HYPERSENSITIVE REACTION OF TEPARY BEAN UPON INOCULATION WITH THE COMMON BEAN BLIGHT PATHOGEN

Tepary bean, Phaseolus acutifolius A. Gray, has been suggested as a so urce of genetic resistance to Xanthomonas campestris pv, phaseoli (Xcp ) for beans. However, the mechanism of resistance has not been elucida ted. The tepary bean reaction to inoculation with Xcp suggests an indu ced 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 detach ed leaves the response after 48 h was chlorosis of the inoculated tiss ues, but after 72 h a rapid collapse of the tissues surrounding the in oculated area was observed. The reaction resembles a hypersensitive re sponse (HR) in attached and detached leaves and cotyledons. HR was mor e evident in younger leaves than in older leaves and was found associa ted 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 a mong virulent strains of Xcp.