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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
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Diagnostic manual for plant diseases in Vietnam6
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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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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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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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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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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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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Diagnostic manual for plant diseases in Vietnam18
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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Diagnostic manual for plant diseases in Vietnam20
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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Diagnostic manual for plant diseases in Vietnam22
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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Diagnostic manual for plant diseases in Vietnam24
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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Diagnostic manual for plant diseases in Vietnam26
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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Section 2. General plant health 27
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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Diagnostic manual for plant diseases in Vietnam28
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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Section 2. General plant health 29
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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Diagnostic manual for plant diseases in Vietnam30
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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Section 2. General plant health 31
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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Diagnostic manual for plant diseases in Vietnam32
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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Diagnostic manual for plant diseases in Vietnam34
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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Section 3. Te diagnostic process 35
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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Diagnostic manual for plant diseases in Vietnam36
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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Section 3. Te diagnostic process 37
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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Section 3. Te diagnostic process 39
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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Section 3. Te diagnostic process 41
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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Diagnostic manual for plant diseases in Vietnam44
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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Section 4. Symptoms of disease 45
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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Diagnostic manual for plant diseases in Vietnam46
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