Rieger begonia are collectively referred to as a begonia hybrid group. Its global annual sales is 90,000,000 cutting seedlings. It is one of the top ten potted plants. In the summer of 2011, serious outbreaks of a suspected bacterial leaf spot disease were observed on five Rieger begonia cultivars (Dark Britt, Rebecca, Blitz, Barkos, and Borias). These plants were grown for potted cutting seedling production in commercial nurseries located in Shilin county of Yunnan Province, China. The initial symptoms of the disease were small circular or polygonal water-soaked needle spots on leaf margin that later these spots expanded and joined together, forming bigger inverted V-shaped necrotic specks (4). Yellow-pigmented bacterial colonies were consistently isolated from diseased leaves and stems on NA agar medium and incubated at 28°C. Twelve bacterial strains were isolated and used for further studies. All the isolates were Gram-negative, rod-shaped, motile, aerobic, and non-sporing. All of the bacterial strains isolated in the present study were identified as Xanthomonas campestris pv. begoniae (Xcb) based on biochemical and physiological identification (Biolog carbon source utilization analysis) and 16S rDNA sequences analysis and further pathogenicity determination (1). The results show that the sequence homology rate of HT1-1 (GenBank Accession No. JN648097) and X. euvesicatoria (syn. X. campestris pv vesicatoria) (GeneBank Accession No. AM039952) is 99%. This strongly suggests that the Rieger begonia isolates belong to X. campestris pv. begoniae (2). For Koch's postulates, 10 surface-disinfected young leaves from five susceptible Rieger begonia plants (cv. Dark Britt) were inoculated by spraying a phosphate-buffered saline suspension of each bacterial isolate (3.0 × 108 CFU/ml) onto the leaves (3). Controls were inoculated similarly with phosphate-buffered saline solution. All inoculated plants were covered with polyethylene bags for 24 h at 25°C and then put in the greenhouse. After inoculation, water-soaked and necrotic symptoms were observed on inoculated Rieger begonia leaves within 7 to 9 days. No symptoms were observed on controls. Bacteria were reisolated and confirmed to be identical to the original isolates by the methods described above. To our knowledge, this is the first report of Xcb causing leaf spot on Rieger begonia plants in China. The infection process of Xcb on Rieger begonia plants and rapid detection of this pathogen are underway.
References: (1) M. R. Gillings et al. PNAS 12:102, 2005. (2) C. L. Oliver et al. Plant Dis. 4:96, 2012. (3) H. Ornek et al. New Dis. Rep. 13:40, 2006. (4) O. Pruvost et al. Plant Dis. 4:96, 2012.