Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/22/2004
Publication Date: 9/16/2004
Citation: Scandiani, M., Ruberti, D., O Donnell, K., Akoi, T., Pioli, R., Giorda, L., Luque, A., Biasoli, M. 2004. Recent outbreak of Soybean Sudden Death Syndrome caused by Fusarium virguliforme and F. tucumaniae in Argentina. Plant Disease. 88:1044. Available: http://www.apsnet.org/pd/searhnnotes/2004/0714-01n.asp. Interpretive Summary: Based on pathogenicity experiments conducted within greenhouses within the United States, the economically devastating disease of soybean called sudden death syndrome (SDS) has been shown to be caused by the filamentous fungus Fusarium virguliforme. In this study, fungal isolates from soybeans exhibiting typical SDS symptoms were characterized using morphological and DNA data to determine what pathogen or pathogens were inducing the disease in Argentina. Results of these analyses demonstrated that two species are responsible to the soybean SDS disease in Argentina, F. virguliforme and F. tucumaniae. Based on the results of greenhouse-based pathogenicity experiments, isolates of both species were able to induce the SDS disease. This is the first study to demonstrate that isolates of F. tucumaniae are capable of causing soybean SDS.
Technical Abstract: Sudden death syndrome (SDS) of soybean was detected initially in Argentina in 1991/92 in the Pampas Region and in 1992/93 in the Northwest Region. The first report of the fulfillment of Koch´s postulates of SDS caused by Fusarium solani f. sp. glycines (FSG) in Argentina was published in 2003(1). Subsequently, analyses have shown that FSG represents several morphologically and phylogenetic distinct species, including Fusarium tucumaniae in Argentina and Fusarium virguliforme in the U.S.(2). Isolations were made from plants that exhibited typical SDS symptoms from different fields in Santa Fe and Buenos Aires Provinces in 2001, 2002 and 2003. To determine what species are responsible for SDS within Argentina, cultures of eight slow growing isolates, which developed bluish pigmentation and produced abundant macroconidia in sporodochia on potato dextrose agar, were subjected to morphological and molecular phylogenetic analyses and pathogenicity tests. Morphological analyses demonstrated that three of the isolates were F. virguliforme and five were F. tucumanaie. Isolates of F. tucumaniae produced longer and narrower sporodochial conidia than F. virguliforme, while the latter species produced diagnostic comma-shaped conidia, too. Molecular phylogenetic analyses of DNA sequences from multiple loci confirmed the morphology-based identifications, and showed that the soybean SDS pathogen in the U.S., F. virguliforme, was also present in Argentina. This is the first report of F. virguliforme in Argentina and the first report of this pathogen outside the U.S. Five isolates of F. tucumaniae and three isolates of F. virguliforme were used for artificial inoculations. F. virguliforme isolate 171 provided by J. Rupe (University of Arkansas, Fayetteville) was used as a positive control. Each of the isolates were inoculated separately on soybean cultivars Ripley, RA 702, Pioneer 9492, Spencer and A-6445RG in a greenhouse, using the soil infestation and toothpick method. All eight isolates produced foliar SDS symptoms 15-25 days after inoculation. Lesions produced on leaves averaged 3.3 for the three isolates of F. virguliforme, 2.6 for the five isolates of F. tucumaniae and 3.3 for the positive control, using a disease severity scale where 1 = no symptoms and 5 = severely infected or dead plants. Under these conditions, F. virguliforme appeared to be more virulence than F. tucumaniae. Uninoculated controls remained symptomless. Koch´s postulates were confirmed using soybean cultivars RA 702 and A6445RG. Colonies recovered from symptomatic plants inoculated by the soil infestation and toothpick method were identical to those from which the inoculum was prepared. Strains were recovered 100% and 60%, respectively, from plants inoculated by the toothpick and soil infestation method. This is the first report of the fulfillment of Koch´s postulates for F. tucumaniae and F. virguliforme in Argentina. (1) M. Scandiani et al. Plant.Dis. 87:447, 2003. (2) T. Aoki et al. Mycologia 95:660-684, 2003.