Hypersensitive reaction of tepary bean upon inoculation with the common bean blight pathogen

Tepary bean, Phaseolus acutifolius A. Gray, has been suggested as a source of genetic resistance to Xanthomonas campestris pv, phaseoli (Xcp) for beans. However, the mechanism of resistance has not been elucidated. The tepary bean reaction to inoculation with Xcp suggests an induced resistance mechanism. A brown discoloration at the scratched site and surrounding tissue appeared on cotyledons after 48 h. It was more pronounced with the high virulent than low virulent strains. On detached leaves the response after 48 h was chlorosis of the inoculated tissues, but after 72 h a rapid collapse of the tissues surrounding the inoculated area was observed. The reaction resembles a hypersensitive response (HR) in attached and detached leaves and cotyledons. HR was more evident in younger leaves than in older leaves and was found associated with viable cells, but not with dead cells.This study suggests the use of the tepary bean and its HR as a key test for differentiating among virulent strains of Xcp.