First report of black rot caused by <i>Xanthomonas nasturtii</i> on watercress in Spain and Portugal

Xanthomonas nasturtii was first identified as the cause of black rot watercress (Nasturtium officinale) on plants grown in Florida, USA (Vicente et al., 2017). Similar symptoms had been reported earlier Hawaii (McHugh & Constantinides, 2004) and an unidentified sp. isolated Portugal 2003 from wild (Cruz Since 2017, crops southern Spain have occasionally exhibited small yellow leaf lesions around hydathodes, spots, V-shaped lesions, wilt, distortion senescence (Fig. 1). Symptoms were more frequent during mild humid periods November to February, reducing yields by up 60% leading rejection some (Lascelles, 2019). In 2021, seen produced sold UK. Isolations made leaves showing typical between 2017 2021 excising macerating tissue fragments sterile water plating King's medium B (KB). Seed tested using a method based International Testing Association for detection campestris pv. campestris. Mixtures bacterial colonies observed plates after incubation 48–72 hours at 28°C resembling (yellow, mucoid) sub-cultured. Isolates purified stored -76°C nutrient broth with 150 g/l glycerol 2017) and/or -80°C Protect® (Protect System, UK). contrast type strain (NCPPB 4600) isolates did not browning KB 2). Pathogenicity three accessions Savoy cabbage cv. Wirosa F1 (Bejo Zaden B.V., The Netherlands) stabbing insect pin charged bacteria Xanthomonas-like pathogenic all 3) whereas no cabbage. To complete Koch's postulates, re-isolations symptomatic inoculated WHRI 10015A (from Spain) 10049A Portugal). same morphological characteristics obtained. Fatty acid profiling (Weller 2000) performed trypticase soy agar 48 hours; comparisons libraries that include X. nasturtii, indicated highest matches 0.38 0.68 axonopodis DNA extracted outlined Vicente al. (2017). partial 16S rRNA gene amplified sequenced confirmed their identity species. gyrB selected according Parkinson (2007) (Table phylogeny 4) belong shows there is genetic diversity amongst them. This report Europe. We suspect may be widespread production areas Iberian Peninsula and, crop beds, pathogen likely spread relatively easily. Based seed lot Spain, borne therefore it important test avoid introducing disease into clean beds new areas/countries. work funded Royalcress S.A., Watercress Company, UKRI, Defra Scottish Government.