Despite the importance of potato the late blight disease, caused by the oomycete Phytophthora infestans is most destructive disease for potato production. The study was conducted to identify virulent races and spatial distribution of Phytophthora infestans populations in major potato growing areas of northwestern Ethiopia in 2018. Seventy-four isolates (samples) were collected and multiplied on potato tuber slices and virulence was assessed on Black’s potato differentials. Isolates reaction to differentials revealed 6, 11, and 16 race types at Awi, South Gondar and West Gojam, respectively. Shannon diversity index was 0.80 for the entire isolates but it was 0.75, 0.84 and 0.92 for isolates collected from Awi, South Gondar and West Gojam, respectively. Tewnty-seven physiological races were detected in the pooled population when the 74 isolates from the three populations were combined. Absence of race complexity was found in isolates of South Gondar and West Gojam while the highest race complexity with virulence factors of 10 was found in South Gondar. Differentials, R3, R5, R8, and R9 had larger contribution to the total variability of isolates and R9 and R5 genes were most effective withstood 95% and 92% of isolates. It is concluded that potato varieties resistant to major genes are hardly possible to be used as disease management option due to the absence of R genes resistant to all isolates, the complexity, and distribution of races in the region.
