Bacterial leaf streak (BLS) caused by Xanthomonas oryzae pv. oryzicola is prevalent in Asia where it can decrease yield by as much as 30%. In Africa, BLS has been reported in Madagascar, Nigeria, Senegal, and recently in Mali (1). The pathogen is seed transmitted and rice seeds can be a source of primary inoculum (3). In October 2009, leaf streak symptoms were observed on 3-month-old field rice grown in three regions of Burkina Faso (Haut-Bassin, Cascades, and East Center). Disease was found on cultivated Oryza sativa (varieties TS2, FKR19, and FKR56N), wild rice species (O. longistaminata and O. barthii), and weeds. Symptoms consisted of water-soaked lesions that developed into translucent, yellow streaks with visible exudates at the leaf surface. Yellow-pigmented Xanthomonas-like colonies were isolated on PSA semiselective medium (peptone 10 g, sucrose 10 g, bacto agar 16 g, distilled water 1,000 ml, actidione 50 mg liter–1, cephalexin 40 mg liter–1, and kasugamycin 20 mg liter–1). A multiplex PCR developed for the identification of Xanthomonas oryzae pathovars (2) was used to check the identity of Xanthomonas-like isolates. X. oryzae pv. oryzicola strains BLS256 from the Philippines and CFBP 7331 from Mali were used as positive controls. Three expected DNA fragments (331, 691, and 945 bp) corresponding to X. oryzae pv. oryzicola were obtained from all isolates using the multiplex PCR. No fragment was observed for negative controls (distilled water as the template). Five X. oryzae pv. oryzicola isolates were further analyzed by sequence analysis using portions of the gyrB housekeeping gene together with reference strains. Two sequence types were identified among Burkinabe isolates differing by only one nucleotide. When compared with the nucleotide database with BLAST, three isolates (BAI6, BAI15, and BAI19) were 100% identical to the type culture strain X. oryzae pv. oryzicola BLS256 (gyrB sequence was obtained from GenBank AAQN01000001.1) while the other two (BAI5 and BAI20) demonstrated 99% sequence similarity. The nucleotide sequence of isolate BAI5 was submitted to GenBank (HQ112342). Pathogenicity tests were performed on greenhouse-grown 3-week-old rice plants cv. Nipponbare. Cultures were grown overnight in PSA medium and adjusted in sterile water to 1 × 108 CFU/ml and inoculated into rice leaves with the blunt end of a 1-ml syringe. Four infiltrations were done per isolate per leaf and two leaves were inoculated per plant. Control plants were inoculated with sterile water. After 15 days of incubation in the greenhouse at 27 ± 1°C with a 12-h photoperiod, inoculated leaves exhibited water-soaked lesions with yellow exudates that were identical to symptoms seen in the field. Control plants remained symptomless. Colonies with morphology typical of Xanthomonas were recovered from the symptomatic leaves and typed using multiplex PCR to fulfill Koch's postulates. Three isolates have been deposited in the Collection Française de Bactéries Phytopathogènes (CFBP) and identified as X. oryzae pv. oryzicola strains CFBP7341–43. To our knowledge, this is the first report of X. oryzae pv. oryzicola in Burkina Faso. Further surveys and strain collection will be necessary to evaluate the geographic distribution and prevalence of BLS in Burkina Faso and neighboring countries.
References: (1) C. Gonzalez et al. Mol. Plant-Microbe Interact. 20:534, 2007. (2) J. Lang et al. Plant Dis. 94:311, 2010. (3) G. Xie and T. Mew. Plant Dis. 82:1007, 1998.