Characterization of New Races of pv. in Mali Informs Resistance Gene Deployment.

Bacterial leaf blight caused by pv. represents a severe threat to rice cultivation in Mali. Characterizing the pathotypic diversity of bacterial populations is key to the management of pathogen-resistant varieties. Forty-one pv. isolates were collected between 2010 and 2013 in the major rice growing regions in Mali. All isolates were virulent on the susceptible rice variety Azucena; evaluation of the isolates on 12 near isogenic rice lines, each carrying a single resistance gene, identified six new races (A4 to A9) and confirmed race A3 that was previously reported in Mali. Races A5 and A6, isolated in Office du Niger and Sélingué, were the most prevalent races in Mali. Race A9 was the most virulent, circumventing all of the resistance genes tested. Xa3 controlled six of seven races (i.e., 89% of the isolates tested). The expansion of race A9 represents a major risk to rice cultivation and highlights the urgent need to identify a local source of resistance. We selected 14 isolates of pv. representative of the most prevalent races to evaluate 29 rice varieties grown by farmers in Mali. Six isolates showed a high level of resistance to pv. and were then screened with a larger collection of isolates. Based on the interactions among the six varieties and the pv. isolates, we characterized eight different pathotypes (P1 to P8). Two rice varieties, SK20-28 and Gigante, effectively controlled all of the isolates tested. The low association observed among races and pathotypes of pv. suggests that the resistance observed in the local rice varieties does not simply rely on single known genes. pv. is pathogenically and geographically diverse. Both the races of pv. characterized in this study and the identification of sources of resistance in local rice varieties provide useful information to inform the design of effective breeding programs for resistance to bacterial leaf blight in Mali.