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Late blight caused by the oomycete Phytophthora infestans Montagne de Bary is a devastating disease on potatoPeduction.A total of six parameters affecting the PCR system in P. infestans were investigated based on the template DNA of theisolate HH06-23 and EST-SSR primer pair Pi08N.The results showed that the optimal annealing temperature was 63℃,and the optimum PCR system of EST-SSR was 25 ng template DNA,0.5 mmol·L-1 dNTPs,2μL 10×Buffer(Mg2+ free),1.75 mmol·L-1 MgCl2,15 pmol primer,and 1.2 U Tap DNA polymerase in total 20 μL reaction system.The PCR program was initial denaturation at 94℃for 2 min,foilowed by 35 cycles of 94℃ for 30 s,63℃ for 30 s,and 72℃ for 30 S,then a final extension step was 72℃ for 7 min,and held at 4℃.In addition,using the optimal PCR system,a total of 20 isolates of P.infestans were used for testing the stability andpolymorphism of the PCR amplification.The clarity and abundant polymorphism indicated that this system was stable and suitable forresearching the genetic diversity of P. infestans population.