|Xiang, Yiwen - University Of Illinois|
|Scandiani, Mercedes - National University Of Rosario|
|Herman, Theresa - University Of Illinois|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/24/2014
Publication Date: 4/1/2015
Publication URL: http://handle.nal.usda.gov/10113/61161
Citation: Xiang, Y., Scandiani, M.M., Herman, T.K., Hartman, G.L. 2015. Optimizing conditions of a cell-free toxic filtrate stem cutting assay to evaluate soybean genotype responses to Fusarium species that cause sudden death syndrome. Plant Disease. 99:502-507.
Interpretive Summary: A soil fungus named Fusarium virguliforme causes the soybean disease sudden death syndrome. The fungus produces a toxin or possibly toxins both in infected plants and when grown in a pure culture in the laboratory. When the fungus is grown in liquid culture, filtering out the live fungal cells from the filtrate produces cell-free culture filtrates. This filtrate is used to test its toxicity on soybean stem-cuttings, which if toxic, produce foliar symptoms similar to those obtained from root inoculations of whole plants and those observed in production fields infected with the fungus. The objectives of this study were to (i) optimize the production conditions for the fungus to produce toxic culture filtrates and optimize the incubation conditions of the stem-cutting assay used to test the toxicity of the fungal culture filtrates, and (ii) to compare the optimized assay and a whole plant root inoculation assay. The results provided information on how to optimize toxicity of the culture filtrate by altering incubation conditions, like temperatures, time of incubation, and culture media. Symptoms of the disease were similar when using the culture filtrate and whole plant root inoculations. This report will be of interest to soybean pathologists, those studying fungal toxins, and other scientists interested in host-pathogen interactions.
Technical Abstract: Cell-free toxic culture filtrates from Fusarium virguliforme, the causal fungus of soybean sudden death syndrome (SDS), cause foliar symptoms on soybean stem-cuttings similar to those obtained from root inoculations in whole plants and those observed in production fields. The objectives of this study were to (i) optimize the production conditions for F. virguliforme cell-free toxic culture filtrates and the incubation conditions of the stem-cutting assay used to test the toxicity of the cell-free toxic culture filtrates, and (ii) use the optimized assay and a whole plant root inoculation assay to compare four SDS-causing isolates on a panel of selected soybean genotypes. Area under the disease progress curve (AUDPC) values were highest (P = 0.05) when cuttings were immersed in culture filtrate of fungus grown in soybean dextrose broth, in filtrate produced from the fungus grown for 18 or 22 days, and when stem-cuttings were incubated at 30°C. AUDPC values and shoot dry weights from the whole plant root inoculations and the AUDPC values from the stem-cutting assay differed (P < 0.05) among nine soybean genotypes tested with F. virguliforme and F. tucumaniae isolates, and the AUDPC values from the two assays were positively correlated (r = 0.44 at P < 0.0001).