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Borage (Borago officinalis) is a New Host of Pseudomonas cichorii in the Ebro Valley of Spain

Borage (Borago officinalis L.) is an important vegetable crop for consumption in the Ebro Valley of northeastern Spain. During the autumn and spring of the last 10 years, black necrotic lesions were observed in leaf petioles of white-flowered borage plants in greenhouses and seedbeds in Zaragoza, Spain. These lesions developed from the soil line and spread upward to the central vein of leaves. Severely infected leaves of mature and occasionally young plants become rotted. Longitudinal sections through the crown revealed severely necrotic cortical, vascular, and pith tissues. Isolations from infected roots, petioles, and leaves onto King's B medium yielded a gram-negative, rod-shaped bacterium with colonies that were fluorescent under UV light. Bacterial colonies were purified and characterized. The isolates were strictly aerobic, negative for levan production, soft rot of potato, and arginine dihydrolase activity, oxidase positive, and induced hypersensitive reaction in tobacco leaves (2). The bacteria were further identified as Pseudomonas cichorii by comparison of the 49 carbohydrate utilization profiles, API 50 CH (bioMérieux, Marcy l'Etoile, France), with the reference strain ICPPB 2827. Ten lettuce plants used as indicators and borage plants were inoculated by root and petiole injections of bacterial suspensions (10⁸ CFU/ml) of the borage strains and the P. cichorii reference strain ICPPB 2827. Inoculated plants and controls were maintained in a growth chamber at 20 to 25°C with nearly 100% relative humidity. Symptoms of varnish spot, described in lettuce (1), and the black lesions initially observed in borage roots, petioles, and leaves were evident on all inoculated plants at 7 and 5 days after inoculation, respectively. No symptoms developed on control plants. A bacterium with identical characteristics to those described above was reisolated from the black lesions on inoculated plants. To our knowledge, this is the first report of P. cichorii as a pathogen of B. officinalis. Successful infection of borage plants was dependent on high humidity conditions, which is present because of the greater density of mature crops.

References: (1) R. G. Grogan et al. Phytopathology 67:957, 1977. (2) R. A. Lelliot et al. J. Appl. Bacteriol. 29:470, 1966.