PLANT PROTECTION 4

22 The diagnostic road map
WHAT IS DIAGNOSIS?
Diagnosis is essentially a problem-solving exercise involving the initial recognition of a problem, then
gathering and sorting information about the affected plants and the nearby environment so that causes can be
identified. From prior knowledge or experience and information available, a diagnosis may be possible. If the
problem is still unresolved, expert advice may be needed to reach a diagnosis.
A FEW TERMS
THE
DIAGNOSTIC
ROAD MAP
THE 7 STEPS
It is helpful to have an understanding of terms associated with diagnosis:
Host – a plant on, or in which, a pest or parasite lives. In this book the term
‘affected plant’ is used to include host plants and plants affected by non-parasitic
agents, eg nutrient deficiencies.
Pest – a term used to include ‘any species, strain or biotype of plant, animal or
pathogenic agents injurious to plants or plant products’ (International Plant Protection
Convention). This book follows this convention so the word ‘pest’ can mean pest,
disease or weed unless otherwise specified.
Identification – to identify a plant, pest, disease or weed or other organism by its
scientific name and or its accepted/standard common name.
Diagnosis – identification of a disease by investigation of its symptoms and history
(Oxford Concise Dictionary).
Detection – to detect the presence of an organism or some other causal agent. In
some cases this will provide a basis for diagnosis. However, just because you have
detected and identified something does not mean that it is the cause of the problem.
Monitor – a program of sampling, inspecting and recording to detect, locate, identify
and quantify potential pests, diseases and weeds and their natural enemies.
Tests – in this book the term diagnostic test or test is used to describe any method or
procedure which detects or identifies an organism or causal agent.
Pest information sheets – documents which contain details of the scientific name,
legislative requirements, host range, diagnostic descriptions and tests, pest cycle,
spread, conditions favouring and recommended control methods. Pest information
sheets are sometimes called pest prescription sheets, fact sheets or other names.
The ‘pest signature’ – the information in a pest information sheet which may be
used to provide, confirm or reject a diagnosis. It may eliminate certain possibilities.
The degree to which a suspect pest matches its ‘pest signature’ varies. In some cases
a single component is sufficient for an accurate diagnosis, eg a diagnostic test, but in
most cases, several components are needed, eg symptoms and pH test.
Proof of diagnosis – written evidence that confirms or rejects a conclusion. You
need to provide evidence of how you reached your diagnosis. For some problems
this is easy, eg matching a description of a citrus butterfly or spores of a fungus to
illustrations and descriptions; for complex causes, this is more difficult.
The diagnostic road map is presented as 7 separate steps and offers a systematic and
adaptable approach to making a diagnosis (see Fig. 5 opposite).
Some steps in this guide may be bypassed, combined or revisited.
– Remember, at any step the cause(s) may be identified. With a bit of experience
you may be able to answer the client’s enquiry immediately!
– You may decide to send a sample directly to a diagnostic service or one of the
various hotlines set up by plant quarantine and grower groups.
– You may need to use references to identify the affected plant or you may need to
send it to a plant identification service.
– Samples may not be available for examination at the beginning of an
investigation, but they could be requested. Additional questions may be asked.
Advantages of a systematic, adaptable approach:
– Improves the likelihood of a correct diagnosis being obtained efficiently.
– Embraces new technologies, which improves diagnostics and control methods.
New techniques are continually being developed to identify diseases that are
difficult to identify by conventional methods.
– Follows a well established routine in which possible causes can be progressively
eliminated. With experience, the diagnostic process becomes semi-automatic
or instinctive but control is still retained by the diagnostician (like driving a car).
– Makes the task of a novice diagnostician less intimidating, easier and faster by
avoiding too many sidetracks.
– Helps the more accomplished diagnostician avoid the bias of looking only for
the familiar problems and stopping the investigative process too early.
– Encourages the keep up-to-date status required of IPM and BMP (see page 18).
– Ensures records are kept. Diagnosis involves detective work and information
gathering which means taking notes and keeping records of the enquiry, how the
diagnosis was arrived at and any advice and information provided. These records
provide proof of diagnosis to your client and should follow the diagnostic road
map illustrated opposite (see also pages 62, 72, 105, 121).