Abstract
The succinate dehydrogenase inhibitor (SDHI) fungicide, fluopyram, is used as a soybean seed treatment to manage Fusarium virguliforme, the casual agent of sudden death syndrome (SDS). More recently, other species within clade 2 of the Fusarium solani species, F. tucumaniae in South America and F. brasiliense in America and Africa, have been recognized as additional agents capable of causing SDS. To determine if fluopyram could be used for management of SDS caused by these species, in vitro sensitivity tests of the three Fusarium species to fluopyram were conducted. The mean EC50 values of F. brasiliense and F. virguliforme strains to fluopyram were 1.96 and 2.21 μg ml-1, respectively, but interestingly F. tucumaniae strains were highly sensitive (mean EC50 = 0.25 μg ml-1) to fluopyram compared to strains of the other two species. A sequence analysis of Sdh genes of Fusarium strains revealed that the F. tucumaniae strains contain an arginine at codon 277 in the SdhB gene instead of a glycine as in other Fusarium species. Replacement of glycine to arginine in SdhB-277 in a F. virguliforme wild-type strain Mont-1 through genetic transformation resulted in increased sensitivity to two SDHI fungicides, fluopyram and boscalid. Similar to a F. tucumaniae strain, the Mont-1 (SdhBG277R) mutant caused less SDS and root rot disease than Mont-1 on soybean seedlings with the fluopyram seed treatment. Our study suggests the amino acid difference in the SdhB in F. tucumaniae results in fluopyram being efficacious if used as a seed treatment for management of F. tucumaniae, which is the most abundant SDS causing species in South America. The establishment of baseline sensitivity of Fusarium species to fluopyram will contribute to effective strategies for managing Fusarium diseases in soybean and other pathosystems such as dry bean.
Highlights
Soybean sudden death syndrome (SDS) caused by Fusarium virguliforme, is an economically devastating disease in North America
Our findings indicate that F. tucumaniae is the most sensitive species to fluopyram among the three main causal species of Fusarium responsible for soybean SDS
The greater sensitivity in F. tucumaniae was attributed to the amino acid (277R) in SdhB, which was confirmed by F. virguliforme SdhBG277R mutants displaying increased sensitivity to fluopyram and boscalid
Summary
Soybean sudden death syndrome (SDS) caused by Fusarium virguliforme, is an economically devastating disease in North America. SDS ranked among the top forth yield-suppressing soybean diseases and 209.7 million bushels of soybean yield loss were attributed to SDS during 2010-2014 in the United States and Ontario, Canada (Allen et al, 2017). A recent survey of SDS symptomatic soybean fields in Michigan found that F. brasiliense was present in the U.S (Wang et al, unpublished). All of these SDS causing species belong to clade 2 of the Fusarium solani species complex (FSSC), which contains species capable of causing root rot, but possibly little to no SDS (O’Donnell et al, 2010)
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