Robert Koch (1884-1890) forwarded four essential procedural steps called postulates for correct diagnosis of a disease and its actual causal agent.
The postulates are:
1. Recognition: The pathogens must be found associated with the disease in the diseased plant. The symptom of the disease should be recorded.
2. Isolation: The pathogen should be isolated, grown in pure culture in artificial media. The cultural characteristics of the pathogen should be noted.
3. Inoculation: The pathogen of pure culture must be inoculated on healthy plant of same species/variety. It must be able to reproduce disease symptoms on the inoculated plant identical to step 1.
4. Re-isolation: The pathogen must be isolated form the inoculated plant in culture media. Its cultural characteristics should be similar to those noted in step 2 (This step was added by E.F. Smith).
If all the postulates are proved true, then the isolated pathogen is identified as the actual causal organism responsible for the disease.