Abstract
One of the major contraints to rice production is diseases caused by bacteria of Xanthomonas genus. Bacterial bligth (BB) and baterial leaf streak (BLS) are two emerging diseases in West Africa, caused respectively by Xanthomonas oryzae pv. Oryzae (Xoo) and Xanthomonas oryzae pv. oryzicola. BB was first reported in Senegal in the 1980s by Trinh et al. But no strain of Xoo was isolated and no other reports were made. However, BLS has not been reported in Senegal. The choice of resistance genes against BB and BLS is based on their effectiveness compared to the most virulent widespread strains of Xoo and Xoc in the area. The lack of studies on Senegalese Xanthomonas means that no strategy has yet been envisaged to control these diseases. Variety resistance is one of the most effective ways to control these diseases. Characterization of the diversity of X.oryzae strains as well as identifying sources in local varieties for resistance are essential conditions for sustainable control. Thus objectives of our study were: (i) to clarify BB and BLS status in Senegal through surveys in rice fields and samples whose presence or not will have to be confirmed; (ii) to build a collection of X. oryzae strains to characterize; (iii) to identify resistance genes to fight BB and BLS; (iv) to identify sources of resistance by conducting an assessment of the resistance of rice accessions grown in Senegal for BLB resistance. Prospecting, isolation campaigns and confirmation by PCR have resulted in a collection of 43 strain of Xoo and 91 Xoc Senegalese strains. Screening of IRBBs quasi-isogenic lineages identified 6 races from 3 different sites, including 2 races already described in Mali (A 3) and Burkina Faso (A1). Of 13 resistance genes tested, only 2 are able to control the 6 Xoo races, one dominant (Xa1) and one recessive (xa5). On the other hand, the screening of 22 varieties of rice grown in Senegal found 4 varieties highly resistant to all identified races. Kitaake RXo1, Carolina Gold, and IRBB1 phenotyping identified Xo1 and Xa1 as effective genes to control Xoc. On the other hand RXo1 is bypassed by the majority of Xoc tested with 81.32% of Xoc not having AvRXo1. The MLVA analysis showed that there is a significant diversity of haplotypes in Senegal and the presence of a common haplotype in 3 regions could indiquâtes the exchange of infected material between remote regions. Yield loss assessment of 3 varieties grown in Senegal showed that yield losses confered to Xoo can vary between 5% and 55% depending on the variety grown and the race present.