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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