Submitted to: Tropical Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2011
Publication Date: 5/1/2011
Citation: Scandiani, M.M., Ruberti, D.S., Giorda, L.M., Pioli, R., Luque, A., Bottai, H., Leiva, M., Ivancovich, J.J., Aoki, T., O Donnell, K. 2011. Comparison of inoculation methods for characterizing relative aggressiveness of two soybean sudden-death syndrome pathogens, Fusarium virguliforme and F. tucumaniae. Tropical Plant Pathology. 36(3):133-140. Interpretive Summary: The economically devastating disease of soybean called sudden death syndrome (SDS) has become a constraint to its production within the United States and various countries in South America. Recent molecular studies have revealed that this disease can be induced by four closely related species of Fusarium whose evolutionary origins appear to be South America (O’Donnell et al.  Phytopathology in press). To date, no detailed comparative studies have been conducted to evaluate which method of inoculation is best for testing pathogenicity on soybean under greenhouse conditions. Therefore, the present study was initiated to evaluate the three most commonly used inoculation methods. These methods included one employing inoculation via mycelium infested toothpicks and two methods in which infested sorghum grain was used as the inoculum. Isolates of the two most important SDS pathogens, F. virguliforme (Fv) and F. tucumaniae (Ft), were used in the experiment. Previous field experiments have shown that the isolates of Fv included in this study are more aggressive and pathogenic than those of Ft. In contrast to the toothpick method, in which isolates of Fv and Ft could not be distinguished, both variations of the soil infestation method was able to differentiate the more aggressive and pathogenic isolates of Fv from Ft. These results should benefit plant breeders who are focused on developing soybean cultivars with broad based resistance to SDS.
Technical Abstract: Fusarium tucumaniae and F. virguliforme are the primary etiological agents of sudden-death syndrome (SDS) of soybean in Argentina and the United States, respectively. Five isolates of F. tucumaniae and four of F. virguliforme were tested for pathogenicity to soybeans, by comparing a toothpick method with two variations of the soil infestation inoculation method. Fusarium virguliforme isolate 171 was used as a positive control for pathogenicity and SDS symptoms. Partially resistant soybean cultivar RA629 and susceptible cultivar A6445RG were inoculated separately with each of the nine isolates. Foliar symptoms were assessed at four time intervals after inoculation. Disease incidence was used to determine the area under the disease progress curve (AUDPCi). All nine isolates produced typical SDS foliar symptoms. Significant interaction was found in soybean cultivar x inoculation method x experiment (p < 0.0001) and isolate x inoculation method x experiment (p < 0.0060). Soybean cultivar RA629 yielded a lower AUDPCi and foliar disease severity rating than A6445RG using each of the three inoculation methods. Both variations of the soil infestation method differentiated F. virguliforme from F. tucumaniae, in that isolates of F. virguliforme tested were more aggressive and pathogenic than those of F. tucumaniae. Aggressiveness and pathogenicity of these two species, however, were not distinguished using the toothpick method.