Soybean SDS in South Africa is caused by Fusarium brasiliense and a novel undescribed Fusarium sp.

Authors: TEWOLDEMEDHIN, Y - Plant Protection Institute - South Africa; LAMPRECHT, S - Plant Protection Institute - South Africa; Vaughan, Martha; Doehring, Gail; O Donnell, Kerry

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/2/2016
Publication Date: 10/14/2016
Citation: Tewoldemedhin, Y.T., Lamprecht, S.C., Vaughan, M.M., Doehring, G., O'Donnell, K. 2017. Soybean SDS in South Africa is caused by Fusarium brasiliense and a novel undescribed Fusarium sp. Plant Disease. 101(1):150-157.

Interpretive Summary: Sudden death syndrome (SDS) of soybean has become a contraint to the production of this agronomically important crop in North and South America. Characteristics of the disease include wilting and discoloration of the leaves, roots, stems, and ultimately death. Four closely related Fusarium species have been shown to induce soybean SDS in South America, but only one of these pathogens is responsible for this disease in North America. Soybean SDS was detected in South Africa for the first time during pathogen surveys conducted in 2013-2014. Because this disease is responsible for economically devastating reductions in yields of this crop in North and South America, this study was initiated to identify the pathogen using genetic data, assess whether the isolates could induce the disease in a pathogenicity experiment, and to test whether cell free culture filtrates of the isolates could induce typical SDS symptoms on susceptible soybean cultivars in a whole seedling assay. The genetic data revealed that two pathogens are responsible for SDS in South Africa, F. brasiliense and a novel, undescribed Fusarium sp. Both species induced typical SDS symptoms in a pathogenicity experiment. Similarly, culture filtrates of both species induced SDS symptoms in the whole seedling assay, which suggests the filtrates contained compounds toxic to soybean. The discovery of two soybean pathogens in South Africa will be of interest to quarantine officials and plant disease specialists charged with preventing the introduction and spread of foreign pathogens into new production areas and to soybean breeders focused on developing cultivars with broad based resistance to this economically destructive disease

Technical Abstract: Soybean sudden death syndrome (SDS) was detected in South Africa for the first time during pathogen surveys conducted in 2013-2014. The primary objective of this study was to characterize the 16 slow-growing Fusarium strains that were isolated from the roots of symptomatic plants. Molecular phylogenetic analyses of a portion of translation elongation factor 1-a (TEF1) and the nuclear ribosomal intergenic spacer region (IGS rDNA) indicated the etiological agents were F. brasiliense and a novel, undescribed Fusarium sp. This is the first report of F. brasiliense outside of Brazil and Argentina and the novel Fusarium sp. causing soybean SDS. Koch’s postulates were completed for both fusaria on seven soybean cutivars that are commercially available in South Africa. Results of the pathogenicity experiment revealed that the strains of F. brasiliense and Fusarium sp. differed in agressiveness to soybean, as reflected in differences in foliar symptoms, root rot, and reduction in shoot length. Cell-free culture filtrates of the two soybean SDS pathogens from South Africa, and two positive control strains of F. virguliforme from the United States, induced typical SDS symptoms on susceptible soybean cultivars in a whole seedling assay, indicating that they contained phytotoxins.