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First Report of Bacterial Blight of Rutabaga (Brassica napus var. napobrassica) Caused by Pseudomonas syringae pv. alisalensis in California

In 2005, commercial, organically grown rutabaga (Brassica napus var. napobrassica) in San Benito County, CA showed symptoms of a previously undescribed disease on approximately 30% of the plants. Initial symptoms consisted of small (1 to 2 mm in diameter), angular, water-soaked flecks that often were surrounded by chlorotic haloes. These flecks enlarged and coalesced into large, irregularly shaped, gray brown lesions that could be as long as 10 mm. Lesions were visible from both adaxial and abaxial leaf surfaces and generally retained the chlorotic borders. A blue-green fluorescing pseudomonad was consistently isolated from lesions on King's medium B. Eight isolates were characterized and were levan positive, oxidase negative, and arginine dihydrolase negative. Isolates did not rot potato slices but induced a hypersensitive reaction in tobacco (Nicotiana tabacum cv. Samsun). These data indicated that the bacteria belonged to Lelliot's LOPAT group 1 (2). This was confirmed with data from fatty acid methyl ester analysis (MIS-TSBA version 4.10; MIDI, Inc., Newark, DE) that showed that the isolates were highly similar (similarity = 0.922 or greater) to Pseudomonas syringae. Amplification of repetitive bacterial sequences (rep-PCR) using the BOXA1R primer and the polymerase chain reaction resulted in identical banding patterns for the rutabaga isolates and the P. syringae pv. alisalensis pathotype strain. Pathogenicity was demonstrated by growing inocula of six isolates in nutrient broth shake cultures for 48 h (24°C), adjusting the bacterial suspension to 10⁶ CFU/ml, and misting the resulting suspensions onto rutabaga (cv. American Purple Top). Plants were enclosed in plastic bags for 24 h and then incubated in a greenhouse (24 to 26°C). Control plants were misted with sterile water and treated the same way. After 5 to 7 days, foliar symptoms similar to symptoms seen in the field developed on all inoculated plants, and reisolated bacteria were characterized and found to be P. syringae pv. alisalensis. Control plants remained symptomless. The results of two sets of pathogenicity tests were the same. To our knowledge, this is the first report of commercially grown rutabaga as a host of P. syringae pv. alisalensis and the first report of a B. napus host of this pathogen. This bacterial pathogen has previously been reported on commercial plantings of arugula (Eruca sativa), broccoli (Brassica oleracea var. botrytis), and broccoli raab (Brassica rapa var. rapa) in California and under experimental conditions it causes disease on additional hosts, including members of the Poaceae (1).

References: (1) N. A. Cintas et al. Plant Dis. 86:992, 2002. (2) R. A. Lelliott. J. Appl. Bacteriol. 29:470, 1966.