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First Report of Burkholderia glumae Causing Bacterial Panicle Blight on Rice in Ecuador

Rice (Oryza sativa L.) is one of the leading crops and the basis of most diets in Ecuador and other countries. Diseases such as bacterial panicle blight (BPB), also known as seedling rot or grain rot, have the potential to threaten rice production worldwide. Burkholderia glumae, a causal agent of BPB, has severely affected the rice industry in many countries of Africa, Asia, and the Americas (1,2,4), but no report of this bacteria in Ecuador can be found in the literature. Rice plantations showing BPB-like symptoms including upright panicles with stained and vain grains were spotted in Palestina city, one of Ecuador's most extensive rice areas, in July 2013, but similar symptoms have been observed in the region since early 2012. Six symptomatic plants from two different groves were collected. Samples were plated on the semi-selective medium S-PG (KH₂PO₄ 1.3 g, Na₂HPO₄ 1.2 g, (NH₄)₂SO₄ 5 g, MgSO₄·7H₂O 0.25 g, Na₂MoO₄·2H₂O 24 mg, EDTA-Fe 10 mg, L-cystine 10 μg, D-sorbitol 10 g, pheneticillin potassium 50 mg, ampicillin sodium 10 mg, cetrimide 10 mg, methyl violet 1 mg, phenol red 20 mg, agar 15 g/liter distilled water) and axenic colonies were transferred to potato dextrose agar (PDA) to test for fluorescence (3). Colonies of the potential pathogen were 1 mm, circular, entire margin, with a smooth and shiny surface. When cultured in PDA, isolates showed a moist texture, dull yellow color, and displayed fluorescence with exposure to UV light. Cells were bacterial gram-negative rods of 1 to 2 × 0.5 μm. Twelve presumptive isolates were submitted to biochemical tests (API 20NE). The biochemical profile (APIWEB) showed that all the isolates belonged to the Burkholderia genus with a 99.9% similarity. To determine the bacterial species, colonies were submitted to ELISA tests using specific antibodies for B. glumae from Agdia, Inc. The two isolates that were positive for B. glumae were sequenced using a part of the 16s rDNA amplified by the primers 536F: 5′-GTGCCAGCMGCCGCGGTAATAC-3′ and 1492R: 5′-GGTTACCTTGTTACGACTT-3′. The obtained sequences (deposited into GenBank as KF601202) shared 100% similarity with several B. glumae strains after a BLAST query. Isolates were then diluted to 10⁸ UFC/ml and used to inoculate healthy rice plants. Inoculated plants produced BPB-like symptoms including upright panicles with stained vain grains and the bacterium was re-isolated from symptomatic plants. To the best of our knowledge, this is the first report of B. glumae in Ecuador. Further research is ongoing to identify and determine the pathogenicity of the remaining Burkholderia strains that tested negative for B. glumae.

References: (1) J. Luo et al. Plant Dis. 91:1363, 2007. (2) R. Nandakumar et al. Plant Dis. 93:896, 2009. (3) T. Urakami et al. Int. J. Syst. Bacteriol. 44:235, 1994. (4) X.-G. Zhou. Plant Dis. 98:566, 2014.