Italian dandelion (Cichorium intybus L.) is a leafy chicory plant grown as a fresh vegetable in salads. In August 2011, necrotic lesions were observed on leaves of about 20% of the Italian dandelion plants in an experimental field at the Tropical Research and Education Center (TREC) of the University of Florida in Homestead, FL. The lesions were dark brown with a yellow halo and most were irregular in shape. Some lesions formed along the leaf edge and developed into large lesions up to 20 to 30 mm long. Symptomatic leaf tissues were cut into small pieces (2 to 3 mm), sterilized in 10% Clorox (0.6% sodium hypochlorite) for 2 min following surface disinfection in 70% ethanol for 30 s, placed on nutrient agar (NA), and incubated at 28°C in the dark for 4 to 5 days. A bacterium was consistently isolated from the leaf lesions. Colonies of these bacteria were raised, opaque, and round with a slightly irregular, colorless margin while the center was light brown. The colonies were gram-negative with yellow fluorescence appearance on Pseudomonas F agar (Becton, Dickinson and Company, Sparks, MD). LOPAT tests were carried out to further identify the species of this fluorescent pseudomonad (2). The bacterium was levan negative, oxidase positive, potato rot negative, arginine dihydrolase negative, and tobacco hypersensitivity positive. Results from this test indicated that the bacterium belongs to LOPAT group III of fluorescent pseudomonads. Bacterial identity was further confirmed as Pseudomonas cichorii by amplifying the 16S rRNA gene with the universal bacterial primers 8F and 1492R (1) and sequence analysis (GenBank Accession No. KC311733). The partial 1,361-bp 16S rRNA gene sequences had 100% identity with P. cichorii (JF951725.1), which was isolated from infected soybean plants. Pathogenicity of the isolate was tested twice by spraying a suspension of the bacterium (107 CFU/ml) onto three 4-week-old plants until runoff. The control plants were sprayed with sterile distilled water. All plants were covered with plastic bags to maintain moisture for 48 h in an air-conditioned greenhouse (22 ± 1°C). Similar lesions were observed on the inoculated plant leaves 5 to 7 days after inoculation, whereas no lesions were developed on the control plants. Bacterial colonies with same morphology were recovered from the lesions, fulfilling Koch's postulates. P. cichorii is widespread worldwide with an important economic impact on celery, chrysanthemum, and lettuce. In South Florida, P. cichorii was reported as the causal agent of bacterial leaf spot on escarole (Cichorium endivia L.) (3). To our knowledge, this is the first report of bacterial leaf spot on Italian dandelion caused by P. cichorii in Florida.
References: (1) J. P. Galkiewicz and C. A. Kellogg. Appl. Environ. Microbiol. 74:7828, 2008. (2) T. Goszczynska et al. Introduction to Practical Phytobacteriology. Isteg Scientific Publications, Irene, South Africa, 2000. (3) K. Pernezny and R. N. Raid. Plant Dis. 85:1208, 2001.