Multiple Introductions Without Admixture of Colletotrichum truncatum Associated with Soybean Anthracnose in Brazil

Knowledge of the population structure, genetic diversity, and reproductive mode of plant pathogens can help to implement effective disease management strategies. Anthracnose is one of the most prominent diseases in soybean and is mainly associated with the species Colletotrichum truncatum. However, the genetic structure of C. truncatum populations associated with soybean remains unknown. We collected C. truncatum isolates from 10 sites representing two Brazilian states (Mato Grosso and Goias) and used 13 highly polymorphic microsatellite markers to investigate the population genetic structure of the pathogen. Analyses revealed high gene and haplotypic diversity within populations, as well low genetic differentiation and sharing of multilocus haplotypes among populations and regions. Bayesian and multivariate analysis revealed the presence of three distinct genetic clusters with at least two coexisting in all locations, and all of them coexisting in eight locations. We found limited evidence for admixture between clusters, with only two isolates showing nonzero membership with a second cluster. Analyses of linkage disequilibrium rejected the hypothesis of random mating in all clusters, but values of the index of association were low and not consistent with long-term lack of sexual reproduction. Our findings suggest that Brazilian C. truncatum populations resulted from at least three founder events that led to three genetic clusters that spread throughout the country, raising questions with respect to the factors allowing their maintenance in syntopy without evidence of admixture between them.