Phenotypic and Genotypic Diversity of Potato Late blight Pathogen (Phytophthora infestans) in South Korea Jinhee Seo*, Janggyu Choi, Jihong Cho, Yeongeun Park, Hyeonjin Park, Gyubin Lee and Younggyu Lee Highland Agriculture Research Institute, National Institute of Crop Science, Rural Development Administration, Pyeongchang 25342, Republic of Korea Phytophthora infestans is endemic to south korea and can infect potato crops at any stage in the growing season. Little is known about P. infestans population in south korea. The objectives of this study were to analyze genotypc and phenotypic changes of the population of P. infestans in South korea from 2009 to 2020, and to examine genotypic diversity of P. infestans populations at the same regions. 86 number of samples were collected from 11 locations in the 6 provinces. 8 of the samples that either amplified with CAPS markers and check through pairing test were the A2 mating types. A1 isolates were found every year and prevailed (8 out of 86 isolates). In the A2 mating type, isolate was found in Miryang in 2011, 2013. A2 has not been found since 2013, but was discovered in Buan and Miryang in 2020. In the race resistance test, the majority of isolates were virulent against R4. The virulence against R6 and R8 was moderately frequent. Virulence against R1, R3, R5, R9, R10 and R11 was generally rare of isolates collected in 2019. Area and mating type seem to be irrelevant. The KR_1_A1, known as the standard P. infestans genome of South Korea, had 12 types of core RxLR effectors. Three candidates were found in the avr3a gene, and the ATG05134 gene was 83.8% identical to the previously known avr3a gene. Since the avr3a gene is known to highly pathogenic, this gene mutation will affect the pathogenicity. Key words: Potato late blight, Phytophthora infestans, RxLR Effector