Phenotypic and Genotypic Structure of Phytophthora infestans Populations Infecting Tomato in Türkiye

Phytophthora infestans , causing late blight, remains a major constraint to tomato production due to its high genetic variability, rapid adaptation and evolving fungicide resistance. We investigated the population structure of P. infestans isolates from South Marmara, Türkiye, during the 2020–2022 growing seasons using clone‐corrected population genetic analysis. Isolates were characterized by mating type, in vitro mefenoxam sensitivity, 12 polymorphic simple‐sequence repeat (SSR) markers, and virulence assays using potato differential lines. Overall, 89% of the isolates were A1 mating type and 11% were A2. Mefenoxam assays showed 56% of isolates were sensitive, 21% intermediate and 23% resistant. SSR genotyping revealed extremely high clonality (93.9% clonal fraction) with five distinct clonal lineages among 82 isolates. The population was dominated by the 1_A1_TR clonal lineage (73.2%, n = 60), followed by 13_A2_TR, 23_A1_TR1, 23_A1_TR2 and 39_A1_TR. These lineages are first reported here in association with tomato late blight in Türkiye. Isolates from Çanakkale exhibited the highest genetic diversity with four lineages, while isolates found in Balıkesir had only 1_A1_TR present. Clone‐corrected analysis of molecular variance revealed no significant genetic differentiation among provinces. Solanum demissum virulence assays revealed common phenotypes overcoming multiple potato‐specific R‐genes ( R1–R11 ), absent in agricultural tomatoes. A minimum spanning network illustrated the central role of the dominant 1_A1_TR lineage and its genetic relationships to minor lineages. Thus, the population structure of P. infestans in Türkiye is predominantly clonal, probably influenced by migration from Europe and local clonal expansion, with context‐dependent fitness advantages under regional production systems.