PLANT PROTECTION 4

HOW DEFINITE DOES THE DIAGNOSIS NEED TO BE?
PRELIMINARY,
USEFUL,
ACCURATE,
DEFINITE OR
INCONCLUSIVE?
Pest information
sheet
Common name
Scientific name
Causes
Significance
Host range
Description
Pest cycle
Overwintering
Spread
Conditions favouring
Control
Diagnosis may be:
Preliminary
Useful
Accurate
Definite positive
Definite negative
Inconclusive
One should be cautious about being too definite
How definite does the diagnosis need to be? The client or situation determines what is
possible or most appropriate. Courts of law and quarantine require different levels of
diagnosis than a home gardener.
The degree to which a suspect pest matches the information in the pest information
sheet, ie its pest signature, will determine the level of diagnosis. In some cases a
single component is sufficient for an accurate diagnosis, eg a diagnostic test, but in
most cases, several components are needed. It may also eliminate certain possibilities.
Diagnoses may be classified into different levels, an example of which is presented
below. Levels may comprise of 4 levels of certainty; a 5 th level recognises that some
investigations will be inconclusive.
A preliminary diagnosis is based mainly on knowledge and experience in
recognizing previously described signs and symptoms and their similarity to published
descriptions and illustrations. A preliminary diagnosis requires a grasp, even if
incomplete, of growing plants and the causes of signs and symptoms produced by
pests, their life cycles, how they spread and conditions that favour them.
– Experienced diagnosticians in plant clinics can satisfactorily diagnose 50-70% of
the samples submitted. The low percentage is due to the poor quality of some
specimens, lack of background information, time constraints, or a perception that an
enquiry is unimportant, eg homeowner’s potted African violet!
– A preliminary diagnosis can be confirmed or rejected by further investigations to
produce a useful, accurate or definite diagnosis.
A useful diagnosis is the level at which many diagnosticians operate. Important
components of the pest signature must be consistent with the sample or specimen.
An accurate diagnosis is usually acceptable to plant pathologists, growers,
quarantine and courts of law. For an accurate diagnosis all facets of the pest
signature must be present or their absence explained.
– For parasitic pests their presence in, or in association with, the plant must be
verified.
– For non-parasitic agents the presence of the causal agents at the appropriate time
and place must be established. A site visit and appropriate questions increases the
reliability of the diagnosis.
A definite diagnosis.
26 The diagnostic road map
– A definite positive diagnosis requires the identification of both the causal agent
and the symptoms it causes. Most pests and diseases are known and well studied
so that descriptions, information about their life cycles and diagnostic tests are
available and can be used to confirm or reject a diagnosis.
– A definite negative diagnosis is often all a diagnostician can conclude, ie what the
problem is not and the diagnostician’s report may read ‘no evidence of infectious
disease’. Suspected problems are eliminated because critical components of the
pest signature are not present, eg a diagnostic test is negative. Many causes may be
eliminated by noting recent weather, determining soluble salt levels in soil and
noting the appearance of roots and internal tissues of stems. A negative diagnosis is
still useful. Clients are often pleased to know that symptoms do not confirm the
presence of the exotic fireblight disease of apples, hormone herbicide injury or a
fungal disease. Such information can allow certain fungicides to be omitted from a
spray program.
An inconclusive diagnosis. For a variety of reasons, many investigations will be
inconclusive, eg samples are too small, inability to recognise exotic pests or symptoms
are non-specific (indistinct), undescribed or of unknown causes. The time and effort
devoted to some investigations may not be justifiable and it may be unlikely that a
definite diagnosis can be reached.