Evaluation of weeds as alternative hosts of Xanthomonas phaseoli pv. manihotis, the causal agent of cassava bacterial blight in Burkina Faso

Cassava bacterial blight (CBB), caused by Xanthomonas phaseoli pv. manihotis (Xpm), is one of the most important diseases threatening cassava and has recently been reported in Burkina Faso. Better management of CBB calls for the combined use of genetically resistant varieties and appropriate cultural practices. The objectives of this study were to evaluate the potential role of weed plants present in cassava fields as inoculum reservoirs for CBB. An inventory of weeds growing in eight cassava fields at four locations in Burkina Faso, and the cultivation of soil samples from the same eight fields, indicated the existence of 27 species, with a prevalence reaching 28.32 plants of a given species per square meter. Field characterization led to the identification of seven species of weeds displaying symptoms reminiscent of Xpm infection, but no Xpm could be isolated from or detected in those plants. Artificial inoculations with Xpm resulted in 21 species producing symptoms of necrosis around the inoculation points. The presence of Xpm in the leaves of all these weed species was confirmed by re-isolation of the pathogen two weeks after inoculation. Thus, weeds that can harbor Xpm were prevalent in the cassava fields we studied, but their role as potential sources of Xpm inoculum for CBB epidemics remains to be demonstrated.