Abstract
Bacterial blight (BB) caused by X. oryzae pv. oryzae is a destructive disease of rice. Experiments were
conducted to know the status (incidence and severity) of bacterial blight (BB) in the thirty
agroecological zones of Bangladesh to assess the pathotypes of X. oryzae pv. oryzae using Near
Isogenic lines along with assessment of the R gene performances using pyramid lines and to
characterize X. oryzae pv. oryzae pathotypes using Bangladeshi differential variety (BDVs). Finally,
molecular characterization of X. oryzae pv. oryzae pathotypes or races were carried out using RFLP
analysis with avrBs3 repeat domain as probe and compared these genetic variations among the
pathotypes of X. oryzae pv. oryzae with rep-PCR (REP, ERIC, BOX AIR) analyses. In rainfed condition,
the highest bacterial blight incidence and severity were recorded in hybrid varieties Aloron from
Netrokona (AEZ 9) and the lowest incidence was observed in local variety Sughondi from Jalhokhati
(AEZ 13). In irrigated condition, the highest bacterial blight incidence was recorded in hybrid variety
Hera-2 from Netrokona (AEZ 9) and the lowest incidence was observed in local variety from
Hobiganj (AEZ 20, 21, 22). Variety Swarna, Puja, BRRI dha28, BRRI dhan29, BRRI dhan49 showed а
moderate resistant and hybrid Hera-2, Tej, Partex, Agrodhan, Aloron, ACI-2, Dhani gold showed
susceptibility against BB. Pathogenic bacteria from all groups produced round ended, mucoid
colonies and yellow in colour on NBY medium indicated that all bacterial isolates are BB pathogen X.
oryzae pv. oryzae. The results of species-specific confirmation test of X. oryzae pv. oryzae with primers
XOR-F and XOR-R2 revealed that all the tested representative isolates produced desired 470bp
fragment length. Pathotypic analyses of 239 X. oryzae pv. oryzae isolates were conducted based on
their reactions against 10 Near Isogenic Lines (NILs) and a total of eight pathotypes were determined.
These pathotypic analyses explained the pathogenic relatedness of X. oryzae pv. oryzae populations
from diverse geographic areas in Bangladesh. Among eight pathotypes, pathotypes IV and V were
considered as major containing maximum number of isolates, (30.13% and 23.01% respectively),
whereas Pathotype VIII considered as minor containing only 2.51% isolates recorded from Khulna
and Barishal (AEZ 13). Pathotype I showed highest virulence or aggressiveness compatible with all
NILS, whereas pathotype VIII exposed lowest virulence to NILSs and performance of individual Rgenes against the X. oryzae pv. oryzae isolates, R-genes Xa1 (75.00%), Xa11 (62.50%) and Xa21 (50.00%)
Xa8 and Xa10 performed resistance where Xa4 worst as compared to all others genes. In pyramid
lines with a newly evolved field isolates showed high susceptibility to bacterial blight in IRBB61
(Xa4+Xa5+Xa7), IRBB60 (Xa4+Xa5+Xa13+Xa21), IRBB54 (Xa5+Xa21), and IRBB53 (Xa4+Xa21) and
highly resistance, to IRBBB63 (Xa5+Xa7+Xa13) and IRBB57 (Xa5+Xa7 +Xa21). Based on the reaction
on four Bangladeshi popular varieties, X. oryzae pv. oryzae were classified into four pathotypes that
were classified into eight pathotypes based on the reaction on NILs. The frequent distributions of
major four pathotypes were recorded from Dinajpur, Gaibandha, Natore, Bhola, Hobiganj districts.
BB resistance is breaking down in many countries because Xoo generates new effector(s) in a new
dominant race to overcome specific recognition by the matching R gene in plants. Virulence
differentiation of X. oryzae pv. oryzae is thought to be mediated by different members of the avrBs3
family effectors in different strains or races. Isolates of X. oryzae pv. oryzae representing all eight
pathotypes were analyzed by RFLP with avrBs3 repeat domain as probe and the results conceded that
Bangladeshi X. oryzae pv. oryzae strains seems carrying a minimum of two and maximum of nine
avrBs3 family genes. The resistance phenotype on IRBB7 and IRBB10 indicated some pathotypes
possess avrxa7 and some possess avrXa10 genes. Phylogenetic relationship revealed that X. oryzae pv.
oryzae isolates distributed into two RFLP haplotypes and these haplotypes are widely distributed all
over Bangladesh. Phylogenetic analyses based on rep-PCR (REP, ERIC and BOX) revealed two main
haplotypes of X. oryzae pv. oryzae races. Therefore, both RFLP and rep-PCR analyses of X. oryzae pv.
oryzae field populations distinguished two lineages. Relationship between pathotypes and molecular
haplotypes of X. oryzae pv. oryzae in Bangladesh indicated that the same lineage possess different
pathotypes and different lineage possesses different pathotypes. The results indicated that eight
different pathotypes might have originated from common ancestral haplotype.