Sudden-death syndrome of soybean is caused by two morphologically and phylogenetically distinct species within the Fusarium solani species complex—F. virguliforme in North America and F. tucumaniae in South America

Abstract

Soybean sudden-death syndrome has become a serious constraint to commercial production of this crop in North and South America during the past decade. To assess whether the primary etiological agent is panmictic in both hemispheres, morphological and molecular phylogenetic analyses were conducted on strains selected to represent the known pathogenic and genetic diversity of this pathogen. Maximum-parsimony analysis of DNA sequences from the nuclear ribosomal intergenic spacer region and the single copy nuclear gene translation elongation factor 1-α, together with detailed morphological comparisons of conidial features, indicate that SDS of soybean in North and South America is caused by two phylogenetically and morphologically distinct species. Fusarium virguliforme sp. nov., formally known as F. solani f. sp. glycines, is described and illustrated for the SDS pathogen in North America, and F. tucumaniae sp. nov. is proposed for the South American pathogen. The molecular phylogenetic results challenge the forma specialis naming system because pathogenicity to soybean might have evolved convergently in F. tucumaniae and F. virguliforme. Phylogenetic evidence indicates the two SDS pathogens do not share a most recent common ancestor, since F. tucumaniae was resolved as a sister to a pathogen of Phaseolus vulgaris, F. phaseoli comb. nov. All three pathogens appear to have evolutionary origins in the southern hemisphere since they are deeply nested within a South American clade of the F. solani species complex.

• Argentina
• conidiogenesis
• Fusarium phaseoli
• Glycine max
• Phaseolus vulgaris
• SDS

The authors thank Prof. Walter Gams for preparing the Latin diagnoses of F. tucumaniae and F. virguliforme. Special thanks are due to all members of the soybean research group at INTA-EEA, Marcos Juárez, Sr. Francisco H. Fuentes, INTA-EEA, Famailla, and members of the Phytopathology section, EEAOC, Tucumán, Argentina, for their assistance in obtaining fresh soybeans exhibiting typical SDS symptoms and for providing invaluable information on the occurrence of SDS in Argentina.