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Background and Aims Tyloses and embolisms have been reported to impair water transport during the development of Pierce's disease ( PD), caused by the xylem-dwelling pathogen X ylella fastidiosa. This work investigates the relative importance of these xylem conduit obstructions in stems of inoculated V itis vinifera plants. Methods and Results After 18 weeks in a greenhouse, internodes from control and needle-inoculated Chardonnay vines were assessed for embolisms using non-destructive magnetic resonance imaging ( MRI). Tyloses were identified by subsequent destructive histology and light microscopy examination. Embolism of individual or small sectors of vessels was detected by MRI analysis of a control shoot, usually in the proximity of the pith; virtually no tyloses were observed in the corresponding histological sections. From a symptomatic (inoculated) shoot, MRIs revealed large areas of embolised vessels extending radially towards the epidermis in most of the internodes; several individual or groups of two to three vessels appeared to be occluded with tyloses. Conclusions In all assessed internodes, MRI-detected embolisms were found more frequently than tyloses, suggesting that over the course of PD embolism formation precedes tylose development. Significance of the Study Embolisms appear to be the first and predominant type of occlusion in stems during early PD progression; nevertheless, their importance for explaining water transport impairment has been overlooked in most studies. [ABSTRACT FROM AUTHOR]