Fourth Edition
This fourth edition first published 2020
© 2020 John Wiley & Sons Ltd
First Edition 1977 by Blackwell Science Ltd
Second Edition 1982
Third Edition 1998
Reprinted 2002
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Library of Congress Cataloging‐in‐Publication Data
Names: Lucas, J. A. (John Alexander), 1948– author.
Title: Plant pathology and plant pathogens / prof. John A. Lucas, Rothamsted Research, West Common, Harpenden, Hertfordshire, UK.
Description: Fourth edition. | Hoboken : Wiley‐Blackwell, 2020. | Includes bibliographical references and index.
Identifiers: LCCN 2019051991 (print) | LCCN 2019051992 (ebook) | ISBN 9781118893869 (paperback) | ISBN 9781118893821 (adobe pdf) | ISBN 9781118893852 (epub)
Subjects: LCSH: Plant diseases. | Phytopathogenic microorganisms–Control. | Plant‐pathogen relationships.
Classification: LCC SB731 .D5 2020 (print) | LCC SB731 (ebook) | DDC 632–dc23
LC record available at https://lccn.loc.gov/2019051991
LC ebook record available at https://lccn.loc.gov/2019051992
Cover Design: Wiley
Cover Image: Confocal microscopy image of a GFP‐labelled strain of the cereal eyespot fungus Oculimacula colonizing a wheat coleoptile. Source: Bowyer et al. 2000.
Plant pathology is an applied science concerned primarily with practical solutions to disease problems in agriculture, horticulture, and forestry. The study of plant disease and the development of methods for its control continue to be vital elements in the drive to improve crop productivity and ensure global food security. Projected world population growth and the impacts of climate change make this a priority. From a more strategic perspective, the experimental analysis of the interactions between plants and pathogens has become fertile territory for scientists interested in emerging areas of plant biology, including recognition and response systems, signal pathways, and innate plant immunity. The use of molecular genetic techniques has provided new insights into how pathogens cause disease and how plants defend themselves against attack. In turn, this new understanding is suggesting novel ways in which plant disease might be controlled.
This book provides an introduction to the main elements of plant pathology, including the applied aspects of disease identification, assessment and control, and fundamental studies of host–pathogen interactions. The aim is to place the subject not only in the context of agricultural science but also in the wider spheres of ecology, population biology, cell biology, and genetics. The impact of molecular biology and genomics on the diagnosis and analysis of plant disease is a recurrent theme.
A large number of people have helped me in the production of this new edition. First, I should acknowledge the invaluable help of many of my colleagues at Rothamsted Research who have generously provided advice, assistance, and materials for use in the book. They include Kim Hammond‐Kosack, Jason Rudd, Kostya Kanyuka, Nicola Hawkins, Ana Machado, Bart Fraaije, Jon West, Stephanie Heard, Anastasia Sokolidi, Kevin King, John Jenkyn, Roger Plumb, James Bell, David Hughes, Mike Adams, Graham Shephard, and Sally Murdoch. Joe Helps advised on mathematical modeling as well as replotting data. The library staff, Tim Wales, Catherine Fernhead, and Chris Whitfield, helped with tracing publications and scanning figures. Special thanks are due to Lin Field and Sheila Bishop who supported me as a visiting worker in the Biointeractions and Crop Protection department. Numerous others in the plant pathology and crop protection community have also supplied images, articles, advice, and data for use in figures and tables, including Bruce McDonald, Mike Coffey, George Sundin, Pamela Gan, Richard O'Connell, Petra Boetink, Yaima Arocha‐Rosete, Geert Haesaert, David Cooke, Paul Birch, Diane Saunders, Judith Turner, Moray Taylor, Sarah Holdgate, Nicola Spence, Ana Lopez, Hans Cools, Mike Davey, Jurriaan Ton, Stephen Rolfe, Paul Bowyer, Clive Brasier, Joan Webber, John Mansfield, Lili Huang, Victoria Routledge, Lucy Carson‐Taylor, Jason Pollock, Dierdre Haines, David Whattoff, Graham Matthews, Haydn Beddows, Dennis Gonsalves, and Malcolm Briggs. Thanks go to Holly Regan‐Jones for her skilful copy editing and Dave Gardner who re‐drew many of the Figures. Vimali Joseph provided vital support during the final production process. My appreciation should also be extended to previous colleagues at the University of Nottingham, especially Alison Daniels and the late Brian Case, who took many of the studio photos featured in this edition. Space does not allow me to acknowledge the many other colleagues who agreed to the use of published results.
Finally, thanks are also due to my daughter Maria Gabriella, whose grasp of digital technology greatly exceeds my own, and my wife Adélia, not only for her forbearance throughout the time taken to complete this project but also for her active contribution searching out all manner of useful information on the internet.
This book is accompanied by a companion website:
The URL is www.wiley.com/go/Lucas_PlantPathology4
The website includes:
Figures from the book
We see our cattle fall and our plants wither without being able to render them assistance, lacking as we do understanding of their condition.
(J.C. Fabricius, 1745–1808)
The health of green plants is of vital importance to everyone, although few people may realize it. As the primary producers in the ecosystem, green plants provide the energy and carbon skeletons upon which almost all other organisms depend. The growth and productivity of plants determine the food supply of animal populations, including the human population. Factors affecting plant productivity, including disease, therefore affect the quantity, quality, and availability of staple foods throughout the world. Nowadays crop failure, due to adverse climate, pests, weeds, or diseases, is rare in developed agriculture, and instead there are surpluses of some foods. Nevertheless, disease still takes a toll, and much time, effort, and money are spent on protecting crops from harmful agents. In developing countries, the consequences of plant disease may be more serious, and crop failure can damage local or national economies, and lead directly to famine and hardship. Improvements in the diagnosis and management of plant disease are a priority in such instances. Furthermore, the pressures on plant productivity are increasing. The area of cultivated land available per person on the planet declined from around 0.4 ha in the 1960s to less than 0.3 by the year 2000 (FAO data), and as the human population continues to multiply the area will further decrease.
As well as supplying staple foods, plants provide many other vital commodities such as timber, fibers, oils, spices, and drugs. The use of plants as alternative renewable sources of energy and chemical feedstocks is becoming more and more important, as other resources such as fossil fuels are depleted and the need to mitigate climate change becomes a priority. Finally, the quality of the natural environment, from wilderness areas to urban parks, sports fields, and gardens, also depends to a large extent on the health of plants.
Healthy plants provide a series of benefits for the farmer, food chain, and the environment (Table 1). The yield and quality of crop products are ensured, and healthy plants are more efficient at using precious resources such as water and nutrients. In doing so, they also prevent losses of nitrogen and other nutrients to the wider environment, and reduce pollution of rivers and ground water, which in many areas is a potential problem for drinking supplies. Vigorous, healthy plants are more competitive with weeds, and are easier to harvest than crops that are stunted or collapsed. Plant root systems play an important role in reducing soil erosion, and thereby help to conserve another precious resource. Finally, given the mounting concerns about climate change, it should be noted that healthy crops have a lower carbon footprint than diseased crops, due to their greater productivity and more efficient use of inputs per area of cultivated land.
Table 1 Benefits of healthy plants
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The science of plant pathology is the study of all aspects of disease in plants, including causal agents, their diagnosis, physiological effects, population dynamics and control. It is a science of synthesis, using data and techniques from fields as diverse as agriculture, microbiology, meteorology, engineering, genetics, genomics, and biochemistry. But first and foremost, plant pathology is an applied science, concerned with practical solutions to the problem of plant disease. Part of the appeal of the subject is to be found in this mixture of pure and applied aspects of biology.
The scope of plant pathology is difficult to define. On a practical level, any shortcoming in the performance of a crop is a problem for the plant pathologist. In the field, he or she may well be regarded in the same way as the family doctor – expected to provide advice on all aspects of plant health! A distinction is often drawn between disease caused by infectious agents and disorders due to noninfectious agents such as mineral deficiency, chemical pollutants, or adverse climatic factors. The main emphasis of this book is on disease caused by plant pathogenic microorganisms such as fungi, oomycetes, bacteria, and viruses. Under favorable conditions, these pathogens can multiply and spread rapidly through plant populations to cause destructive disease epidemics. Many of the principles discussed apply equally well, however, to other damaging agents such as insect pests and nematodes.
A fundamental concept in plant pathology is the disease triangle (Figure 1) which shows that disease results from an interaction between the host plant, the pathogen, and the environment. This can be enlarged to include a further component, the host–pathogen complex (Figure 1), which is not simply the sum of the two partners, as the properties of each are changed by the presence of the other.
A comprehensive analysis of plant disease must take all four components into account. Obviously, one needs to be familiar with the characteristics of the host and the pathogen in isolation. The successful establishment of a pathogen in its host gives rise to the host–pathogen complex. Unraveling the dynamic sequence of events during infection, the molecular “cross‐talk” taking place between the partners, is one of the most challenging problems in experimental biology. In addition, the effects of the environment on each of other components must be understood. This includes not only physical and chemical factors but also macro‐ and microbiological agents. The two‐way arrow between the host and the environment in Figure 1b should not be overlooked, as populations of crop plants often have important effects on their surrounding microclimate. For example, the relative humidity within a crop canopy is higher than that outside and this will favor the development of some microorganisms. Plants attacked by pests and pathogens often produce volatile compounds that can act as signals sensed by neighboring plants. Effects of pathogens on the environment are more subtle but may be significant; some fungi, for instance, produce the volatile hormone ethylene, which can in turn affect the development of adjacent host plants.
Figure 1 The disease triangle (a) incorporating the host–pathogen complex (b).
This book is intended to provide an outline of the main elements of modern plant pathology. The approach is designed to achieve a balance between laboratory and field aspects of the subject, and to place the phenomenon of plant disease in a wider biological context. Research in plant pathology can be broadly divided into tactical and strategic aspects. The former is concerned with providing solutions to disease problems by identifying causal agents and evaluating the most cost‐effective options for their control. The latter is longer term and aims to understand fundamental aspects of plant disease such as pathogen ecology, population biology, host–pathogen interactions, and plant immunity. This knowledge can then be applied to devise improved methods of disease control.
The book is divided into three parts. The first focuses on the problem of plant disease, the causal agents and their significance, disease diagnosis, and the development of epidemics in plant populations. This account highlights the influence of environmental factors on the multiplication and spread of pathogens, and the use of climate data in disease prediction. The second part deals with host–pathogen interactions: how pathogens gain entry to the host, how their growth and development in the plant lead to disease symptoms, and how the plant responds. The outcome of any host–pathogen confrontation depends on a dynamic interplay between factors determining microbial pathogenicity and the active mechanisms of plant immunity. This interaction ultimately determines host–pathogen specificity, whereby any pathogen is able to cause disease in only a restricted range of host plants. Plant and molecular biologists are studying host–pathogen systems as experimental models to probe the mechanisms of gene expression and regulation, and to reveal details of the evolutionary “arms race” taking place between plants and pathogens. Part III deals with the practical business of disease control, often described as crop protection. This covers the management of disease by means of chemicals, breeding for resistance, and alternative biological approaches. Finally, the combined use of cultural practices and all these other measures to provide sustainable, integrated systems for disease control is described.
A comprehensive treatment of individual diseases and the methods used in their control is beyond the scope of a text of this length. For the sake of brevity, specific pathogens or the diseases they cause are often mentioned without further explanation. This approach may be likened to that adopted in many ecology texts, in which the reader is expected to be familiar with most of the higher plants or animals discussed therein. There is, however, an appendix listing all the pathogens and diseases mentioned in the book, together with brief details which will enable the reader to obtain further information about particular diseases. More detail concerning specific aspects of pathology may be obtained by consulting the recommended further reading.
Many scientific journals contain reviews and research papers relevant to plant pathology. One especially useful source is the Annual Review of Phytopathology. Others include:
Advances in Botanical Research
Annals of Applied Biology
Crop Protection
European Journal of Plant Pathology
Fungal Genetics and Biology
Journal of Phytopathology
Molecular Plant–Microbe Interactions
Molecular Plant Pathology
Mycological Research
New Phytologist
Pest Management Science
Plant Disease
The Plant Cell
The Plant Journal
Physiological and Molecular Plant Pathology
Phytopathology
Plant Pathology
PLOS Pathogens
American Phytopathological Society: www.apsnet.org
British Society of Plant Pathology: www.bspp.org.uk
European Foundation for Plant Pathology: www.efpp.net
Review of Plant Pathology, an abstracts database of plant pathology research: www.cabi.org/publishing‐products/online‐information‐resources/review‐of‐plant‐pathology