Histopathology of susceptible and resistant Capsicum annuum cultivars infected with Ralstonia solanacearum

The vascular colonisation of resistant and susceptible hot chilli (Capsicum annuum) cultivars by Ralstonia solanacearum was examined using transmissio n electron microscopy. Tap roots of artificially-inoculated plants, grown i n sterilised soil were investigated to observe the morphological barriers i nvolved in the restriction of bacterial spread. In the resistant cultivar, several responses induced in response to bacterial infection, were observed . First, a cell wall coating material developed together with swelling of t he primary wall of the xylem vessels, limiting the bacterial spread. Second , formation of various types of vesicles in the vascular parenchyma cells, which enveloped the bacterial mass and also partly restricted the pathogen spread. Third, induction of hypersensitive reaction in the xylem vessels re sulted in the distortion and lysis of the bacteria. In the susceptible cult ivar, Vascular coating, production of vesicle and induction of hypersensiti ve reaction were not observed and bacterial spread was not limited. Rapid v ascular colonisation of the susceptible cultivar seemed to be generalised w hich resulted in the rapid wilting of affected plants. Other reactions involved in both resistant and susceptible cultivars includ e disorganisation of cytoplasm of parenchyma cells, disintegration of nucle i, and rupturing of xylem vessel walls. The restriction of pathogen spread associated with the resistance in C, annuum to bacterial wilt was mainly at tributed to some induced, morphological and physical barriers.