We report on strong site-selective enhancement of the emission of localized excitons in an InxGa1-xN film, induced by resonant coupling to a single plasmon confined in a gold nanoparticle. The particle was attached to an atomic-force-microscope probe and placed at the near-field distance of the surface. The observation is explained by the enhancement of the spontaneous emission recombination rate of the excitons due to the local increase in the photonic mode density near the metal particle. The interpretation is consistent with the intensity increase and lifetime shortening of the emission observed in the same film with deposited Au clusters. We show that the nanoscale roughness of the film is an important prerequisite of the efficient plasmonic enhancement.