diagnostic manual for plant diseases in viet 13726

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Diagnostic manual for plant diseases

in Vietnam


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Australian Centre for International Agricultural ResearchCanberra 2008

Diagnostic manual for plant diseases

in Vietnam

Lester W. Burgess

imothy E. Knight

Len esoriero

Hien Tuy Phan


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Te Australian Centre or International Agricultural Research

(ACIAR) was established in June 1982 by an Act o the Australian

Parliament. Its primary mandate is to help identiy agricultural

problems in developing countries and to commission collaborative

research between Australian and developing-country researchers in

fields where Australia has special competence.

Where trade names are used this does not constitute endorsement o

nor discrimination against any product by the Centre.

Tis series contains the results o original research supported by

ACIAR, or material deemed relevant to ACIARs research and

development objectives. Te series is distributed internationally,with an emphasis on developing countries.

Australian Centre or International Agricultural Research 2008

GPO Box 1571

Canberra AC 2601

Australia

Internet: http://www.aciar.gov.au

Email: [email protected]

Burgess L.W., Knight .E., esoriero L. and Phan H.. 2008. Diagnostic

manual or plant diseases in Vietnam. ACIAR Monograph No. 129,

210 pp. ACIAR: Canberra.

ISBN 978 1 921434 18 1 (print)

ISBN 978 1 921434 19 8 (online)

echnical editing by Biotext Pty Ltd

Design by Clarus Design Pty LtdPrinting by Goanna Print Pty Ltd
http://www.aciar.gov.au/http://www.aciar.gov.au/


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Foreword 3

Foreword

Plant diseases continue to cause significant crop losses in Vietnam and otherregions o tropical South-East Asia. Te recent epidemic o rice grassy stunt

virus and rice ragged stunt virus in southern Vietnam highlighted the significant

socioeconomic effects o crop diseases at a national level.

Outbreaks o disease o valuable cash crops can also have a major impact on

small armers in localised areas where there are ew suitable alternative cropsan

example being ginger wilt complex in Quang Nam province.

Te accurate diagnosis o the cause o a disease is essential to the success o

control measures. However, many diseases produce similar symptoms, making

diagnosis in the field difficult or impossible. Hence, diagnostic laboratories are anessential component o a plant protection network. Staff assigned to diagnostic

work require intensive training at the undergraduate and graduate level in both

field and laboratory skills, and in the basic concepts o plant disease and integrated

disease management.

Accurate diagnosis o diseases is also essential to the development o a scientifically

sound national database on plant diseases. A database on diseases in Vietnam will

be a critical part o successul plant quarantine operations. Furthermore, a national

database is a critical element o the biosecurity measures that relate to trade in

agricultural products, especially or members o the World rade Organization.

Tis manual is designed to help plant pathologists develop basic skills in the

diagnosis o the cause o diseases, ocusing on ungal diseases o the roots and

stems. Tese diseases are insidious, and cause significant socioeconomic losses

in Vietnam.


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Diagnostic manual for plant diseases in Vietnam4

Te content o this manual is based on the experience o the authors and many

colleagues in Australia and Vietnam in training programs associated with various

projects unded by the Australian Centre or International Agricultural Research

(ACIAR), AusAID Capacity Building or Agriculture and Rural Development, and

Academy o echnological Sciences and Engineering Craword Fund.

Te manual complements other publications produced by ACIAR and various

colleagues in Vietnam.

Peter CoreChie Executive Officer

Australian Centre or International Agricultural Research


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Contents 5

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.1 Reerences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2 General plant health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.1 Weeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.2 Pests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.3 Pesticides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.4 Nutrition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.5 Soil conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.6 Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.7 Crop history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3 Te diagnostic process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.1 Case studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4 Symptoms of disease. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.1 Common symptoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.2 Diseases o oliage, flowers or ruit . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.2.1 Spore production on diseased oliage . . . . . . . . . . . . . . . . . . 46

4.2.2 Foliar ungal and ungal-like pathogens difficult or

impossible to grow in culture . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.2.3 Pathogens that produce sclerotia on inected tissue . . . . . . 48

Contents


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4.3 Diseases o roots, crown and stem. . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.4 Reerences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5 In the field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.1 Field equipment or diagnostic studies. . . . . . . . . . . . . . . . . . . . . . . . 545.2 Conducting a field survey. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

6 In the laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.1 Laboratory examination o the samples . . . . . . . . . . . . . . . . . . . . . . . 59

6.1.1 Wilting and stunting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

6.1.2 Lea diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

6.2 Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

6.2.1 Using a dissecting microscope . . . . . . . . . . . . . . . . . . . . . . . . 61

6.2.2 Using a compound microscope . . . . . . . . . . . . . . . . . . . . . . . 62

6.2.3 Preparing slides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

6.3 Isolating ungal pathogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

6.3.1 Isolation rom leaves and stems . . . . . . . . . . . . . . . . . . . . . . . 66

6.3.2 Isolation rom small, thin roots . . . . . . . . . . . . . . . . . . . . . . . 68

6.3.3 Isolation rom woody roots and stems . . . . . . . . . . . . . . . . . 69

6.3.4 Soil baiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

6.3.5 Soil dilution plate method . . . . . . . . . . . . . . . . . . . . . . . . . . . 716.4 Subculturing rom isolation plates . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

6.5 Purification o cultures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

6.5.1 Single sporing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

6.5.2 Hyphal tip transer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

6.6 Recognising pure cultures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

6.7 Identification o ungal pathogens. . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

6.8 Reerences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

7 Fungal taxonomy and plant pathogens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

7.1 Key eatures o ungi and ungal-like organisms . . . . . . . . . . . . . . . . 83

7.2 Classification o plant pathogenic ungi . . . . . . . . . . . . . . . . . . . . . . . 84

7.3 Reerences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

8 Pathogenicity testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

8.1 echniques o pathogenicity testing . . . . . . . . . . . . . . . . . . . . . . . . . . 89

8.1.1 Stem and oliar inection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90


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Contents 7

8.1.2 Soil inoculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

8.2 Preparation o inoculum or pathogenicity testing. . . . . . . . . . . . . . 92

8.2.1 Spore suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

8.2.2 Millet seed/rice hull medium (50:50 by volume) . . . . . . . . 92

9 Integrated disease management. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

9.1 Crop rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

9.2 Crop management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

9.2.1 Good drainage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

9.2.2 Flooding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

9.3 Pathogen-ree transplants, seed, and other planting material. . . . 100

9.4 Quarantine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

9.5 Resistant or tolerant cultivars. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

9.6 Grafing to resistant rootstock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

9.7 Fungicides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

9.8 Hygiene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

9.9 Reerences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

10 Root and stem rot diseases caused by pathogens that survive in soil. . 105

10.1 Sclerotinia sclerotiorum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

10.2 Sclerotium rolfsii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

10.3 Rhizoctonia species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

10.4 Phytophthoraand Pythium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

10.4.1 Asexual reproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

10.4.2 Sexual reproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

10.4.3 Identiying and differentiating Phytophthora

and Pythium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

10.4.4 Oomycete disease cyclePhytophthoraand Pythium . . . 119

10.4.5 Pythiumspecies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

10.4.6 Phytophthoraspecies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

10.5 Fusariumspecies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

10.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

10.5.2 Fusariumpathogens in Vietnam . . . . . . . . . . . . . . . . . . . . . 126

10.5.3 Fusarium wilt isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

10.5.4 Fusarium oxysporumand Fusarium solanikey

morphological eatures or identification . . . . . . . . . . . . . . 132


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10.6 Verticillium albo-atrumand V. dahliaeexotic ungal

wilt pathogens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

10.7 Plant parasitic nematodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

10.7.1 Nematode extraction rom soil and small roots. . . . . . . . . 139

10.8 Diseases caused by bacterial pathogens . . . . . . . . . . . . . . . . . . . . . . 142

10.8.1 Bacterial wilt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

10.8.2 Isolation o bacterial plant pathogens . . . . . . . . . . . . . . . . . 144

10.9 Diseases caused by plant viruses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

10.10 Reerences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

11 Common diseases of some economically important crops . . . . . . . . . . 151

11.1 Common diseases o chilli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

11.2 Common diseases o tomato . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

11.3 Common diseases o peanut. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

11.4 Common ungal diseases o onions . . . . . . . . . . . . . . . . . . . . . . . . . 158

11.5 Common ungal diseases o maize . . . . . . . . . . . . . . . . . . . . . . . . . . 160

12 Fungi, humans and animals: health issues. . . . . . . . . . . . . . . . . . . . . . . . . 162

12.1 Key mycotoxigenic ungi in Vietnam . . . . . . . . . . . . . . . . . . . . . . . . 164

12.2 MycotoxigenicAspergillusspecies . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

12.2.1 Aspergillus flavus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

12.2.2 Aspergillus niger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

12.2.3 Aspergillus ochraceus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

12.3 Mycotoxigenic Fusariumspecies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

12.3.1 Fusarium verticillioides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

12.3.2 Fusarium graminearum . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

13 Te diagnostic laboratory and greenhouse . . . . . . . . . . . . . . . . . . . . . . . . 171

13.1 Te diagnostic laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17113.1.1 Location o the laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . 171

13.1.2 Preparation room . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

13.1.3 Clean room . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

13.2 Laboratory layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

13.3 Laboratory equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

13.3.1 Equipment or the clean room . . . . . . . . . . . . . . . . . . . . . . . 174

13.3.2 Equipment or the preparation room . . . . . . . . . . . . . . . . . 176


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Contents 9

13.4 Greenhouse or plant disease studies . . . . . . . . . . . . . . . . . . . . . . . . 177

13.4.1 Preparation area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

13.4.2 Potting mixture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

13.4.3 Greenhouse hygiene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

13.4.4 Plant management and nutrition . . . . . . . . . . . . . . . . . . . . . 181

Appendix 1 Making a flat transer needle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

Appendix 2 Health and saety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

Appendix 3 Acronyms and abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

Bookshelf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

ables

able 8.1 echniques o plant pathogenicity testing . . . . . . . . . . . . . . . . . . . . 89

able 10.1 Features o common crop pathogens that survive in soilin Vietnam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

able 10.2 Characteristics o Sclerotinia sclerotiorum . . . . . . . . . . . . . . . . . . . 109

able 10.3 Characteristics o Sclerotium rolfsii. . . . . . . . . . . . . . . . . . . . . . . . . 112

able 10.4 Characteristics o Rhizoctonia species . . . . . . . . . . . . . . . . . . . . . . 115

able 10.5 Characteristics o Pythiumspecies . . . . . . . . . . . . . . . . . . . . . . . . . 122

able 10.6 Characteristics o Phytophthoraspecies . . . . . . . . . . . . . . . . . . . . . 123

able 10.7 Fusarium oxysporum(vascular wilts) . . . . . . . . . . . . . . . . . . . . . . . 128

able 10.8 Characteristics o Fusarium wilts . . . . . . . . . . . . . . . . . . . . . . . . . . 130

able 10.9 Hints or differentiating between Fusarium oxysporum andFusarium solani . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

able 10.10 Characteristics o Verticillium albo-atrum and V. dahliae . . . . . . 136

able 11.1 Common diseases o chilli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

able 11.2 Common diseases o tomato . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

able 11.3 Common diseases o peanut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

able 11.4 Common ungal diseases o onions . . . . . . . . . . . . . . . . . . . . . . . . 158

able 11.5 Common ungal diseases o maize . . . . . . . . . . . . . . . . . . . . . . . . . 160

able 12.1 Key mycotoxigenic ungi in Vietnam . . . . . . . . . . . . . . . . . . . . . . . 164

able A3.1 Commonly used antibiotics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188


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able A3.2 Required times or sterilisation using moist and dry heatover a range o temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

able A3.3 Suggested times or sterilisation o different volumeso liquid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

Figures

Figure 2.1 Key actors in maintaining plant health . . . . . . . . . . . . . . . . . . . . . . 25

Figure 2.2 Invertebrate pest damage: (a) white grub (inset) damage tomaize roots, (b) wilting maize plant affected by white grub,(c) aphid inestation, (d) typical bronzing o lea caused bymites eeding on the underside o the lea (inset) . . . . . . . . . . . . . . 27

Figure 2.3 Nutrient deficiencies causing disease-like symptoms: (a)

blossom end rot due to calcium deficiency o tomato, (b)potassium deficiency o crucier, (c) boron deficiencyo broccoli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Figure 2.4 Lateral root growth caused by a hard layer in the soil profile(plough pan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figure 2.5 Ageratum conyzoides: (a) blue flowered variety, (b) whiteflowered variety, (c)Ageratum conyzoidesroot affected by

Meloidogynespp. (nematodes) causing root knot symptoms,(d) wiltingAgeratum conyzoidescaused by Ralstoniasolanacearum(a bacterium), (e) aster yellows-like symptoms

onAgeratum conyzoides(inset: the aster Callistephuschinensisshowing aster yellows symptoms) . . . . . . . . . . . . . . . . . . 31

Figure 3.1 A flow diagram o the diagnostic process . . . . . . . . . . . . . . . . . . . . 33

Figure 3.2 Steps involved in the isolation, purification, identificationand pathogenicity testing o the pineapple heart rotpathogen, Phytophthora nicotianae. . . . . . . . . . . . . . . . . . . . . . . . . . 34

Figure 3.3 Discussions with armers on ginger wilt . . . . . . . . . . . . . . . . . . . . . 36

Figure 3.4 A ginger wilt survey in Quang Nam in January 2007: (a)ginger with symptoms o quick wilt, (b) ginger plants with

yellowing, a symptom o slow wilt, (c) adjacent crops, onecrop with quick wilt, the other symptomless, (d) and (e)plants being removed careully using a machete, keepingthe root systems intact, () sample bag labelled with sitenumber, armers name and date . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Figure 3.5 Preparation and examination o plants with ginger wilt orthe laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Figure 3.6 Isolation procedure or potential plant pathogenicorganisms rom ginger rhizome . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39


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Contents 11

Figure 3.7 Isolation o Fusarium oxysporumrom some segments oginger rhizomes on selective isolation medium (peptonepentachloronitrobenzene agar) or Fusarium . . . . . . . . . . . . . . . . . 40

Figure 3.8 Bioassay procedure or isolating Ralstonia solanacearumrom diseased ginger rhizome: (a) chilli and tomato cuttings

in control (lef) and wilted cuttings in water extract romrhizome segments (right), (b) wilted chilli cutting showing

vascular browning, (c) isolation o R. solanacearumromchilli cutting, (d) and (e) pathogenicity test in bitter melono bacterium isolated in the bioassay . . . . . . . . . . . . . . . . . . . . . . . . 41

Figure 4.1 Formation o conidia on oliage by various ungal pathogens . . . 46

Figure 4.2 Fungal and ungal-like pathogens o the oliage: (a) powderymildew on a cucurbit, (b) white blister on Brassica sp., (c)Cercospora lea spot and rust on peanut, (d) downy mildewon cabbage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Figure 4.3 Sclerotial ormation by (a) Rhizoctonia solani, (b) Sclerotiumrolfsii and (c) Sclerotinia sclerotiorum . . . . . . . . . . . . . . . . . . . . . . . . 48

Figure 4.4 Diseases o the crown, roots and stem: (a) club rooto cruciers, (b) wilting o cruciers (healthy [lef] anddiseased [right]) caused by club root (Plasmodiophorabrassicae), (c) Fusarium wilt o asters (note the productiono sporodochia on the stem), (d) spear point caused byRhizoctonia sp., (e) Phytophthora root rot o chilli, ()Phytophthora root rot o chilli causing severe wilt, (g)Pythium root and pod rot o peanuts, (h) perithecia oGibberella zeae causing stalk rot o maize . . . . . . . . . . . . . . . . . . . . 50

Figure 5.1 alking with armers in the field . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Figure 5.2 Suggested equipment or use in the field . . . . . . . . . . . . . . . . . . . . . 55

Figure 6.1 Examination o colonies under a dissecting microscope . . . . . . . . 62

Figure 6.2 Examination o ungal spores under a compound microscope . . 63

Figure 6.3 Components o a compound microscope . . . . . . . . . . . . . . . . . . . . 64

Figure 6.4 echnique or isolating plant pathogens rom woody tissues:(a) cutting off lateral roots, (b) washing the sample, (c)

removing the lower section o the stem at the soil line, (d)spraying the sample with 70% alcohol, (e) allowing thealcohol to evaporate, () cutting segments o stem tissue . . . . . . . 70

Figure 6.5 Baiting soil or Phytophthora using flower petals and leaves . . . . 71

Figure 6.6 Diagram o dilution series used or dilution plating . . . . . . . . . . . 72

Figure 6.7 Dilution plate containing Fusariumspp. on peptone PCNBagar (ideally the number o colonies should be between 10and 30) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73


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Figure 6.8 Diagram o a root isolation plate showing (inset) multipleungi growing rom the same root section . . . . . . . . . . . . . . . . . . . 75

Figure 6.9 Common contaminants ound on culture plates: (a)Penicilliumsp. (airborne contamination), (b) Cladosporiumsp. (in pure culture), (c) Trichodermasp. (developing rom a

diseased root segment) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Figure 6.10 Steps in the single sporing process . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Figure 6.11 Te single sporing procedure, showing correct selection oan individual spore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Figure 6.12 Hyphal tip transer, example o tip removal rom a slopedwater agar plate o Rhizoctoniasp. . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Figure 6.13 Colonies o common ungal pathogens on potato dextrose agar . . 80

Figure 8.1 Stem inoculation technique or pathogenicity testing:(a) piercing the lower stem, (b) transerring the pure cultureto the wound site, (c) wrapping the wound site in plastic,(d) mycelium developing on soil surace rom diseasedstem, (e) an inoculated plant (lef) and an uninoculatedcontrol (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Figure 8.2 Different methods or inoculating soil to produce diseasein the glasshouse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

Figure 8.3 An inoculum flask . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Figure 8.4 Preparation o millet seed/rice hull medium in flasks . . . . . . . . . . 94

Figure 8.5 Preparation o millet seed/rice hull medium orpathogenicity testing: (a) millet seed and rice hulls thathave been soaked in distilled water or 24 hours, (b)thorough mixing o inoculum medium components,(c and d) transer o medium to conical flasks using amakeshif unnel, (e) flask plugged with cotton woolwrapped in muslin, () flask covered with aluminium oilready or autoclaving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Figure 9.1 Diagrammatic summary o appropriate control measures orcommon groups o diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

Figure 9.2 Chipping weeds rom a drainage urrow to improvedrainage in a black pepper crop affected by Phytophthoraroot rot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

Figure 9.3 Measures or preventing transer o contaminated soilon ootwear: disposable synthetic overshoes (lef) anddisinecting shoes afer inspecting a crop affected by apathogen which survives in soil (right) . . . . . . . . . . . . . . . . . . . . . 103

Figure 10.1 Sclerotinia sclerotiorumdisease cycle . . . . . . . . . . . . . . . . . . . . . . . 110


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Contents 13

Figure 10.2 Sclerotinia sclerotiorumaffecting: (a) long beans, (b)lettuce, (c) cabbage (wet rot), (d) cabbage; (e) apotheciarom sclerotia in soybean residue; () apothecium next toshort bean; (g) long bean (sclerotia produced on bean); (h)germinated sclerotium producing apothecia . . . . . . . . . . . . . . . . . 111

Figure 10.3 Sclerotium rolfsii: (a) in pathogenicity test (note hyphalrunners), (b) on decaying watermelon, (c) basal rot with theormation o brown spherical sclerotia . . . . . . . . . . . . . . . . . . . . . 113

Figure 10.4 Examples o Rhizoctonia diseases: (a) spear pointsymptoms on diseased roots, (b) Rhizoctonia sheath blighton rice, (c) sclerotia o Rhizoctoniaon diseased cabbage,(d) Rhizoctonia disease on maize hull . . . . . . . . . . . . . . . . . . . . . . 114

Figure 10.5 Sporangium o Pythiumillustrating zoospore releasethrough a vesicle (lef), and zoospore release directly romPhytophthorasporangium (right) . . . . . . . . . . . . . . . . . . . . . . . . . . 116

Figure 10.6 Diagram illustrating sexual reproduction in Pythium,involving contact between an antheridium and anoogonium to orm an oospore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

Figure 10.7 Pythiumsp. (lef) and Phytophthorasp. (right), showingthe characteristic aster growth and aerial mycelium on thePythium plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

Figure 10.8 Simplified disease cycle o an oomycete plant pathogen . . . . . . . 120

Figure 10.9 (a) Oogonium o Pythium spinosum showing attached lobeo an antheridium, (b) mature oospore o P. mamillatum, (c)

sporangium o P. mamillatum showing discharge tube andvesicle containing developing zoospores, (d) sporangium oP. irregulare showing mature zoospores in thin walled vesicleprior to release, (e) digitate sporangia in P. myriotilum, ()distinct sporangiophore and sporangia o Phytophthorasp. . . . . 121

Figure 10.10 Pythium diseases on peanuts: (a) Pythium rootlet rot andstem rot o peanut seedling grown under very wet conditions,(b) comparison o two mature peanut plants, healthy plant(lef), stunted plant with severe Pythium root rot (right), (c)severe Pythium pod and tap root rot o peanuts . . . . . . . . . . . . . . 123

Figure 10.11 Diseases caused by Phytophthora palmivoraon durian: (a)tree yellowing, (b) canker on trunk, (c) ruit rot. Diseasescaused by P. palmivoraon cocoa: (d) seedling blight, (e)black pod symptoms. Root rot (quick wilt) o black peppercaused by P. capsici: () lea drop, (g) wilting. Disease causedby P. infestans: (h) late blight o potato. . . . . . . . . . . . . . . . . . . . . . . 125


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Figure 10.12 Diseases caused by Fusariumspecies: (a) Fusariumoxysporum . sp.pisi causing wilt on snowpeas, (b) F.oxysporum . sp. zingiberi sporodochia on ginger rhizome,(c) stem browning caused by F. oxysporum, (d) Perithecia oF. graminearum on maize stalk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

Figure 10.13 Fusarium wilt o banana caused by F. oxysporum. sp. cubense:(a) severe wilt symptoms, (b) stem-splitting symptom,(c) vascular browning. Fusarium wilt o asters caused by F.oxysporum. sp. callistephi: (d) severe wilt causing death,(e) wilted stem with abundant white sporodochia on thesurace. Fusarium wilt o snowpeas caused by F. oxysporum. sp.pisi: () field symptoms o wilt (note patches o deadplants), (g) vascular browning in wilted stem. . . . . . . . . . . . . . . . . . . .129

Figure 10.14 Four-day-old cultures o Fusarium oxysporum (lef) and F.solani (right), in 60 mm Petri dishes on potato dextrose agar . . . . 132

Figure 10.15 Differentiating between Fusarium oxysporum (lef)and F. solani(right): (a) and (b) macroconidia, (c) and(d) microconidia and some macroconidia, (e) and ()microconidia in alse heads on phialides (note the shortphialide in F. oxysporum and the long phialide typical o F.solani) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

Figure 10.16 Chlamydospores o Fusariumsolaniin culture on carnationlea agar (CLA) (F. oxysporum chlamydospores look the same) . . 134

Figure 10.17 Verticillium dahliae:(a) culture on potato dextrose agar(cultures grow slowly), (b) microsclerotia on old cottonstem, (c) hyphae in inected xylem vessels, (d) wiltedpistachio tree affected by V. dahliae, (e) and () wilted leaveso eggplant inected by V. dahliae . . . . . . . . . . . . . . . . . . . . . . . . . . 135

Figure 10.18 Nematodes: (a) plant parasitic with piercing stylet (mouthspear), (b) non-plant parasitic with no stylet . . . . . . . . . . . . . . . . 137

Figure 10.19 Damage to a plant root system caused by: (a) root knotnematode, (b) root lesion nematode, both diseases resultingin stunting and yellowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

Figure 10.20 Root knot nematode symptoms: (a) swollen root (knot)

symptoms, (b) emale nematodes ound withinroot knots (galls) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

Figure 10.21 Schematic illustration o common procedures or extractingnematodes rom roots or soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

Figure 10.22 Baerman unnel apparatus or nematode extraction . . . . . . . . . . 140

Figure 10.23 Whitehead tray apparatus or nematode extraction . . . . . . . . . . . 141


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Contents 15

Figure 10.24 Diseases caused by bacterial pathogens: (ac) Bacterialwilt o bitter melon, (d) bacterial lea blight, (e) Ralstoniasolanacearum causing quick wilt o ginger, ( ) bacterialsof rot o chinese cabbage caused by Erwinia aroideae, (g)Pseudomonas syringae on cucurbit lea . . . . . . . . . . . . . . . . . . . . . 143

Figure 10.25 echnique or isolating Ralstonia solanacearum rom aninected stem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

Figure 10.26 Diagram o bacterial streak plating, showing order ostreaking and flaming between each step . . . . . . . . . . . . . . . . . . . 146

Figure 10.27 Bacterial streak plate afer 2 days growth at 25 C . . . . . . . . . . . . 146

Figure 10.28 Maceration o roots or rhizome or use in bacterial streakplating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

Figure 10.29 Virus diseases: (a) tomato spotted wilt virus on chilli, (b)beet pseudo-yellows in cucumbers, (c) yellow lea curl virus

in tomato, (d) turnip mosiac virus on leay brassica (right),healthy plant (lef), (e) virus on cucumber, () crumplecaused by a virus in hollyhock (Althaea rosea) . . . . . . . . . . . . . . . 149

Figure 11.1 Diseases o chilli: (a) healthy chilli plant (lef) andwilted (right), which can be caused by several diseases,(b) stem browning, a typical symptom o bacterial wiltcaused by Ralstonia solanacearum, (c) basal rot causedby Sclerotium rolfsii, (d) Phytophthora root rot caused byPhytophthora capsici, (e) chilli affected by tomato spottedwilt virus, () chilli ruit affected by anthracnose, caused by

Colletotrichumsp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153Figure 11.2 omato diseases: (a) tomato showing symptoms o yellow

lea curl virus in new growth, (b) tomato ruit showingbacterial speck lesions caused by Pseudomonas syringae, (c)root knot nematode caused byMeloidogynesp., (d) velvetlea spot caused by Cladosporium fulvum, (e) target spotcaused byAlternaria solani . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

Figure 11.3 Peanut diseases: (a) peanut rust caused by Pucciniaarachidis, (b) Cercospora lea spot (Cercospora arachidicola)and rust, (c) peanuts affected by root rot showing yellowing

and stunting symptoms, (d) eeder root rot and pod rotcaused by Pythiumsp., (e) necrotic peanut cotyledonshowing abundant sporulation o the pathogenAspergillusniger, () Pythium root rot on peanut seedling, (g) healthypeanut plant (lef) and stunted root rot affected plant (right) . . . 157

Figure 11.4 Diseases o onion: (a) Stemphylium lea spot, (b) downymildew caused by Peronosporasp., (c) symptoms o pinkroot rot caused by Phoma terrestris. . . . . . . . . . . . . . . . . . . . . . . . . 159


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Figure 11.5 Diseases o maize: (a) common (boil) smut on maize cobcaused by Ustilago maydis, (b) banded sheath blight causedby Rhizoctonia solani, (c) white mycelial growth on inectedcob caused by Fusarium verticillioides . . . . . . . . . . . . . . . . . . . . . . 161

Figure 12.1 Corn kernels inected with Fusarium graminearum and a

diagrammatic illustration o the diffusion o mycotoxinsrom ungal hyphae into kernel tissue . . . . . . . . . . . . . . . . . . . . . . 163

Figure 12.2 Aspergillus flavus sporulating on inected peanut seeds onisolation medium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

Figure 12.3 Aspergillus flavus, three colonies on Czapek yeast autolysateagar (lef), conidia produced abundantly on heads onconidiophore (centre), conidia (right) . . . . . . . . . . . . . . . . . . . . . . 165

Figure 12.4 Aspergillus niger, three colonies on Czapek yeast autolysateagar (lef), conidia produced abundantly on heads on long

conidiophore (centre), conidia (right) . . . . . . . . . . . . . . . . . . . . . . 166Figure 12.5 Aspergillus ochraceus, three colonies on Czapek yeast

autolysate agar (lef), conidia produced abundantly on headson conidiophore (centre), conidia (right) . . . . . . . . . . . . . . . . . . . 168

Figure 12.6 Fusarium cob rot caused by Fusarium verticillioides(lef),and pure cultures on potato dextrose agar (right) . . . . . . . . . . . . 169

Figure 12.7 Fusarium cob rot caused by F. graminearum (lef), and purecultures on potato dextrose agar (right) . . . . . . . . . . . . . . . . . . . . 170

Figure 13.1 ypical arrangement o equipment in a diagnostic

laboratory (laboratory in Nghe An PPSD): (a) and (b) twoviews o clean room, (c) and (d) two views o preparation room. 172

Figure 13.2 Floor plan o diagnostic laboratory, indicating suggestedlayout o equipment and benches . . . . . . . . . . . . . . . . . . . . . . . . . . 173

Figure 13.3 Essential instruments or isolation, subculturing,purification and identification o ungal and bacterial plantpathogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

Figure 13.4 Diagrammatic illustration o a suggested design or agreenhouse suitable or pathogenicity testing and otherexperimental work with plant pathogens . . . . . . . . . . . . . . . . . . . 178

Figure 13.5 Plant pathology greenhouse at Quang Nam PPSD: (a)general view o greenhouse showing insect-proo screens,(b) shade cloth sun screen and flat polycarbonate roofingwith wind driven ventilator units . . . . . . . . . . . . . . . . . . . . . . . . . . 178

Figure 13.6 Preparation o commercial ertiliser or greenhouse use . . . . . . . 181

Figure A1.1 A step-by-step guide to making a flat transer needle . . . . . . . . . 184


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Preface 17

Preface

Tis manual is designed to provide a basic introduction to diagnosing ungaldiseases o crops in Vietnam. Te content is based primarily on experience gained

during two Australian Centre or International Agricultural Research (ACIAR)

projects in northern and central Vietnam.1It takes into account other manuals

published or in press.

Four low-cost diagnostic laboratories were established in the central provinces o

Vietnam during the current ACIAR project.2Tese laboratories are located at the

Plant Protection Sub-departments (PPSDs) in the provinces o Quang Nam, Tua

Tien Hue and Nghe An, and at Hue University o Agriculture and Forestry. Tey

have the equipment needed to isolate and identiy common genera o ungal andbacterial pathogens that persist in soil, and common oliar ungal and bacterial

pathogens. Tey also have acilities or pathogenicity testing newly recognised

pathogens in Vietnam. Te staff in these laboratories have had basic laboratory

training through workshops at Hanoi Agricultural University and in the Quang

Nam PPSD, where a teaching laboratory has been established. Staff have also

been involved in regular field surveys o disease and have diagnosed diseases

collected by armers.

Each laboratory has a small library and a computer or accessing web-based

inormation, which are essential resources or diagnostic plant pathologists.

Small greenhouses have been established in each province, both or pathogenicity

testing and or the evaluation o ungicides and soil amendments or disease

suppression. Te design and operation o greenhouses or experimental work

1 CS2/1994/965 Diagnosis and control o plant diseases in northern Vietnam(19982001) and CP/2002/115 Diseases o crops in the central provinces o Vietnam:diagnosis, extension and control (20052008).

2 CP/2002/115 Diseases o crops in the central provinces o Vietnam: diagnosis,

extension and control (20052008).


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and the production o pathogen-ree planting material have been the subject o

training activities in Vietnam and Australia. Dr Ngo Vinh Vien, Director o the

Plant Protection Research Institute, has recommended that all staff receive training

and proessional development in these areas. Te team rom the current ACIAR

project visited nurseries in Dalat as part o the activities.

Te integration o English teaching with training in plant pathology has been a

critical aspect o staff development in the current project. Many o our colleagues

in the current project can now seek advice by email (with the aid o digital images)

on new disease problems.

Colleagues rom Vietnam and Australia have contributed images and text or this

manualthese contributions are acknowledged individually.

Diagnostic work provides a basis or designing field trails on disease control, and

developing control measures or extension purposes. Te accurate diagnosis o a

wide range o diseases and the identification o pathogens to species level dependson broad experience over many years. We hope this manual will assist our early-

career Vietnamese colleagues with their first field and laboratory studies on plant

disease diagnosis.


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Acknowledgments 19

Acknowledgments

Te authors sincerely thank Dr .K. Lim or suggesting the concept o a diagnosticmanual or plant disease in Vietnam and the Australian Centre or International

Agricultural Research (ACIAR) or financial support or the initiative. Te senior

author also acknowledges the invaluable support and encouragement provided

by ACIAR or diagnostic, research and capacity building activities in Vietnam or

over 12 years.

Te authors also sincerely thank successive rectors, our colleagues in plant

pathology and staff o the international office at Hanoi Agricultural University or

their support since 1992. Similarly the authors are indebted to staff o the Plant

Protection Research Institute or guidance and support, especially the Director,Dr Ngo Vinh Vien.

Te assistance o staff at Te University o Sydney, Royal Botanic Gardens and

Domain rust, and the New South Wales Department o Primary Industries with

teaching and research activities in Vietnam is also grateully acknowledged.

We are also indebted to the generosity, hospitality and support provided by

colleagues in the Plant Protection Sub-departments in Quang Nam, Tua Tien

Hue, Nghe An, Quang ri and Lam Dong, the Hue University o Agriculture and

Forestry, Centre or Plant Protection Region 4, and the collaborating armers in

these and other provinces. Our current project has been especially rewarding toall concerned.

Te ollowing colleagues in Vietnam and Australia have contributed to this manual

through images o plant diseases, associated comments and editorial advice.

However, the authors bear final responsibility or the content and illustrations.

AustraliaBarry Blaney, Julian Burgess, Eric Cother, Norma Cother, Nerida

Donovan, Phillip Davies, Mark Fegan, Col Fuller, David Guest, Ailsa Hocking,

Greg Johnson, Edward Liew, Suneetha Medis, Dorothy Noble, ony Pattison, Brett

Summerell and Ameera Yousiph.


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VietnamDang Luu Hoa, Dau Ti Vinh, Ho Dac To, Hoa Pham Ti, Hoang Ti

Minh Huong, Huynh Ti Minh Loan, Luong Minh am, Ngo Vinh Vien, Nguyen

Kim Van, Nguyen Ti Nguyet, Nguyen ran Ha, Nguyen Vinh ruong, Pham

Tanh Long, ran Kim Loang, ran Ti Nga and ran Ut.


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Section 1. Introduction 21

1 Introduction

Plant diseases cause serious income losses or many armers in Vietnam, byreducing crop yields and the quality o plant products. Te costs o control

measures such as ungicide can urther reduce a armers income.

Some diseases are caused by ungi that produce mycotoxins, such as aflatoxin,

which can contaminate ood products (e.g. maize and peanuts). Contamination by

mycotoxins can have adverse effects on human and animal health.

Occasionally diseases spread in devastating epidemics through major crops.

Such epidemics can have serious economic and social impacts on an entire

region or country. In 2006, or example, rice grassy stunt virus and rice ragged

stunt virus caused major losses to rice crops in the Mekong delta, affecting onemillion hectares across 22 provinces. Tis epidemic directly affected millions o

arming amilies.

Te Vietnamese Ministry o Agriculture and Rural Development has long

recognised the importance o plant disease in agriculture. It has an extensive

network o research centres and a network o plant protection staff at provincial

and district levels across Vietnam. Tese resources provide diagnostic support

and inormation on control measures or disease. Tis service is a major

challenge, given the diversity o crops and diseases, and the range o climatic

regions in Vietnam.

Successul control o disease depends on accurate identification o the pathogen

and the disease. Some common diseases can be diagnosed accurately in the field

by visual symptoms. For example, boil smut o maize, Sclerotinia stem rot, root

knot nematode, club root and peanut rust all have symptoms that are distinct and

obvious to the unaided eye. However, there are many diseases that have similar

non-specific symptoms (e.g. wilting, stunting, lea yellowing). Some o these can be

identified accurately in the laboratory by examining samples using a microscope.

Many ungal pathogens and parasitic nematodes can be identified in this way.


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However, some ungal and bacterial pathogens can only be identified by isolation

into pure culture. Once isolated, pure cultures can be identified using a microscope

and, i necessary, identification can be confirmed using molecular and other more

costly techniques. Most o the ungal pathogens that cause root and stem rots can

only be identified by isolation o the pathogen into pure culture. Most plant virus

diseases can only be identified accurately in a virology laboratory. Diagnostic kitsare available that enable ast and accurate diagnosis o some viral and bacterial

diseases in the field; however, these kits are relatively expensive.

Tis manual was designed to assist in the establishment and operation o small

laboratories or diagnosing common ungal diseases at a provincial level in

Vietnam. It is particularly concerned with the ungal root and stem rot diseases

that cause significant losses to many Vietnamese armers every year. Many o these

diseases are yet to be properly identified.

In this manual the terms ungi and ungal are generally used in the traditional

sense as is common practice in Vietnam at present. Tus these terms are used to

reer to the true ungi as well as ungal-like filamentous species in the Oomycetes,

and the endoparasitic slime moulds. However the importance o understanding

the modern approach to the taxonomic treatment o these organisms is

emphasised in the text. An outline o one o the modern taxonomic systems o

classification o these various organisms is included in the manual.

Fungal diseases are useul or diagnostic training. Te Australian Centre or

International Agricultural Research (ACIAR) has supported the establishment o

our diagnostic laboratories at the provincial level, including considerable training

in the field and laboratory or staff. Tere has been encouraging progress, althoughit takes many years o experience and practice to become amiliar with diagnosing

diseases caused by all plant pathogensungi, bacteria, viruses, mollicutes

and nematodes.

Te staff in a diagnostic laboratory must keep accurate records o diagnoses

in an accession book and every sample should be recorded. Inormation on

the occurrence o diseases can then be entered into a national database on

diseases, which is a key element o biosecurity processes supporting the export

o agricultural produce. Te national database will be very important now that

Vietnam has joined the World rade Organization. A national database o plantdiseases and a network o diagnostic laboratories will help Vietnam to meet the

challenges o establishing and maintaining biosecurity. Ideally, laboratories should

maintain a reerence culture collection and a herbarium o disease specimens (see

Shivas and Beasley 2005).

Disease is only one actor affecting plant health and, consequently, crop yields.

It is important or the diagnostic plant pathologist to be aware o all the actors

that affect plant health and interact with diseasepests, weeds, pesticide use, soil

characteristics, local climate and other environmental actors.


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Section 1. Introduction 23

Te successul diagnosis and control o disease is acilitated by close collaboration

between plant protection staff and armers. Farmers can be very observant and can

provide important inormation to assist in diagnosis rom their own observations

and experience.

Tis manual is organised into the ollowing sections: general plant health and factors that can aect it

eld and laboratory procedures for diagnosing the causes of a disease

symptoms of plant disease

procedures and equipment for working in the eld

procedures and equipment for working in the laboratory

a brief introduction to fungal taxonomy

methods for pathogenicity testing

integrated disease management

diseases caused by fungal pathogens that live in soil

common diseases of some economically important crops

health implications of fungal pathogens

design, development and operation of diagnostic laboratories and greenhouses

appendixes on making a at transfer needle, maintaining health and safety

procedures, as well as recipes or media, sterilisation methods, and methods or

preservation o ungal cultures a suggested reference library for diagnostic laboratories.

1.1 References

Shivas R. and Beasley D. 2005. Management o plant pathogen collections.

Australian Government Department o Agriculture, Fisheries and Forestry. At:

.


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2 General plant health

Plant health is a determining actor in crop yield and consequently in the incomeo the armer. Tereore, it is very important to manage the health o the crop so

that profits are maximised.

increased

YIELD

greater

FARMER INCOME

improved

PLAN HEALH

Disease is only one o the actors that can affect the health o crop plants. Other

actors include pests, weeds, nutrition, pesticides, soil conditions and the

environment (Figure 2.1). All o these actors must be considered during thediagnostic process as each can affect the plant and cause symptoms similar to

those caused by disease. Each actor can also potentially affect the development o

disease in the plant.

Diagnostic plant pathologists should have an understanding o all o the actors

that affect plant health and disease. In the field, the pathologist should record

inormation on all o the relevant actors (see field sheet in Section 5), and discuss

the history o the field and crop management with the armer.

Vietnam has a wide range o agroclimatic regions. For example, the central and

northern provinces experience a cool to cold winter that avours temperate

pathogens. Te low temperatures inhibit growth o some crops making them more

susceptible to seedling and other diseases. Furthermore, the yearly weather cycle

includes very wet as well as dry periods. Such weather can also lead to crop stress

and avour some diseases, especially o the roots and stems caused by pathogens

that survive in soil. Indeed waterlogging and poor drainage are major actors

avouring these diseases in Vietnam. Tereore high raised beds and good drainage

are critical practices in integrated disease management. A diagnostic pathologist

must understand these effects.


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Section 2. General plant health 25

2.1 Weeds

Many pests and pathogens persist on weed hosts when the susceptible crop host isabsent. Tereore, effective weed control is an important control measure and a key

part o integrated disease management (IDM). In addition, weeds growing with

a crop will compete or water, nutrients and light, which will stress the crop and

increase disease severity.

Figure 2.1 Key factors in maintaining plant health


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2.2 Pests

Feeding by invertebrate pests can cause damage to the plant similar to diseasesymptoms (Figure 2.2). For example, aphids, lea hoppers, thrips, mites and

whiteflies can cause damage to the lea similar to the symptoms o some oliar

diseases. Tese pests also can act as vectors o viruses and bacteria. Stem borers

and root grubs affect water uptake and can cause wilting that is similar to wilting

caused by vascular wilt and root rot diseases.

2.3 Pesticides

Te application o pesticides can cause lea damage, such as lea burn and lea

spots. Tese symptoms can be conused with symptoms o lea blight and lea spots

caused by many ungal and bacterial pathogens. Herbicides may stress plants,

affecting their susceptibility to a pathogen.

2.4 Nutrition

Poor nutrition commonly causes stunting and poor root growth (Figure 2.3).

Tese symptoms are also caused by root rot pathogens. Other signs o mineral

deficiencies and toxicities can also be similar to the symptoms o some diseases.

For example, nitrogen deficiency causes lea yellowing, particularly o the lower

leaves. Lea yellowing is also a symptom o root disease, which can also disrupt the

uptake o nitrogen. Mineral deficiencies or toxicities can affect the susceptibility o

plants to some pathogens.


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Figure 2.2 Invertebrate pest damage: (a) white grub (inset) damage to maize roots, (b) wilting maize

plant affected by white grub, (c) aphid infestation, (d) typical bronzing of leaf caused by mites feeding

on the underside of the leaf (inset)


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2.5 Soil conditions

Waterlogging (poor drainage), poor soil structure, hard clay soils and plough pans

(hard layers in the soil profile) can interere with root growth. Stunting o the roots

decreases the uptake o water and nutrients, causing stress on the whole plant.

Stunting o the roots can also cause wilting and yellowing o the leaves, changes

which are similar to the symptoms o many plant diseases. A plough pan can causeroots to grow laterally (turn sideways) (Figure 2.4), reducing root unction and

growth; this stresses the plant, leading to avourable conditions or some pathogens.

Figure 2.3 Nutrient deficiencies causing disease-like symptoms: (a) blossom end rot due to calcium

deficiency of tomato, (b) potassium deficiency of crucifer, (c) boron deficiency of broccoli


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2.6 Environment

A variety o weather conditions can cause damage and stress to plants, and thus be

detrimental to plant health. Tese conditions, including extremes o temperature,

humidity and rain, as well as hail, flooding, drought and typhoons, lead to

increased disease incidence and severity. High temperatures, low humidity and

drought can cause severe wilting and plant death. Wet windy conditions acilitateinection and the spread o many ungal and bacterial lea pathogens. Wet soil

conditions avour Phytophthora and Pythium root rot diseases. Drought stress

acilitates some root diseases, and stem and stalk rot problems. Te combination o

root rot disease and dry soil can kill plants.

Tere is evidence that typhoons or gale-orce winds that severely shake trees

cause damage to the tree root systems. Such damage can acilitate higher levels

o inection by root rot pathogens and cause decline and death o the trees. For

example, typhoons or high winds are the suspected cause o tree decline in some

coffee and lychee trees in Vietnam.

Figure 2.4 Lateral root growth caused by a hard layer in the soil profile (plough pan)


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2.7 Crop history

An understanding o the history o the crop can help with the diagnosis o adisease. For example, the origin o the seed and whether it was treated with

ungicide can provide an indication o whether a seed-borne pathogen may be

affecting the crop. As discussed above it is important to understand the history o

weather conditions prior to a disease outbreak. Cool wet conditions avour many

root rot pathogens but the plant may tolerate some damage to the roots under

these conditions as transpiration rates are low. However, i the weather turns hot

and transpiration rates are high, the diseased plant can quickly wilt and die.

An earlier inestation o a virus vector in a crop could indicate that a virus carried

by the vector has inected the crop and is responsible or the symptoms observed.Knowledge o the previous crops and their diseases can also provide a guide to

potential diseases in the current crop. For example, some rotations will increase

the severity o particular diseases caused by soil-borne pathogens. For example,

successive crops in the amily Solanaceae are likely to increase bacterial wilt caused

by Ralstonia solanacearum.


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Weeds as alternative hosts forAgeratum conyzoides

Weeds can act as alternative hosts o many important crop pathogens.

Ageratum conyzoides is a common weed in Vietnam (Figure 2.5), growing within

crops, in allow areas between crops and alongside ootpaths. It is an alternative

host o several important pathogens and provides a source (reservoir) o inoculum

o these pathogens to inect new crops. I this weed is present, the armer can lose

the benefit o crop rotation or controlling pathogens in the soil.

Ageratum conyzoides is a host o Ralstonia solanacearum (which causes bacterial

wilt), root knot nematode and possibly aster yellows, which is a disease caused by a

phytoplasma transmitted by lea hopper vectors to susceptible crops such as asters,

potatoes, carrots and strawberries.

Controlling weeds acting as alternative hosts is extremely important.

Figure 2.5 Ageratum conyzoides: (a) blue flowered variety, (b) white flowered variety,

(c)Ageratum conyzoidesroot affected by Meloidogynespp. (nematodes) causing root knot

symptoms, (d) wiltingAgeratum conyzoidescaused by Ralstonia solanacearum(a bacterium),

(e) aster yellows-like symptoms onAgeratum conyzoides(inset: the aster Callistephus chinensis

showing aster yellows symptoms)


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3 Te diagnostic process

Te main activities involved in the diagnostic process are:

collecting diseased plants in the eld

examining collected plants in the laboratory

pathogenicity testing

disease diagnosis.

Tese activities are shown in Figure 3.1.

3.1 Case studies

In this section, two case studies are presented to provide an illustrated overview o

the diagnostic process:

diagnosing the cause of pineapple heart rotPhytophthora nicotianae

surveying a complex diseaseginger wilt caused by bacterial and Fusarium

wilts.


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Section 3. Te diagnostic process 33

Figure 3.1 A flow diagram of the diagnostic process

Obtain information

alk to farmer, examine

diseased plants, take notes,collect samples

Detailed examination of

samples in laboratory

Possible fungal or bacterial

diseases

Isolation and purification

Disease thought to be

caused by plant pathogenic

nematodes or viruses

Identification

Morphological identification

of fungal pathogens that

cannot be cultured

Complete diagnosis

Pathogenicity testing

Well-known pathogen

Advise farmer of best

control / IDM strategy

Confirmation of a new

disease by national or

international centre

Isolation and detection of

nematodes

Identification by Plant

Protection Research

Institute or other centre


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Diagnosing the cause of pineapple heart rot

Phytophthora nicotianae

Figure 3.2 provides an example o the steps to ollow during the diagnostic process.

Figure 3.2 Steps involved in the isolation, purification, identification and pathogenicity testing of the

pineapple heart rot pathogen, Phytophthora nicotianae (Images provided by Dang Luu Hoa)

Field Survey

Pathogenicity esting

Collect diseased plant

samples

Field survey: note and record symptoms,

disease distribution and other details

Phytophthora nicotianae

growing on millet seed

Fungal inoculum mixed into

pathogen-free soil

Healthy plant potted in

inoculated soil

Reproduction of disease symptoms

as seen in the field

Completion of

Kochs Postulates


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Wash and surface

sterilise samples

Plate segments on

selective medium

Incubation of plates

Selection of material on

margin of necrotic tissue

Identification of pure culture

from hyphal tip (P. nicotianae)

Colonies growing

from segments

Small segments cut and

transferred aseptically

LaboratoryIsolation and Purification

Subculture of fungal colony and

hyphal tipping on water agar


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Surveying a complex diseaseginger wilt caused by bacterial and Fusarium wilts

Introduction

Ginger wilt was first recorded officially in Quang Nam in 2000. Te disease has caused

severe losses, with many armers losing 100% o their crop. A preliminary study in 2006

indicated that bacterial wilt and Fusarium wilt were involved. A systematic survey o

the disease complex was made in January 2007, as a part o the Australian Centre or

International Agricultural Research project CP/2002/115, Diseases o crops in the central

provinces o Vietnam: diagnosis, extension and control (20052008).

Te objective was to isolate and identiy potential pathogens associated with diseasedginger plants, and determine their relative importance. en diseased plants were collected

rom each o 10 crops rom two districts, Phu Ninh and ien Phuoc, which are the main

areas o ginger production in Quang Nam. Crops were selected on an ad hoc basis or

sampling beore inspection.

In the field

Inormation was collected rom the armers at each crop site (Figure 3.3). Te armers

maintained that there were two types o wilt: quick wilt and slow wilt. Te leaves o plants

with quick wilt appeared to have been boiled in water and were translucent. In contrast,

the leaves o plants with slow wilt appeared yellow (Figure 3.4). Tese comments suggested

that two diseases were involved and the symptoms described were assumed to correspond

to bacterial wilt (quick wilt) and Fusarium wilt (slow wilt).

Figure 3.3 Discussions with farmers on ginger wilt


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Figure 3.4 A ginger wilt survey in Quang Nam in January 2007: (a) ginger with symptoms of quick

wilt, (b) ginger plants with yellowing, a symptom of slow wilt, (c) adjacent crops, one crop with quick

wilt, the other symptomless, (d) and (e) plants being removed carefully using a machete, keeping the

root systems intact, (f) sample bag labelled with site number, farmers name and date


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()

In the laboratory

Specimen preparation

Plant roots were washed careully to remove soil. Te plant was then examined and small

samples rom diseased areas on the plant were taken into the laboratory or microscopic

examination and isolation o the pathogens (Figure 3.5).

Figure 3.5 Preparation and examination of plants with ginger wilt for the laboratory

Gently washing the soil

from the roots

Examining the plant (leaves,

shoot, rhizome, roots) and

recording symptoms

Root knot nematode

(Meloidogynesp.) present in

some root systems

Cut rhizome from root

and stem


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Isolation of potentially pathogenic organisms from diseased tissue

Ginger rhizomes were surace sterilised, peeled and surace sterilised again. A disc was

removed rom each rhizome, rom which segments were cut and plated on peptone PCNB

(pentachloronitrobenzene) agar and Phytophthoraselective medium. Another segment was

macerated and streaked on a plate o Kings B medium to isolate bacteria (Figure 3.6).

Figure 3.6 Isolation procedure for potential plant pathogenic organisms from ginger rhizome

Te remaining segment was

macterated in sterile water on a

glass slide and streaked onto a plate

of Kings B medium for isolation of

bacteria

Five small segments were cut from

the disc and two were plated on

PPA and two on PSM

Peeled rhizome was then flamed

and a disc from the top of the

rhizome was then aseptically

removed

Rhizome was peeled (outer

layer removed) and then surface

sterilised again in 70% ethyl alcohol

Rhizome was surface sterilised in

70% ethyl alcohol for 5 seconds andflamed


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()

Recovery of potential pathogens

Fusariumspp. were isolated rom rhizomes (Figure 3.7) rom plants identified as having

slow wiltyellowing and root knot nematode symptomsrom some sites. Te isolates

were purified by single spore isolation and identified as Fusarium oxysporum.

Te colonies o F. oxysporumwere identical in culture on carnation lea agar and potato

dextrose agar, indicating that they could be pathogenic. (Cultures o saprophytic strains o

F. oxysporumare usually quite variable in culture.)

Phytophthoraspecies were not isolated rom the ginger rhizome.

A Pocket Diagnostic Kit test or Ralstonia solanacearum was positive or rhizomes rom

three plants identified as having quick wilt, which indicates that this bacterium was present

in the rhizome tissue. Te kit test was negative or a rhizome rom a plant identified as

having slow wilt symptoms (yellowing)F. oxysporum was isolated rom this rhizome.

A variety o bacterial colonies were isolated on Kings B medium and it was not possible toidentiy putative colonies o R. solanacearum with confidence. Because the plants that were

sampled had obvious to severe symptoms and had been subjected to very wet soil prior to

sampling, conditions would have avoured colonisation o diseased tissue by non-pathogenic

bacteria.

Consequently, a bioassay procedure was used in an attempt to recover cultures o

R. solanacearum(Figure 3.8). Te inner tissue o additional rhizomes rom plants previously

sampled at six sites was cut into segments and shaken with 30 mL o sterile water. Te water

was then poured into small containers with reshly cut tomato and chilli stem cuttingsbait

Figure 3.7 Isolation of Fusarium oxysporumfrom some segments of ginger rhizomes on selective isolation

medium (peptone pentachloronitrobenzene agar) for Fusarium


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seedlingswhich were kept in a greenhouse at 2530 C. Within 48 days, some o the

cuttings in the water extracts wilted and some o these showed signs o bacterial ooze.

Control seedlings in sterile water remained healthy.

Figure 3.8 Bioassay procedure for isolating Ralstonia solanacearumfrom diseased ginger rhizome:

(a) chilli and tomato cuttings in control (left) and wilted cuttings in water extract from rhizome segments

(right), (b) wilted chilli cutting showing vascular browning, (c) isolation of R. solanacearumfrom chilli

cutting, (d) and (e) pathogenicity test in bitter melon of bacterium isolated in the bioassay


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()

A stem section rom each wilted bait seedling was macerated in sterile water and streaked

on Kings B medium. A single colony rom this medium was then subcultured orpurification and also inoculated into the stem o a 6-week-old bitter melon plant, to assess

virulence. Some o the bitter melon plants showed severe wilting and the isolation process

was repeated with these stems to obtain pure cultures o R. solanacearumor identification

at an international reerence centre.

Confirming identification with pathogenicity tests

Fusarium oxysporum . sp. zingiberihas been reported widely in many countries as a cause

o Fusarium wilt o ginger. However, it was essential that the isolates o F. oxysporum rom

ginger rom Quang Nam were tested or pathogenicity in local ginger cultivars, to provethat they were pathogenic isolates and not saprophytic strains. Tereore, representative

isolates were grown on millet seed/rice hull medium or use in pathogenicity tests.

Because R. solanacearum is also known to cause bacterial wilt o ginger, pure cultures o

R. solanacearum were also tested or pathogenicity to local cultivars o ginger to complete

Kochs postulates (criteria used to establish a causal relationship between pathogen and

a disease).

For precise identification, pure cultures o R. solanacearum were also sent to an

international reerence laboratory. Tis species is variable, including a number o races,

which differ in host range and require different crop rotations or control.

Samples oMeloidogyne galls were also orwarded to a reerence laboratory or precise

identification o species.

After proof of pathogenicity is obtained

Once it was established that wilt pathogens are responsible or the wilt disease on the ginger

plants, staff developed a supply o pathogen-ree ginger rhizomes or planting in small

demonstration plots using the pathogen-ree rhizomes and soil. Te soil was deemed to be

ree o the pathogen where crops resistant to bacterial wilt (and root knot nematode) had

been grown previously. Note that R. solanacearum has a wide host range.

Although F. oxysporum. sp. zingiberionly causes disease on ginger, it may persist on roots

o symptomless non-host plants. Good crop hygiene is important so that soil rom diseased

fields is not introduced into disease-ree fields on shoes and digging implements.


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Section 4. Symptoms of disease 43

4 Symptoms of disease

Diagnosis begins with careul observation o all parts o the diseased plantoliage, flowers, ruit, stems and roots. It can be difficult to identiy the pathogen

responsible because many plant pathogens cannot be identified with the unaided

eye. Te visible effects o the pathogen on the plantsymptomscan help in

determining the type or types o pathogens present. Symptoms o disease may be

caused by:

damage to the plant tissues

disruption of the normal physiological functions of the plant:

water and nutrient uptake

photosynthesis

growth.

Symptoms o disease should be recorded careully in a field notebook and in

photographs using a digital camera (i possible).

4.1 Common symptoms

Common non-specific symptoms may be caused by many different types o

pathogens. Wilting, yellowing and stunting are common non-specific symptoms

(see wilt pathogen sections). Wilting is commonly caused by vascular wilt

pathogens, root rots and root galls, collar rots, stem rots and by dry soil. Tese and

many other diseases also cause stunting and yellowing. Tereore, it is important to

examine all parts o the plantmost importantly the roots.


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Pathogens that damage roots or stem tissuestypically ungi and nematodes

disrupt the absorption o water and nutrients. Tis can cause wilting, yellowing

and stunting, beginning with stunting o the plant and, as the disease progresses,

wilting, yellowing and plant death. Bacterial wilts cause wilting and plant death

and some virus diseases also cause wilting, stunting and plant death.

Lea disease symptoms may be caused by ungal pathogens (lea spot and blight),

bacterial pathogens (lea spot and blight) and plant pathogenic viruses (mosaics

or mottling, lea dwarfing and lea rolling). Viruses may also cause non-specific

symptoms (yellowing, wilting and stunting).

Plant parasitic nematodes mainly affect plant root systems, causing root lesions,

root galls, and root prolieration in association with cysts. Tis leads to stunting,

yellowing, wilting and sometimes plant death.

Some symptoms can help in determining the cause o a disease as they are specific

to certain pathogens. For example, the distinctive root galls caused byMeloidogynespp. (root knot nematodes) or club root caused by Plasmodiophora brassicae,

enable an accurate diagnosis in the field.

Legumes such as peanuts and soybeans have root nodulessmall swellings on the

roots caused by inection with nitrogen-fixing bacteria such as Rhizobiumspp.

Tese are beneficial and important to the health o these plants. Healthy nodules

are usually pink when cut with a knie.

Due to the range o possible symptoms that may be observed in the field, plant

pathologists should examine diseased plants careully and take clear notes to help

in making an accurate diagnosis. In addition, plant pathologists should question

armers to get inormation about the history o the crop and the development o

the disease.

A single plant may be affected by several different pathogens, causing a range o

disease symptoms.

Diseases thought to be caused by plant viruses should be sent to a laboratory

or examination by virologists experienced in identiying viruses. Te accurate

identification o plant parasitic nematodes, plant pathogenic bacteria and related

pathogens also requires expert examination.

In contrast, many ungal pathogens can be identified successully in the field or in

a basic laboratory, provided adequate reerence materials are available.


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Seek help i you are not confident in determining the cause o a disease. Do not

guessthe armers income will be affected by your diagnosis and advice.

Plant pathogens are identified as the likely cause o disease in only about hal

o plant samples sent to diagnostic laboratories. Plants may be affected by other

actors such as pesticides or environmental stress.

4.2 Diseases of foliage, flowers or fruit

Many ungal pathogens cause diseases o the oliage (leaves, petioles and stems),flowers or ruit, and usually produce spores rom spore-orming structures on

the diseased tissue. Te spores are carried by wind or splashed by rain onto

other plants, which spreads the disease. Fungal pathogens can rapidly build up

disease levels and cause epidemics under weather conditions suitable or the

specific pathogen.

Te presence o spore-orming structures such as pycnidia or perithecia,

conidiophores or sporangiophores are the basis or morphological identification o

ungal pathogens, and the diagnosis o plant ungal disease.

Use a hand lens to assist with the identification o ungal structures in the field.

However, or accurate identification o spores and spore-orming structures, use a

microscope in the laboratory.

Many ungal pathogens cause spots (lesions) on the leaves, flowers or ruit. Yellow

spots are chlorotic lesions and brown spots are necrotic lesions. Brown spots are

called necrotic because the pathogen has caused death, or necrosis, o the tissue.

Fungal pathogens that cause lesions on leaves, flowers and ruit can ofen beisolated and grown on culture media.

Saprophytic ungi also grow on diseased plant tissues as secondary colonisers.

For example, saprophytic species oAlternariaand Pestalotiacommonly grow on

diseased lea tissue and orm spores on the tissue. Tese saprophytic species can

lead to an incorrect diagnosis as they are usually visible under the microscope and

grow rapidly on the isolation medium.


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4.2.1 Spore production on diseased foliage

Fungi may orm spores asexually rom specialised hyphae called conidiophores.

In the species Stemphylium, Botrytis,Aspergillus, Penicillium, Bipolaris,Alternaria

and Cercospora, conidiophores develop on the diseased tissue rom hyphae

(Figure 4.1). Some species (e.g. Septoria, Diaporthe, Didymella,AscochytaandPhoma) orm conidia in specialised structures called pycnidia, others (e.g.

Colletotrichum, Cylindrosporium) in structures called acervuli.

Figure 4.1 Formation of conidia on foliage by various fungal pathogens

Conidia formed on an acervulus

Colletotrichum

Cylindrosporium

Conidia formed in pycnidia on tissue

Septoria

Diaporthe

Didymella

Ascochyta

Phoma

Conidia formed in conidiophores on surface of tissue

Stemphyllium

Botrytis

Aspergillus

Penicillium

Bipolaris

Alternaria

Cercospora


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Some ungal pathogens also produce spores rom sexual reproductive structures

on leaves, stalks, stems or ruit. For example, Fusarium graminearum (Gibberella

zeae) orms ascospores in an ascus within perithecia on mature diseased stalks and

cobs o maize. Tis ungus causes stalk and cob rot o maize.

Perithecia are similar in shape to pycnidia, but pycnidia orm conidia and do not

have asci.

4.2.2 Obligate foliar fungal and fungal-like pathogens

Te downy mildews, powdery mildews and rusts are caused by obligate plant

parasites. Tese pathogens are only able to inect, grow and produce spores in

living host tissuethey cannot be isolated and grown in cultureand are usuallyonly able to inect one or two host species or varieties (cultivars). For example,

peanut rust can only inect peanuts.

Figure 4.2 Fungal and fungal-like pathogens of the foliage: (a) powdery mildew on a cucurbit, (b) white

blister on Brassica sp., (c) Cercospora leaf spot and rust on peanut, (d) downy mildew on cabbage


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Figure 4.3 Sclerotial formation by (a) Rhizoctonia solani, (b) Sclerotium rolfsiiand (c) Sclerotinia sclerotiorum

Te symptoms o downy mildews, powdery mildews and rusts are usually obvious

(Figure 4.2). Tey absorb nutrients rom living plant cells, which commonly leads

to yellowing o the tissue. Photosynthesis in the lea tissue is reduced, leading to

reduced plant growth.

4.2.3 Pathogens that produce sclerotia on infected tissue

In humid conditions, some ungal pathogens produce hyphae and/or sclerotia on

inected plant suraces. Hyphae o some Rhizoctoniaspecies grow on inected stem

bases and leaves. In Vietnam, some Rhizoctoniaspecies produce irregularly shaped

brown sclerotia on diseased tissue on maize leaves and cabbages (Figure 4.3a).

Sclerotium rolfsii causes stem base rot on many annual vegetable and field crops

in hot wet weather in Vietnam. S. rolfsii orms obvious white hyphae (mycelium)

on the inected stem base and small round brown sclerotia (Figure 4.3b) rom

these hyphae.

Sclerotinia sclerotiorum produces white mycelium and large black sclerotia on

the stems and oliage o many broad-leaed crops, such as long and short beans,

tomatoes, cabbages, potatoes and lettuce (Figure 4.3c).


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4.3 Diseases of roots, crown and stem

Fungal pathogens can cause serious diseases o the roots, crown (base o the stem)or the stem. Some ungal pathogens only affect seedlings, killing them beore or

afer emergence; others only affect the eeder rootlets, and some species only affect

the stem (e.g. Sclerotinia sclerotiorum and Sclerotium rolfsii).

Root symptoms may be non-specific. Rots o the eeder rootlets, main roots, crown

or stem disrupt the uptake o water and nutrients, causing stunting and yellowing

o the leaves, wilting and sometimes death o the plant.

Genera that commonly cause these diseases in Vietnam are Phytophthora,

Pythium, Fusarium, Sclerotinia, Sclerotium, Rhizoctonia and Phoma(Figure 4.4).

One root parasite, Plasmodiophora brassicae, which causes club root o cruciers,is an obligate parasite and cannot be grown on culture media. Tere are also

Basidiomycota pathogens o the trunk and roots o perennial tree crops (Shivas

and Beasley 2005), which are generally difficult to isolate into pure culture.

Root rot pathogens can be difficult to isolate as there can be many saprophytic

ungi and bacteria in diseased root tissues, which

diagnostic manual for plant diseases in viet 13726

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