Microscopic study of the establishment of Xanthomonas campestris pv. manihotis on in vitro microcuttings of cassava

The establishment of 2 strains of Xanthomonas campestris pv.manihotis (i.e. strain CIAT1111M, non aggressive and strain ORST 17 aggressive) on vitro plants of cassava was monitored by scanning electron microscopy (SEM). The inoculum was deposited on leaf surfaces of in vitro plants with a fine brush. Immediately after inoculation the appearance and the distribution of bacteria were similar for both strains. Within 6 days the aggressive strain increased rapidly, masses of bacteria were visible near stomata and lesions developed on leaves. On the other hand, the non-aggressive strain did not lead to visible alterations in leaf tissues. SEM revealed bacterial cells under a thin layer of unknown composition. Using SEM examinations, the localization of bacterial cells and their spreading on the leaf surface were more accurately defined. Our results suggest the relative importance of colonization of stomata in aerial bacterial infection. A relation was reported between strain aggressiveness and colonization of cassava leaves. In vitro culture provides a material suitable for studying different steps of host pathogen interactions.