Identification of Pythium species inducing common bean (Phaseolus vulgaris L.) root rot symptoms and development of backcrosses to improve the level of varietal resistance to this disease

The common bean (Phaseolus vulgaris L.) is the most important food legume grown worldwide. It is a prioritary crop in various countries of East Africa and is grown mainly by small farmers for home consumption, the excess being sold in markets.
Important yield losses of common bean induced by root rot diseases have been identified in several countries in East Africa including Rwanda. This is partly explained by the intensification of the cultivation of beans, the absence of rotations, the practice of continuous cultivation of this legume, and decline in soil fertility. Although bean root rot symptoms are caused by a number of soil borne pathogens depending on environmental conditions, Pythium spp. are the fungal pathogens most frequently associated with severe epidemics in eastern Africa. Studies on root rot have indicated that continuous cropping of beans, a common practice in eastern Africa, exacerbates the problem.
This work was undertaken to improve the varietal resistance of common bean to limit the damages caused by Pythium in Rwanda. An analysis of the diversity of Pythium species associated with root rot was conducted through collect of samples of common bean plants throughout the country and through the characterization of the Pythium species causing root rot.
The collected samples were used to isolate 96 typical Pythium colonies which were classified into 16 Pythium species according to their respective molecular sequences of the ribosomal ITS fragments. Molecular characterization using the ITS-DNA sequences was also carried out on samples isolated on infected beans roots. The study of the distribution of each species identified within the samples analyzed, revealed that Pythium vexans is the most widespread taxon in the different common bean producing areas in Rwanda.
Pathogenicity tests of the 16 identified Pythium species were performed on a set of 10 common bean varieties. The results showed that all identified Pythium species were pathogenic to common bean: they all induce symptoms of root rot under controlled conditions. However, the common bean varieties used in this investigation showed differences in their reaction to inoculation with the 16 Pythium species.
At the end of this work of the characterization of Pythium species isolated in Rwanda, a scheme for improving varietal resistance has been implemented. It is based on a backcross protocol assisted by molecular marker (PYAA 19800). Recurrent parents are composed of three common bean varieties traditionally grown in Rwanda while the two donor parents are a resistant variety of the Mesoamerican gene pool and a resistant variety of Andean pool.
The progeny obtained after the backcrossing program was subjected to the pathogenicity trials by inoculating with a strain of Pythium ultimum in controlled conditions in order to verify the effectiveness of this improvement protocol. These trials have shown that in the offspring all the individuals showing the presence of the marker gene were also resistant to Pythium with very low levels of severity at the end of inoculation tests.