Comparison of inoculation methods for characterizing relative aggressiveness of two soybean sudden-death syndrome pathogens, Fusarium virguliforme and F. tucumaniae

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 isolates of F. virguliforme were tested for relative aggressiveness to soybean, using a toothpick inoculation method and two versions of a soil infestation inoculation method. Partially resistant soybean cultivar RA629 and susceptible cultivar A6445RG were inoculated separately with each of the nine isolates. Two experiments for each inoculation method were performed. Analysis of variance identified a significant three-way interaction of soybean cultivar*experiment*SDS pathogen (P = 0.01) using the different methods. When the two soil infestation methods were used, F. virguliforme was more aggressive than F. tucumaniae; however, when using the toothpick method, isolates of F. virguliforme and F. tucumaniae were equally aggressive. Although all three methods discriminated levels of partial resistance of the genotypes to SDS, results of the present study indicated that soil inoculations with sorghum infested grain represent the best method for evaluating soybean cultivar resistance to SDS. The existence of interactions among the host, pathogen and environmental conditions highlights the need for additional studies to improve the reproducibility of tests for screening soybean germplasm for resistance to SDS.