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

Abstract

A new bacterial disease of citrus was recently identified in Florida and is here named bacterial brown leaf spot (BBLS) of citrus. BBLS-infected citrus leaves from the field displayed circular, brownish, flat lesions with slightly raised and water-soaked margins surrounded by a chlorotic halo. Based on Biolog carbon source metabolic "fingerprinting", fatty acid analysis, and sequence analysis of partial 16S rDNA, gyrB, and rpoD genes, the causal agent of the disease was identified as Burkholderia andropogonis. Pathogenicity of these B. andropogonis isolates taken from multiple citrus leaves with BBLS was tested by various inoculation methods on three species of citrus as well as on carnation, corn, and sorghum. All isolates infected carnation, corn, and sorghum with varying degrees of pathogenicity. Variation among citrus isolates in pathogenicity was also observed in high titer (108 CFU/ml) inoculations of citrus leaves, ranging from a hypersensitive-like response to canker-like lesions. When the inoculum concentration was low (106 CFU/ml), only necrotic spots or small lesions slowly developed with all strains. Growth of B. andropogonis in citrus was relatively slow, tissue wounding appeared necessary for symptom appearance with many isolates, and field samples were recovered only after severe storms, indicating that this wide-host-range bacterium is a weak, opportunistic pathogen of citrus.