Watercress (Nasturtium officinale) is a rapidly growing member of the Brassicaceae family that is mainly grown in water beds and is one of the oldest known leaf vegetables. Xanthomonas nasturtii causes black rot of watercress, producing lesions around the hydathodes, yellowing and wilting of leaves. This disease was first described in 2017 in samples from Florida, USA, although it has probably been present in crops for much longer. Subsequently, the disease was also found in Hawaii and in Europe, in Spain and Portugal. Isolates were obtained from leaf samples from farms, shop-bought bagged leaves and from seed. Characterisation of isolates was completed using fatty acid profiling, gyrB partial gene sequencing and pathogenicity tests. Three watercress accessions were susceptible to all tested isolates, but no symptoms were observed in savoy cabbage. Whole-genome sequence comparisons showed genetic diversity amongst the isolates, with isolates from Spain being the most diverse. There is some evidence for the presence of genes linked to heavy metal tolerance in some isolates that could be linked to the extensive application of copper based treatments in some farms. Using the genome sequences obtained in this study, we are developing new markers for diagnostics of X. nasturtii. Control of the disease is difficult as the bacteria can probably be easily transmitted through water in the water beds. Seed testing should be used to select clean seed lots for each new crop.