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First Report of Bacterial Leaf Spot Caused by Pseudomonas syringae pv. viburnii on Viburnum sargentii in Italy

During the spring of 2003, plants of Viburnum sargentii, a species mostly used in gardens as low-maintenance hedges, showing symptoms unlike those of known diseases were observed in some private gardens in the Biella area (northern Italy). Lesions on leaves were the only symptoms seen. Lesions started as water-soaked, circular areas that in 4 days developed into irregular, shrunken, brown spots from 2 to 4 mm in diameter. The core of older lesions appeared somewhat transparent. Leaves dried up completely 3 weeks after symptoms were first seen. No fungal structures were observed in lesions. Microscopic examination of affected leaf tissues revealed abundant bacterial ooze from the cut margin of lesions. Small fragments of tissue from affected leaves were macerated in nutrient yeast dextrose broth (NYDA) and dilutions of the resulting suspension were streaked onto NYDA and potato dextrose agar (PDA). Isolations were made from at least 25 leaves. Plates were maintained at 22 ± 1°C for 48 h. Slightly yellow colonies typical of Pseudomonas species were consistently isolated on NYDA. No fungi were isolated from the spots on NYDA or PDA. Levan production, oxidase production, pectinolytic activity, arginin dihydrase production, and tobacco hypersensitivity (LOPAT) were tested. Strains were positive for levan and negative for oxidase, arginine dihydrolase, and nitrate reductase. Strains did not rot potato slices but induced a hypersensitive reaction on tobacco leaves. Protein analysis (1) indicated that the bacterium isolated was similar to Pseudomonas syringae pv. viburnii NCPPB 1921. The pathogen was identified as Pseudomonas syringae pv. viburnii (2,3). Pathogenicity of 10 colonies was tested by growing inoculum in nutrient broth shake cultures for 48 h, suspending bacterial cultures in water, diluting to 10⁶ CFU/ml, and spraying five 1-year-old healthy plants of Viburnum sargentii. Five control plants were sprayed with sterile nutrient broth. Inoculated and control plants were kept covered with plastic bags for 72 h. After 7 days in a growth chamber at 20 ± 1°C, leaf spots identical to those observed in the field developed on leaves of inoculated plants. Control plants remained symptomless. The pathogenicity test was repeated once. Strains were isolated from the spots and identified as P. syringae pv. viburnii. To our knowledge, this is the first record of bacterial leaf spot of Viburnum sargentii in Europe. A bacterial spot on Viburnum opulus, V. tomentosum, and V. dentatum was reported in the United States (4).

References: (1) D. H. Bergey et al. Bergey's Manual of Determinative Bacteriology. Williams and Wilkins, Baltimore, MD, 1994. (2) J. W. Pscheidt et al. Diseases of Woody Ornamentals and Trees in Nurseries. The American Phytopathological Society, St. Paul, MN, 2001. (3) N. W. Schaad. Laboratory Guide for Identification of Plant Pathogenic Bacteria. The American Phytopathological Society, St Paul, MN, 1998 (4) H. H. Thornberry and H. W. Anderson. Phytopathology 21:907, 1931.