Introduction to Plant Pathology Introduction to Plant Pathology Richard N. Strange University College London Copyright ß 2003 by John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England National l01243 779777 International (þ44) 1243 779777 e-mail (for orders and customer service enquiries): [email protected] Visit our Home Page on http://www.wiley.co.uk or http://www.wiley.com All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency, 90 Tottenham Court Road, London, UK W1P 9 HE, without the permission in writing of the publisher. 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SB731-S868 2003 632’-3–dc22 2003016087 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 470 84972 X (Hardback) 0 470 84973 8 (Paperback) Typeset in 10.5/12.5 pt Times by Kolam Information Services Pvt. Ltd, Pondicherry, India Printed and bound in Great Britain by TJ International Ltd., Padstow, Cornwall This book is printed on acid-free paper responsibly manufactured from sustainable forestry, in which at least two trees are planted for each one used for paper production. To my dear wife, Lilian, with love and appreciation for her patience and support over the last several months. Contents Preface xv 1 The Causal Agents of Plant Disease: Identity and Impact 1 Summary 1 1.1 Introduction 1 1.2 Establishing the cause of disease 2 1.3 The range of organisms that cause plant disease 2 1.3.1 Parasitic angiosperms 2 1.3.2 Fungi 4 1.3.3 Nematodes 12 1.3.4 Algae 14 1.3.5 Oomycetes 15 1.3.6 Plasmodiophoromycetes 16 1.3.7 Trypanosomatids 16 1.3.8 Bacteria 17 1.3.9 Phytoplasmas 28 1.3.10 Viruses 29 1.3.11 Viroids 30 2 The Detection and Diagnosis of Plant Pathogens and the Diseases They Cause 33 Summary 33 2.1 Introduction 33 2.2 Host range and symptomatology 35 2.3 Morphology of the causal organism 38 2.4 Selective media 39 2.5 Biochemical markers 40 2.5.1 Substrate metabolism 41 2.5.2 Fatty acid profiles (FAME analysis) 42 2.5.3 Protein analysis 42 2.5.4 Serological techniques 43 2.5.5 Nucleic acid techniques 46 2.5.6 Choice of diagnostic techniques 57 2.5.7 Ralstonia solanacearum – a case study 58 viii CONTENTS 3 Epidemiology 61 Summary 61 3.1 Introduction 61 3.2 Theories of epidemic development 62 3.2.1 Development of disease in time 63 3.2.2 Development of disease in space 66 3.2.3 Fitting disease progress curves to epidemiological data 69 3.3 The role of the pathogen 70 3.3.1 Sources of inoculum 70 3.3.2 Vectors 75 3.4 The role of the host 80 3.4.1 Host-plant distribution 80 3.4.2 The effect of host resistance on inoculum multiplication 81 3.5 The role of the environment 81 3.5.1 The soil 82 3.5.2 The atmosphere 84 4 The Measurement of Inoculum and Disease Severity and their Effects on Crop Yields 89 Summary 89 4.1 Introduction 90 4.2 Parameters of disease and their measurement 91 4.2.1 Measurement of disease pressure in the soil 91 4.2.2 Measurement of pathogen populations in the soil 92 4.2.3 Measurement of pathogen populations in the air 95 4.2.4 Measurement of pathogens in the plant 96 4.3 Measurement of symptoms 98 4.4 Measurement of yield and quality 102 4.5 Establishing the relation between disease and yield 102 4.5.1 Critical-point models 103 4.5.2 Multiple-point models and area under the disease progress curve 104 4.5.3 Coupling disease progress curves with plant growth 105 4.5.4 Coupling disease predictions with economic loss 106 5 Inoculum Control 107 Summary 107 5.1 Introduction 107 5.2 Reducing the sources of inoculum 109 5.2.1 Eradicating pathogens from seed and propagative material 109 5.2.2 Eradicating sources of inoculum by roguing and pruning 110 5.2.3 Reducing the role of crop residues as sources of inoculum 114 CONTENTS ix 5.2.4 Eradicating inoculum from the soil 114 5.2.5 Control of soil-borne plant disease through the addition of minerals and amendments and the alteration of pH 115 5.2.6 Control of soil-borne plant pathogens by flooding 118 5.3 Reducing inoculum multiplication 118 5.4 Reducing the effectivenes of inoculum 119 5.4.1 The role of forecasting 121 5.4.2 Biological control 121 5.5 Screening, development and application of biological control agents 133 5.5.1 Screening for biological control agents 133 5.5.2 The development of biological control agents 135 5.5.3 The application and establishment of biological control agents 137 5.6 Reducing the spread of inoculum 138 5.6.1 Controlling the spread of wind-borne inoculum 138 5.6.2 Controlling the spread of water-borne inoculum 140 5.6.3 Controlling the spread of soil-borne inoculum 141 5.6.4 Controlling the spread of inoculum in infected propagative material: the role of quarantine 143 5.6.5 Controlling the spread of inoculum by vectors 144 6 Locating, Penetrating and Colonizing the Host 147 Summary 147 6.1 Introduction 148 6.2 The physical and chemical characteristics of materials that cover plants 149 6.3 The physical and chemical characteristics of plant cell walls 150 6.4 Chemotaxis, encystment and chemotropism 153 6.5 Passive entry through natural openings 157 6.6 The roles of physical and chemical signals in the germination of propagules of plant pathogens and the differentiation of infection structures 158 6.6.1 Hydrophobicity 158 6.6.2 Hardness 159 6.6.3 Chemical signals 159 6.6.4 Topographical features 163 6.7 Adhesion 164 6.8 Breaching the cell wall by mechanical force 165 6.9 Breaching the cell wall by the production of degradative enzymes 167 x CONTENTS 6.9.1 Enzymes that degrade the surface layers of plants: waxes, cutin and suberin 168 6.9.2 Pectolytic enzymes 170 6.9.3 Cellulases and xylanases 175 6.9.4 Enzymes involved in the degradation of lignin 176 6.9.5 Proteases 176 6.9.6 Membranlytic enzymes 177 6.10 Global regulation of degradative enzymes 177 6.11 Nutrition of the pathogen 178 6.12 Movement of viruses through the plant 179 7 Subverting the Metabolism of the Host 181 Summary 181 7.1 Introduction 182 7.2 Biochemistry and mechanism of action of hormones in the healthy plant 183 7.2.1 Auxins 183 7.2.2 Cytokinins 186 7.2.3 Gibberellins 187 7.2.4 Ethylene 188 7.2.5 Abscisic acid (ABA) 190 7.2.6 Jasmonates 192 7.2.7 Brassinosteroids 193 7.2.8 Peptide hormones 194 7.3 The role of altered hormone levels in symptom expression 194 7.3.1 Abnormal growth 194 7.3.2 Redirection of nutrients 197 7.3.3 Stunting 197 7.3.4 Chlorosis and necrosis 198 7.3.5 Epinasty 199 7.3.6 Abscission 199 7.4 Crown gall 200 8 Killing the Host – the Role of Toxins 205 Summary 205 8.1 Introduction 205 8.2 Macroscopic symptoms 206 8.3 Bioassay 208 8.4 Host-selective toxins (HSTs) 210 8.4.1 Host-selective toxins from Alternaria species 210 8.4.2 Host-selective toxins from Helminthosporium species and Mycosphaerella zeaemaydis 214 8.4.3 Host-selective toxins from other species 221 8.5 Non-host-selective toxins (NSTs) 223 8.5.1 NSTs from bacteria 224 CONTENTS xi 8.5.2 NSTs from Oomycetes 230 8.5.3 NSTs from fungi 231 8.6 Control of toxin biosynthesis 238 9 The Plant Fights Back – 1. Constitutive Defence Mechanisms 241 Summary 241 9.1 Introduction 241 9.2 Physical barriers 242 9.3 Chemical barriers 243 9.3.1 Low molecular weight compounds 243 9.3.2 High molecular weight compounds 254 10 The Genetics of Compatibility and Incompatibility 261 Summary 261 10.1 Introduction 262 10.2 Pioneering experiments 263 10.3 Some experiments which led to the formulation of the gene-for-gene concept 265 10.4 Variations on the gene-for-gene concept 268 10.5 Molecular corroboration of the gene-for-gene concept 270 10.5.1 Cloning, structure and expression of avirulence genes 271 10.5.2 Function and organization of avirulence genes 276 10.5.3 Cloning, structure and expression of resistance genes 277 10.5.4 Function and organization of resistance genes 283 10.6 The co-evolution of avirulence and resistance genes 286 10.6.1 The evolution of avirulence genes 286 10.6.2 The evolution of resistance genes 288 10.7 Hrp genes 290 11 The Plant Fights Back – 2.