• Biological tests

To confirm a diagnosis and prove that the isolated pathogenic agent is really responsible for the symptoms observed, artificial inoculations of the bacteria are done on a host plant in conditions favourable to the infection

Koch’s postulate:

Koch’s postulate specifies the successive steps that have to be validated to establish a causal relationship between a disease and a micro-organism.

 

The steps of Koch’s postulate applied to phytopathology are as follows:

 

1. The micro-organism must be present in affected plants, and absent in unaffected ones;

 

2. It must be possible to isolate the micro-organism from the diseased plants and grow it in an axenic culture;

 

3. When the micro-organism in pure culture is inoculated to a normal plant, it must induce symptoms characteristic of the disease ;

 

4. It must be possible to re-isolate the initial micro-organism as from plants infected experimentally.

 

This postulate relies on a series of techniques :

  • of observation (microscopy), necessary for the detection and identification of the parasite in situ,
  • of isolation of the pathogen from the tissues of the host ,
  • of methods of production of the inoculum in pure culture
  • and of inoculation of host plants in laboratory.

In practice, the diagnosis of several known diseases do not necessitate all the stages of the Koch’s postulate. In each case, it is possible to stop at the observation or isolation stage. (Extract from Phytopathologie, De Boeck)

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  • Isolation : the initial step towards diagnosis

    • The environment used for isolation must satisfy the energetic and nutritive needs of the bacteria to be cultivated. Those nutritive preparations, liquid or solid, are meant for the development of microorganims in laboratories.

  • Identification by morphological and cytological characteristics

    • Once the bacteria is isolated, the observation of its morphological and cytological characteristics can orientate the diagnosis and help the identification of the bacteria responsible for the disease.