The European Commission requested EFSA’s Panel on Plant Health to perform the pest categorisation for Clavibacter michiganensis subsp. insidiosus. The identity of the bacterium responsible for the bacterial wilt of lucerne is clearly defined. C. michiganensis subsp. insidiosus is present in only a few MSs in the EU and it is listed in the Annex IIAII of the Directive 2000/29/CE. Only sporadic disease outbreaks occur, and not in countries where lucerne production is of importance. The pathogen causes yield and quality loss only if susceptible cultivars are grown and conditions are favourable for disease expression. The pathogen is not reported in the main lucerne-producing MSs. There are no indications that in last decade the pathogen has a high impact on lucerne production in the EU, possibly because of the use of bacterial wilt-resistant varieties. C. michiganensis subsp. insidiosus is seed-borne and probably seed-transmitted, although with some uncertainty. The main pathway for long-distance dispersal of this pathogen is very likely via seeds, while machines and contaminated hay may also potentially play some role in the dissemination of the pathogen. The pathogen can be easily detected and identified on the basis of various microbiological and molecular tests and disease symptoms, including leaf mottling, reduction in plant height, and “witches’ broom” syndrome. Effective management strategies are available and include the use of resistant cultivars and, probably, the use of pathogen-free seeds. Finally, the Panel concluded that severe consequences, in terms of yield and quality losses, are expected for lucerne only if bacterial wilt-susceptible varieties are grown and if weather conditions are conducive to the disease.