Trichothecene Genotype of Fusarium graminearum Isolates from Soybean (Glycine max) Seedling and Root Diseases in the United States.

Fusarium graminearum is an economically important pathogen that causes Fusarium head blight of wheat, barley, and oat, and Gibberella ear and stalk rot of maize. More recently, F. graminearum was reported as a soybean seedling and root pathogen in North America (1,5), causing seed decay, damping-off, and brown to reddish-brown root rot symptoms. Type B trichothecene mycotoxins are commonly produced by F. graminearum, which can be categorized into three trichothecene genotypes; those that produce 3-acetyldeoxynivalenol (3-ADON), 15-acetyldeoxynivalenol (15-ADON), or nivalenol (NIV). The 15-ADON genotype is dominant in populations from small grains and maize in North America (4), but the 3-ADON genotype has recently been found (4). F. graminearum was known as a pathogen of wheat and maize in North America for over a century before it was reported as a soybean pathogen. Therefore, we hypothesized that recent reports on soybean could be associated with the appearance of the 3-ADON genotype. The objective of this research was to determine the trichothecene genotype of F. graminearum isolates from soybean in the United States. Thirty-eight isolates from soybean were evaluated. Twenty-seven isolates came from a 3-year survey for Fusarium root rot from 2007 to 2009 in Iowa. Other isolates (Ahmad Fakhoury, Southern Illinois University, Carbondale) were collected from soybean seedlings during a multi-state survey in 2012, and included three isolates from Illinois, three from Indiana, and five from Nebraska. Species identification and lineage of F. graminearum were confirmed by sequencing the translation elongation factor gene (EF1-α) using EF-1H and EF-2T primers. A maximum likelihood analysis of the EF1-α, including voucher strains from nine lineages of F. graminearum (2), placed all 38 isolates into lineage 7, F. graminearum sensu stricto (representative GenBank accessions KJ415349 to KJ415352). To determine the trichothecene genotype of each isolate we used three multiplex PCR assays. The first two assays targeted a portion of trichothecene biosynthesis genes Tri3 and Tri12 (4), while the third assay targeted portions of the Tri3, Tri5, and Tri7 genes (3). The PCR for the first two assays was conducted as described by Ward et al. (4) using four sets of primers: 3CON, 3NA, 3D15A, and 3D3A; and 12CON, 12NF, 12-15F, and 12-3F for the Tri3 and Tri12 genes, respectively. The PCR for the third assay was conducted as described by Quarta et al. (3) using the following primers: Tri3F971, Tri3F1325, Tri3R1679, Tri7F340, Tri7R965, 3551H, and 4056H. The amplification products were analyzed by gel electrophoresis. All 38 isolates produced amplicons consistent with the 15-ADON genotype; ~610 and 670 bp for the Tri3 and Tri12 genes, respectively (4), and two amplicons of ~708 and 525 bp for the Tri3/Tri5 genes (3). Our results indicated that the dominant trichothecene genotype among isolates of F. graminearum from soybean is 15-ADON, and the introduction of 3-ADON isolates does not explain the recent host shift of F. graminearum to soybean in North America. To our knowledge, this is the first assessment of trichothecene genotypes in F. graminearum populations from soybean from the United States. References: (1) K. E. Broders et al. Plant Dis. 91:1155, 2007. (2) K. O'Donnell et al. Fungal Gen. Biol. 41:600, 2004. (3) A. Quarta et al. FEMS Microbiol. Lett. 259:7, 2006. (4) T. D. Ward et al. Fungal Gen. Biol. 45:473, 2008. (5) A. G. Zue et al. Can. J. Plant Pathol. 29:35, 2007.