Abstract
Angular leafspot (ALS), caused by the fungus, Pseudocercospora griseola, is one of the most important disease of common bean in Tanzania. Breeding for resistance to this disease is complicated by the variable nature of the pathogen. In Tanzania, a thorough analysis of the variability of this pathogen is lacking which limits breeding for durable resistance. This work aimed at characterizing P. griseola in relation to its host in Tanzania. A sample collection of both P. griseola and common bean was conducted in the 2013 and 2014 growing seasons from nine regions. Single spore isolation was performed for P. griseola isolates and DNA was extracted from both P. griseola mycelium and bean leaves from which the pathogen was collected. For characterization of the gene pool origin of the host, Phaseolin DNA marker was evaluated and for the pathogen, the Internal Transcribed Spacer region (ITS) and the Actin gene sequences were evaluated. Phylogenetic analysis showed the presence of 69.7% Andean and 30.3% Mesoamerican strains of P. griseola in Tanzania. The common bean host genotypes showed a similar distribution with 84.2% Andean and 15.8% Mesoamerican. In both cases, Andean strains of the pathogen and Andean bean genotypes outnumbered Mesoamerican. In relation to the common bean genotypes, Andean genotypes were more susceptible to ALS as compared to Mesoamerican genotypes. There were few strains that were of Andean origin but were pathogenic on Mesoamerican common bean genotypes, a group that has previously been termed Afro-Andean. Geographically, most of the regions of Tanzania had only Andean strains except for Kagera where 60% were Mesoamerican strains, and in Arusha and Tanga, where 50 and 33% were Mesoamerican, respectively. Only three regions, Kagera, Mbeya and Rukwa, were found to grow Mesoamerican beans. The findings of this study are important in setting basic objectives for breeding for angular leaf spot disease in Tanzania. Key words: Actin gene, angular leaf spot (ALS), Internal Transcribed Spacer region (ITS), Phaseolin protein, Phaseolus vulgaris.
Highlights
Angular leaf spot (ALS) of common bean
The bean genotypes from which ALS isolates were collected were found to be of Mesoamerican and Andean origin the gene pool composition differed from region to region of Tanzania
A total of 76 common bean genotypes were characterized using the phaseolin DNA marker (Figure 2) and of these 64 (84%) were of Andean origin and the other 12 (16%) were of Mesoamerican origin. These results suggest that most of the common bean genotypes that are cultivated in Tanzania are of Andean origin with few of Mesoamerican origin from this relatively small sample
Summary
Angular leaf spot (ALS) of common bean (Phaseolus vulgaris. L.) caused by the fungus, Pseudocercospora griseola (Sacc) Crous and U. The pathogen causes significant yield losses of up to 80% of common bean in Africa and in other parts of the world (Muthomi et al, 2011). This disease primarily affects aerial parts of the plant and is more destructive in warm, humid production zones (Crous et al, 2006). The disease causes premature defoliation (Correa-Victoria et al, 1989) and it causes the reduction of seed quality, in cases where the disease invades the seeds, and affects the marketability of bean seed across production zones of the world (Pastor-Corrales et al, 1998)
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