Abstract
A survey on the occurrence of brown spot of rice caused by Bipolaris oryzae was carried out across Bangladesh covering major rice based systems and stated the brief result of the survey. Biological variations of B. oryzae covering different sites of Bangladesh were investigated in this research project. Three hundred and six isolates of the pathogen were collected from seven locations representing five agro-ecological zones of the country. Two representative sets of 87 and 152 isolates were characterized for phylogenetic identity through morphology and DNA fingerprints respectively. There were 10 groups of B. oryzae based on growth rate, and two groups each based on colony shape and colony consistency. Characterization by using variable number tandem repeat (VNTR) and amplified fragment length polymorphism (AFLP) analysis of DNA showed considerable diversity among the isolates, being separated into 12 and 6 groups with 75% and 76% similarity level, respectively. A representative set of 20 isolates consisting of nine fingerprinting types out of 12 was selected for pathogenicity test on BR24. The pathogenicity tests showed 20.5-84.5% germination failure, 15.84-38.34% post-emergence seedling death and 18.75-22.50% yield loss. Significant differences in virulence and aggressiveness were observed among the isolates of the same fingerprinting types. The pathogen produced moderately susceptible to highly susceptible reaction on BR24. Tracking of seed-borne pathogen by using DNA marker indicated an average of 20% transmission of seed inoculum of the pathogen to the seed of the next crop. Using apparently healthy seed with bright appearance could take care of the seed inoculum potential, which is already included in the seed certification therefore no seed health standard for B. oryzae was suggested.