Chapter 1
Plant Pathology and Historical Perspectives
Mark T.Windham and Alan S.Windham
CONCEPTS
• Plant pathology is composed of many other disciplines such as botany, microbiology, nematology,
virology, bacteriology, mycology, meteorology, biochemistry, genetics, soil science, horticulture, agronomy, and forestry.
• Plant pathology is the study of what causes plant diseases, why they occur, and how to control them.
• Plant pathologists are usually interested in populations of diseased plants and not in individual diseased plants.
• Plant diseases have had a major impact on mankind. Diseases such as ergotism and late blight of potato have led to the deaths of thousands of people.
• Diseases such as coffee rust have changed the way people behave and/or their customs.
• Diseases such as southern corn leaf spot, chestnut blight, and dogwood anthracnose have appeared suddenly and caused millions of dollars to be lost in damage as the pathogens of the diseases spread through the ranges of the hosts.
BACKGROUND
Plants are the foundation of agriculture and life on this planet. Without plants, there would be nothing to feed livestock or humans. Plants are a primary component in building shelter and making clothing. Like humans and animals, plants are plagued with diseases, and these diseases may have devastating consequences on plant populations. Plant pathology is not a pure discipline such as chemistry, mathematics, or physics, but it embodies other disciplines such as botany, microbiology, nematology, virology, bacteriology, mycology, meteorology, biochemistry, genetics, soil science, horticulture, agronomy, and forestry. Plant pathology encompasses the study of what causes a plant disease; how a pathogen attacks the plant at the molecular, cellular, tissue, and whole plant levels of organization; how the host responds to attack; how pathogens are disseminated; how the environment influences the disease process; and how to manage plant pathogens and thereby reduce the effects of the disease on plant populations. Unlike physicians or veterinarians, who emphasize treatment of individuals, plant pathologists are usually interested in populations of plants and not individuals. An individual wheat plant has little worth to a farmer. If it dies from a disease, the plants on either side of it will grow into its space and their increased yield will compensate for the loss of the diseased plant. However, if entire fields become diseased or fields in a region are devastated by disease, economic losses can be staggering. The exception to emphasizing populations of plants to individual plants is specimen plants that include large shade trees or trees planted by a historical figure, such as an oak planted by George Washington at Mt. Vernon, or a southern magnolia planted on the White House lawn by Andrew Jackson. Extraordinary measures may be taken to protect or treat plants of high value or historical significance .Because of the diversity of questions that plant pathologists are called on to answer, plant pathologists are a heterogeneous group of scientists. Some plant pathologists spend most of their time in the field studying how pathogens move over a large area and what environmental factors play a role in development of epidemics or determining which management tactics are most effective in controlling or reducing the impact of a disease. Other plant pathologists are interested in the processes by which a pathogen induces a disease or may look for genes that confer resistance in a plant and complete most of their professional activities in a laboratory. Some plant pathologists work in outreach programs such as the extension service or in private practice and diagnosis disease problems for producers and home gardeners and make recom0-8493-1037-7/04/$0.00+$1.50© 2004 by CRC Press LLC mendations as to how plant diseases may be managed. Still other plant pathologists work for private companies and are responsible for development of new products (biological control agents, chemicals, and new plant varieties) that reduce the impact of plant diseases on producers and consumers (Appendix I).
IMPACT OF PLANT DISEASES ON MANKIND
Plant diseases have impacted man’s ability to grow plants for food, shelter, and clothing since he began to cultivate plants. Drawings and carvings of early civilizations in Central America depict corn plants with drooping ears and poor root systems. Crop failures for ancient man and through the middle ages were common, and plant diseases were often attributed to displeasure of various deities. The Roman god Robigus was thought to be responsible for a good wheat harvest and Romans prayed to him to prevent their wheat crop from being blasted with fire (rust). In more modern times (since 1800), plant diseases have destroyed military plans of monarchs; changed cultures; caused mass migrations of people to avoid starvation; resulted in loss of major components of forest communities; and bankrupted thousands of planters, companies, and banks. Some examples of the effects of some plant diseases on the history of mankind and the environment are illustrated in the following paragraphs.
ERGOTISM
Ergotism is caused by eating rye bread contaminated with sclerotia (hard survival structures shaped like the spur of a rooster) of Claviceps purpurea. Sclerotia are formed in the maturing heads of rye and may contain alkaloids, including LSD, a strong hallucinogenic compound. Symptoms in humans eating contaminated bread include tingling of extremities, including fingers, hands, and feet; a high fever; hallucinations; mental derangement; abortion; and loss of hands, feet, and legs due to restricted blood flow and subsequent gangrene. Death often follows consumption of large quantities of contaminated grain. In livestock fed contaminated grain, heifers may abort fetuses and livestock lose weight, quit giving milk, and lose hooves, tails, ears, etc., following gangrene. As in humans, death is likely when exposed to high doses of ergot. In the middle ages, thousands of people died from this disease in Europe and the disease was referred to as the Holy Fire, due to the high fever and the burning and tingling sensations in hands and feet of victims. An outbreak in France lead to the name St. Anthony’s Fire, presumably because monks of the Order of St. Anthony successfully treated inflicted people by feeding them uncontaminated rye bread. The disease continued in Europe for centuries. A number of authors have concluded that the Salem Witch Trials were due to an outbreak of ergotism in the early American colonies because rye was the primary grain grown in the New England region. The behavior of the accused “witches” was similar to behavior associated with an outbreak of ergotism in human and livestock populations. In the 1950s, an outbreak of ergotism occurred in several small villages in France and demonstrated that even when it is known how ergotism is caused and how the sclerotia are introduced into grain, epidemics of ergotism are still possible.
IRISH POTATO FAMINE
Potatoes were one of the treasures taken from the New World back to Europe and were readily adapted to European farming practices. By 1840, potatoes had become the staple food crop in Ireland, and the average Irishman ate approximately seven pounds of potatoes daily. Because so many potatoes could be grown on a relatively small plot of land, the population of Ireland increased dramatically during the first four decades of the 19th century. In the early 1840s, an epidemic of a new potato disease was documented in the U.S., but little attention was paid to it in Europe. In 1845, an epidemic of potato cholera, later named late blight of potato and attributed to the pathogen Phytophthora infestans, swept across Europe. Although starvation was common at this time in Europe, it was spared the devastation that was found in Ireland because most of Europe had more diversity in its agricultural production and did not depend on one crop for survival as the Irish did. In Ireland, more than a million people starved to death because of an almost total destruction of the potato crop. Another million people migrated to the U.S., taking whatever jobs they could find in the New World. In cities such as Boston and New York, many of the jobs they took were low-paying, dangerous jobs, such as policemen and firemen. To this day, generations of descendants of these immigrants continue in the same line of work.
COFFEE RUST
In the 1700s and early 1800s, coffee was an expensive drink because of the monopoly Arab traders had on the coffee trade and the careful attention they paid to ensure that viable coffee beans (seeds) did not leave their domain. In the mid-1800s, some coffee beans were smuggled to Ceylon (present-day Sri Lanka) and the British began growing coffee. Coffee became the preferred drink of British citizens, and coffee houses became as common as pubs. By 1870, more than 400 plantations of coffee, comprising at least 200,000 ha, were found in Ceylon. In the 1870s, a new disease, coffee rust, caused by the fungal pathogen Hemileia vastatrix, struck Ceylon, resulting in terrible consequences and the destruction of Ceylon’s coffee trade. Planters, banks, and shipping companies went bankrupt and caused widespread panic in British financial markets. By 1880, tea plants replaced 140,000 ha of the destroyed coffee trees. Great Britain became a country of tea drinkers, and this custom remains to date.
CHESTNUT BLIGHT
When the first colonists arrived in the New World, they found forests of eastern North America populated with American chestnut. Chestnut wood was resistant to decay, and the bark contained tannins that made the production of leather from animal hides feasible. In many areas, one of every four trees in the forest was an American chestnut. The nut crop from these trees was so prolific that the ground could be covered by nearly a foot of nuts. Nuts not only served as a food source for the colonists but were also a major mast crop for wildlife that the colonists depended on for meat. Many of the ships of the American shipping industry in the 19th century were made of rot resistant chestnut timber. In the early 1900s, a new disease of chestnut, now known to be caused by Cryphonectria parasitica, was discovered in the northern Atlantic states and named chestnut blight. The disease spread rapidly south and westward, destroying chestnut stands as it went. The disease finally reached the southern and western extent of the chestnuts’ range in the 1950s. By this time, millions of trees had been destroyed, which represented billions of dollars in lost timber. The effects of the disease on wildlife populations were also dramatic, as wildlife had to adapt to less reliable and nutritional mast crops such as acorns. There have been intensive breeding efforts to incorporate resistance to chestnut blight from Chinese chestnut into American chestnut. Resistant hybrids that have been backcrossed for some generations with American chestnut have resulted in a tree that is resistant to chestnut blight and that strongly resembles the American chestnut. Unfortunately, it will take more than a century before we see forests with the stately giants that Americans marveled at before the onset of chestnut blight.
SOUTHERN CORN BLIGHT
After the advent of hybrid seed corn, corn yields began to skyrocket to unheard of yields and came to dominate the corn seed market. To reduce labor costs in producing hybrid seed corn, seed companies began using breeding lines containing a sterility gene that was inherited through the cytoplasm of the female parent. The trait or gene was named the Texas cytoplasmic male sterility (cms) gene. Using this gene in the female parent meant substantial cost savings for the seed companies because they did not have to remove the tassels by hand when producing hybrid seed corn. This system worked for several years until an outbreak in 1970 of a new race of the fungus that is currently named Cochliobolus heterostropus (see Plate 7C following page 80). This new race caused a disease on corn carrying the male sterility gene (practically all hybrid seed corn at that time), resulting in tan lesions that covered the leaves. Stalks, ear husks, ears, and cobs were also attacked and destroyed by the pathogen. The disease first appeared in Florida, spread northward, and destroyed approximately 15% of the U.S. corn crop. Losses were estimated to be in excess of $1 billion. Experts warned the country that nearly the entire U.S. corn crop would be lost in 1971 if substantial changes were not quickly made in the way hybrid seed corn was produced. Commercial seed companies leased almost all the available space in South America in the winter of 1970 and were able to produce enough hybrid seed corn that did not contain the Texas cms gene to save the corn crop of 1971.
DOGWOOD ANTHRACNOSE
Flowering dogwood, Cornus florida, is a popular tree in landscapes throughout much of the U.S. and is worth more than $100 million in wholesale sales to the U.S. nursery industry. It is also an important natural resource, and its foliage, high in calcium, is the preferred browse of lactating deer in early spring in the eastern U.S. Its bright red berries are high in fat and are an important mast crop to wildlife, including black bears, squirrels, turkeys, and more than 40 species of neotropical song birds. In 1977, a new fungal disease was reported in Seattle, WA, on flowering dogwood and Pacific dogwood, C. nuttallii. The following year, the disease was reported on flowering dogwood in the Brooklyn Botanical Garden in New York. The origin of this disease organism is unknown; however, genetic data suggest that the disease-causing fungus is exotic to the North American continent. Since these reports, dogwood anthracnose, caused by Discula destructiva, has destroyed millions of dogwoods on both coasts. In some areas of the Appalachians, flowering dogwood has nearly disappeared where it was once a common understory tree. The disease epidemic continues to spread, with new reports occurring in the Midwestern U.S. in 2001 through 2002.
CAUSES OF PLANT DISEASES
Plant diseases are caused by fungi, bacteria, mollicutes, nematodes, viruses, viroids, parasitic seed plants, algae, and protozoa. The largest group of plant pathogens is the fungi. This differs considerably from human pathogens, where the most common pathogen groups include bacteria and viruses. This is not to imply that other groups such as bacteria, mollicutes, nematodes, viruses, viroids, and parasitic seed plants do not cause important and destructive diseases—they do. For example, Striga species (witchweed) is the limiting factor in sorghum, sugarcane, and rice production in Africa, Asia, and Australia. Dwarf mistletoe, Arceuthobium species, severely limits conifer production in some areas of the western U.S. Millions of dollars are lost each year to nematode diseases such as root knot, bacterial diseases such as soft rot and crown gall, and viral diseases such as tobacco mosaic virus and impatient necrotic spot virus.
ABIOTIC STRESSES (ABIOTIC DISEASES)
Some abiotic stresses such as air pollution and nutrient deficiencies were earlier referred to as abiotic diseases. However, this terminology is no longer used in modern plant pathology. Plant stresses such as those previously listed above and others such as extremes in temperature, moisture, pH, and light levels, and exposure to herbicides are now referred to as abiotic stresses or environmental stresses that result in disease-like symptoms. Sometimes the symptoms that these stresses cause in plants, for example, chlorosis, wilting, necrosis, leaf spots, and blights, look like symptoms of diseases caused by plant pathogens.
WHERE TO GO FOR MORE INFORMATION ABOUT PLANT DISEASES
Most plant pathologists belong to professional societies such as the American Phytopathological Society, the Nematology Society, the Mycology Society of America, and the American Society of Horticultural Science. The most prominent society for plant pathology is the internationally recognized American Phytopathological Society. The society’s web page (http://www.apsnet.org/) is a clearing house of information on new and emerging disease problems, careers in plant pathology, a directory of plant pathology departments at universities in the U.S., and featured articles on plant diseases. The society also publishes several plant pathology journals such as Phytopathology, Plant Disease, Molecular Plant-Microbe Interactions, and Plant Health Progress, and other publications such as books and compendia on specific diseases or diseases affecting specific hosts. It also publishes the monthly newsletter Phytopathology News. Membership is open to professionals interested in plant pathology and to students at a very reduced rate.
Refrences
Agrios, G.N. 1997. Plant Pathology, 4th ed. Academic Press, New York, 635 pp.
Campbell, C.L., P.D.Petersen and C.S.Griffith. 1999. The Formative Years of Plant Pathology in
the United States. APS Press, St. Paul, MN, 427 pp.
Carefoot, G.L and E.R.Sprott. 1967. Famine on the Wind. Longmans, Ontario, 231 pp.
Horsfall, J.G. and E.Cowling (Eds.). 1977. Plant Disease: An Advanced Treastise, Vols. 1–5.
Academic Press, New York.
Large, E.C. 1940. The Advance of the Fungi. Henry Holt, New York, 488 pp.
Lucas, G.B., C.L.Campbell and L.T.Lucas. 1992. Introduction to Plant Diseases: Indentification
and Management, 2nd ed. Van Nostrand Reinhold, New York, 364 pp.
Schumann, G.L. 1991. Plant Diseases: Their Biology and Social Impact. APS Press, St. Paul, MN, 397 pp.
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