In 2008, field-grown cabbage (Brassica oleracea var. capitata L., cv. Grenader) from Monterey County, California showed symptoms on the wrapper leaves of immature plants that had formed heads. Initial symptoms consisted of small, brown, water-soaked flecks surrounded by chlorotic haloes. These flecks later coalesced into large, irregularly shaped, brown-black, necrotic lesions with chlorotic haloes visible on both adaxial and abaxial surfaces of the leaf. This disease resulted in lower quality and reduced marketability of the cabbage. Five gram-negative, blue-green fluorescing bacteria were isolated from separate lesions on different plants on King's medium B agar. The isolates were positive for levan formation and negative for oxidase and arginine dihydrolase. The isolates did not cause soft rot on potato slices but did induce a hypersensitive reaction in tobacco (Nicotiana tabacum L. cv. Samsun). These data indicated that the bacteria belonged to Lelliot's LOPAT group 1 (2). Repetitive extragenic palindromic sequence (rep)-PCR using the BOXA1R primer resulted in identical DNA fragment banding patterns for the cabbage isolates and the pathotype of Pseudomonas cannabina pv. alisalensis (formerly P. syringae pv. alisalensis). Additionally, both P. cannabina pv. alisalensis and the five cabbage isolates were sensitive to bacteriophage PBS1 while the pathotype strain of P. syringae pv. maculicola was not (1). Pathogenicity of the five cabbage isolates was evaluated in two independent experiments. Inoculum was prepared by growing the bacteria on nutrient agar for 48 h (27°C), suspending the bacteria in 0.01 M phosphate buffer (pH 7.0), adjusting each suspension to 0.6 OD at 600 nm (approximately 108 CFU/ml), and adding three to five drops of Tween 20. In each experiment, two cabbage, broccoli raab (Brassica rapa subsp. rapa cv. Sorrento), and oat (Avena sativa cv. Montezuma) plants were inoculated for each isolate by spraying until runoff. Positive control plants were inoculated with the pathotype strain of P. cannabina pv. alisalensis and negative control plants were treated with sterile 0.01 M phosphate buffer. The plants were placed in a mist chamber for 48 to 72 h and then in a greenhouse (20 to 25°C). After 7 to 10 days, foliar symptoms similar to symptoms observed on the original diseased cabbage plants developed on all inoculated plants including the positive control plants inoculated with P. cannabina pv. alisalensis. Additionally severe symptoms on broccoli raab and minor symptoms on oats developed on plants inoculated with cabbage strains or P. cannabina pv. alisalensis. For each experiment, bacteria reisolated from symptomatic tissue were identical to the bacteria used to inoculate the plants and to P. cannabina pv. alsialensis for rep-PCR DNA fragment banding pattern and sensitivity to phage PBS1. There were no symptoms on any of the cabbage and oat negative controls. Additionally, there were no symptoms on any broccoli raab negative controls in the first experiment; however, in the second experiment, a small (<1 mm) lesion was detected on one leaf of one plant. To our knowledge, this is the first report of P. cannabina pv. alisalensis causing bacterial blight of cabbage in California. This disease may have significant impact because of the large acreage of cabbage grown in California (approximately 5,666 ha annually).
References: (1) C. T. Bull et al. Syst. Appl. Microbiol. 33:105, 2010. (2) R. A. Lelliott. J. Appl. Bacteriol. 29:470, 1966.
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