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Poster: Epidemiology: Population Biology Genetics


Exploring genetic diversity in Macrophomina phaseolina, the causal agent of charcoal rot on soybean
V. ORTIZ LONDONO (1), M. Chilvers (1), K. Wise (2) (1) Michigan State University, U.S.A.; (2) Purdue University, U.S.A.

Macrophomina phaseolina is a soil and seed-borne fungal pathogen infecting more than 500 plant species and causing charcoal rot disease on soybean (Glycine max). Recently, a set of Macrophomina isolates collected from three plant species in Senegal, morphologically similar and phylogenetically closely related to M. phaseolina, were described as a new species, M. pseudophaseolina. However, occurrence of M. pseudophaseolina is unknown outside of Senegal. In this study, 36 putative M. phaseolina isolates, collected from soybean grown in the northern and the southern US were identified to the species level. ITS, EF-1α and ACT loci were sequenced and analyzed. Additional sequences were obtained from GenBank. The Incongruence Length Difference (ILD) test was not significant (p=0.95) and supported a decision to combine the sequences. Phylogenetic relationships of the isolates were explored using Maximum Parsimony (MP), Neighbor-Joining (NJ), Maximum Likelihood (ML) and Bayesian Inference (BI). The MP, ML, NJ and BI phylogenies resulted in two separate clades corresponding to M. phaseolina and M. pseudophaseolina and placed all 36 isolates from soybeans in the U.S. in M. phaseolina. The Shimodaira-Hasegawa test indicates the ML tree as the best, but there were not significant differences among any of them. The ML tree reconstructed the M. phaseolina and the M. pseudophaseolina clades with a bootstrap support value of 100%.