Three semi-selective media for Pseudomonas syringae pv. maculicola and P. cannabina pv. alisalensis

Three semi-selective media, DTarTA, SPbc, and SPamt, were developed and tested to isolate Pseudomonas syringae pv. maculicola (Psm) and P. cannabina pv. alisalensis (Pca) from Raphanus sativus seeds. DTarTA contained D-tartaric acid as a carbon source and potassium tellurite, ampicillin sodium, and methyl violet as antibiotics. DTarTA suppressed growth in 19 of the 24 pathovars from the P. syringae complex, whereas Psm and Pca grew and formed gray to black colonies. SPamt contained sucrose and peptone as nutrient sources and was supplemented with bromothymol blue and the same antibiotics present in DTarTA and Psm and Pca formed yellowish to dark brown colonies on the SPamt medium. SPbc contained sucrose and peptone and was supplemented with cephalexin and boric acid as antibiotics and Psm and Pca formed semi-translucent to white colonies on the SPbc medium. SPamt and SPbc suppressed the growth of several plant-associated bacteria (except the P. syringae complex). The growth of saprophytic bacteria in seeds on the different media was compared with that on King's B medium, using five types of commercially available Raphanus sativus seeds. The suppression rate of DTarTA was 85-99% and was lower for seeds with more saprophytic bacteria. The suppression rates of SPamt and SPbc were 90-99%. In detection tests using 10,000 seed samples mixed with Pca or Psm-contaminated seeds, it was possible to selectively isolate Psm and Pca using SPamt and SPbc, even when the colony numbers of the target bacterium constituted less than 10% of the total colonies. KEY POINTS: • Bacterial leaf spot and blight pathogens were selectively isolated from seeds. • DTarTA medium distinguishes these pathogens from P. syringae complex pathovars. • SPamp and SPbc media have different selectivity for plant-associated bacteria.