Plant diseases have been known and observed by humans since ancient CHAP T E R The rusts, mildews, times. and blights were familiar to the Greeks, Romans, Hebrews, Chinese, and Indians.
Plant Diseases and the Major Pathogen Groups Ancient Plant Pathology Plant diseases have been known and observed by humans since ancient times. The rusts, mildews, and blights were familiar to the Greeks, Romans, Hebrews, Chinese, and Indians. However, they did not understand the true nature of the diseases and instead ascribed blame to supernatural forces. As humans began to practice agriculture in small, concentrated areas, they became dependent on this sedentary practice, rather than hunting and gathering, for survival. This practice also made them more susceptible to starvation and famine if the crops became diseased. Some early Greek philosophers included botany among their interests, and it is through their surviving writings that we find the most extensive early accounts of plant diseases. Aristotle (384–322 BCE), a student of Plato (427–347 BCE), is credited with writing about plant diseases as early as 350 BCE. Aristotle’s protégé, Theophrastus (371–287 BCE), was the first Greek to describe the fungi, and his writings clearly demonstrate the accuracy of his observations of disease etiology and the predisposing factors that appeared to promote disease. Theophrastus also suggested potential techniques for relieving the effects of disease. He was one of the most influential botanists of antiquity and is often referred to as the “Father of Botany.” The Roman writer Gaius Plinius Secundus, also known as Pliny the Elder (23–71 CE), included numerous plant disease references in his Naturalis historia. He was more concerned about control methods for disease than was Theophrastus, yet many of his suggested remedies were based on superstitions, reflecting the common beliefs of the day. Cleidemus, a Greek author of the mid-fourth century BCE, was considered by E. L. Greene to be the
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first plant pathologist, making observations on diseases of the fig, olive, and grapevine. Greene was an early U.S. botanist known for his prolific publication record and best known for his two-part work Landmarks of Botanical History. The works of Cleidemus have not survived, but we are aware of him through the writings of both Aristotle and Theophrastus.
Diversity of Plant Diseases Thousands of species of wild and cultivated plants found in gardens, fields, and forests are attacked by numerous and diverse pathogens, resulting in tens of thousands of distinct plant diseases. English names for some of these diseases include leaf spots, cankers, root rots, blights, wilts, galls, rusts, mildews, smuts, and molds. There are known to be more than 3,000 different kinds of plant rusts on grains, grasses, and dicots (Fig. 1.1), and at least 1,500 powdery mildew diseases (Fig. 1.2) occur on trees, woody ornamentals, annual grains, vegetables, and hundreds of other different kinds of plants.
Fig. 1.1. Rust disease on dry bean, caused by Uromyces appendiculatus. (Courtesy R. M. Harveson)
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Fig. 1.2. Powdery mildew disease on sunflower, caused by Golovinomyces orontii. (Courtesy R. M. Harveson)
Although diseases are widespread among plants, both cultivated and wild, the many kinds of pathogens exhibit differing habits and behaviors. Some pathogens are common on many kinds of plants, while others are very host specific, attacking only a few species or even varieties of cultivated plants. Some pathogens are relatively benign and cause little damage, while others are extremely destructive. Some spread slowly; others cause widespread damage in astonishingly short periods of time. Most plant disease epidemics are caused by members of four major pathogen groups: nematodes,
Fig. 1.3. Relative size of plant cell compared with various types of pathogens. (This illustration was published in Plant Pathology, 3rd ed., by G. N. Agrios, Figure 1-2, p. 7, Copyright Elsevier 1988. Drawing by Nancy Haver)
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Part One • Introduction
fungi, bacteria, and viruses (Fig. 1.3). In general, the pathogens belonging to these groups were first recognized and investigated in order of their decreasing size.
Nematodes Nematodes are unsegmented, elongated, tubelike animals that resemble earthworms but are not closely related to them (��������������������� Fig. 1.4������������� ). Most nematodes range in size from 0.5 mm (1 ⁄50 inch) to 6 mm (¼ inch). Lucas et al. (1992) provide an intriguing example of a plant-pathogenic nematode to illustrate its minute size: “Approximately 80 lesion nematodes (Pratylenchus spp.) could lie side by side inside this hyphen: - .” These tiny animals have become adapted to diverse environments worldwide, ranging from deserts to mountains and oceans. They reside in huge numbers in both soils (estimated to occur in densities of 30 million individuals per square meter) and water, both fresh and marine (representing 90% of the life forms found on the ocean floor). In fact, nematodes often outnumber other animals and are second only to insects in diversity. Many are free living and nonparasitic (40% of described species) and active as decomposers, but others are parasitic on animals (44% of described species) and plants (15% of described species).
Fig. 1.4. Adult sugar beet cyst nematode with two eggs (inset). (Courtesy H. J. Smith; reprinted from R. M. Harveson, L. E. Hanson, and G. L. Hein, eds. 2009. Compendium of Beet Diseases and Pests, 2nd ed. American Phytopathological Society, St. Paul, MN [Fig. 150])
Fungi Fungi are a very diverse group of eukaryotic (possessing a nucleus and other intracellular organelles) organisms, found in every ecosystem on Earth, ranging from the deserts to the polar and temperate regions to the tropics. Although nobody knows for certain how many species exist, David L. Hawks worth has estimated that at least 1.5 million are present on Earth, and 75,000–80,000 have been scientifically identified. Fungal cells lack chlorophyll, have true nuclei within walled cells, and contain membrane-bound organelles. They typically reproduce by spores, and the life cycles of most species include some type of sexual mechanism. In most known species, cells are arranged in linear arrays, appearing slender and threadlike. The threadlike hyphae are usually branched (Fig. 1.5) and form cell walls composed of glucans and either cellulose (the oomycetes—properly classified among algae) or chitin (true fungi). Constantine J. Alexopoulus stated that fungi are so important that “scarcely a day passes during which all of us are not benefited or harmed directly or indirectly by these inhabitants of the microcosm.” Fungi play major beneficial roles as scavengers and decomposers of organic matter and cellulose (few other organisms degrade cellulosic material), and as such, they negatively affect humans by destroying food, leather, fabrics, and other materials. Fungi are essential in fermentation processes (such as those used in making bread, wine, and beer) and in producing some cheeses. Many are edible (many mushrooms, truffles, and the true morels of the genus
Fig. 1.5. Tube-like vegetative fungal growth (hyphae). (Courtesy R. M. Harveson)
Chapter 1 • Plant Diseases and the Major Pathogen Groups
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TOPICAL FOCUS
B O X 1 .1
Biological Taxonomy and Nomenclature
Fig. 1.6. Morel “mushroom.” This edible species is actually an ascomycete. (Copyright © 2009 Mary Smiley [ladyflyfish]; reproduced according to terms of the Creative Commons Public License [http:// creativecommons.org/licenses/by- sa/3.0/legalcode])
Morchella) (Fig. 1.6), while others are important to the industrial manufacturing of organic acids and pharmaceuticals (including antibiotics).
Bacteria Bacteria are prokaryotic, single-celled organisms with rigid cell walls, and they lack membrane-bound nuclei and organelles. Their genetic material consists of circular, single-stranded DNA in the cytoplasm (not organized into a nucleus). Bacteria are much smaller than fungi; a typical bacterial cell measures about 1 ����������������������������������������� µ���������������������������������������� m (one-millionth of a meter), while fungal spores often range from 5 to 200 ����������� µ���������� m in diameter. Some species of bacteria are small enough that 25,000 cells laid end to end equal about 1 inch. Another more whimsical example illustrating their tiny size was provided by E. F. Smith in 1920: “If the inhabitants of the United States or of Great Britain were reduced to the size of the smaller bacteria the entire population could occupy the surface of a silver dollar or of an English penny—all that too without crowding!” Bacteria are classified into four main groups based on shape: spherical (coccus), rod or cylindrical (bacillus), spiral (spirillum), and thread shaped (filamentous). This system of classification was developed by the German botanist Ferdinand Cohn in the 1860s, and he also described six genera based on these groups. This system was widely accepted and represented the first attempt to classify bacteria, and these fundamental divisions still serve as the basis for today’s taxonomic nomenclature (Box 1.1). 6
Taxonomy is the science of grouping biological organisms on the basis of shared characteristics. The term nomenclature refers to the naming of organisms according to a binomial system developed by Carl Linnaeus (see Chapter 2, Biographical Profile 2.1). Thus, according to this system, each organism possesses a species name composed of a genus (identifying the genus to which it belongs) and a species epithet (which when combined with the genus name is unique to that particular organism). Strict international rules are in place for how all organisms should be named, and this process is regulated by a code consisting of principles, rules (also referred to as articles), and recommendations put forth by different international groups. Each pathogen group is governed by a distinct code: bacteria by the International Code of Nomenclature of Bacteria; fungi by the International Code of Nomenclature of Algae, Fungi, and Plants; and nematodes by the International Code of Zoological Nomenclature. Viruses are slightly different and are named according to the code set by a group founded in the 1970s, the International Committee on Taxonomy of Viruses. This code functions similarly to the other three codes in several but not all respects (see Chapter 16, Topical Focus 16.2). To get a proposed name for a new organism accepted, a scientific paper on the proposed species must be published in accordance with rules of the appropriate code. Each code also refers to types (i.e., well-preserved specimens that assist in defining a given species).
Viruses Viruses are very small (too small to be seen directly with a light microscope), infectious agents that replicate only within the cells of living hosts. They are assemblages of molecules, including nucleic acid (RNA or DNA) and a protective protein coat or envelope. Viruses are ubiquitous on Earth and infect all kinds of organisms, including algae, fungi, bacteria, plants, and animals. Since the discovery of Tobacco mosaic virus (TMV) in the last decade of the nineteenth century, more than 5,000 viruses have been identified and described in detail.
Part One • Introduction
Viruses vary in size from 10 to 300 nm (1 nm = one-millionth of a millimeter). As a rule of thumb, viruses are approximately 100 times smaller than bacteria. As a demonstration of how small viruses are, consider that TMV is about 300 nm in length. Thus, it would be possible to line up more than 3,300 TMV particles end to end in the space of 1 mm, which would also approximate the size of this hyphen: - .
References and Further Reading Agrios, G. N. 2005. Plant Pathology. 5th ed. Elsevier Academic Press, Burlington, MA. Ainsworth, G. C. 1976. Introduction to the History of Mycology. Cambridge University Press, Cambridge. Ainsworth, G. C. 1981. Introduction to the History of Plant Pathology. Cambridge University Press, Cambridge. Alexopoulos, C. J. 1962. Introductory Mycology. 2nd ed. Wiley, New York.
Dugan, F. M. 2008. Fungi in the Ancient World: How Mushrooms, Mildews, Molds, and Yeast Shaped the Early Civilizations of Europe, the Mediterranean, and the Near East. American Phytopathological Society, St. Paul, MN. Greene, E. L. 1909. Landmarks of botanical history: A study of certain epochs the development of the science of botany. Part I– Prior to 1562 A.D. Smithsonian Misc. Coll. 54:1-329. Hawksworth, D. L. 2004. Fungal diversity and its implications for genetic resource collection. Stud. Mycol. 50:9-18. Lucas, G. B., Campbell, C. L., and Lucas, L. T. 1992. Introduction to Plant Diseases, Identification and Management. 2nd ed. Van Nostrand Reinhold, New York. Orlob, G. B. 1971. History of plant pathology in the middle ages. Annu. Rev. Phytopathol. 9:7-20. Orlob, G. B. 1973. Ancient and Medieval Plant Pathology. Pflanzenschutz-Nachrichten Bayer. Leverkusen, Germany. Smith, E. F. 1920. An Introduction to Bacterial Diseases of Plants. W. B. Saunders, London.
Chapter 1 • Plant Diseases and the Major Pathogen Groups
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