Greenhouse-grown Brussels sprouts (Brassica oleracea L. var. gemmifera) transplants from Monterey County, California showed symptoms in 2006 of a previously undescribed disease. Initial symptoms consisted of small (1 to 2 mm in diameter), angular, water-soaked flecks, some of which were surrounded by chlorotic haloes. These flecks coalesced into large, irregularly shaped, gray brown lesions as large as 10 mm. Lesions were visible from both adaxial and abaxial leaf surfaces and generally retained chlorotic borders. This disease resulted in decreased quality and reduced marketability of the transplants. Gram-negative, blue-green fluorescing bacteria were consistently isolated from lesions on King's medium B agar. Ten isolates were selected and used in further studies. Isolates were levan positive, oxidase negative, and arginine dihydrolase negative. Isolates did not rot potato slices but induced 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 Brussels sprouts isolates and the pathotype of Pseudomonas cannabina pv. alisalensis (formerly P. syringae pv. alisalensis). Like P. cannabina pv. alisalensis, the isolates from Brussels sprouts were sensitive to bacteriophage PBS1 (1). All 10 isolates were used in two independent pathogenicity experiments. Inoculum for pathogenicity studies 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), and adjusting each suspension to 0.6 OD at 600 nm (approximately 108 CFU/ml). In each experiment, six Brussels sprouts plants were inoculated for each isolate by spraying until runoff or by swabbing a suspension of the appropriate bacterial isolate to which Carborundum had been added. Additionally, four Brussels sprouts isolates were used to spray inoculate six broccoli raab (Brassica rapa subsp. rapa) plants. Positive control plants were inoculated with the pathotype of P. cannabina pv. alisalensis, and the negative control plants were inoculated with sterile buffer or sterile buffer with Carborundum. Inoculated plants were placed in a mist chamber for 48 h and then in a greenhouse (20 to 25°C). After 5 to 7 days, foliar symptoms similar to symptoms observed on the original diseased Brussels sprouts plants developed on all inoculated plants, including the positive control plants inoculated with P. cannabina pv. alisalensis. Negative control plants remained symptomless. In each experiment, bacteria reisolated from symptomatic tissue were identical to the bacteria used to inoculate the plants and to P. cannabina pv. alisalensis for LOPAT reactions, REP-PCR DNA fragment banding pattern, and sensitivity to phage PBS1. To our knowledge, this is the first report of P. cannabina pv. alisalensis causing bacterial blight of Brussels sprouts.
References: (1) C. T. Bull et al. Syst. Appl. Microbiol. 33:105, 2010. (2) R. A. Lelliott. J. Appl. Bacteriol. 29:470, 1966.
