Detection of the black rot pathogen (Xanthomonas campestris pv. campestris) and other xanthomonads in Zimbabwean and imported Brassica seed

Zimbabwean and imported Brassica seed were investigated for the presence of the black rot pathogen, Xanthomonas campestris pv. campestris (Xcc), and other plant pathogenic xanthomonads. Washings from 111 seed lots were plated on three semi-selective agar media. Bacterial colonies similar to three reference strains of Xcc were isolated from six of nine lots from Zimbabwe and seven of 102 imported lots. These strains were further characterized by means of viscosity, conjugated Staphylococcus aureus slide agglutination, pathogenicity, and the Biolog GN system. The most efficient semi-selective media were FS and NSCAA followed by NSCAA. All strains were positive in the viscosity test, and while most were positive in slide agglutination test, some gave negative or weak reactions. The Biolog GN MicroPlate system identified the strains as follows: Xcc (3 strains), Xcc Biolog type A (4), Xcc Biolog type B (1), X.c. pv, aberrans (1), X.c. pv. armoraciae (1) and Stenotrophomonas maltophilia (syn. Xanthomonas maltophilia) (1) and a Xanthomonas-like strain (1). All except S. maltophilia and the Xanthomonas-like strain were pathogenic to cabbage seedlings. This study shows that identification of Xcc-like colonies isolated on semi-selective media must be confirmed by other tests. Either viscosity or Biolog testing were found to be highly effective methods for presumptive identification of suspected colonies of X.c. pv. campestris, pv. armoraciae and pv. raphani, however, final confirmation of these pathogens should be by pathogenicity tests. This is the first report on the occurrence of X.c. pv. armoraciae and X.c. pv. raphani in Brassica seed produced in Zimbabwe.