Abstract: The possible carryover effect of three fungicides and one insecticide on results of PCR and Multiplex-PCR for detection of bacterial pathogens was studied. The potato brown rot caused by phylotype II, sequevar I of Ralstonia solanacearum and the potato ring rot bacteria Clavibacter michiganensis ssp. sepedonicus were considered in this study. Under optimized conditions for PCR activity, the potato extract prepared from the Cara potato cultivar did not interfere with band development of DNA’s extracted from the organisms in concern. Regardless of their mode(s) of action, the studied pesticides exerted different effect(s) on band development according to their chemical grouping. The dimethomorph (Acrobat), systemic fungicide, caused complete inhibition of band development of DNA extracted from either potato brown rot or potato ring rot bacteria which may be considered as false negative reaction(s). The propamocarb carbamate (Previcur N), systemic fungicide, did not inhibit band development of the organisms in concern. The dithiocarbamate plus zinc and manganese (Diathane M45), contact fungicide however, caused a complete band inhibition of ring rot DNA contrary to that being reported for brown rot, indicating a preferential polymerase activity. The organophosphate (Malathion), penetrative non-systemic insecticide, did not inhibit DNA band development for both organisms. Multiplex-PCR of mixed DNA’s of R. solanacearum and C. michiganensis ssp. sepedonicus caused a false negative reaction through inhibition of band development of R. solanacearum but not for C. michiganensis ssp. sepedonicus, under the effect of propamocarb carbamate (Previcur N) fungicide and organophosphate (Malathion) insecticide. It could be concluded that the pesticides carryover effect on PCR results of potato brown rot and ring rot bacteria are being variable according to the pesticide grouping. The dimethomorph (Acrobat) fungicide caused false negative effect with both organisms under study followed by dithiocarbamate (Diathane) fungicide in case of ring rot bacteria. The false negative multiplex-PCR is being evident only in case of R. solanacearum under the stress of both propamocarb carbamate (Previcur N) fungicide and on the organophosphate (Malthion) insecticide. The mode of action of the used pesticide, either contact or systemic, had no effect on PCR or multiplex-PCR results. Further investigations are needed with different groups of pesticide.