PLANT DISEASE

an impairment of the normal state of a plant that interrupts or modifies its vital functions. All species of plants, wild and cultivated alike, are subject to disease. Although each species is susceptible to characteristic diseases, these are, in each case, relatively few in number. The occurrence and prevalence of plant diseases vary from season to season, depending on the presence of the pathogen, environmental conditions, and the crops and varieties grown. Some plant varieties are particularly subject to outbreaks of diseases; others are more resistant to them. Additional reading General works include G.C. Ainsworth, Introduction to the History of Plant Pathology (1981), a review of the developments in the field of plant pathology and the influence of plant diseases on history; Gail L. Schumann, Plant Diseases: Their Biology and Social Impact (1991), a discussion of the social and cultural influence of plant diseases; and E.C. Large, The Advance of the Fungi (1940, reissued 1962), a popular account of plant disease epidemics and how they have influenced economic and political history.Compilations of practical information are A. Johnston and C. Booth (eds.), Plant Pathologist's Pocket Book, 2nd ed. (1983), on the identification, isolation, and culture of plant pathogens; Michael D. Smith (ed.), The Ortho Problem Solver, 3rd ed. (1989), a handbook for indoor and outdoor plants; Westcott's Plant Disease Handbook, 5th ed. rev. by R. Kenneth Horst (1990), a comprehensive reference covering plants grown in the United States, for professional and amateur gardeners; Louis Pyenson, Plant Health Handbook (1981), a guide for the amateur gardener; G.R. Dixon, Plant Pathogens and Their Control in Horticulture (1984), with both host and microorganism/disease indexes; I.M. Smith et al. (eds.), European Handbook of Plant Diseases (1988), a reference for professional plant pathologists and for advanced study in the field, covering economically important diseases of crops and forest trees in Europe; Pascal P. Pirone, Diseases and Pests of Ornamental Plants, 5th ed. (1978), an excellent reference for gardeners and landscape professionals; Rubert Burley Streets, The Diagnosis of Plant Diseases: A Field and Laboratory Manual Emphasizing the Most Practical Methods for Rapid Identification (1982); William R. Jarvis, Managing Diseases in Greenhouse Crops (1992); and H. David Thurston, Tropical Plant Diseases (1984), a discussion of important diseases of major tropical crops. The Compendium of Plant Diseases is an outstanding series of well-illustrated books by experts in each field, designed to assist in the identification, prevention, and control of major plant diseases and disorders of specific crops.College-level texts include George N. Agrios, Plant Pathology, 3rd ed. (1988), a comprehensive discussion of parasitism and pathogenicity and the biochemistry of host-pathogen relationships; J.G. Manners, Principles of Plant Pathology, 2nd ed. (1993), an investigation of the physiology and genetics of host-pathogen interactions, disease epidemiology, and control; Daniel A. Roberts and Carl W. Boothroyd, Fundamentals of Plant Pathology, 2nd ed. (1984), an examination of causal agents, symptoms, and control of plant diseases; C.H. Dickinson and J.A. Lucas, Plant Pathology and Plant Pathogens, 2nd ed. (1982), an overview of plant disease with extensive coverage of host-pathogen interactions at both the cellular and subcellular levels; and George B. Lucas, C. Lee Campbell, and Leon T. Lucas, Introduction to Plant Diseases: Identification and Management, 2nd ed. (1992), a survey of the causes, impact, and management of plant diseases. Specific aspects are treated in depth in the following references: Kurt J. Leonard and William E. Fry (eds.), Plant Disease Epidemiology, vol. 1 (1986), covering population dynamics and management of disease-causing agents; Jrgen Kranz (ed.), Epidemics of Plant Diseases, 2nd completely rev. ed. (1990), a presentation of the latest mathematical and statistical methods in use for analysis and modeling of plant disease epidemics; R.K.S. Wood and G.J. Jellis (eds.), Plant Diseases: Infection, Damage, and Loss (1984), a comprehensive assessment; William F. Bennett (ed.), Nutrient Deficiencies & Toxicities in Crop Plants (1993), an examination of the role of nutrients on the health of major crop plants; R.D. Durbin (ed.), Toxins in Plant Disease (1981), on the role of microbial toxins in the plant disease cycle; P.G. Ayres (ed.), Effects of Disease on the Physiology of the Growing Plant (1981), a compilation of seminar papers; George W. Bruehl, Soilborne Plant Pathogens (1987); David F. Farr et al., Fungi on Plants and Plant Products in the United States (1989), a comprehensive discussion; Masao Goto, Fundamentals of Bacterial Plant Pathology (1992), a discussion of the morphology, taxonomy, and physiology of phytopathogenic bacteria; J.F. Bradbury, Guide to Plant Pathogenic Bacteria (1986), identifying phytopathogenic bacteria and the diseases they cause; David C. Sigee, Bacterial Plant Pathology: Cell and Molecular Aspects (1993), including discussions of interactions with host cells, virulence factors, and genetics; R.E.F. Matthews, Plant Virology, 3rd ed. (1991), a comprehensive text covering all aspects of plant-infecting viruses, and Diagnosis of Plant Virus Diseases (1993), a discussion of strategies; R.T. Plumb and J.M. Thresh (eds.), Plant Virus Epidemiology (1983), a collection of works by international authorities in the field of plant virology, including several case histories of particularly important diseases; Karl Maramorosch (ed.), Plant Diseases of Viral, Viroid, Mycoplasma, and Uncertain Etiology (1992); Karl Maramorosch and S.P. Raychaudhuri (eds.), Mycoplasma Diseases of Crops (1988), a collection of discussions regarding detection of mycoplasmas, their interactions with plants, insects, and viruses, and their control; Victor H. Dropkin, Introduction to Plant Nematology, 2nd ed. (1989), on the classification and characterization of plant-parasitic nematodes; M. Wajid Khan (ed.), Nematode Interactions (1993); Kerry F. Harris and Karl Maramorosch (eds.), Pathogens, Vectors, and Plant Diseases: Approaches to Control (1982), on the identification and control of insect and nematode vectors of plant diseases; Anne R. Leslie and Gerrit W. Cuperus (eds.), Successful Implementation of Integrated Pest Management for Agricultural Crops (1993); H. David Thurston, Sustainable Practices for Plant Disease Management in Traditional Farming Systems (1992), a discussion of integrated control of phytopathogenic microorganisms; Richard N. Strange, Plant Disease Control: Towards Environmentally Acceptable Methods (1993), an ecologically sensitive analysis of current methods of disease prevention and control; R. James Cook and Kenneth F. Baker, The Nature and Practice of Biological Control of Plant Pathogens (1983), a discussion of the influence of environment on the interactions among microorganisms and crop plants; Arthur W. Engelhard (ed.), Soilborne Plant Pathogens: Management of Diseases with Macro- and Microelements (1989), on the effects of fertilization, nutrition, and pH on diseases caused by soilborne plant pathogens; P.R. Day and G.J. Jellis (eds.), Genetics and Plant Pathogenesis (1987), covering the genetic aspects of disease and pest resistance; and M.S. Wolfe and C.E. Caten (eds.), Populations of Plant Pathogens: Their Dynamics and Genetics (1987), a collection of essays with ideas and concepts relevant to the long-term development of disease-control methods based on population dynamics. Michael J. Pelczar, Jr. Rita M. Pelczar Classification of plant diseases by causal agent Plant diseases are often classified by their physiological effects or symptoms. Many diseases, however, produce practically identical symptoms and signs but are caused by very different microorganisms or agents, thus requiring completely different control methods. Classification according to symptoms is also inadequate because a causal agent may induce several different symptoms, even on the same plant organ, which often intergrade. Classification may be according to the species of plant affected. Host indexes (lists of diseases known to occur on certain hosts in regions, countries, or continents) are valuable in diagnosis. When an apparently new disease is found on a known host, a check into the index for the specific host often leads to identification of the causal agent. It is also possible to classify diseases according to the essential process or function that is adversely affected. The best and most widely used classification of plant diseases is based on the causal agent, such as a noninfectious agent or an infectious agent (i.e., a virus, viroid, mycoplasma, bacterium, fungus, nematode, or parasitic flowering plant). Noninfectious disease-causing agents Noninfectious diseases, which sometimes arise very suddenly, are caused by the excess, deficiency, nonavailability, or improper balance of light, air circulation, relative humidity, water, or essential soil elements; unfavourable soil moisture-oxygen relations; extremes in soil acidity or alkalinity; high or low temperatures; pesticide injury; other poisonous chemicals in air or soil; changes in soil grade; girdling of roots; mechanical and electrical agents; and soil compaction. In addition, unfavourable preharvest and storage conditions for fruits, vegetables, and nursery stock often result in losses. The effects of noninfectious diseases can be seen on a variety of plant species growing in a given locality or environment. Many diseases and injuries caused by noninfectious agents result in heavy loss but are difficult to check or eliminate because they frequently reflect ecological factors beyond human control. Symptoms may appear several weeks or months after an environmental disturbance. Injuries incurred from accidents, poisons, or adverse environmental disturbances often result in damaged tissues that weaken a plant, enabling bacteria, fungi, or viruses to enter and add further damage. The cause may be obvious (lightning or hail), but often it is obscure. Symptoms alone are often unreliable in identifying the causal factor. A thorough examination of recent weather patterns, the condition of surrounding plants, cultural treatments or disturbances, and soil and water tests can help reveal the nature of the disease. Ecology Plant geography Plants occur over the surface of the Earth in well-defined patterns that are closely correlated with both climate and the history of the planet. Forests are the most important of these natural communities from the standpoint of area, carbon content, annual carbon fixation, the cycling of nutrient elements, and influence on energy and water budgets, as well as being the principal reservoir of biotic diversity on land. The most extensive forests are the boreal coniferous forests of North America, Scandinavia, northern Europe, and northern Asia. The moist forests of the tropics are the most diverse, often containing as many as 100 species of trees per hectare (1 hectare = 2.47 acres) and occasionally many more. Forests are commonly distinguished from woodlands and savannas in having a closed canopy of trees that may be 10 metres to more than 50 metres in height. At their wetter and cooler limits, forests are replaced by tundra, a community of low-growing trees and shrubs with a ground cover of Sphagnum and other mosses, dwarfed shrubs, and perennial herbs. The transition from forest to tundra occurs at high latitudes and elevations and may extend over a considerable distance, throughout which trees are scattered, restricted to sheltered areas, and stunted in growth. Such taiga stands are common at the northern limit of trees throughout the Northern Hemisphere. The alpine transition tends to be more abrupt. Both are often determined locally by the distribution of snow. Forests yield at their warmer and drier margins to grassland (called prairie in North America and steppe in Asia). In North America the rich grasslands of the eastern mid-continent, which is affected by moisture moving northward from the Gulf of Mexico, once supported a diversity of herbaceous plants, including grasses that might reach several feet in height, and were known as tall-grass prairie. Westward, where water was less abundant, the grasses diminished in diversity and size to form the short-grass plains. Southward and westward the plains yielded under increasing aridity to desert, as old as any other zone in an evolutionary sense and very diverse in species. Similar patterns of vegetation occur across the broad waist of Asia. The distinction between grassland and forest is blurred in many places, especially in the semiarid subtropics and in other regions where fires may be common. This transitional zone is savanna, an open, grassed woodland. The southeastern coastal plain of North America was originally such a region, a pine savanna that produced plants adapted to, in fact dependent on, burning for survival. The long-leaf pine (Pinus palustris), for instance, has a grass stage, which lasts for several years of early growth, with the bud protected at the very surface of the ground by a thick tuft of long, grasslike leaves that shield it from the heat of a fire. Once sufficient energy has been stored in the taproot and short stem, the tree virtually explodes into growth and passes rapidly through a period of vulnerability to fire. As a tree of 3 metres (10 feet) or more in height, it is safe from all but the hottest of fires. Oaks and other species are replacing pines as savannas disappear from the southeastern United States because of the continuing division of land ownership and protection from fire. In contrast, the extensive savannas of eastern Africa have been maintained through climate, fires, and grazing and support an extraordinary diversity of migratory animals that have evolved to use different parts of the ecosystem. The patterns of use are complex and phased in time, and they work to maintain both the diversity and productivity of the savanna. These African populations are the last of the large and diverse mammalian and avian populations that once also grazed similar areas of North America, Europe, and Asia. The forests of the moist tropics remain the most complex and fascinating of terrestrial ecosystems. They extend from mountain slopes to river swamps and occurred throughout the tropics wherever precipitation allowed trees to survive. Human activities have virtually eliminated forests from vast areas of the tropics and threaten to destroy the residual forests globally. The largest areas of tropical forest occur now in the Amazon basin of South America and in the Congo basin of west-central Africa. The extensive tropical forests of Southeast Asia have been reduced to fragments of the area they occupied as recently as 1950. The forests of the Amazon basin have evolved as a part of a river system whose water level fluctuates annually by as much as 15 metres (50 feet) or more along the middle and lower Amazon. There are substantial further differences in the quality of water. The Negro River, for example, drains an area of sands low in nutrient elements, where organic matter has accumulated sufficiently in soils to produce the humic acids that give the river its dark colour and sufficient acidity to affect the plant and animal life it can support. The areas flooded in the annual cycle are forested and are known by the Portuguese word vrzea. Trees in this zone survive flooding for several weeks annually and provide the basis of a food web that includes fish adapted for grazing on tree fruits and seeds. The grazing fish possess large flat molars adapted for masticating seeds and other coarse organic matter, and they compete for seeds dropped from pods in vrzea trees on the river. The distribution of the various zones of vegetation over the Earth has changed with climates throughout time. The greatest changes in the recent past have been due to the periods of glacial advance and retreat over the last several hundred thousand years. As recently as 15,000 years ago glacial ice covered much of eastern North America as far south as the present Hudson estuary and covered as well all of Scandinavia and northern Europe. Sea level was tens to hundreds of feet lower at various times, and extensive coastal areas now flooded were exposed. Such drastic changes in climate have sorted and resorted plant species, allowed the establishment of a diversity of forest types over time, and worked against the establishment or preservation of the mutual dependencies among species so common in the tropics. These higher-latitude forests, while similar in form and genera and sometimes in species around the world, are apparently highly mutable, the antithesis of the stable and self-perpetuating organism they were once thought to be. Nonetheless, the concept of the plant community as an organism remains viable and useful in ecology and is the basis of much of the most progressive analysis of how to manage the Earth for successful support of large numbers of people. Succession and zonation It is known from studies of plant residues and pollen preserved in the highly acid sediments of bogs and from observations of contemporary glaciers that the vegetation southward from the glacial front in the Northern Hemisphere was banded in much the same way the vegetation is zoned today: tundra occurred in a zone closest to the ice; coniferous forests occurred in a warmer and drier zone southward; and deciduous forests occurred still farther southward. As the habitat changedthat is, as the glacial ice melted and the glacial front retreatedthe vegetation migrated onto the new landscape: first the pioneer species of the tundraa few hardy low-growing or crustose lichens and mosses of small staturefollowed by dwarfed willows and birch and, ultimately, the full panoply of tundra plants. As the climate ameliorated further, the forest followed, always according to pattern, with a few pioneers followed by the full array of species characteristic of the forest. The process of invasion of a new landscape not previously occupied by plants has long been called primary succession. In this case the succession was in response to both the availability of a new habitat and a climatic warming that allowed the replacement of tundra by forest and of the coniferous forest ultimately by deciduous forest as the warming continued. Further warming might result in the replacement of deciduous forest by grassland, as has occurred worldwide at the steppe- or prairie-forest border in response to climatic changes over recent centuries. This boundary is strongly influenced by the fires that have swept grasslands throughout time. Disturbances such as clearings for agriculture, fires, diseases, and storms severe enough to open gaps in a forest may start secondary successional changes that also reestablish the normal vegetation for that climatic zone. Throughout much of the tropics, where forests have been destroyed over large areas to make pasture, conditions appropriate for what might be a normal succession within disturbance-caused gaps in an otherwise intact forest do not exist, and the sites are permanently impoverished. The process generates grassland, shrublands, or sometimes bare earth. One-third of the land area of India has been impoverished in this way and is lost to agriculture, forests, or forestry. This class of land is the most rapidly growing class globally, according to interpretations of statistics of the United Nations Food and Agriculture Organization in 1985. Evolution and paleobotany The evolutionary history of plants is recorded in fossils preserved in lowland or marine sediments. Some fossils preserve the external form of plant parts, others show cellular features, and still others consist of microfossils such as pollen and spores. In rare instances, fossils may even display the ultrastructural or chemical features of the plants they represent. The fossil record reveals a pattern of accelerating rates of evolution coupled with increasing diversity and complexity of biological communities that began with the invasion of land and continued with the progressive colonization of the continents. At present, fossil evidence of land plants dates to the Ordovician Period (505 to 438 million years ago) of the Paleozoic Era. By far the most diverse and conspicuous living members of the plant kingdom are vascular plants (tracheophytes), in which the sporophyte phase of the life history (see above Reproduction and life histories: Life histories) is dominant. Fossil remains of vascular plants provide evidence for evolutionary changes in the structure of the plant body (sporophyte and gametophyte), in the variety of plant forms, in the complexity of the life history, in the tolerance for ecological conditions, and in systematic diversity. Nonvascular plants, or bryophytes (mosses, liverworts, and hornworts), are much smaller and less diverse than vascular plants. The first evidence for liverworts is from rocks of the Devonian Period (408 to 360 million years ago), while the earliest moss fossils are from the Permian Period (286 to 245 million years ago). In contrast to tracheophytes, most fossil bryophytes are relatively similar to living forms. Understanding of the evolution of nonvascular plants is, therefore, less complete than for tracheophytes. Evolution of land plants in the Ordovician through Middle Devonian periods Botanists now believe that plants evolved from the algae; the development of the plant kingdom may have resulted from evolutionary changes that occurred when photosynthetic multicellular organisms invaded the continents. The earliest evidence for land plants consists of isolated spores, tracheid-like tubes, and sheets of cells found in Ordovician rocks. The abundance and diversity of these fossils increases into the Silurian Period (438 to 408 million years ago), where the first macroscopic (megafossil) evidence for land plants has been found. These megafossils consist of slender forking axes that are only a few centimetres long. Some of the axes terminate in sporangia that bear trilete spores (i.e., spores that divide meiotically to form a tetrad). Since a trilete mark indicates that the spores are the product of meiosis, the fertile axes may be interpreted as the sporophyte phase of the life cycle. Fossils of this type could represent either vascular plants or bryophytes. Another possibility is that they are neither but include ancestors of vascular plants, bryophytes, or both. The earliest fossils also include at least one or more additional plant groups that became extinct early in the colonization of the land and therefore have no living descendants. By the Early Devonian Period (408 to 360 million years ago), some of the fossils that consist of forking axes with terminal sporangia also produced a central strand of tracheids, the specialized water-conducting cells of the xylem. Tracheids are a diagnostic feature of vascular plants and are the basis for the division name, Tracheophyta. The simplest and presumably most primitive vascular plants from the Late Silurian and Early Devonian periods (421 to 387 million years ago) were the Rhyniopsida. They included plants such as Cooksonia and Rhynia, which were herbaceous colonizers of moist habitats. Most were less than 30 centimetres (12 inches) tall. The plant body was not differentiated into stems, leaves, and roots; rather, the forking, above-ground axes bore terminal sporangia and produced stomata which demonstrate that the plants were green and photosynthetic. Surface or underground axes served to root the plant and were anchored by rhizoids. Because such plants produced only one type of spore, they were nonseed plants with a homosporous life cycle and free-living gametophytes. A small number of such gametophytes have been described from Devonian deposits. Some plants of the Early Devonian had multicellular emergences of tissue along their above-ground axes, which are thought to have increased the light-capturing surface of the photosynthetic tissue. Such emergences (enations) gave rise to the leaves (microphylls) of the Lycopsida, thus producing an above-ground shoot system that consisted of branching stems with leaves. Underground axes that lacked leaves would have become the roots. Lycophytes were the first plants with well-differentiated shoot systems, and they are the most ancient groups of vascular plants with living representatives. The leaves of several other plant groups were derived from modifications of the forking axes. There was a variety in structure among Devonian plants. The axes of some plants forked equally, while other plantse.g., Drepanophycuswere more specialized, comprising a large, centrally located axis as well as smaller axes borne laterally. In plants where the lateral systems branched in only one plane, the side branches were flat like leaves. Filling in (webbing) of the spaces between forks of the laterals with photosynthetic tissue produced leaves called megaphylls. There is evidence for the evolution of megaphylls in several plant groups of the Late Devonian Period (374 to 360 million years ago) and Early Carboniferous Period (360 to 320 million years ago). Although most of these groups have no living representatives, by the Carboniferous Period they had given rise to homosporous ferns, sphenopsids (horsetails), and seed plants (gymnosperms). As Devonian plants with microphylls and those with specialized branching systems diversified, many grew to the size of shrubs. By the Middle Devonian Period (387 to 374 million years ago) there were shrub-size representatives of several lineages, but a further increase in the size of herbaceous plants was restricted by the limited diameter that above-ground stems and rooting systems could achieve. The development of lateral (secondary) growth overcame this size restriction. The ability to produce secondary growth evolved independently in several groups. In the lycophytes, much of this secondary growth occurred in cortical tissues; in the ancestors of seed plants and several other lineages, however, the production of wood accounted for most of the growth in stem diameter. About the same time, downward-growing, central rooting systems evolved independently in lycophytes and other plant groups. As a result there were forests with a canopy of giant lycophyte and gymnosperm trees near the beginning of the Early Carboniferous Period. As plants developed more complex growth forms, they also underwent systematic diversification and evolved more specialized modes of sexual reproduction. The most primitive vascular plants had a homosporous life cycle, in which reproduction and dispersal involved a single type of spore. Extant homosporous plants include most ferns and many lycophytes. The homosporous life cycle is an effective means for long-distance dispersal of species. Although it permits the fertilization of an egg by a sperm from the same gametophyte plant, genetic recombination, considered important for more rapid evolution, is not possible. Moreover, because the gametophytes of homosporous plants are exposed to the environment for an extended period of time, mortality is relatively high. By the Middle Devonian Period (387 to 374 million years ago), the heterosporous life cycle had evolved independently in several groups, including lycophytes and the ancestors of seed plants. In heterosporous plants there are two sizes of spores; the smaller (a microspore) produces a male gametophyte, and the larger (a megaspore) produces a female gametophyte. The incidence of genetic recombination is increased by this obligate cross-fertilization. Both types of gametophytes develop quickly within the protective spore wall. Compared with homosporous plants, reproduction is more rapid and mortality is reduced in heterosporous plants. By the end of the Devonian, heterosporous plants became the dominant species in most wetland environments; however, the need for an abundant source of water from the environment to effect fertilization prevented the heterosporous plants from establishing communities in drier habitats.