cells in, and unable to transport water and minerals, the infected area soon dies. As in the case of the Chestnut, the internal nature of the infection made treatment by spraying with a fungicide, impractical.

After the tree dies, the fungus does not move onto another tree, it is able to live as a saprobe and will grow throughout the trunk of the dying tree. Spores continue to be produce on the tree in tiny tunnels that have been dug out by female Elm Bark Beetles. These tunnels are where the beetles lay their eggs. The larval stage feeds under the bark and after going through the pupa stage, it will have metamorphosed, into the winged adult beetle and begin to crawl its way through the tunnels. In doing so, it will continually brush its body against the conidia of the Dutch Elm fungus, and after emerging from the tunnels, the adult beetle will further spread the disease.

Today, there are few elm trees left in the world. The elms would be extinct now but for their prolific seed production. However, even so, there are now relatively few elm trees compared to the large forests that once covered North America, England and Europe. The few trees that are left in the United States require very intensive care to prevent infection by the disease. Injection of fungicide has been tried, but these are very expensive. Microorganisms have been tested to determine if some sort of biocontrol can rid the disease from the elms. So far results have proved negative or are not yet approved by Environmental Protection Agency. It would not be impossible to imagine this tree becoming extinct despite the efforts to control the disease.

Like the Chestnut, the Elm was a tree that was important not only for its quality of wood, as an ornamental that lined many a city streets as a shade tree. Today, the only hope seems to be hybridization between the various elms and the Asiatic elm since it is the only elm which is immune to the Dutch Elm Disease. For that reason, it is believed that the Dutch Elm Disease is native to China and Japan.

There are few books that tell the stories of the above plant diseases. Should you be interested in reading these stories in more detail, the following books may be consulted:

Carefoot, G.L. and E.R. Sprott. 1969. Famine on the Wind. Angus & Robertson Ltd., London.

Kavaler, L. 1965. Mushrooms, Molds, and Miracles. The John Day Company, New York.

Absentee Landlords: With respect to the potato famine, landowner who rented to the Irish while remaining a resident of England. This was especially a problem during the famine since that meant all of the good produced from the land was exported. Currently, it is a term used to describe a person who owns and rents property in order to earn a profit, but does not live within the property's local economic region.

Anton de Bary: Considered to be the father of plant pathology and the person who demonstrated that the Late Blight of Potato was caused by a fungus that he named Phytophthora infestans. Genus literally means plant eater.

Bordeaux Mixture: A copper sulfate and lime solution first used to control Plasmopara viticola, the cause of Downy Mildew of Grapes, but also found to be effective against Phytophthora infestans, the cause of Late Blight of Potato. Mixture named for the university in which it was developed.

Corn Act: English law during the 1800's that prohibited the import of foreign grains. Act was repealed during the potato famine.

Cryphonectria parasitica: Species of Ascomycota that causes the Chestnut Blight disease. Disease is responsible for loss of the Castanea dentata (American Chestnut) forest throughout North America.

Cultivar: A short hand for (culti)vated (var)iety of plants that have been developed from a natural species and maintained under cultivation

European Elm Bark Beetle: Beetle responsible for the dispersal of spores of Ophiostoma ulmi, the cause of the Dutch Elm Disease.

Hypovirulent Strain of Cryphonectria parasitica: Strain of Cryphonectria parasitica that are weaker than the common strains of this species and will not cause the death of death of Chestnuts. When mycelia of this strain come in contact with the virulent (=common) strains of this species, it is rendered hypovirulent.

John Lindley: Professor of Botany at University College, in London, and editor of the Gardener's Chronicle and Agricultural Gazette, during the potato famine. Believed that Late Blight of Potato was due to excess water that had been sucked up through the roots of the potatoes. Unable to get rid of the excess, the plant tissue became swollen and their tissues rotted away.

Ophiostoma ulmi: Species of Ascomycota that is the cause of the Dutch Elm disease that is responsible for the elm forests in North America and Europe.

Phylloxera vastatrix: Species of aphid, native to North America, that is parasitic on roots of grape plants. First discovered in France in 1862 and almost destroyed wine industry.

Phytophthora infestans: Species of "fungus" causing Late Blight of Potato.

Pierre Millardet: Professor of Botany, University of Bordeaux, during late 1800s. Credited with discovery of Bordeaux Mixture that controlled Plasmopara viticola, and saved wine industry of France.

Plant Quarantine Act: Following the introduction of the Cryphonectria parasitica, law enacted that would require plants coming into country to go through a quarantine period to ensure that they are not carrying diseases.

Reverend Dr. Miles J. Berkeley: Amateur mycologist who believed that the fungi found on the blighted potatoes during the potato famine was the cause rather than the result of the disease.

Vegetative Propagation: The utilization of cells from an existing plant in order to reproduce more plants. This means of reproduction is asexual and as such produces clones of the parent plant. This is the means of reproduction when growing potatoes.

Seed Potato: The specific means of vegetative reproduction utilized when growing potatoes. The seed potato is not actually a seed, but the "eye" of a potato, which is actually a bud that will give rise to the potato plant.
spontaneous generation

Virulent Strain of Cryphonectria parasitica: Common strain of Cryphonectria parasitica that normally will eventually kill Chestnut once infection occurs.