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The potato late blight pathogen, <i>Phytophthora infestans</i>, is able to rapidly evolve to overcome resistance genes. The pathogen accomplishes this by secreting an arsenal of proteins, termed effectors, that function to modify host cells. Although hundreds of candidate effectors have been identified in <i>P. infestans</i>, their roles in pathogenicity or virulence remains basically unknown. The potato <i>RB</i> gene, derived from the wild species <i>Solanum bulbocastanum</i>, confers resistance to most <i>P. infestans</i> strains through recognition of members of the pathogen effector family IPI-O. While the majority of IPI-O proteins are recognized by RB to elicit resistance (e.g. IPI-O1, IPI-O2), some family members are able to elude detection (e.g. IPI-O4). Our results showed that IPI-O4 functions to turn off resistance mediated by the potato gene <i>RB</i>. This effector accomplishes this by directly interacting with the RB protein, which likely modifies its ability to turn on host resistance. Further molecular analysis identified two amino acids within the effector that determine interaction, which can assist in developing appropriate disease control strategies.<p><p>Keywords: Oomycete, Root-Tuber Crops, Potato