Bacterial Blight, a New Disease of Lobelia ricardii Caused by Pseudomonas cichorii

Lobelia ricardii is a small perennial bedding plant grown for its abundant purple flowers. During April 1998, young lobelia plants with severe leaf blight were observed in a commercial nursery in Washington State. Symptoms consisted primarily of watersoaking of leaves, followed by collapse of leaves, watersoaking of stems, and eventual dieback of all foliage. Isolations from affected tissues were made on King's medium B (KMB). A fluorescent, gram-negative, oxidase-positive, arginine dihydrolase- and potato rot-negative, levan-producing bacterium was recovered that produced a hypersensitive response in tobacco. The bacterium was identified as Pseudomonas cichorii by carbohydrate utilization profiling (Biolog, Hayward, CA). A single-colony isolate of the bacterium was raised on KMB. A bacterial suspension was made from a 24-h agar culture of the isolate, using 0.1 M phosphate buffer, pH 6.2, with 0.2% gelatin (PBG). The suspension concentration was adjusted to 1 × 10⁷ cells per ml by direct enumeration. The suspension (3 ml) was atomized onto leaves of five potted 5-month-old lobelia plants that were then bagged in plastic and maintained at 22°C. Bags were removed after 2 days. Eight days after inoculation, foliage was extensively watersoaked, and plants showed symptoms similar to those observed in the nursery. The plants later died. P. cichorii was reisolated from all inoculated plants. Control plants, which were treated with PBG, did not develop any symptoms nor was P. cichorii isolated from them. This is the first report of P. cichorii as a pathogen of L. ricardii.