Bacterial Brown Leaf Spot of Citrus, a New Disease Caused by Burkholderia andropogonis

Burkholderia andropogonis was first described as the causal agent of stripe disease of sorghum and leaf spot of velvet bean in 1911 (20); at that time, it was classified as Pseudomonas andropogonis. Pseudomonas woodsii was also described, in the same publication, as an important pathogen of carnation. These two pathogens were reclassified to the genus Burkholderia based on DNA-rRNA hybridizations (13) and were shown to be synonymous based on phenotypic, genotypic, and chemotaxonomic evidence (9,13). Symptoms of B. andropogonis–infected leaves include brown, water-soaked, circular lesions with chlorotic halos. These lesions can coalesce, resulting in severe blight or leaf drop. When infected leaves are dissected in water, a stream of bacterial ooze is observed. B. andropogonis is now known to cause leaf spot, streaks, and stripe on a wide variety of host plants covering 52 species from 15 families of unrelated monocotyledons and dicotyledons, including ornamentals such as white clover, carnation (26), and bougainvillea (29), and economically important crops such as corn, coffee, and chick pea (15). Recent reports show B. andropogonis to be a causal agent of diseases of jojoba (10), orchids (23), and golden cane palms (32). It can also survive on weeds such as Johnsongrass and sudangrass (8). Since 2002, on both residential and commercial citrus plants, citrus leaves with flat, circular, brownish lesions, with slightly raised and water-soaked margins, surrounded by a chlorotic halo have been observed by citrus canker inspectors with the Division of Plant Industry, Florida Department of Agriculture and Consumer Services (DPI-FDAC) (21). The disease was observed only after storms that caused a high degree of damage and wounding to citrus trees. The flat lesions with slightly raised margins appeared somewhat similar to citrus canker (Xanthomonas citri pv. citri) and were therefore sent to the DPIFDAC laboratory for further analysis and identification. However, the lesions were different from those of citrus canker disease, and were confirmed negative for X. citri by polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA). Under the microscope, a bacterial stream was consistently observed from leaf tissue slices cut from these lesions, and Gram-negative bacteria producing whitish colonies were consistently isolated on nutrient agar plates. All isolates, when inoculated into tomato and pepper, caused hypersensitive reactions (HR), indicating they were likely phytopathogenic. Further testing was conducted to confirm and to characterize the causal agent of what appeared to be a new citrus bacterial disease. In this paper, we describe bacterial brown leaf spot (BBLS), a new, opportunistic bacterial disease of citrus in Florida, and identify the causal agent to be B. andropogonis. Furthermore, we have developed a simple and rapid genetic typing method for differentiating B. andropogonis strains using a simple PCR test based on a single primer.