Molecular Markers Dispute the Existence of the Afro-Andean Group of the Bean Angular Leaf Spot Pathogen, Phaeoisariopsis griseola

Abstract

Coevolution of the angular leaf spot pathogen, Phaeoisariopsis griseola, with its common bean host has been demonstrated, and P. griseola isolates have been divided into Andean and Mesoamerican groups that correspond to defined bean gene pools. Recent characterization of P. griseola isolates from Africa has identified a group of isolates classified as Andean using random amplified polymorphic DNA (RAPD), but which are able to infect some Mesoamerican differential varieties. These isolates, designated Afro-Andean, have been identified only in Africa. Random amplified microsatellites, RAPD, and restriction digestion of amplified ribosomal intergenic spacer region were used to elucidate the relationships among the Afro-Andean, Andean, and Mesoamerican groups of P. griseola. Cluster and multiple correspondence analysis of molecular data separated isolates into Andean and Meso-american groups, and the Afro-Andean isolates clustered with Andean isolates. Analysis of molecular variance ascribed 2.8% of the total genetic variation to differences between Afro-Andean and Andean isolates from Africa. Gene diversity analysis revealed no genetic differentiation (G(ST) = 0.004) between Afro-Andean and Andean isolates from Africa. However, significant levels of genetic differentiation (G(ST) = 0.39) were observed between Afro-Andean or Andean isolates from Africa and Andean isolates from Latin America, revealing significant geographical differentiation within the Andean lineage. Results from this study showed that Afro-Andean isolates do not constitute a new P. griseola group and do not represent long-term evolution of the pathogen genome, but rather are likely the consequents of point mutations in genes for virulence. This finding has significant implications in the deployment of resistant bean genotypes.

Characterization of the common bean host and Pseudocercospora griseola, the causative agent of angular leaf spot disease in Tanzania

“…The distribution of these two gene pools across bean growing regions show that 75% of the total Mesoamerican bean samples were collected from the Kagera region, while very few were collected from Rukwa (17%) and Mbeya (8%) (Figure 3). These results from Tanzania confirm previous findings indicating that both Andean and Mesoamerican beans are grown within some African countries (Wortmann et al, 1998;Mahuku et al, 2002). In Tanzania, it has been found that farmers grow both Andean and Mesoamerican beans and in most cases they mix the two together in the same plot of land as a strategy of risk management since some cultivars fail and others are tolerant to biotic and abiotic factors (Blair et al, 2010).…”

“…Because Andean isolates that were virulent on Mesoamerican genotypes were found, this suggests that in Tanzania, both common bean genetic backgrounds need to be improved so as to attain durable resistance. Similar findings have been reported from pathogenic characterization using differential cultivars (Ddamulira et al, 2014); however, in this study, no differential cultivars were used and instead virulence grouping was employed as described by Mahuku et al (2002) in which the affected genotypes were characterized. Further, in this study, there is interaction between Andean isolates with Mesoamerican genotypes where some Andean isolates were shown to be virulent on Mesoamerican genotypes ( Figure 5).…”

“…Apart from these two distinct sets of host and pathogen based on geographical origin, another group was found peculiar to Africa designated as Afro-Andean. This group has characteristics typical of the isolates of Andean origin but it was found to be pathogenic on Mesoamerican common bean which is unusual (Mahuku et al, 2002;CIAT, 1997).…”

“…Further, in this study, there is interaction between Andean isolates with Mesoamerican genotypes where some Andean isolates were shown to be virulent on Mesoamerican genotypes ( Figure 5). Using RAPD markers, the Afro-Andean group was included within Andean isolates (Mahuku et al, 2002). This implies that, there is still within group evolution over time which indicates the continued need to …”

“…Despite the existence of genetic resistance, the variable nature of the pathogen makes achieving durable resistance a challenge (Mahuku et al, 2002;Pastor-Corrales et al, 1998). Therefore, knowledge on the races of the pathogen to be overcome in a region is crucial when durable resistance is sought, as well as the monitoring of the evolution of the pathogen over time (Mahuku et al, 2002). This pathogen monitoring is even more important when the pathogen shows high levels of variability as is the case for the ALS pathogen.…”