Plant Diseases Research Paper

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The relationship between humans and the diseases that afflict plants is intertwined throughout history. Humans were often the ones who unintentionally introduced or spread certain types of disease among plants, which resulted in food shortages and famine. Thus the study of plant diseases remains crucial; whatever threatens crops threatens the health and survival of humans.

Preliterate peoples as well as some literate peoples believed that spirits cause disease. Greek physicians dismissed this notion and instead insisted that disease had physical rather than supernatural causes. In the fifth century BCE, the Greek physician Hippocrates taught that an imbalance of fluids causes disease in humans, a claim that left the cause of disease in plants both ignored and unexplained. In the nineteenth century, the German botanist Anton de Bary, the German bacteriologist Robert Koch, and the French chemist Louis Pasteur swept aside the ideas of Hippocrates. De Bary, working with the potato, and Pasteur and Koch, working with cattle, demonstrated that pathogens (parasitic microbes) cause disease. The germ theory of disease is the foundation of modern medicine.

The focus on human diseases should not deflect attention from plant diseases. Despite a perception to the contrary, plants suffer from more diseases than humans do and for an obvious reason. Plants colonized the land 410 million years ago, whereas modern humans made their appearance only 130,000 years ago. The pathogens that attack plants have had some 400 million more years to evolve new types by mutation than those that attack humans.

Plants diseases often affected history when they provoked famines, but human actions also affected plants from very early times, when our ancestors learned to control fire and began to burn dry vegetation to help their hunting. Subsequently, humans carried seeds to new locations when they began to plant fields of grain and other food, and eventually they carried favored crops all around the world. They also spread plant diseases unintentionally together with weeds, as well as animals and insect pests. Plants and humankind, in effect, interacted so closely that they began to evolve together. Even as nomadic foragers, humans depended on plants for sustenance. The rise of agriculture in western Asia some 10,000 years ago and its spread throughout the world have wedded the destiny of humans to that of crops (domesticated plants). Whatever has threatened crops has threatened the health and survival of humans.

Diseases of the Staple Grasses

Grasses such as wheat, rice, and rye have been the staple crops that have sustained populations over the centuries; thus the diseases associated with them have great impact on humans, potentially affecting not only food supplies but their health as well.

Wheat Rust

Wheat rust is one of the oldest plant diseases. Some scholars believe that a passage in Genesis records an outbreak of rust in the Levant that caused famine so severe it forced the Hebrews to migrate to Egypt, the granary of the ancient Mediterranean world. If these scholars are right, this text is the earliest written account of a plant disease.

Only in the fourth century BCE did Theophrastus, a Greek botanist and pupil of Aristotle, coin the term rust for this disease because of its reddish hue on the leaves and stem of wheat plants. Theophrastus wrote that wheat planted in valleys and other low ground suffered from rust more often and more acutely than wheat planted on high ground, though he could not explain this fact.

That insight came to the Romans. As early as 700 BCE, they identified the reddish hue on wheat plants as the mark of rust. At that time they began to worship Robigus; historians identify Robigus as the god of rust, a fair statement so long as one remembers the Greek rather than Roman origin of the term rust. The idea that a god unleashed rust on the Romans underscores their belief that rust had a supernatural cause. Trade with the Greek city-states led the Romans to abandon a supernatural explanation of plant diseases. In the first century BCE, the naturalist Pliny the Elder made the crucial link between moisture and the onset and spread of rust, writing that rust afflicted wheat grown in areas where fog and dew were common in morning and evening. Pliny’s insight into the role of water in spreading rust was prescient because rust, like all fungal diseases, spreads in wet environments. The rust fungus needs water to produce the millions of spores that are the next generation of fungi. Two centuries later, the agricultural writer Columella warned farmers against staking their livelihood on wheat. The only protection against rust was to grow a diversity of crops. Columella recommended cultivation of chickpeas and lentils because of their immunity to rust.

Columella had reason to worry: the first three centuries of the Common Era were unusually wet in the lands along the Mediterranean Sea, bringing rust to wheat fields throughout the Roman Empire.

In the seventh and eighth centuries, Arabs brought the barberry bush with them as they swept across North Africa and into Spain. Neither Arabs nor Europeans understood that the bush harbors rust fungi because the fungi inhabit the bush without harming it, much as the pathogens that cause malaria and yellow fever live in the gut of the female mosquito without harming her. A barberry bush that harbors rust fungi has no symptoms of disease. Only in the seventeenth century did Europeans begin to suspect the bush to be a Trojan horse. In 1660 France enacted the first law to eradicate the bush. Other European nations passed similar laws, as did the American colonies in the eighteenth century.

These measures were not enough to stop the spread of rust. Plant breeders in the nineteenth century began to search for rust-resistant wheat to cross with high-yielding but susceptible varieties; this effort accelerated during that century as England, France, the German states, and the United States poured money into agricultural science. Around 1900 agronomists at the U.S. Department of Agriculture identified an Italian durum wheat suitable for pasta and a Russian emmer wheat suitable for bread. These were the first of innumerable resistant wheat varieties that give humans the best, if incomplete, protection against failure of the wheat crop from rust.

Rice Stunt Disease

Chinese records first mention the cultivation of rice four thousand years ago, though its cultivation began earlier in southeastern Asia. By 500 BCE, farmers grew rice in China, the Korean peninsula, and the swath of land between modern Vietnam and India. By the first century CE, farmers were growing rice in Japan, Indonesia, and the Philippines. The people of these regions were nearly as dependent on rice as the Irish would be on potato in the nineteenth century. True, farmers also grew soybeans throughout Korea and China, and wheat grown along the Indus River reached the people of central and southern India by trade, but soybeans and wheat were minor supplements to a diet of rice.

Roughly forty diseases afflict rice, making difficult the task of sorting among them difficult, as well as among climatic factors, to explain the 1,800 famines that Chinese documents have recorded since 100 BCE and the seventy in India since 33 CE. Because rice needs more water than any other grain to thrive, Chinese and Indian texts often attributed crop failures to inadequate or impure water.

In the sixth century CE, a Japanese text mentions stunted (short or dwarf) rice plants that bore little or no rice. The condition baffled farmers for 1,200 years. In 1733, 12,000 Japanese died of famine when stunt destroyed their rice crop, yet no one was any closer to understanding what stunted rice plants. Unlike Europe, Asia never developed science in its modern form but only gradually assimilated it from Europeans during the eighteenth and nineteenth centuries. The people of Japan and continental Asia did, however, have a tradition of careful observation. This tradition led one Japanese farmer in 1874 to study the feeding habits of leafhoppers on rice plants. He doubted that insect bites alone could arrest plant growth and instead proposed that leafhoppers carried a pathogen that they transmitted to rice plants by bite. The pathogen, not leafhoppers, stunted rice plants.

The idea was as novel as it was correct. Leafhoppers carry within their gut Rice Dwarf Virus just as, one may recall, various species of mosquitoes carry the pathogens for malaria and yellow fever. Rice Dwarf Virus remains virulent throughout a leafhopper’s life, and female leafhoppers pass the virus to their offspring, multiplying it with each generation. When the leafhopper population is large, as it must have been in Japan in 1733, the virus becomes widespread enough to cause failure of the rice crop even though the leafhopper is an inefficient flier.

The discovery of insect transmission of a pathogen opened a new field in the study of plant diseases by uniting entomology, the study of insects, with plant pathology. Scientists came quickly to understand that control of insect populations is essential if one hopes to minimize crop damage from an insect-borne pathogen. It was no longer enough for the plant pathologist to understand diseases. He now had to understand the feeding and mating habits of insects and their distribution in areas of disease. The need to combat insects accelerated the study and development of insecticides as a branch of applied chemistry in the twentieth century. The study of insect-borne viruses and the development and use of insecticides would later be crucial in fighting corn diseases in the United States.

Rye Ergotism

The importance of wheat and rice to the sustenance of Europeans and Asians has deflected attention from rye and its diseases. The Germanic tribes that settled the lands that are today France and Germany began growing rye in the second century CE. Wheat always commanded a higher price than rye, making rye bread the staple of the poor until the even cheaper potato spread through Europe between the sixteenth and nineteenth centuries.

The diseases of rye thus afflicted the poor rather than the rich. Particularly serious was ergotism, a fungal disease that fills rye grains with a toxin that in sufficient quantities causes convulsions and death in humans. Unlike most plant diseases, ergot of rye threatens humans by poisoning them rather than by causing famine. The agony of death from ergot toxicity led medieval Europeans to attribute the disease to God’s wrath, hence the name “Holy Fire.” Medieval chronicles cite the first outbreak of Holy Fire in the eighth century. In 857 CE, thousands died in the Rhine Valley, with smaller outbreaks throughout France and Germany.

One may recall that fungi spread in wet environments. Evidence from dendrology and medieval chronicles suggests that after 1000 CE, Europe’s climate turned wet and cool, hastening the spread and severity of ergotism in northern and western Europe. An outbreak in 1039 was the first in a series of virulent outbreaks between the eleventh and eighteenth centuries. Ergotism, along with famine in the early fourteenth century, may explain the high mortality of the Black Death.

Diseases of the Staple Crops Indigenous to the Americas

Like grasses in Europe and Asia, dependence on certain crops can be dangerous—culturally and economically—if they become susceptible to disease. Potatoes and corn are two stale crops that have been catastrophically affected.

Late Blight of Potato

The fungus that causes late blight of potato is at the center of a tragedy, the Irish Potato Famine. The tragedy has its roots not in Europe but in the Andes Mountains, where the natives of Peru domesticated the potato. The Spanish conquered Peru in the sixteenth century. In search of gold, they found a more valuable commodity, the potato. From Peru the potato reached Spain by ship around 1570, reaching Ireland before 1650.

By 1800 the Irish, squeezed by their lack of land and high rents, had no choice but to embrace the potato for sustenance because it yielded more food per unit of land than any grain. Reliance on a single crop is always risky, as Columella had emphasized in the first century. The potato posed risks far greater than the Irish could have imagined. The Spanish had brought little more than a few handfuls of potatoes with them. These potatoes were of the same stock and thus genetically uniform. Because the potato propagates by shoots, new potatoes, barring mutation, are genetic equivalents of the parent potato. With all potatoes near carbon copies of one another, any disease that threatens one potato threatens all.

With the potato vulnerable, catastrophe struck in 1845. Six weeks of rain hastened the spread of the blight fungus across Ireland. The plants died and potatoes rotted in the ground. Blight struck again in 1846. Their staple gone, one million starved and 1.5 million fled Ireland in the next five years.

The tragedy galvanized scientists throughout Europe into action. In 1861 Anton de Bary isolated the culprit, a fungus he named Phytophthora infestans, and by spreading it on healthy potato plants demonstrated that it caused blight. The Potato Famine had spurred de Bary’s discovery, which marked the beginning of plant pathology as a science.

Corn Diseases

As with potato diseases, scientists know little about corn diseases during the pre-Columbian period. What is clear, however, is that corn, unlike potato, is a cross-pollinating plant that produces plants with genetic diversity. Crossbreeding achieves diversity by reshuffling chromosomes in any plant or animal, including humans. This diversity should minimize the occurrence of epidemics, for within a heterogeneous population some individuals, in this case corn plants, should be resistant to disease.

Corn Viruses

Since the 1920s, corn breeders have reduced the genetic diversity by breeding a small number of high-yielding corn plants of roughly uniform genotypes, making corn vulnerable to epidemics. An outbreak of corn stunt disease along the lower Mississippi valley in 1945, reminiscent of the stunt disease that ravaged rice in Asia, presaged corn’s vulnerability to an epidemic. As is true of rice stunt, a virus causes corn stunt disease and is spread by an insect, in this case a species of aphid.

Worse was to follow. A few stunted corn plants in Portsmouth, Ohio, erupted in an epidemic that engulfed the Ohio and Mississippi valleys in 1963 and 1964, costing farmers who had planted on some lands along these rivers their entire corn crop. The culprit was not the corn stunt virus as scientists first thought but two viruses: Maize Dwarf Mosaic Virus (MDMV) and Maize Chlorotic Dwarf Virus (MCDV). The initial confusion among scientists slowed their response, opening the entire Midwest and South to the danger of an epidemic.

The method of virus transmission saved corn growers. A species each of aphid and leafhopper transmits MDMV and MCDV, respectively, by bite. Both feed primarily on Johnsongrass that grows along the Ohio and Mississippi rivers. Both viruses inhabit Johnsongrass, as the ergot fungus inhabits the barberry bush, without signs of disease. But neither insect is a strong flier, and, unlike the leafhopper that carries Rice Dwarf Virus, neither aphid nor leafhopper retains MDMV and MCDV in virulent form more than fortyfive minutes, limiting the range of both viruses.

Once scientists had fingered the aphid, leafhopper, and Johnsongrass as culprits, the U.S. Department of Agriculture launched a campaign to kill aphids and leafhoppers by insecticide and Johnsongrass by herbicide in late 1964. The expenditure of chemicals and money ended the threat of these viruses and led scientists to believe they held the upper hand against corn diseases. Success prevented all but a few scientists from questioning the wisdom of growing genetically uniform corn throughout the Midwest and South.

Southern Corn Leaf Blight

Catastrophe struck in 1970 as Southern Corn Leaf Blight, a fungal disease, swept the United States, destroying 710 million bushels of corn, 15 percent of the corn crop that year. From Texas to Georgia and Florida, farmers lost half their corn crop. These losses cost farmers $1 billion, and the collapse of farm commodity prices cost investors billions more. In a single summer, one corn fungus threatened financial ruin.

Plant pathologists identified a single female parent of corn (a type of corn that produced no pollen and so was male sterile) as susceptible to Southern Corn Leaf Blight. By dropping it from the corn pedigree, agronomists bred new varieties of corn resistant to South Corn Leaf Blight, but corn remains as genetically uniform today as it was in 1970.

Future Prospects

The imperative to minimize crop losses from diseases will only intensify in the future as the human population grows exponentially. At their origin 130,000 years ago, modern humans cannot have numbered more than a few thousand. Only around 1800 did the human population number 1 billion. The population doubled by 1940 and again by 1975. Today, more than 6 billion humans crowd the earth, and demographers fear our numbers may swell to upwards of 9 billion by 2045.

To avert famine on an unprecedented scale, farmers must triple food production by then. To fall even 2 percent short of the goal, demographers believe, will condemn some 270 million people to starvation. Only with the highest yielding varieties of potatoes, corn, soybeans, wheat, and rice—the sustenance of humanity—can humans hope to avert widespread starvation. But only a small number of varieties of any crop can yield enough food for a hungry world. The future will only exacerbate the problem of genetic homogeneity. Crops may become more rather than less vulnerable to epidemics.


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