Diversity of Xanthomonas oryzae pv. Oryzae on susceptible and resistant rice lines in bacterial blight hot spot areas of the Philippines

Rice bacterial blight is caused by Xanthomonas oryzae pv. oryzae (Xoo), which is one of the most common diseases in most rice-growing countries. The study aims to determine the diversity of Xoo races on resistant and susceptible rice lines during the peak of the epidemic in two hot spot areas in the province of Laguna, Philippines. Knowledge of pathogen diversity is important to deploy suitable rice varieties that would prolong their resistance and prevent disease outbreaks. Rice leaf samples with typical bacterial blight lesions were collected from the susceptible IR24 and the resistant IRBB57 lines within an experimental plot. Leaf samples were taken at five growth stages of rice, i.e., booting (62-66 days), heading (69-73 days), milky (76-80 days), dough (83-87 days), and maturity (90-95 days). Identification of Xoo race was conducted by molecular analysis using race-specific SNP markers. Three Xoo races (Race-2 complex, Race 8, and Race-9 complex) were identified on susceptible rice IR24. Only one race of Xoo (Race-9 complex) was identified on resistant rice IRBB57. Race 8 was the most abundant in IR24 rice population (51%), followed by Race-9 complex and Race-2 complex with 33% and 16%, respectively. On rice IR24, the presence of Xoo races varied in every rice stage. Race-9 complex was predominantly detected in all growth stages of IRBB57, but the infection in this resistant line did not progress during rice growth. The Xoo pathogen was still present on the resistant rice but could not develop into a disease. Race 8 was first detected in the irrigated lowland rice-growing areas in Laguna province and deserves further study.