YSSM-XP MEDIUM FOR XANTHOMONAS-CAMPESTRIS PV PHASEOLI

A starch-based agar medium (YSSM-XP) was developed for isolation of Xanthomonas campestris pv. phaseoli and 'fuscans' strains from dry bean plants and seeds. All strains of X. c. phaseoli and 'fuscans' grew on this opaque medium in 3-4 days and produced a clear zone of starch hydrolysis around the colonies. Zones were persistent and allowed for enumeration of colonies for an extended period in contrast to other starch-based media. The 'fuscans' strains produced the brown diffusing fuscous pigment in 6-9 days. Most other X. campestris pathovars grew on this medium but the ubiquitous Erwinia herbicola, Pseudomonas fluorescens, and other plant pathogenic bacteria tested did not. The number of contaminants was low when isolations were made from leaf or seed samples of dry bean. YSSM-XP was prepared by mixing 200 mL of a stock salt solution (NH4H2PO4 2.5 g, K2HPO4 2.5 g, MgSO4.7H2O 1.0 g, NaCl 25 g, distilled water 1 L), yeast extract 5.0 g, and distilled water 800 mL, adjusting pH to 7.4 and then adding soluble potato starch 10 g and Difco Bacto agar 15 g with vigorous shaking. After autoclaving, the medium was cooled to 50-degrees-C and the following ingredients were added before pouring plates: cephalexin 20 mg, chlorothalonil 15 mg, kasugamycin 20 mg, brilliant cresyl blue 1 mg, and methanol 20 mL. Plates were stored at 4-5-degrees-C for a minimum of 4 days to render the medium opaque by starch retrogradation and then dried before use.