The aim of this study was to identify the potato spindle tuber viroid( PSTVd) by reverse transcriptase-PCR . among all plant diseases which are caused viroids, PSTVd was the first viriod which is recognized by plant pathologist . suspected potato tubers from Razavi and Northern Khorasan provinces collected and cultured . leaves of cultured tubers were used for RNA extraction by PEG6000 precipitation method. subsequent RT-PCR reaction has been carried out using specific primers. A 359 bp fragment has been appeared after electrophoresis which has not appeared in healthy tuber samples.

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