Molecular characterization, comparison of screening methods, and evaluation of cross‐pathogenicity of black rot (Xanthomonas campestris pv. campestris) strains from cabbage, choy sum, leafy mustard and pak choi from Taiwan

Abstract

Choy sum (Brassica rapa var. parachinensis), leafy mustard (Brassica juncea) and pak choi (B. rapa var. chinensis) are highly nutritious components of diets in Taiwan and other Asian countries, and bacterial black rot caused by Xanthomonas campestris pv. campestris (Xcc) is a major biotic constraint in these crops. As very little was known about the Xcc strains from these crops in these regions, including their cross‐pathogenicity and aggressiveness on different hosts, Xcc strains were obtained from cabbage (Brassica oleracea var. capitata), choy sum, leafy mustard and pak choi crops in Taiwan. Two previously published PCR‐based assays reliably distinguished the Xcc strains from other Xanthomonas species and subspecies. Phylogenetic analysis based on repetitive sequence‐based PCR assays placed the Xcc strains in a clade distinct from other Xanthomonas species, and also showed host specificity. Although all of the Xcc strains from the different host species were pathogenic on all five Brassica test species in both a detached leaf assay and an intact plant assay, in the intact plant assay they showed differences in virulence or aggression on the different test hosts. The Xcc strains from leafy mustard and pak choi were consistently highly aggressive on all the test host genotypes, but the strains from choy sum and cabbage were less aggressive on leafy mustard and choy sum. The intact plant assay proved more discriminating and reliable than the detached leaf assay for comparing the aggressiveness of Xcc strains on different host genotypes, and so, with the new Xcc strains isolated in this study, will be useful for screening leafy brassica germplasm accessions for resistance to black rot.