A semiselective medium for detecting epiphytic and systemic populations of Pseudomonas savastanoi from oleander.
A new medium, oleander knot agar (OKA), was developed to isolate P. [syringae pv.] savastanoi from Nerium oleander plants. OKA contained (in g/litre) agar (14.0), smallcap˜L-serine (2.0), yeast extract (1.0), NH4H2PO4 (0.5), K2HPO4.3H2O (0.5), NaCl (5.0), MgSO4.7H2O (0.2), boric acid (1.0), vancomycin (0.15), cephalexin (0.05), bacitracin (0.05), ampicillin (0.015), novobiocin (0.02) and cycloheximide (100). All 39 strains of P. syringae pv. savastanoi from different geographical areas grew on OKA, and the quantitative recovery ranged from 5.7 to 93.6% (mean=43.6%). OKA was more selective than other media tested, including BCBRVB, M-71, KBBC, MSP, proline agar and PVF-1, and inhibited 89-96.7% of saprophytic bacterial strains recovered from N. oleander tissues. Of other plant-pathogenic bacteria tested, 44% did not grow on OKA and 13% grew sparingly. OKA was used to detect epiphytic populations of P. syringae pv. savastanoi from galls, leaves, stems and flowers and to detect systemic movement of the bacterium in N. oleander stems. The pathogen was present in wash water from 26 to 34 apparently healthy N. oleander plants. Of these plants, 31 subsequently developed galls within 1 year. The bacterium was also capable of systemic movement up to 25 cm above and 20 cm below the site of inoculation.