Optimization of the concentrations of three primer pairs used for the multiplex RT-PCR assay of PRSV, PLDMV, and PapMV. The amplified products of PRSV, PLDMV, and PapMV are indicated by the sizes of 613 bp, 355 bp, and 205 bp, respectively. Lanes 1–10 indicate the concentration combinations of three primers specific for PRSV, PLDMV, and PapMV: (1) 0.4:0.4:0.4 μM; (2) 0.4:0.4:0.2μM; (3) 0.4:0.2:0.4 μM; (4) 0.4:0.2:0.2 μM; (5) 0.4:0.2:0.1 μM; (6) 0.4:0.1:0.1 μM; (7) 0.4:0.1:0.2 μM; (8) 0.2:0.2:0.2 μM; (9) 0.2:0.1:0.1 μM; (10) 0.1:0.1:0.1 μM. Lane 11: negative control (ddH2O); Lane M: DL2000 DNA Marker.