Defeated Stacked Resistance Genes Induce a Delay in Disease Manifestation in the Pathosystem Solanum tuberosum-Phytophthora infestans

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.