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First Report of Bacterial Leaf Spot Caused by Pseudomonas cichorii on Phlox paniculata in Italy

Phlox paniculata L. (fall phlox) is a perennial garden species belonging to the Polemoniaceae family. During the spring of 2003 and 2004, leaf spot symptoms were observed on fall phlox plants in some private gardens in the Biella area (northern Italy). Lesions were first observed on leaves at the collar level and later on the entire plant. Lesions started as water-soaked areas, which in few days developed on the upper side of the leaves into irregular, shrunken, reddish brown spots from 1 to 2 mm in diameter. Lesions on the lower surface sometimes had a translucent halo. In many cases, the leaves were completely withered. Disease was particularly severe during the spring and fall and its incidence ranged from 10 to 25%. No fungal structures were observed within the 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. No fungi were isolated from the spots on NYDA or PDA. Colonies typical of Pseudomonas species were consistently isolated on NYDA. Isolates were negative for levan, potato soft-rot (pectolytic activity), and arginine dehydrolase while positive for oxidase and hypersensitivity on tobacco leaves. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of whole-cell protein analysis (1) indicated that the bacterium isolated was similar to Pseudomonas cichorii (Swingle) Stapp NCPPB 943 and 3283 strains. On King's medium B, (2) a typical fluorescent pigment was produced. The pathogen was identified as Pseudomonas cichoriii. 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 10 1-year-old healthy plants of P. paniculata. Ten control plants were sprayed with sterile nutrient broth. Inoculated and control plants were kept covered with plastic bags for 72 h. After 8 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. Bacteria were reisolated from the spots and identified as P. cichoriii. To our knowledge, this is the first record of bacterial leaf spot of Phlox paniculata in Italy as well as in the world.

References: (1) D. H. Bergey et al. Bergey's Manual of Determinative Bacteriology. Williams and Wilkins, Baltimore, MD, 1994. (2) E. O. King et al. J. Lab. Clin. Med. 44:301, 1954.