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First Report of Bacterial Blight Caused by Pseudomonas syringae pv. syringae on Common Vetch in Spain

Common vetch (Vicia sativa L.) is an important legume crop used for livestock feed in the Mediterranean Area. This crop has an important role for sustainable agriculture in dryland rotations in Spain, where the Castilla y León Region is the major production area. During the springs of 2007 and 2008, necrotic lesions on stems, leaves, and flowers were observed in five different common vetch plots around Medina de Rioseco (Castilla y León). Four of the plots were sown with cv. Buza. No information was available about the cultivar in the fifth plot. In many cases, lesions had expanded into the stems causing complete wilting. Disease incidence was estimated at approximately 20%. Two symptomatic plants per plot were sampled. One section per plant was individually surface disinfested in 0.5% NaOCl for 1 min, followed by three washes in sterile water. Macerates were plated in King's B medium (KB) agar (24°C for 48 h) (2). Colonies from all isolations on KB agar were pale yellow and blue-green fluorescent under UV light, as typical of fluorescent Pseudomonas spp. (2). Ten isolates (one per section) were characterized. These were identified as Pseudomonas syringae by the LOPAT scheme (2) and Hugh-Leifson reaction and all utilized erythritol, l-lactate, and dl-homoserine, but not l(+)-tartrate, as carbon sources. They were positive for aesculin and gelatine hydrolysis. The 10 isolates caused severe necrotic lesions when they were puncture inoculated (10⁸ CFU/ml suspension, 50 μl per wound, and two replicates) on immature lemon fruits (Citrus × limon, cv. Primofioro), bean pods (Phaseolus vulgaris L., cv. Ancha Lisa), and pea pods (Pisum sativum L., cv. Ucero). PCR amplification of a 752-bp syrB fragment (3) was positive for all isolates. On the basis of these tests, the 10 isolates were identified as P. syringae pv. syringae (2). Subsequently, each isolate was inoculated into two sets of 10 plants of V. sativa cv. Buza by injecting 200 μl of a bacterial suspension (10⁸ CFU/ml) into the stem (2); 10 plants were injected with sterile water as controls. Ten days after inoculation, necrotic symptoms were observed on all plants, and 1 week later, all plants were completely wilted and dead. These symptoms were similar to those observed in the field. Control plants remained symptomless. Isolations were made from two inoculated plants per each original isolate, and all reisolates were identical to the original isolates in the above biochemical tests and PCR of the syrB gene. P. syringae pv. syringae reference strains, CFBP1768 and CFBP1769 (Collection Française de Bactéries Phytopathogènes), gave the same results in all biochemical, pathogenicity, and PCR tests. To our knowledge, this is the first report of bacterial blight caused by this pathogen on vetch in Spain. This pathogen had been previously identified in this crop in France (4) and in V. villosa (a closely related species) in the United States (1). Therefore, to prevent the spread of this pathogen, research on efficient preventive and control measures is needed.

References: (1) G. L. Ercolani et al. Phytopathology 64:1330, 1974. (2) N. W. Schaad et al., eds. Laboratory Guide for the Identification of Plant Pathogenic Bacteria. 3rd ed. The American Phytopathological Society, St. Paul, MN, 2001. (3) K. N. Sorensen et al. Appl. Environ. Microbiol. 64:226, 1998. (4) C. Tourte and C. Manceau. Eur. J. Plant Pathol. 101:483, 1995.