Bacterial pathogens of rice in the Kingdom of Cambodia and description of a new pathogen causing a serious sheath rot disease

A study of rice diseases in Cambodia from 2005 to 2007 showed widespread occurrence of diseases caused by Acidovorax avenae subsp. avenae, Burkholderia gladioli, B. cepacia and Pantoea ananatis. This is the first report of these pathogens in Cambodia. Additionally, a pseudomonad causing a widespread disease similar to sheath brown rot (caused by Pseudomonas fuscovaginae) was isolated. The studied strains were pathogenic to rice cvs Sen Pidau and IR 66, producing similar, though slightly less severe, symptoms to those observed in the field. Based on comparative 16S rDNA gene sequence analysis, combined with cell wall fatty acid analysis and metabolic profiles, the isolated strains were allocated to the genus Pseudomonas. The novel species were differentiated from Pseudomonas fuscovaginae and P. putida by their inability to metabolize D-fructose, D-galactose, D-galactonic acid lactone, D-galacturonic acid, D-glucosaminic acid, D-glucuronic acid, p-hydroxy phenylacetic acid, D-saccharic acid and urocanic acid. The major fatty acids were C-16:0, summed feature 3 (C-16:1 omega 7c and C-16:1 omega 6c) and summed feature 8 (C-18:1 omega 7c), representing 80% of the total. Partial 16S rRNA gene sequences (1460 bp) were identical, except for two nucleotide changes amongst the six strains. Alignment of the causal strains within type-culture databases revealed similarities of 99.7% with Pseudomonas parafulva AJ 2129(T), 99.2% with P. fulva IAM 1592(T), 98.9% with P. plecoglossicidia FPC 951(T), and 98.1% with P. fuscovaginae MAFF 301177(T). On the basis of data from this polyphasic study, it is proposed that the unknown strains isolated from rice represent a novel species of the genus Pseudomonas.