Bacterial streak and bulb rot of onion .1. A diagnostic medium for the semiselective isolation and enumeration of Pseudomonas viridiflava

A semiselective, diagnostic agar medium (T-5) and low temperature incubation technique were developed for recovering Pseudomonas viridiflava from the environment or plant material. Medium T-5 contains the following per liter: NaCl, 5.0 g; NH4H2PO4, 1.0 g; K2HPO4, 1.0 g; MgSO4 . H2O, 0.2 g; D-tartaric acid, 3.0 g; phenol red, 0.01 g; agar, 20.0 g; bacitracin, 10 mg; vancomycin, 6 mg; cycloheximide, 75 mg; novobiocin, 45 mg; penicillin G, 5 mg. The pH is adjusted to 7.4. Antibiotics are added aseptically after autoclaving. P. viridiflava recovery from artificially infested, field-soil (Tifton loamy-sand), with a cropping history of no onion production, was high, with a corresponding reduction of 99.99% of nontarget bacteria. However, soils from fields with a long history of onion production, near Vidalia, Georgia, contained significantly larger populations of background microflora that grew on medium T-5. Incubation at 5 degrees C reduced contaminating microflora 1,000- to 10,000-fold with no reduction in recovery of the target organism. However, this low temperature incubation required an increased incubation period of 3 weeks and reduced the level of fluorescence of P. viridiflava.