First Report of the Fusarium tricinctum Species Complex Causing Fusarium Head Blight of Wheat in Brazil

Abstract

Fusarium graminearum species complex (FGSC) is the main cause of Fusarium head blight (FHB) in Brazil. However, during surveys conducted from 2008 to 2011, 26 wheat kernel-borne or airborne (air above wheat canopy) strains failed using a FGSC-specific multilocus genotyping assay (Del Ponte et al. 2015). Eight spike-borne strains originating from south of Parana (PR) state (two locations), 11 airborne strains from one field at Passo Fundo (RS), and seven kernel-borne strains each from a different location in northern RS state were subjected to amplification and sequencing of the translation elongation factor 1-alpha gene (primers EF1/EF2, O’Donnell et al. 1998). Maximum parsimony- and maximum likelihood-based methods were fitted using MEGA X (Kumar et al. 2018). Eleven strains were also sequenced for the second largest subunit of RNA polymerase II (primers 5f2/7cr, O’Donnell et al. 2008). The genetic analyses used either the single- or the two-gene datasets. Sequences were deposited in GenBank (MK572748 to MK572784). Three species of the Fusarium tricinctum species complex (FTSC) were identified: 15 F. avenaceum, nine FTSC 11, and two F. reticulatum. Although the two most frequent species were either airborne or kernel-borne, F. reticulatum was only airborne. Among the strains from PR, FTSC 11 was represented by one strain and the rest by F. avenaceum. In RS, FTSC 11 and F. avenaceum, airborne or kernel-borne, were in similar frequency. Eighteen strains representative of all species were subjected to pathogenicity assays on an FHB-susceptible spring wheat (BR18). Two standard FGSC strains, F. graminearum (CML 3066) and F. meridionale (CML 3344), were included for comparison. Three potted wheat plants (replicates) were used. Fungi were grown on Spezieller Nahrstoffarmer agar for 7 days at 25°C with a 12-h photoperiod. A macroconidia suspension (1 × 10⁴ macroconidia/ml) was applied uniformly onto wheat heads at flowering using a household handheld sprayer (2 ml/head). Plants were covered with a plastic bag for 48 h and remained in the greenhouse until harvest (35 days). Disease severity was assessed visually at 4 and 8 days postinoculation (dpi) on each inoculated head. The assay was repeated once. All strains were pathogenic to wheat, and the reisolation of at least two strains from infected spikes confirmed their identity. FHB severity at 4 dpi ranged from 11.7 to 82.8% for F. avenaceum, from 14.1 to 46.2% for F. reticulatum, and from 14.1 to 90.2% for FTSC 11. Similarly, a large intraspecies variation of mean FHB severity was observed at 8 dpi for F. avenaceum (33 to 94.5%), F. reticulatum (32.6 to 79.6%), and FTSC 11 (42.2 to 97.8%). Although evidence for a species effect was not strong (P = 0.08), some strains of all species induced FHB at levels similar to FGSC. FTSC members are known to prevail in cooler climates such as Canada and northern Europe (Bottalico and Perrone 2002; Grafenhan et al. 2013). They were reported in Kentucky (U.S.A.) wheat, including F. reticulatum strains that were weakly pathogenic toward wheat (Bec et al. 2015). In Brazil, barley from the 2016 growing season was found to harbor FTSC strains (Piacentini et al. 2019). Further work is needed to understand whether FTSC is an emergent FHB pathogen, especially because of their known ability to produce enniatins and beauvericin mycotoxins, which are not regulated in Brazil.