Phylogeny of Pseudocercospora griseola from Puerto Rico, Central America and Tanzania confirms the existence of an Afro-Andean clade

Abstract

Angular Leaf Spot (ALS), caused by the fungus Pseudocercospora griseola, is an important disease of common bean (Phaseolus vulgaris). Previous studies demonstrated that P. griseola and Phaseolus vulgaris have co-evolved, resulting in the classification of the fungus into Middle American and Andean populations that reflect the evolutionary history of the host. However, an unusual group of P. griseola isolates found in Africa, termed Afro-Andean, was previously found to be pathogenic on Middle American cultivars. This study evaluated the diversity of P. griseola isolates from four countries and confirmed the existence of an Afro-Andean clade using molecular methods. A total of 171 P. griseola isolates from Puerto Rico, Honduras, Guatemala and Tanzania were evaluated. Sequence from four nuclear genes (β-tubulin, actin, the internal transcribed spacer (ITS) region of the ribosomal DNA, and the small subunit rRNA gene) were used to construct phylogenetic trees by Bayesian inference. Phylogenetic trees grouped all isolates from Puerto Rico (43), Honduras (25) and Guatemala (14) in the Middle American clade. Of the 89 Tanzanian isolates, 37 were Middle American and 40 Andean, and a third population with 12 isolates was identified as Afro-Andean. Models using isolation with migration estimated that the divergence between Middle American and Andean P. griseola populations occurred approx. 100,000 years ago, and that the Afro-Andean clade is a sister clade of the Andean population that diverged approx. 40,000 years ago. The results provide an understanding of the co-evolution of beans and the ALS pathogen, and help inform current and future bean breeding efforts.