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Abstract Potato virus Y(PVY,Potyviridae) is among the most important viral pathogens of potato. The potato resistance geneNy_tbrconfers hypersensitive resistance to the ordinary strain of PVY (PVY^O), but not the necrotic strain (PVY^N). Here, we unveil that residue 247 of PVY helper component proteinase (HCPro) acts as a central player controllingNy_tbrstrain‐specific activation. We found that substituting the serine at 247 in the HCPro of PVY^O(HCPro^O) with an alanine as in PVY^NHCPro (HCPro^N) disruptsNy_tbrrecognition. Conversely, an HCPro^Nmutant carrying a serine at position 247 triggers defence. Moreover, we demonstrate that plant defences are induced against HCPro^Omutants with a phosphomimetic or another phosphorylatable residue at 247, but not with a phosphoablative residue, suggesting that phosphorylation could modulateNy_tbrresistance. Extending beyond PVY, we establish that the same response elicited by the PVY^OHCPro is also induced by HCPro proteins from other members of thePotyviridaefamily that have a serine at position 247, but not by those with an alanine. Together, our results provide further insights in the strain‐specific PVY resistance in potato and infer a broad‐spectrum detection mechanism of plant potyvirus effectors contingent on a single amino acid residue.