Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/16/2010
Publication Date: 12/20/2010
Citation: Scandiani, M.M., Aoki, T., Luque, A.G., Carmona, M.A., O Donnell, K. 2010. First report of sexual reproduction by the soybean SDS pathogen Fusarium tucumaniae in nature. Plant Disease. 94(12):1411-1416. Interpretive Summary: Sudden death syndrome (SDS) is a devastating root-rot and vascular wilt disease of soybeans caused by several closely related species of the fungus Fusarium. These fungal pathogens are responsible for substantial yield reductions in North and South America. We conducted a survey of SDS pathogen diversity in March and April 2010 in the provinces of Buenos Aires and Santa Fe in Argentina. Examination of field-collected soybean roots under a dissecting microscope in the laboratory resulted in the discovery of reddish-orange fruiting bodies (the sexual stage) on five different roots. Colonies derived from 16 of the fruit body spores produced asexual reproductive structures that matched the published description of the SDS pathogen Fusarium tucumaniae. In addition, 16 cultures derived from asexual spores collected from roots bearing fruit bodies also produced colonies that were morphologically diagnostic of F. tucumaniae. Independent confirmation that the 32 isolates were F. tucumaniae was obtained by running a DNA sample of each isolate through a molecular diagnostic assay for soybean SDS species determination. This study represents the first report of sexual reproduction by a soybean SDS pathogen in nature. Knowledge that F. tucumaniae is capable of reproducing sexually and asexually informs plant breeders, plant pathologists and quarantine officials that this pathogen is much more likely to overcome quantitative resistance in soybean, compared with SDS pathogens that only reproduce asexually.
Technical Abstract: Of the four fusaria that have been shown to cause soybean sudden death syndrome (SDS), field surveys indicate that Fusarium tucumaniae is the most important and genetically diverse SDS pathogen in Argentina. Although none of the SDS fusaria have been shown to produce perithecia in nature, a heterothallic sexual cycle has been demonstrated for F. tucumaniae via laboratory crosses. Herein we report on the discovery of perithecia of F. tucumaniae on soybean in Argentina. Ascospores derived from these perithecia gave rise to colonies that produced sporodochial conidia diagnostic of F. tucumaniae. Sporodochial conidia were longer and narrower than those produced by the other SDS fusaria; these conidia also possessed a diagnostic acuate apical cell and a distinct basal foot cell. Sixteen strains derived from single ascospores subjected to a validated multilocus genotyping assay (MLGT) for SDS species determination, together with 16 conidial isolates from two sites where teleomorphs were collected, independently confirmed the morphological identification as F. tucumaniae. This study represents the first authentic report of sexual reproduction by a soybean SDS pathogen in nature.