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<p>Abstract</p> <p>Background</p> <p>Late blight is the most serious potato disease world-wide. The most effective and environmentally sound way for controlling late blight is to incorporate natural resistance into potato cultivars. Several late blight resistance genes have been cloned recently. However, there is almost no information available about the resistance pathways mediated by any of those genes.</p> <p>Results</p> <p>We previously cloned a late blight resistance gene, <it>RB</it>, from a diploid wild potato species <it>Solanum bulbocastanum</it>. Transgenic potato lines containing a single <it>RB </it>gene showed a rate-limiting resistance against all known races of <it>Phytophthora infestans</it>, the late blight pathogen. To better understand the <it>RB</it>-mediated resistance we silenced the potato <it>Rar1 </it>and <it>Sgt1 </it>genes that have been implicated in mediating disease resistance responses against various plant pathogens and pests. The <it>Rar1 </it>and <it>Sgt1 </it>genes of a <it>RB</it>-containing potato clone were silenced using a RNA interference (RNAi)-based approach. All of the silenced potato plants displayed phenotypically normal growth. The late blight resistance of the <it>Rar1 </it>and <it>Sgt1 </it>silenced lines were evaluated by a traditional greenhouse inoculation method and quantified using a GFP-tagged <it>P. infestans </it>strain. The resistance of the <it>Rar1</it>-silenced plants was not affected. However, silencing of the <it>Sgt1 </it>gene abolished the <it>RB</it>-mediated resistance.</p> <p>Conclusion</p> <p>Our study shows that silencing of the <it>Sgt1 </it>gene in potato does not result in lethality. However, the <it>Sgt1 </it>gene is essential for the <it>RB</it>-mediated late blight resistance. In contrast, the <it>Rar1 </it>gene is not required for <it>RB</it>-mediated resistance. These results provide additional evidence for the universal role of the <it>Sgt1 </it>gene in various <it>R </it>gene-mediated plant defense responses.</p>