For diagnosis of plant disease, it is essential to determine whether the disease is caused by a pathogen or an environmental factor i.e. abiotic factor. It is also important to know whether the disease is infectional or non-infectional.
Diagnosis of a disease caused by either a virus, viroids, rickettsia or protozoa may involve a series of tests. The most common of which are –
- Inoculation of several host plants with the pathogen and comparison of the symptoms on these plant/hosts with those produced on the same host by known pathogen.
- Electromicroscopy of infected tissues & comparison of the morphology of the pathogen in them – if found with that of other known pathogens.
- Application of certain antibiotics on the diseased plant to determine whether the pathogen is susceptible to any of them or not as expressed by recovery of the plant from the disease.
For example – Susceptibility to tetracyclin would tend to indicate a possible MLO etiology. Susceptibility to penicillin (from fungus) would indicate a possible Rickettsia like bacter etiology, while no effect would indicate possible viral etiology.
- Thermotherapy of the disease by exposed diseased plants or parts of them to hot water in high an temperatures for different periods of time. Recovery of symptoms at lower temperature or in shorter periods would tend to indicate MLO or Rickettsia like etiology.
- Serology: If the pathogens can be isolated and purified e.g. Virus, Spiroplasma citrr cultures, antiserum may be produced & diagnostic subsequently used for diagnostic serological test.