Identification, Variability and Control of Brown Leaf Spot Disease Pathogen(S) of Rice.

Abstract

Brown leaf spot disease (BLSD) of rice results in germination failure, seedling mortality, yield and quality reduction. Typical BLSD with serious effect on yield was observed in rice farmers fields in Atwima Nwabiagya District of the Ashanti region. The study therefore sought to isolate, identify and control BLSD of rice pathogen(s). Diseased rice leaves were sampled and cut into pieces, disinfected, and arranged equidistantly on a PDA medium for fungal pathogen growth. The fungal pathogens were identified using morphological features and confirmed by DNA sequencing. The pathogenicity of isolated fungal pathogens were confirmed through Koch postulate. Two (2) epiphytes of rice (Purpureocillium lilacinum and Aspergillus flavus), two (2) plant extracts (Azadirachta indica and Nerium oleander), a positive control (mancozeb) were tested for their ability to control the disease in-vitro and in screenhouse. Molecular identification using BLAST search analysis in the NCBI specifically identified the pathogenic isolates into Bipolaris zeicola, Bipolaris oryzae, Curvularia lunata, Curvularia clavata, Curvularia soli, Curvularia geniculata, Curvularia senegalensis, Curvularia subpapendorfii and Curvularia platzii. All isolates showed symptoms of brown leaf spot similar to what was observed on field of rice plants after confirming through Koch postulate. The investigation on the pathogenicity of the obtained Curvularia isolates displayed the existence of a pathogenic variability among the isolates. Apart from B. oryzae and C. lunata, which were the known pathogen associated with rice brown leaf spot disease in Ghana, new species such as B. zeicola, C. clavata, C. soli, C. geniculata, C. senegalensis, C. subpapendorfii and C. platzii were found to be associated with rice in a disease complex and causing brown leaf disease individually, suggesting together they contribute to disease severity in a complex. Perhaps, it is the first report of B. zeicola, C. clavata, C. soli, C. senegalensis, C. geniculata, C. subpapendorfii and C. platzii associated with brown leaf spot disease of rice in Ghana and have since been deposited at NCBI GenBank (with accession numbers: PQ410456, PP564497, PP564496, PP564502, PP564499, PP564501 and PP564498 respectively). Two rice epiphytes (A. flavus and P. lilacinum) and two botanicals (A. indica and N. oleander) inhibited mycelia growth of brown leaf spot pathogens in an in-vitro condition and subsequently reduced brown leaf spot severity of rice seedlings in screenhouse experiment.