Summary A root knot nematode resistance gene in Nicotiana tabacum, Rk, providing resistance to the nematode parasite Meloidogyne incognita is tightly linked to, or is a pleiotropic gene with a veinal necrosis systemic hypersensitive response to infection by Potato virus Y (PVY) M(s)N(r). The single PVY M(s)N(r) open reading frame was sequenced and found to have 89% protein identity to PVY N. Individual PVY M(s)N(r) polypeptides were deduced and the corresponding cDNA were cloned into a Potato virus X (PVX) based expression vector and used as templates for in vitro transcriptions. Infected plant sap, from N. benthamiana inoculated with infectious RNA, was used to inoculate both root knot nematode (RKN) resistant and susceptible tobacco lines. Lines were then evaluated for the induction of the hypersensitive response. The PVY M(s)N(r) NIb-replicase protein was found to induce a hypersensitive response 10 days post inoculation in nematode resistant tobacco. None of the other PVX/PVY M(s)N(r) constructs induced a hypersensitive response. The NIb-replicase of PVY N, which shares 93% identity to PVY M(s)N(r), did not induce a hypersensitive response when expressed from the PVX vector. This confirmed that the PVY M(s)N(r) NIb-replicase is the elicitor of PVY M(s)N(r) veinal necrosis on RKN plants and thus the first report of a Potyvirus replicase functioning as an avirulence factor.