Characterization of bacterial leaf blight epidemic in the Office du Niger (Mali) and search for a sustainable resistance against the pathogen

Since 2002, plant health monitoring has allowed us to observe the appearance of a new disease whose symptoms are whitish to yellowish lesions on the leaves and partial or complete sterility of spikelets, giving a blackish aspect to the panicle. These symptoms suggest bacterial infection. The objectives of this study were: ∞ To characterize disease epidemic through plant health surveys in order to identify the level and condition of epidemic development and impact on rice production; ∞ To test the resistance potential of rice germplasm, mainly varieties bearing known resistant genes, in order to identify those that can protect rice against bacterial disease. Twenty-nine farmers were surveyed throughout the Office du Niger. The disease incidence and severity were monitored through observations every 10 days with all of these farmers. The disease impact on production was determined through survey plots in attacked and unattacked zones. The first symptoms of the disease appeared between 45 and 147 days after sowing. Disease incidence varied from one area to another and also evolved over time. It increased from the first to the third observation (10‐30 days after first appearance of symptoms) on all plots. From the third to the fourth observation (30‐40 days after first appearance of symptoms), the incidence decreased in the zones of N’Debougou (28.5 to 16%) and Kouroumari (23.25 to 20%). Highest incidences were recorded during the fourth observation in Niono (45.7%) and Molodo. (40.5%). Yield losses up to 80% were recorded. Twenty-four varieties and near-isogenic lines (NILs) were tested for their reaction to infections of bacterial leaf blight (BLB) caused by Xanthomonas oryzae pv. oryzae in five sites in an experimental design in randomized blocks with three replications on small plots of three lines of 1 m long. Reactions of the different NILs and varieties showed a zonal effect on the severity of the disease. Strains in Kayo seemed to be more virulent than those of the other sites. Those in Niono 1 appeared to be less aggressive rarely reaching incidences above 20%. Lines Xa-4/xa-5 and Xa-7 performed better than other lines under conditions of BLB infections in all sites with 16 and 19% incidence on average, respectively.