These methods are frequently used in bacteriological diagnosis. Most of the diagnosis methods finalized are based on genetic amplification.
The diagnosis of the diseases caused by viroids can only be done by serological methods. In this case techniques based on the analysis of nucleic acids in the infected plants are performed by electrophoresis on polyacrylamid gel, or on the characterization of nucleic acids by molecular hybridization. The resort to molecular amplification, through the PCR, has allowed to extend the limits of sensitivity to diagnosis techniques based on the detection of specific sequences of nucleic acids Return to the page "Identifying the phytopathogenic agents : methods of diagnosis”
This method is based on the principle of the formation of DNA double strand (duplex) between a DNA molecule, target of the pathogen fixed on a support of DNA and a strand of complementary nucleic acid which is marked (marker) and which reveals its presence.
Being given the nature and pathological proprieties of viroids, the disease diagnosis that they induce can only be attested by serological methods by reason of the absence of structural proteins as well as antigenic properties. Furthermore biological tests for viroids are time-consuming and can only be reproduced under very strict environmental conditions of the indicator plants.
The polymerase chain reaction is a molecular biological method defined in 1985 by Kary Mullis, who won the Nobel prize in 1993. This method makes it possible to duplicate a DNA fragment and, by repeating the procedure to obtain billions of copies (2 N after N cycles).
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