The distribution and virulence of common bacterial blight pathogen of beans in Uganda

Abstract

Common bacterial blight caused by Xanthomonas axonopodis pv. phaseoli and Xanthomonas fuscans subsp. fuscans is one of the foliar diseases causing significant yield losses to beans worldwide. Its management involves mainly use of disease-free seed, resistance, field sanitation and crop rotation. These measures have unfortunately not been effective in Uganda due to the subsistence nature of farming systems that relies heavily on home saved seed. CBB resistant bean cultivars are rare and often compromised by emergence of more virulent and aggressive strains of the pathogen. Unfortunately, areas of low CBB pressure have not been mapped in Uganda and there was limited knowledge on strain diversity and virulence when compared to the rest of the world. The aim of this PhD research was to study the distribution of CBB in different agro-ecologies of Uganda and to isolate and characterize common bacterial blight causing pathogen for virulence. A mapping survey was carried out across the country in seven agro-ecologies during the first and second season of 2016. During the survey, leaf samples were also collected for isolation of CBB causing pathogen. The isolated pathogens were characterized for pathogenicity, virulence and phylogenetic relationship with those isolated previously from Uganda. Areas of low CBB pressure were found in North west farmland and South West farmland but were not correlated with rainfall and temperature patterns. The study further identified a more adapted, virulent isolate of Xanthomonas citri pv fuscans (current classification), with additional type III effectors not previously reported for this pathogen. The study findings further suggest that virulent isolates are endemic and could have provided selection pressure for local varieties over the years. Local varieties with moderate resistance were found that could be associated with sources of resistance different from what is currently known.