Duplex RT-PCR Detection of Potato spindle tuber viroid with an Internal Control of Potato ND2 mRNA Duplex RT-PCR Detection of Potato spindle tuber viroid with an Internal Control of Potato ND2 mRNA Development of cDNA Dimeric Probes and Its Application in Diagnosis ofPotato spindle tuber viroid Comparison of different size of probes,labels and detection substrates for diagnosis of Potato spindle tuber viroid by nucleic acid spot hybridization IDENTIFICATION OF POTATO SPINDLE TUBER VIROID(PSTVD)AND ITS CONTROL