Plant Pathology Concepts and Laboratory Exercises

Bonnie H. Ownley, Robert N. Trigiano, Bonnie H. Ownley, Robert N. Trigiano

• Publisher: CRC Press
• Year: 2016
• ISBN:
  9781498774727
• Edition: 3
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

Continuing in the tradition of its predecessors, this new edition combines an informal, easy to read style with a thorough introduction to concepts and terminology of plant pathology. After reviewing fundamental concepts, the book discusses groups of plant pathogens and molecular tools for studying them, pathogen interactions, epidemiology and disease control, and special topics in plant pathology. The book details various disease-causing organisms, including viruses, fungi, prokaryotics, nematodes, and various biotic agents. It also examines various plant-pathogen interactions, molecular attack strategies, extracellular enzymes, host defenses, and disruption of plant function.

New in the Third Edition

• Molecular plant-fungal interactions
• Expanded treatment of molecular tools
• Advanced biocontrol concepts
• How to use and care for microscopes

Part I

Introductory Concepts

1	What Is Plant Pathology?
	H. David Shew and Barbara B. Shew

CONCEPT BOX

• Most plants are healthy most of their lives; disease is the exception.

• The science of plant pathology had its beginnings in the late blight epidemics of the 1840s in Ireland and Europe.

• The germ theory of disease is the foundation of plant pathology.

• Plant disease is the result of a continuous interaction between a plant and a pathogen in a favorable environment.

• A disease cycle is the series of steps in the interaction of a host and pathogen from inoculation through pathogen reproduction and survival.

• Major pathogens of plants include fungi, fungus-like organisms, bacteria, viruses, nematodes, and parasitic seed plants.

• Diagnosis is the art of identifying disease based on symptoms and signs and associated factors.

• Koch’s postulates are a set of rules to establish if a pathogen is the cause of a disease.

• Disease impacts include making plants and plant products scarce, dangerous to consume, and more costly to obtain.

CONCEPT OF PLANT HEALTH

Most plants, for most of their lives, are healthy! This is fortunate, because healthy plants are the foundation of the earth’s terrestrial ecosystems. They are the source of the nutrients that sustain the interdependent organisms that together make up a stable ecosystem. Plants capture energy from the sun, and this energy provides food for large and small herbivores, the carnivores that eat the herbivores, and the scavengers that degrade the remains, including those of the plants themselves. Plants also provide energy for a variety of microorganisms that live in and on them, some of which are parasites that cause disease.

The plants that we observe in natural ecosystems are a product of natural selection. They have adapted to the biotic and abiotic environments of the ecosystem that they support by growing and reproducing more efficiently than their competitors. However, evolution is a dynamic process, and ecosystems are subject to changes due to climate change, introduction of new plant and animal species, introduction of exotic pathogens to the undisturbed ecosystem, and adaptation by existing microorganisms. In fact, coevolution with microorganisms, including those that are capable of causing diseases, is an important part of the long-term adaptation of a plant species to its environment. In this dynamic interplay between plants and their microbial companions, the pathogen sometimes gains the advantage and epidemics flare. In agroecosystems, many of the natural checks and balances of natural ecosystems are removed, so epidemics may occur more often and become very severe unless disease management practices are implemented.

In the following chapters, you will be introduced to the broad scientific discipline known as plant pathology. The primary goal of the text and the associated laboratory experiments is to raise your awareness of the importance of plant pathogens and plant diseases. The chapters will introduce you to the vast array of organisms that cause plant diseases and will allow you to experience the dynamic nature of the interactions between microbes and plants and to understand how we have successfully and unsuccessfully attempted to manage the organisms that cause plant diseases.

DEVELOPMENT OF PLANT PATHOLOGY

Plant pathology is a very broad and diverse scientific discipline. It integrates information from all of the core disciplines dealing with plant biology, plant production, microbial biology, and ecology to understand the dynamic interactions that result in disease. The concept that links all of these disciplines together within the science of plant pathology is the concept of disease: how it starts, develops, and spreads, and how it is prevented or managed. Literally, plant pathology is the science that studies plant suffering (pathology: pathos = suffer, and logy = study of). Plant pathologists attempt to improve plant health and crop productivity through the study of plant diseases, so that the severity and impact of diseases (suffering) can be alleviated.

As people moved from foragers to cultivators and began to rely on harvests of food and fiber from cultivated crops, their awareness of plant diseases must have increased. Much like today, the diseases that early agrarians observed no doubt ranged from minor to devastating. There are numerous references to blights and mildews in religious texts, including the Hebrew Bible, and in early Greek and Chinese writings. However, over several thousand years, there was little advancement in the understanding of the causes of disease and in the development of disease management strategies, largely because the biological basis of a disease was unknown. A disease was due to bad weather, toxic air, celestial events, imbalances of the sap, or divine intervention. The fact that microorganisms were only first observed in the seventeenth century, following the invention of the microscope, is hardly surprising. Even then, it would be nearly 200 years before the relationship between diseases and microorganisms was conclusively established. With few exceptions, people, including most scientists, instead believed in spontaneous generation. In the case of plant diseases, this belief or theory held that microbes were the result and not the cause of disease or decay.

It took multiple epidemics of the late blight disease of potato in Ireland and other areas of Europe in the 1840s, and the tragic events that followed, to provide the impetus for the founding of the science of plant pathology. The germ theory of disease provided the biological basis of the science. Within 15 years of the epidemics of late blight, Julius Kühn published the first textbook of plant pathology, concluding that both parasitic and nonparasitic factors resulted in plant abnormalities (disease). In the 1860s and 1870s, at least five additional textbooks were written about diseases of different groups of plants. The science of plant pathology continued to develop in response to the need of societies to understand the causes of plant diseases and to find the means to control them. It was the successful demonstration that disease problems could be alleviated by applying this new knowledge that led to the rapid development of plant pathology as a science in the late nineteenth and early twentieth centuries.

GERM THEORY OF DISEASE

Germ theory of disease was the single most important discovery in the early development of plant pathology. It states that germs (living organisms) cause diseases. A review of the history of plant pathology reveals that multiple scientists developed early evidence for the role of microbes in disease causality. It is beyond the scope of this chapter to list all those findings, but perhaps Prevost presented the most convincing results in his comprehensive studies of the bunt (smut) disease of wheat in the late eighteenth and early nineteenth centuries. His experimental approach led to a thorough description of the pathogen, its development in the plant, and even the approaches for controlling the disease. His treatise on this disease in 1807 should have provided the evidence needed to establish the germ theory of disease in plants, but his peers rejected the work as unsound (probably meaning too controversial).

It was not until the 1840s that a plant disease drew enough attention from scientists of the day to begin the science of plant pathology. The disease, late blight of potato (Figure 1.1a), ravaged potato crops throughout much of Europe and was especially devastating to the people of Ireland. The suffering that resulted from the consecutive years of epidemics was made worse by the dependence of the Irish population on a single food crop. Ireland was dominated by a landholding arrangement in which poor tenant farmers raised wheat, oats, barley, and other cash crops for export while depending almost exclusively on potatoes for their own sustenance. Even as cash crops continued to be exported, severe epidemics of late blight led to widespread starvation, sickness, and death. The Great Famine resulted in an estimated 1 million deaths and the mass emigration of at least a million more people from Ireland.

The multiple late blight epidemics in the 1840s found a better-prepared scientific community and a more profound need to understand the devastating effects of plant diseases than ever before. Many scientists uncovered important clues in the aftermath of the Great Famine, but the scientific approach used by Anton deBary finally led to the understanding of the true cause of late blight and other plant diseases. In 1861, deBary published his first work on the relationship of the pathogen, Phytophthora infestans, to late blight of potato. Much like Prevost, deBary described the development of the disease, from inoculation to symptom development and production of a new generation of spores on inoculated potato tissues. He also demonstrated the survival of the pathogen in potato tubers. He was able to duplicate all the stages of the disease cycle and repeat these stages in controlled inoculations with the pathogen. In key experiments, deBary inoculated potato plants and compared them under identical conditions with plants that had not been inoculated (controls). When only the inoculated plants became diseased, it was clear that infection by P. infestans was the cause of late blight, and the germ theory was validated. By the time deBary published his work in 1861, many other scientists had begun to support the germ theory of disease on other plants, but deBary is credited with providing the conclusive proof for this theory and is often referred to as the founder of plant pathology.

FIGURE 1.1 Symptoms of diseases studied to confirm the germ theory for different pathogen groups. (a) Destruction of a potato crop by late blight of potato caused by the fungus-like organism Phytophthora infestans. (b) Blighted terminal of ornamental pear caused by the fire blight bacterium, Erwinia amylovora. (c) Alternating light and dark green areas on tobacco leaves caused by tobacco mosaic virus. ([a] Courtesy of Marc Cubeta. With permission. [b, c] Courtesy of H.D. Shew.)

The germ theory for other groups of plant pathogens followed the pivotal work of deBary. Burrill and his student, Arthur, developed the evidence for bacteria a...