Potato is one of the most important agricultural crops in Algeria and worldwide.Each year, potato late blight, caused by Phytophthora infestans, is responsible for significant damage that leads to large production losses, and is thus a direct threat to food security in Algeria. In this study, 131 isolates of P. infestans and 92 DNA fingerprints captured on FTA cards were sampled from commercial and seed production fields in three major potato production regions (western, eastern, and central) during the main-season and late-season in Algeria over six cropping seasons (2010)(2011)(2012)(2013)(2014)(2015)(2016).Genotypes of P. infestans and population genetic diversity were analysed using a 17plex simple-sequence repeat (SSR) marker assay, and the mating type of all isolates was characterized. Both mating types (A1 and A2) were found, and often occurred in the same field. Differences in mating type proportion were observed between regions and between sampling periods. Analysis with SSR markers showed the prevalence of the EU_13_A2 lineage (70%) over EU_2_A1 (16%), EU_23_A1 (10%), and 4% of unknown multilocus lineage (MLL). The EU_13_A2 showed differentiation within the group. EU_23_A1 was found mainly in late-season crops. However, the cropping region did not influence the distribution of lineages due to the dispersal of the pathogen in Algeria by seeds. Genetic structure did not reveal a clear variation in distribution of the three lineages throughout the sampling regions. These data provide important new information on the composition and change over time of P. infestans populations in Algeria and open the way for a better understanding of the local epidemiology of this important pathogen.

Cultivated potato (Solanum tuberosum L.) is one of the most important crops worldwide. Phytophthora infestans (Mont.) de Bary is the oomycete pathogen responsible for the famous Irish famine (1840s). It is still the most important pathogen affecting potato crops, causing the late blight disease on potato and tomato. It is mainly controlled by fungicides. Breeding for disease resistance is the best alternative to chemical control of the disease. One of the strategies used is to stack many resistance genes in the same genotype. Here, we wanted to test the effect of the stacked resistance gene (R) from S. demissum on the infection process by the virulent race EU_13_A2. Four potato genotypes were tested, each one harboring, respectively, one, two, three or four R genes. All the tested genotypes were sensitive. However, the sensitivity degree was negatively correlated with the number of genes harbored by each genotype. There was a delay of two days of symptoms manifestation for the genotype with 4Rs, and the pathogen produced less spores on the detached leaf test. In addition, the amount of phenolic compounds produced is higher in the genotypes with multiple R genes.