The diagnosis of infections in plants has changed immeasurably over the years. Early
references to disease in barley crops date back to the mid-1500s and probably represent
the first true records that we have of symptoms being noted. Early records
merely described the physical appearance of plants and were usually associated with
catastrophic crop failure. By 1870 fungal and bacterial diseases of plants were diagnosed,
but it was not until after 1900 that virus diseases were identified. The science
of plant pathology in its true sense came into being when it became possible to treat
plants to control pathogens or to use husbandry to avoid pathogen problems. Physical
symptoms on plants can be diagnostic, but very often they can be caused by several
pathogens and a more scientific approach is required. Diagnostic methods mainly
developed for the medical and veterinary sciences have now been applied to diseases
of plants and we now have a bewildering assortment of methods at our disposal. Plant
diseases still account for heavy losses in many parts of the world where total crop failure
due to disease can lead to human misery.
In the western world, much emphasis is now placed on effective disease control
by the use of clean seed and appropriate chemical intervention, but both rely on good
diagnostics to establish disease status prior to action being taken. The development
of quick and cheap methods for disease detection ensures that crop plants remain free
of pathogens.
Plant pathology techniques fall into three categories: traditional, serological and
nucleic acid, although some span more than one discipline. Traditional methods include
the use of indicator plants to produce visual symptoms of disease on susceptible hosts
and the use of synthetic media to encourage the growth of microorganisms which can
then be identified by colony morphology. These methods can be coupled with more
advanced techniques where additional information is required for diagnosis.