Xanthomonas nasturtii was first identified as the cause of black rot of watercress (Nasturtium officinale) on plants grown in Florida, USA (Vicente et al., 2017). Similar symptoms had been reported earlier in Hawaii (McHugh & Constantinides, 2004) and an unidentified Xanthomonas sp. was isolated in Portugal in 2003 from wild watercress (Cruz et al., 2017). Since 2017, watercress crops in southern Spain have occasionally exhibited small yellow leaf lesions around the hydathodes, leaf spots, V-shaped leaf lesions, wilt, distortion and senescence (Fig. 1). Symptoms were more frequent during mild and humid periods from November to February, reducing yields by up to 60% and leading to rejection of some crops (Lascelles, 2019). In 2021, symptoms were seen on watercress produced in Portugal and sold in the UK.