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First Report of Syringae Leaf Spot Caused by Pseudomonas syringae pv. syringae on Tomato in Italy

In the spring of 2006 and 2007, grafted and nongrafted tomato plants (scion cv. Cuore di Bue, rootstock Lycopersicon lycopersicum × L. hirsutum cv. Beaufort) displaying stem and petiole necrosis were observed in many commercial greenhouses in the Piedmont of northern Italy. Initial symptoms that developed 2 to 10 days after transplanting consisted of water-soaked circular lesions (2 to 3 mm in diameter) on stems and petioles. These lesions eventually coalesced into brown-to-black areas as much as 1 cm in diameter. In some cases, necrotic areas progressed from stem petioles to leaf tissues. Thereafter, plants wilted and died within a few days. In some greenhouses, more than 80% of young plants exhibited symptoms and production was severely reduced. Two to three sections of symptomatic tissue from stems and petioles from 20 affected plants were surface disinfested in 0.5% NaOCl for 1 min and repeatedly washed in sterile deionized water. Samples were macerated in nutrient yeast dextrose broth, streaked onto nutrient yeast dextrose agar (NYDA), and incubated at 22 ± 1°C for 48 h. Light yellow colonies typical of Pseudomonas spp. were consistently isolated on NYDA. All colonies fluoresced under UV light when grown on King's B medium (3). Colonies were levan positive, oxidase negative, potato soft rot negative, arginine dihydrase negative, and tobacco hypersensitivity positive (LOPAT test; group Ia). In addition, all isolates were positive for arbutin and aesculin hydrolysis and utilized erythitol, but not adonitol, l(+)-tartrate or dl-homoserine as a carbon source. The isolates also caused severe necrotic lesions on lemon fruits and lilac leaves (4). The bacterial colonies were identified as Pseudomonas syringae pv. syringae (1). Also, repetitive-sequence PCR using the BOXA1R primer indicated that the isolates belong to pattern 4 of P. syringae pv. syringae (4). The pathogenicity of three isolates was tested twice by growing the bacterium in nutrient broth shake cultures for 48 h, pelleting the suspension, resuspending the cell pellet in sterile water to a concentration of 10⁶ CFU/ml, and spraying 35-day-old healthy tomato plants (cv. Cuore di Bue) with the inoculum. Ten grafted and 10 nongrafted plants were inoculated, and the same number of plants was sprayed with sterile nutrient broth as a control. After inoculation, plants were covered with plastic bags for 48 h and placed in the greenhouse at 22 ± 1°C. Six days postinoculation, stem lesions, similar to those seen in the field, and leaf spots were observed on all bacteria-inoculated plants, but not on the controls. Leaf tissues did not develop symptoms. Isolations were made from the lesion margins and the resulting bacterial colonies were again identified as P. syringae pv. syringae. To our knowledge, this is the first report of Syringae leaf spot caused by P. syringae pv. syringae in Italy as well as in Europe. A bacterial spot of tomato caused by P. syringae pv. syringae has been reported in the United States (2).

References: (1) A. Braun-Kiewnick and D. C. Sands. Pseudomonas. Page 84 in: Laboratory Guide for the Identification of Plant Pathogenic Bacteria. 3rd ed. N. W. Schaad et al., eds. The American Phytopathological Society, St. Paul, MN, 2001. (2) J. B. Jones et al. Phytopathology, 71:1281, 1981. (3) E. O. King et al. J. Lab. Clinic. Med. 44:301, 1954. (4) M. Scortichini et al. Plant Pathol. 52:277, 2003.