Fluopyram Sensitivity and Functional Characterization of SdhB in the Fusarium solani Species Complex Causing Soybean Sudden Death Syndrome

Abstract

The succinate dehydrogenase inhibitor (SDHI) fungicide, fluopyram, is used as a soybean seed treatment to manage <i>Fusarium virguliforme</i>, the casual agent of sudden death syndrome (SDS). More recently, other species within clade 2 of the <i>Fusarium solani</i> species, <i>F. tucumaniae</i> in South America and <i>F. brasiliense</i> 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, <i>in vitro</i> sensitivity tests of the three <i>Fusarium</i> species to fluopyram were conducted. The mean EC₅₀ values of <i>F. brasiliense</i> and <i>F. virguliforme</i> strains to fluopyram were 1.96 and 2.21 μg ml^-1, respectively, but interestingly <i>F. tucumaniae</i> 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 <i>Sdh</i> genes of <i>Fusarium</i> strains revealed that the <i>F. tucumaniae</i> strains contain an arginine at codon 277 in the <i>Sdh</i>B gene instead of a glycine as in other <i>Fusarium</i> species. Replacement of glycine to arginine in SdhB-277 in a <i>F. virguliforme</i> wild-type strain Mont-1 through genetic transformation resulted in increased sensitivity to two SDHI fungicides, fluopyram and boscalid. Similar to a <i>F. tucumaniae</i> strain, the Mont-1 (SdhB^G277R) 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 <i>F. tucumaniae</i> results in fluopyram being efficacious if used as a seed treatment for management of <i>F. tucumaniae</i>, which is the most abundant SDS causing species in South America. The establishment of baseline sensitivity of <i>Fusarium</i> species to fluopyram will contribute to effective strategies for managing <i>Fusarium</i> diseases in soybean and other pathosystems such as dry bean.