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 o ther plant pathogenic xanthomonads. Washings from 111 seed lots were plated on three semi-selective agar media. Bacterial colonies similar to three re ference strains of Xcc were isolated from six of nine lots from Zimbabwe an d seven of 102 imported lots. These strains were further characterized by m eans of viscosity, conjugated Staphylococcus aureus slide agglutination, pa thogenicity, and the Biolog GN system. The most efficient semi-selective me dia were FS and NSCAA followed by NSCAA. All strains were positive in the v iscosity test, and while most were positive in slide agglutination test, so me gave negative or weak reactions. The Biolog GN MicroPlate system identif ied the strains as follows: Xcc (3 strains), Xcc Biolog type A (4), Xcc Bio log type B (1), X.c. pv, aberrans (1), X.c. pv. armoraciae (1) and Stenotro phomonas maltophilia (syn. Xanthomonas maltophilia) (1) and a Xanthomonas-l ike strain (1). All except S. maltophilia and the Xanthomonas-like strain w ere pathogenic to cabbage seedlings. This study shows that identification o f Xcc-like colonies isolated on semi-selective media must be confirmed by o ther tests. Either viscosity or Biolog testing were found to be highly effe ctive 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 rep ort on the occurrence of X.c. pv. armoraciae and X.c. pv. raphani in Brassi ca seed produced in Zimbabwe.