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Occurrence of Bacterial Leaf Blight of Cocoyam and Characterisation of the Causal Organism in Northern Nigeria

Aim: To determine the occurrence of bacterial leaf blight of cocoyam and to characterize isolates of the causal organism in Northern Nigeria. 
Place and Duration of Study: The study was conducted in cocoyam farmers’ fields in Kaduna and Kano States, Department of Crop Protection, Ahmadu Bello University, Zaria, Nigeria and the Mississippi State University, USA in 2015. 
Methodology: Field surveys were conducted in Kaduna and Kano States of northern Nigeria to determine occurrence of bacterial leaf blight of cocoyam. Sampling was done using quadrat. Incidence and severity of the disease was assessed on the field. Bacterial isolates obtained from the diseased samples were used for pathogenicity and hypersensitive reaction (HR) tests. Biochemical tests such as Gram, catalase, oxidase, pectolytic, amylolytic and production of acid from glycerol were carried out on the isolates. A nearly full length of the 16S rDNA gene of selected isolates was PCR amplified using 16S rRNA primers. The resulting amplicons were sequenced using Sanger sequencing. The 16S rDNA gene sequences were aligned along with other Xanthomonas sequences imported from the NCBI database using muscle tool from MEGA6. 
Results: The results showed Kaduna State had higher incidence and severity (50.2%, 13.5%) than Kano State (20.5%, 5.1%) and that bacterial isolates induced blight symptoms on cocoyam and elicited HR reactions on tobacco. Isolates were Gram negative, catalase positive, oxidase negative, amylolytic, pectolytic and produced no acids from glycerol. BLAST search of sequenced genes showed 98-100% homology to Xad. Maximum likelihood phylogenetic trees constructed for the 16S rDNA gene sequences revealed isolates were identical to the Xad reference strains KP247494, KM576803 and EU203153. Generally, all the isolates obtained were Xad. 
Conclusion: Bacterial leaf blight of cocoyam occurred in Kaduna and Kano States and the bacterial isolates were identical to Xanthomonas axonopodis pv. dieffenbachiae.