undulosa Using a Modification of Wilbrink's Agar Medium'

Wilbrink' s-boric acid-cephalexin agar medium (WBC) was developed and tested for iso­ lating Xalllh omonas campeslris pv. IIndulosa, the causal agent of bacterial leaf streak of wheat or black chaff. from seed . WBC medium is Wilbrink's medium that has been modified by adding 0.75 gil of boric acid and 10 mg/l of cephalexin. Compared to the standard Wilbrink's medium, average plating efficiency of 15 strains of X. c. pv. undulosa isolated from different hosts and locations in Mexico was 100.5% for WBC medium . Recovery of those strains was only 59.7% when 1 mg/l of gentamycin was added to WBC. Recovery was not possible either on WBC amended with 2 mg/l gemamycin or on XTS medium comaining 8 mg/l gentamyc in . About 90% of saprophytic bacteria washed from wheat and triticale seed lots harvested in plot s infested by bacterial leaf streak were eliminated using WBC, making it easier to iden­ tify the pale yellow X. c. pv . undulosa colonies. Using WBC and washing in saline (30 minutes), 1 x 103 to 6 .1 x 106 X. c. pv . undlllosa colony-forming units (CFU) were found per gram of seed, the detection limit being 1 xl 03 CFU . WBC agar is a useful alternative for XTS medium because of the toxic effect of gentamycin observed under our conditions.