Rashidul Islam

Bacterial panicle blight (BPB) of rice, a disease caused by Burkholderia glumae and B. gladioli, threatens global rice yields and has recently emerged in Bangladesh. Three hundred BPB-infected samples from 20 Bangladesh districts were analyzed using S-PG medium and gyrB PCR amplification, identifying 46 B. gladioli and 5 B. glumae potential isolates. Twenty of these isolates were chosen for in-depth characterization. Pathogenicity tests identified BD_21g (B. glumae) as the most virulent strain, followed by BDBgla132A (B. gladioli). Disease severity on rice strongly correlated with onion bulb assays, validating the assay as a rapid virulence-screening tool. Phenotypic characterization of the 20 isolates revealed substantial variation in toxoflavin production, lipase activity, polygalacturonase activity, motility, and type III secretion system. Comparative genomic analysis of virulence-associated genes between BDBgla132A and BD_21g showed high protein sequence identity, particularly in toxoflavin biosynthesis and transport genes, while genes encoding lipase (lipA/lipB), polygalacturonase (pehA/pehB), and those involved in motility, displayed moderate to high identity. Both strains retained virulence-related genes that are homologous to those of B. cepacia but displayed differential virulence strategies. Retrotranscribed-qPCR revealed significantly higher expression of toxoflavin and lipase-encoding genes in BD_21g compared with BDBgla132A, consistent with its elevated enzymatic activities. Conversely, BD_21g showed reduced expression of pectinolytic and flagellar genes over BDBgla132A, consistent with the enhanced pectinolytic activity and motility observed in BDBgla132A. These findings reveal that BD_21g (B. glumae) and BDBgla132A (B. gladioli) rely on different virulence strategies to infect rice, providing critical insights for developing targeted BPB management approaches in Bangladesh.

Bacterial panicle blight (BPB) is one of the emerging diseases occurring in different Agro-Ecological Zones (AEZ) of Bangladesh and can cause up to 75% yield loss. In Bangladesh, the typical symptoms of BPB include sheath rot, panicle blight, grain spotting, and grain rot in both inbred and hybrid rice varieties, which resemble those reported by Zhou (2019). To confirm, 300 field samples of 20 panicles each with typical BPB symptoms from 20 districts (3 locations each district and 5 fields per location) were collected during mid-November 2022 for the causal pathogen(s) isolation. Nearly 70% of the panicles showed a dark brown chaffy appearance in the fields. For identification of the causal pathogen(s), 1 g of rice grains with typical BPB symptoms was surface sterilized by immersing for 15 seconds in 70% ethanol, 1 min in 3% sodium hypochlorite solution followed by rinsing the grains three times, and soaked in 1 mL sterile distilled water for 10 min (Mirghasempour et al. 2018). During grinding using mortar and pestle, 5 mL water was added (Islam et al. 2023) after which the suspension (20 μL) was then streaked onto the selective medium (S-PG) (Tsushima et al. 1986). Purple color colonies on the S-PG medium were selected and purified as candidate pathogens. For further confirmation, the genomic DNA of the bacterial isolates was extracted and amplified by PCR using 16SF (5'-AGAGTTTGATCCTGGCTCAG-3') and 16SR (5'-GGCTACCTTGTTACGACTT-3') (Nandakumar et al. 2009), and glu-FW (5'-GAAGTGTCGCCGATGGAG-3') and glu-RV (5'-CCTTCACCGACAGCACGCAT-3') primers (Maeda et al. 2006). The PCR products were visualized on 1% agarose gel resulting amplicons of 1494bp for 16S-rDNA and 529bp for gyrB. The PCR results revealed 529bp amplification for gyrB gene in one sample that was collected from a field in Natore (24°21'0.00" N 89°04'59.88" E) district cultivating Swarna (a local rice variety), primarily indicating the causal pathogen is Burkholderia glumae. The PCR products were sequenced using both primers and sequence data was analyzed by the BLAST nucleotide program. The obtained partial sequences of 16S rDNA and gyrB were deposited in Genbank (OR573691 and PP332812 respectively). The homology of 16S rDNA resulted over 98% with B. glumae (OK559611 and ON870618.1) and 100% with B. glumae (PP332812 and KX213523) for gyrB gene. To confirm B. glumae by pathogenicity test, 10 mL (108 UFC/ml) suspension of the representative strains, 0.5 mL was then injected into the panicles and sheaths of Horidhan (a susceptible local variety) in greenhouse condition and a control was inoculated with distilled water (Nandakumar et al. 2009). Typical BPB like symptoms were observed after 3 weeks post inoculation. The pathogen was again confirmed by reisolating from the infected spots as B. glumae to fulfill Koch's postulates. This report confirms the presence of B. glumae causing BPB of rice in Bangladesh. Future research for the investigation of BPB and the evolutionary origins of its causal bacteria is necessary to reduce the emergence of the disease and its management in Bangladesh.

Sheath blight is caused by the fungal pathogen Rhizoctonia solani which causes yield losses of 20% up to 70%. While fungicide application is indeed effective against R. solani and can be safe when applied correctly, improper applications can have potentially negative effects on the environment and animals and may lead to fungicide resistance in pathogen subpopulations. The present study aimed to evaluate the efficacy of formulated bacterial bioagents in controlling sheath blight pathogen R. solani and sheath blight disease of rice to assess their performance in increasing rice yield. Out of 23 bacterial isolates, four bacterial isolates of different species viz. BDISO45JoyR (Bacillus subtilis), BDISO49JoyR (B. subtilis), BDISOB219R (Pseudomonas taiwanensis), and BDISOB221R (Pseudomonas sp.) were found to be the most effective against R. solani. These four bacterial isolates were formulated using talcum powder as a carrier material in the laboratory. Under field conditions, the formulated bacterial bioagents significantly reduced the lesion length and tiller infection of sheath blight, with BDISO45JoyR (B. subtilis) showing a maximum reduction of 55.25% and 54.80%, respectively, over the untreated control. The minimum reduction of lesion length and tiller infection was 28.30% and 32.17% was recorded in BDISOB219R (P. taiwanensis). BDISOB221R (Pseudomonas sp.) showed a maximum plant height of 116.5 cm. All the formulated bacterial bioagents showed better performance over control that increased yield. The maximum yield of 5.28 t/ha with a 58.51% increase over control was observed with BDISO45JoyR (B. subtilis) treatment compared to the untreated control. Therefore, bacterial bioagents should be considered for use in disease prevention and for the healthy growth of plants.

Citrus canker, caused by Xanthomonas citri pv. citri (Xcc), is one of the most widespread citrus disease. In this study, 54 isolates of Xcc were obtained from citrus canker-infected leaves, twigs and fruit samples collected from various citrus-growing regions in Bangladesh. These isolates were analysed through pathogenicity tests and PCR amplification. The results revealed varying levels of aggressiveness among the isolates, ranging from weak to moderate and aggressive. No correlation was found between the host plants and their geographical locations. The genetic analysis of Xanthomonas isolates from around the world revealed 100 % similarity. All the Xanthomonas isolates were grouped into 2 major clusters. DNA fingerprinting showed high genetic variability, with no correlation between hosts, geographic locations or aggressiveness levels. This information on the population diversity of the citrus canker pathogen, Xanthomonas citri pv. citri (Xcc), will be valuable for developing sustainable management strategies for citrus cankers in Bangladesh and the broader Indian sub-continent region.

Maize, one of the most important cereal crops in Bangladesh, is severely contaminated by fumonisin, a carcinogenic secondary metabolite produced by Fusarium including Fusarium proliferatum. Biocontrol with Bacillus strains is an effective approach to controlling this F. proliferatum as Bacillus has proven antagonistic properties against this fungus. Therefore, the present study aimed to determine how native Bacillus strains can reduce fumonisin in maize cultivated in Bangladesh, where BDISO76MR (Bacillus subtilis) strains showed the highest efficacy both in vitro in detached cob and in planta under field conditions. The BDISO76MR strain could reduce the fumonisin concentration in detached cob at 98.52% over untreated control, by inhibiting the conidia germination and spore formation of F. proliferatum at 61.56% and 77.01%, respectively in vitro. On the other hand, seed treatment with formulated BDISO76MR showed higher efficacy with a reduction of 97.27% fumonisin contamination compared to the in planta cob inoculation (95.45%) over untreated control. This implies that Bacillus-based formulation might be a potential approach in mitigating fumonisin contamination in maize to ensure safe food and feed.

Rice (Oryza sativa) is the most important food for more than two thirds of the world’s population. Bangladesh is the third largest producer and consumer of rice globally. Recently, several symptoms of Bacterial Panicle Blight (BPB) in rice, including seedling blight, sheath rot, floret sterility, and spotted grains, have been detected in the country. In addition, the presence of the most prevalent and virulent causative agent of BPB, Burkholderia glumae, has been confirmed in rice displaying symptoms of the disease. BPB could become one of the next emerging diseases of rice in Bangladesh, and a complete genome of a B. glumae strain from the country will help clarify its origin and devise proper management systems to continue sustainable rice production. We report the first complete genome sequence of a B. glumae strain (BD_21g) isolated from symptomatic rice grains in Bangladesh (Natore District). The genome contains 2 chromosomes (1 and 2, with 3,417,499 and 3,855,283 bp, respectively) and 4 plasmids (1-4, with 123,248, 46,628, 88,744 and 53,064 bp, respectively).