Bacterial blight of Iris caused by Pseudomonas syringae in Virginia

Iris (Iris germanica) is an important ornamental plant throughout the USA. During the summer of 2007, severe outbreaks of previously unknown bacterial blights on Iris plants were observed at several production nurseries. Initially, both surfaces of leaves showed small, dense water soaked lesions, which later necrotized, desiccated and collapsed. As disease progressed, infected parts of the plant turned black to brown and died. Isolations were made from leaf spots onto nutrient agar and King's B medium. Isolates were subjected to morphological, physiological/biochemical, hypersensitivity and pathogenicity tests (Little et al., 1998; Braun-Kiewnick & Sands, 2001) including the LOPAT tests of Lelliott et al. (1966). Molecular characterization based on 16S rRNA sequencing (MCLab, San Francisco, CA 94080, USA) and fatty acid methyl ester analysis (Microbial ID, Newark, DE 19713, USA) were also carried out. An authentic culture of Pseudomonas syringae pv. syringae from snap bean (isolate 4263, BV 3 B728a) was used as a positive control. Colonies were creamy white, flat, circular on King's B medium, and produced a pale fluorescent pigment under UV light. They were Gram negative, negative for oxidase test, pectolytic activity, and arginine dihydrolase test but positive for gelatin liquefaction, levan production, catalase activity and gave a typical HR reaction within 24h (LOPAT group 1a).

Sequence analysis of a 430bp 16S rDNA fragment amplified by PCR using universal primers gave the closest match to P. syringae pv. syringae ( 79 %) in a BLAST search in GenBank (http://www.ncbi.nlm.nih.gov) with S=302, E=6e-79. The results of FAME analysis indicated the closest match was P. syringae (Similarity Index 0.860). Pathogenicity tests were performed by infiltrating a cell suspension (10⁷ CFU mL^-1) into Iris plants, with appropriate controls. Water soaked areas followed by necrotic symptoms were observed on Iris plants three days after infiltration. These results show that this pathogen belongs to Pseudomonas syringae. Among most Pseudomonas species, P. syringae is unique in its ability to cause disease in over 180 species of plants in several unrelated genera (Braun-Kiewnick & Sands, 2001; Bradbury, 1986) but to our knowledge, this is the first report of this pathogen attacking Iris plants. Pathovar status is yet to be determined.