Pests and Diseases of Potatoes: A Colour Handbook

177 Pages·2016·13.57 MB·English

by  Wale S.

Diseases Pests
,
Disorders
and of
Potatoes
A Colour Handbook
Stuart Wale
Head of Crop Services, SAC (Scottish Agricultural College), Aberdeen, UK
HW (Bud) Platt
Research Scientist, Agriculture and Agri-Food Canada, Crops and Livestock Research Centre,
Charlottetown, PEI, Canada
Nigel Cattlin
Holt Studios: Images of Agriculture and Plant Science
MANSON
PUBLISHING
We dedicate this book to all who are involved with scientific investigation, education, extension,
phytosanitary regulation, management and dealing with the consequences of potato diseases and pests.
It is hoped that the material provided is beneficial.
We also acknowledge the dedicated interest and kind support of Shirley Jo-Ann Platt (1951–2007),
Sue Wale, and others who have, over the years, provided assistance to all those who deal directly with
potato disease and pest issues.
Copyright © 2008 Manson Publishing Ltd
ISBN: 978-1-84076-021-7
All rights reserved. No part of this publication may be reproduced, stored in a retrieval
system or transmitted in any form or by any means without the written permission of the
copyright holder or in accordance with the provisions of the Copyright Act 1956 (as
amended), or under the terms of any licence permitting limited copying issued by the
Copyright Licensing Agency, 33–34 Alfred Place, London WC1E 7DP, UK.
Any person who does any unauthorized act in relation to this publication may be liable
to criminal prosecution and civil claims for damages.
A CIP catalogue record for this book is available from the British Library.
For full details of all Manson Publishing titles please write to:
Manson Publishing Ltd, 73 Corringham Road, London NW11 7DL, UK.
Tel: [phone]
Fax: [phone]
Website: www.mansonpublishing.com
Commissioning editor:Jill Northcott
Project manager:Ayala Kingsley
Copy editor:Derek Hall
Proof-reader:John Forder
Indexer:Jill Dormon
Book design and layout:Ayala Kingsley
Colour reproduction:Tenon & Polert Colour Scanning Ltd, Hong Kong
Printed by:Grafos SA, Barcelona, Spain
Plant Protection Handbook Series
Alford:Pests of Fruit Crops – A Colour Handbook
Alford:Pests of Ornamental Trees, Shrubs and Flowers – A Colour Atlas
Biddle/Cattlin:Pests, Diseases and Disorders of Peas and Beans – A Colour Handbook
Blancard:Cucurbit Diseases – A Colour Atlas
Blancard:Tomato Diseases – A Colour Atlas
Blancard/Lot/Maisonneuve:Diseases of Lettuce and Related Salad Crops – A Colour Atlas
Bridge/Starr:Plant Nematodes of Agricultural Importance – A Colour Handbook
Fletcher/Gaze:Mushroom Pest and Disease Control – A Colour Handbook
Helyeret al:Biological Control in Plant Protection – A Colour Handbook
Koike/Gladders/Paulus:Vegetable Diseases – A Colour Handbook
Murrayet al:Diseases of Small Grain Cereal Crops – A Colour Handbook
Wale/Platt/Cattlin: Pests and Diseases of Potatoes – A Colour Handbook
Williams: Weed Seedlings – A Colour Atlas
Contents
Contributors. . . . . . . . . . . . . . . . 4 Phytophthora infestans CHAPTER 4
Preface. . . . . . . . . . . . . . . . . . . . . . . . . 5 LATEBLIGHT. . . . . . . . . . . . . . . . . . 48 Nematodes
Acknowledgements. . . . . . . . . . . . . 5 Polyscytalum pustulans
SKINSPOT. . . . . . . . . . . . . . . . . . . . 53 Ditylenchus destructor
Introduction . . . . . . . . . . . . . . . . . . . 6 Pythium ultimum POTATOTUBERNEMATODES/
Disease, pest and disorder and otherPythiumspp. POTATOROTNEMATODES . . . . . . . . 93
diagnostics. . . . . . . . . . . . . . . . . . 9 WATERYWOUNDROT/ LEAK . . . . 55 Globodera rostochiensis,
G. pallida
Rhizoctonia solani
CHAPTER 1 RHIZOCTONIADISEASE: STEM, POTATOCYSTNEMATODES . . . . . . 94
Bacterial diseases STOLONANDROOTCANKER/ Meloidogyne hapla, M. incognita,
BLACKSCURF . . . . . . . . . . . . . . . . . 56 M. chitwoodii, M. arenaria,
Clavibacter michiganensis subsp. M. fallax, M. javanica, M. thamesi
Sclerotinia sclerotiorum
sepedonicus WHITEMOULD/ ROOT-KNOTNEMATODES . . . . . . . 96
BACTERIALRINGROT . . . . . . . . . 17 SCLEROTINIASTALKROT . . . . . . . . 60 Paratrichodorusspp.,
Pectobacterium atrosepticum Trichodorus spp.
Spongospora subterranea
Pectobacterium carotovorum subsp f.sp.subterranea STUBBY-ROOTNEMATODES . . . . . . 99
carotovora POWDERYSCAB. . . . . . . . . . . . . . . . 62 Pratylenchus penetrans
Dickeyaspp. and otherPratylenchusspecies
Synchytrium endobioticum
BLACKLEG/ BACTERIALSOFTROT/ includingP. andinus, P. brachyurus,
HARDORPITROT . . . . . . . . . . . . . 20 WART. . . . . . . . . . . . . . . . . . . . . . . 64 P. coffeae, P. crenatus, P. neglectus,
Thecaphora solani
Ralstonia solanacearum P. scribneri, P. thorneyi, P. vulnus
BROWNROT(BACTERIALWILT/ POTATOSMUT. . . . . . . . . . . . . . . . . 66 ROOT-LESIONNEMATODES . . . . . 100
SOUTHERNBACTERIALWILT). . . . . 24 Verticillium albo-atrum, V. dahliae
Streptomyces scabiei VERTICILLIUMWILT/ CHAPTER 5
S. acidiscabiei, S. caviscabiei, POTATOEARLYDYING . . . . . . . . . . 68
Insects and other pests
S. europaeiscabiei, S. luridiscabiei,
S. niveiscabiei, S. puniciscabiei CHAPTER 3 Agriotesspp.,Athousspp.,
S. reticuliscabiei, S. stelliscabiei, Virus and phytoplasma Conoderusspp.,Cteniceraspp.,
S. turgidiscabiei diseases Limoniusspp.Melanotusspp.
COMMONSCAB. . . . . . . . . . . . . . . 26 WIREWORMS . . . . . . . . . . . . . . . . 103
Aster yellows phytoplasma Arionspp.,Milaxspp.,
CHAPTER 2 ASTERYELLOWS. . . . . . . . . . . . . . . 74 Deroceras reticulatum,
Fungal and fungal-like Potato leaf roll virus Limax maximus
diseases POTATOLEAFROLL . . . . . . . . . . . . 75 SLUGS. . . . . . . . . . . . . . . . . . . . . . 105
Potato spindle tuber viroid Empoasca fabae,
Alternaria alternata, A. solani SPINDLETUBER. . . . . . . . . . . . . . . . 77 Macrosteles quadrilineatus,
EARLYBLIGHT/ TARGETSPOT . . . . 28 Potato virus A Aceratagallia sanguinolenta,
Botrytis cinerea POTATOVIRUSA . . . . . . . . . . . . . . 79 Circulifer tenellus
GREYMOULD. . . . . . . . . . . . . . . . . 31 Potato virus X POTATOLEAFHOPPERS . . . . . . . . . 107
Colletotrichum coccodes POTATOVIRUSX . . . . . . . . . . . . . . 80 Epitrix cucumeris, E. tuberis,
BLACKDOT . . . . . . . . . . . . . . . . . . 33 Potato virus Y otherEpitrixspp.,Psylliodesspp.
Fusariumspp. POTATOVIRUSY . . . . . . . . . . . . . . 81 POTATOFLEABEETLES . . . . . . . . . 108
FUSARIUMDRYROT/ WILT . . . . . . 36 Potato yellow vein virus Leptinotarsa decemlineata
Helicobasidium purpureum POTATOYELLOWVEIN. . . . . . . . . . 83 COLORADOPOTATOBEETLE. . . . . 111
VIOLETROOTROT. . . . . . . . . . . . . 39 Potato mop-top virus Liriomyza huidobrensis,
L. cucumifoliae, L. langei,
Helminthosporium solani Tobacco rattle virus
L. dianthi
SILVERSCURF . . . . . . . . . . . . . . . . . 41 SPRAING. . . . . . . . . . . . . . . . . . . . . 85
LEAFMINERFLY . . . . . . . . . . . . . . 114
Phoma exiguavar. exigua Tomato spotted wilt virus
Myzus persicae,
POCKET/ BUTTONROT. . . . . . . . . . 42 TOMATOSPOTTEDWILT. . . . . . . . . 88
Macrosiphum euphorbiae,
Phoma exigua var. foveata Witches’ broom phytoplasma
Aulacorthum solani,
GANGRENE. . . . . . . . . . . . . . . . . . . 44 WITCHES’ BROOM . . . . . . . . . . . . . 90 Aphis nasturtii
Phytophthora erythroseptica APHIDS. . . . . . . . . . . . . . . . . . . . . 118
PINKROT. . . . . . . . . . . . . . . . . . . . 46
4 DISEASES, PESTS AND DISORDERS OF POTATOES
Ostrinia nubilalis
EUROPEANCORNBORER. . . . . . . . 121 CONTRIBUTORS
Phthorimaea operculella
POTATOTUBERMOTH. . . . . . . . . . 124
Canada Peru
Premnotrypesspp.
ANDEANPOTATOWEEVIL. . . . . . . 129 AGRICULTUREANDAGRI-FOOD INTERNATIONALPOTATOCENTER
Symmetrischema tangolias CANADA, CROPSANDLIVESTOCK (CIP), Lima
ANDEAN/ SOUTHAMERICAN RESEARCHCENTRE Jesus Alcázar
POTATOTUBERMOTH. . . . . . . . . . 132 Charlottetown, Prince Edward Research Assistant, Agronomy
Tecia solanivora Island
GUATEMALANPOTATOTUBER Ruth Heinz Veronica Cañedo
MOTH. . . . . . . . . . . . . . . . . . . . . . 136 Research Assistant, Biology
Research Scientist (PDF),
Plant Pathology Jurgen Kroschel
CHAPTER 6
Research Scientist, Entomology
Joe Kimpinski
Non-infectious disorders
Research Scientist, Nematology Norma Mujica
Chemical, nutritional and physical Research Assistant, Agronomy
Rick Peters
CHEMICALDAMAGE. . . . . . . . . . . . . . 141 Research Scientist, Plant Pathology Giovanna Müller
NUTRIENTIMBALANCES Research Assistant, Biology
(Nitrogen deficiency AGRICULTUREANDAGRI-FOOD Willmer Perez
Phosphorus deficiency
Potassium deficiency CANADA, POTATORESEARCH Research Assistant, Plant
Magnesium deficiency CENTRE, Fredericton, Pathology
New Brunswick
Calcium deficiency
Marc Sporleder
Sulphur deficiency Gilles Boiteau
Research Scientist,
Boron deficiency Research Scientist, Entomology
Integrated Crop Management
Iron deficiency
Zinc deficiency Agnes Murphy
Manganese deficiency Research Scientist, Variety ANIMALANDPLANTHEALTH
Manganese toxicity Development and Plant Pathology NATIONALSERVICE(SENASA),
Aluminium toxicity Lima
Rudra Singh
Copper deficiency) . . . . . . . . . . . . 145
Research Scientist, Virology Torres Hebert
STEM-ENDBROWNING . . . . . . . . . . . . 148
Research Assistant, Specialist
TUBERSURFACEINJURIES& CRACKS . 149
AGRICULTUREANDAGRI-FOOD
Environmental CANADA, Vineland Station, USA
AIRPOLLUTIONINJURY. . . . . . . . . . . . 152 Ontario NORTHCAROLINASTATE
FROSTINJURY. . . . . . . . . . . . . . . . . . . 154 Deena Errampalli UNIVERSITY,Raleigh, NC
HEATNECROSIS . . . . . . . . . . . . . . . . . 156 Research Scientist, Plant Pathology George Kennedy
OXYGENDEFICIT/ BLACKHEART . . . . 157 Professor, Entomology
TUBERGREENING. . . . . . . . . . . . . . . . 159 CANADIANFOODINSPECTION
WINDINJURY. . . . . . . . . . . . . . . . . . . 160 AGENCY, UNIVERSITYOFFLORIDA,
CENTREFORANIMALANDPLANT Hastings, FL
Physiological HEALTH, Charlottetown, Prince
D (Pete) Weingartner
COILEDSPROUT/ HAIRYSPROUT . . . . 161 Edward Island
Associate Professor, Plant
DISORDERS. . . . . . . . . . . . . . . . . . . . . 163
Solke H De Boer Pathology
HOLLOWHEART. . . . . . . . . . . . . . . . . 164 Research Scientist, Bacteriology
BLACKSPOT(INTERNALBRUISING). . . 165
INTERNALTUBERSPROUTING. . . . . . . 167 UNIVERSITYOFMANITOBA,
SECONDARYTUBERGROWTH Winnipeg, MB
(Extension of the rose-end
Jelly end rot Fouad Daayf
Gemmation Professor, Plant Pathology,
Chain tuberization/little potato Department of Plant Science
Cracking and hollow heart) . . . . . 168
Glossary . . . . . . . . . . . . . . . . . . . . . 170
Index . . . . . . . . . . . . . . . . . . . . . . . . 172
5
Preface
DESPITE THE FACT that potatoes are immensely pest or disease is involved. Absolute confirmation
important as a food crop worldwide, and that the would depend on reproducing the symptoms, by
diseases, pests and disorders that affect them have infecting a healthy plant with the isolated micro-
received a great deal of investigation, there are few organisms or pest. However, disorders are more
texts that have attempted to describe and encompass difficult to reproduce. This last step of absolute confir-
the crop protection problems in any depth. Only mation is not often undertaken, unless a problem is
relatively recently has it been possible to reproduce perceived as a new one, or a new strain of a disease or
high- quality colour photographs, integrated within pest is suspected. Our intention in this book is to help
text, in books. The Colour Handbookis one of the first readers reach the initial level of diagnosis.
to achieve this for the potato crop, and covers the Once you have confirmed the problem, if you are
majority of diseases, pests and disorders that occur. not familiar with it, some background will be required.
High-quality photographs are important where By supplying information about the economic impor-
accurate visual diagnosis is required. Where possible, tance, life cycle and control of each disease, pest or
and where they were available, ‘typical’ symptoms are disorder, we hope that an appreciation can be achieved.
represented here. Of course, there is always variation The selected references provided can act as a basis for
in the symptoms of any potato problem, but we have further research, especially if there is little access to sci-
tried to find photographs that will help to distinguish entific literature databases. For some diseases and
one disease, pest or disorder from another. pests, scientific knowledge is published continuously,
Identification by comparison with photographs is and it is impossible to include the most up-to-date ref-
only a start to confirming diagnosis. Confirming that erences. For other problems, especially disorders, there
the observed symptoms accord with a verbal descrip- has been little study or published work and, as a result,
tion is an essential next step on the road to correct iden- some of the references may be old.
tification. In this book, each disease, pest or disorder This book is intended as a practical guide and tool.
has a detailed description of the symptoms and, where We expect and hope that it will be carried around and
relevant, the vectors that transmit the disease. referred to in the field, as well as in the laboratory, on
While photographic and written symptoms can an everyday basis. We hope that it will prove a valuable
support an initial diagnosis, its confirmation will asset to all who use it.
depend on isolating the micro-organism/pest involved
or, in the case of disorders, establishing that no living STUARTWALE, BUDPLATT& NIGELCATTLIN
ACKNOWLEDGEMENTS
The authors and publishers would like to thank the from their collections for use in this book. Thanks are
photographic librarians at SASA (Scottish Agricultural also extended to those colleagues and companies who
Science Agency) and CSL (Central Science Laboratory) supplied specimens for photography or contributed
in the UK, who searched for and offered photographs their own existing photographs.
6
Introduction
PRAYFORPEACEand grace and spiritual food for wisdom and guidance, for all these are good.
But don't forget the potatoes.
John Tyler Pettee, 1822–1907
WHATISAYIS, if a man really likes potatoes, he must be a pretty decent sort of fellow.
AA Milne, 1882–1956
We began this venture with great enthusiasm and It is the vegetative propagation of potatoes that exposes
interest in not only bridging the ‘waters’ between our it to challenge from serious pest and disease attack.
respective continents but also to provide a practical and Unlike graminaceous crops, many diseases can be
useful reference tool for anyone involved with the major transmitted in seed potatoes, and there is a continuous
factors that impact on potato production, handling and challenge to generate healthy basic seed. The low mul-
storage worldwide. The format of the book provides a tiplication rate means that several generations of seed
durable tool that is portable for field, vehicle or indoor are needed to reduce the unit seed price to acceptable
reference. It is our sincere hope that as you use this levels. Throughout the seed multiplication phase tubers
book, you will find that we have fulfilled our objectives. are prone to invasion by diseases that are carried from
The World Bank and FAO have ranked the potato generation to generation on, or in, the seed tuber.
crop the fourth most important in the world and the Perhaps the most demonstrable seed-borne agents that
most important non-cereal crop. Potatoes are grown in affect vigour, yield and quality of potatoes are the
every continent and in over 130 countries. Through viruses. These can debilitate potato production, and
breeding and advanced agronomic practices, the crop sophisticated propagation techniques and certification
can be grown under almost all climatic conditions. schemes are in place around the world to ensure
Although potatoes can be grown from true seed, most freedom from viruses and other pathogens.
of the world’s production stems from vegetatively prop- Additional features that distinguish the potato from
agated tubers. This makes it stand apart from other other crops also impact on susceptibility to attack. The
major world crops, such as cereals. Struik and considerable variation in characteristics from tuber to
Wiersema (1999) list seven aspects distinguishing the tuber means that they may differ in susceptibility, and
potato from other major world food crops: therefore consistency in applying control measures can
be difficult. In countries where crops are grown in
• It has a low multiplication rate. different seasons, a different range of pest and disease
• Seed tuber costs are high. problems can occur, and this also presents the crop pro-
• As a vegetative crop it is prone to ‘seed degenera- tectionist with different challenges.
tion’. Another difference between potatoes and gramina-
• Seed tubers usually show innate dormancy after ceous crops is that the storage organ, the tuber, which
harvest. contains around 80% water and an ideal blend of
• Tubers on the same stem show a wide range in size, nutrients, is highly suited as a food source for pests and
dry matter content, dry matter composition and microorganisms. Thus unless great care is taken, there
physiological condition. is potential for huge losses in storage facilities. Losses
• In many regions potatoes are grown in different of 100% have occurred, even in developed countries
seasons of the year. where, for example, bacterial soft rot has taken hold
• A wide range of suitable techniques for the produc- soon after harvest. Whilst storage in some countries is
tion of high-quality seed potato tubers is available. based on simple technology, in other countries great
INTRODUCTION 7
advances including effective insulation, ventilation and scientists. In recent years, this tendency has resulted in
refrigeration have substantially limited the devastating new strains of the pathogen which attack potato crops
losses of the past. across North America (Goodwin et al., 1994a, 1994b,
Despite these technological improvements and 1995, Peters et al., 1998) while, in Europe, more
removal of fears about sufficiency of supply, losses in virulent strains are causing growers in some countries
storage facilities are still high because there has been a to more than double the number of fungicide applica-
change in emphasis toward quality. What concerns tions (Flier, 2002).
potato pathologists and zoologists in the developed There is a worldwide effort to tackle potato late
countries today is the production of potato tubers with blight, and the sharing of knowledge about the biology
excellent skin finish or ideal processing qualities. Many of the pathogen is proceeding at an unprecedented rate.
of the diseases and pests that impact on these quality But other diseases are also of great importance, even if
factors were minor pathogens in the past but can make they are on a more localized scale. For example, brown
the difference between profit and loss now. rot (Ralstonia solanacearum) can be devastating in
That is not to say that major diseases and pests do tropical and sub-tropical countries. Potato cyst
not threaten potato production. Potato late blight nematodes (Globodera rostochiensisandG. pallida)
(Phytophthora infestans) continues to dominate. The have become major pests in parts of Europe. Large
widespread application of fungicides has reduced the quantities of nematicide are used annually to suppress
disease to manageable proportions in developed these pests, with potentially severe environmental con-
countries, but the potential of the pathogen to mutate sequences. Countries free from these devastating
or produce new sexual strains, adapt and overcome nematode problems take immense precautions to avoid
efforts to suppress it has confounded generations of importing them with seed tubers.
8 DISEASES, PESTS AND DISORDERS OF POTATOES
Identification and diagnosis are critical to imple- They are only a starting point for a literature search.
menting effective control measures. Luckily, there are The availability of abstract databases and journals elec-
many technical aids available to scientists, field scouts, tronically has opened opportunities to reach a great
agronomists and farmers to assist identification. An deal of the scientific literature. Even more readily
overview of these is given in the next chapter. However, available on the Internet is a vast amount of informa-
visual diagnosis remains the first line of identification tion on potato diseases, pests and disorders, which can
and is a skill that is honed with training and practice. be accessed through search engines. Some are cited in
With an increasing emphasis on molecular aspects of this book. However, unlike scientific publications in
plant pathology and zoology, there are fewer plant established journals, Internet web sites are usually not
pathologists throughout the world who are field-based peer reviewed, and care is needed when using such
and have developed the skill of diagnostics. We hope information.
that books like this will encourage scientists not to lose Current philosophies in the development of sustain-
touch with field-based plant pathology or zoology. able agricultural systems rely on the implementation of
In the visual identification of potato pests and integrated pest management (IPM) practices. Modern
diseases, a diagnostician needs a clear indication of pest and disease control goals include not only success-
symptoms, both written and visual. It is perhaps sur- ful suppression of causal agents (and therefore disease
prising that so few colour guides to pests and diseases of losses), but a reduction in the use of synthetic pesticides
potatoes are available for this major crop. The and farming methods that negatively impact on the
American Phytopathological Society has released a environment and human health. Successful IPM relies
second edition of its Potato Disease Compendium heavily on early and accurate detection of pests and
(Stevensonet al., 2001). This excellent book contains diseases and their causal agents. In this way, our hope is
good overviews of diseases and disorders. What we that this field guide will contribute to the development
hope makes our book stand out is the inclusion of pests, and implementation of IPM programmes that ensure
the emphasis on symptoms and above all, the quality of sustainable agricultural production, healthy soil, water
the colour photographs. and natural environments, food safety and human
From the start, we did not set out to cover our topic health.
in an exhaustive manner. We listed all known pests and
diseases and ranked them in order of importance, par- Key references
ticularly with respect to the Northern Hemisphere. Flier, W. G. (2002) Impact of new populations of Phytophthora
Those considered of minor importance were omitted. infestanson integrated late blight management. Proceedings
Crop Protection in Northern Britain 2002, Dundee, UK.
No doubt there will be those who will argue with the
217–224.
choices made, but we have attempted to include all the
Goodwin, S. B., Cohen, B. A., Deahl, K. L., Fry, W. E. (1994)
pests, diseases and disorders that are most likely to be Migration from northern Mexico as the probable cause of
recent genetic changes in populations of Phytophthora
encountered in North America or Europe.
infestansin the United States and Canada. Phytopathology,
The book has been prepared using a format that will
84: 553–558.
enable quick access to the most relevant information on Goodwin, S. B., Cohen, B. A., Fry, W. E. (1994) Panglobal
all of the major diseases, pests and disorders of distribution of a single clonal lineage of the Irish potato
famine fungus. Proceedings of the National Academy of
potatoes. Each chapter includes the basic information
Sciences of the USA,91: 11591–11595.
onSymptoms(including photographs), Status of the
Goodwin, S. B., Sujkowski, L. S., Dyer, A. T., Fry, B. A., Fry, W. E.
disease, Pest or Disorder, Life cycle and biology or (1995) Direct detection of gene flow and probable sexual
reproduction of Phytophthora infestansin northern North
Cause,ControlandKey references. To avoid conflicts
America.Phytopathology,85: 473–479.
among crop, disease and pest management treatment
Peters, R. D., Platt, H. W., Hall, R. (1998) Changes in race
recommendations that are often site-specific, the structure of Canadian populations of Phytophthora infestans
chapters include very general information on control based on specific virulence to selected clones. Potato
Research,41: 355–370.
treatments.
Stevenson, W. R., Loria, R., Franc, G. D., Weingartner, D. P.
For each chapter, only a few key references are listed. (Eds.) (2001) Compendium of Potato Diseases. APS Press,
These are selected, in the main, from major journals St. Paul, MN, USA.
that are most likely to be accessible in scientific libraries. Struik, P. C., Wiersema, S. G. (1999) Seed Potato Technology.
Wageningen Pers, Wageningen, Netherlands.
9
Disease, pest and
disorder diagnostics
As much an art as a science plants compared with healthy neighbours. Even here,
Diagnosis of diseases and other problems in potatoes is the diagnostician must be clear about how a healthy
as much an art as a science. Descriptions in books, even crop should develop in his/her locality.
this one, can never fully describe the range of symptoms
that may develop in the field. Photographs often show Field observation aids diagnosis
‘typical’ symptoms, but from this norm variations occur To be certain of diagnosis, access to the problem in the
due to resistance of the host, the prevailing environ- field or potato storage facility is essential. Many clues to
mental conditions or interaction with other pathogens the cause of a problem lie in the distribution of symp-
or pests. An example of this variability is manifest in toms. When a sample is sent to a laboratory for
viruses. In one cultivar, a virus may exhibit severe diagnosis, it may be possible to determine the pest or
mosaic symptoms, yet in another, the symptoms may be pathogen involved but not associated or contributory
milder or non-existent. However, in this second cultivar, factors. A sketch of the distribution in the field or
if an additional virus is present symptoms may be cata- storage facility can help diagnosis in the laboratory
strophic, leading to severe stunting or death. Confusion immensely, but it is no substitute for on-site observa-
in visual diagnosis of symptoms can be overcome by tion. For example, from a sample in the laboratory it
resorting to specific laboratory (or field) diagnostic would be possible to identify and isolate Rhizoctonia
tests. But even then, where a pathogen, pest or disorder solaniand ascribe poor growth to this pathogen. A visit
is confirmed and seems to be the obvious cause, it is to the field might reveal that the presence of the
essential that the diagnostician eliminates other causes pathogen is exacerbated in certain areas by soil com-
or implicatory factors. Where a diagnostician relies paction, or that the problem only occurs in the lightest
solely on visual symptoms, perhaps in the field soil in the field. The use of images taken in situwith
situation, diagnosis may not be clear-cut. A preliminary digital cameras and rapidly dispatched to a diagnostic
diagnosis, before confirmation in the laboratory, may laboratory has both speeded diagnosis and improved
be based on a large proportion of the key diagnostic appreciation of the field situation.
characteristics of a causal agent or the symptoms being
present. Access to all relevant information is crucial
Whether you are attempting to diagnose a potato
Know the appearance and development of healthy problem in the field or in a laboratory, complete diag-
potato crops nosis requires a full knowledge of the field in which the
The first step in diagnosis is to be aware of what healthy crop is grown, the agronomy of the crop itself and as
plants or tubers look like. This may seem obvious, but many observations of the crop as possible. Full back-
there are parts of the world where potato production is ground information is needed if advice is to accompany
so affected by disease or pest problems that the appear- the diagnosis. It may be quite straightforward to
ance of the crops they affect is considered normal. The diagnose a problem, but without clear knowledge of the
poor growth induced by extensive potato cyst background information, advice may be inappropriate
nematode infestation in soil, or 100% virus infection, or misleading. For example, the problem may be very
are such examples. Where widespread problems like minor and action to control it unnecessary. Alternative-
these do not exist, diseases, pests or disorders are ly, the problem may be a symptom of poor husbandry,
usually identified by their relative impact on some in which case control measures should be directed at the