Differential triazole sensitivity among members of the Fusarium graminearum species complex infecting barley grains in Brazil

Authors: MACHADO, FRANKLIN - Universidade Federal De Vicosa; NICOLLI, CAMILA - Universidade Federal De Viçosa; MOLLER, PRISCILLA - Universidade Federal De Viçosa; ARRUDA, RENATO - Universidade Federal De Viçosa; GRUPPI, GUSTAVO - Universidade Federal De Viçosa; Ward, Todd; DEL PONTE, EMERSON - Universidade Federal De Viçosa

Submitted to: Tropical Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2017
Publication Date: 5/16/2017
Citation: Machado, F.J., Nicolli, C.P., Moller, P.A., Arruda, R., Ward, T.J., Del Ponte, E.M. 2017. Differential triazole sensitivity among members of the Fusarium graminearum species complex infecting barley grains in Brazil. Tropical Plant Pathology. 42(3):197-202.

Interpretive Summary: Fungi within the Fusarium graminearum species complex (FGSC) are responsible for Fusarium head blight (FHB) of wheat and barley, and economically destructive diseases of other cereals world-wide. These fungi also contaminate grain with trichothecene mycotoxins that pose a significant threat to food safety and animal health. As part of a project to establish a global picture of FHB pathogen diversity, we determined the prevalence of FGSC species and toxin types associated with barley in Brazil, and also characterized the sensitivity of FGSC isolates to two different triazole fungicides (tebuconazole and metconazole). We identified three distinct species of FHB pathogens among the isolates from barley, with F. graminearum being predominant. However, we observed a greater frequency of nivalenol producing species among barley isolates than we previously observed among wheat isolates from the same region. These results are a potential concern for food safety and animal health, because nivalenol is regarded as the most toxic of the trichothecene mycotoxins produced by species in the FGSC. Although all FGSC isolates were sensitive to both triazole fungicides at concentrations used in field applications, they were significantly less sensitive to tebuconazole as compared with metconazole. The results reported here facilitate monitoring of mycotoxins in barley and the deployment of effective management strategies to reduce FHB and mycotoxin contamination of grain.

Technical Abstract: Fusarium head blight (FHB) is an important disease of small grains and is caused mainly by members of the Fusarium graminearum species complex (FGSC). Barley growers in Brazil rely on fungicides, especially triazoles, to suppress the disease and limit mycotoxin contamination of grain. Information on triazole sensitivity among FHB pathogens from barley is limited and it is unclear whether sensitivity levels differ among FGSC members commonly associated with FHB of barley. To address these questions, grain samples of five barley cultivars were obtained from commercial fields at 21 municipalities in northern Rio Grande do Sul state, Brazil, during 2011. The incidence of FGSC was evaluated and the species identity and trichothecene genotypes of FGSC isolates were determined using a multilocus genotype assay. Sensitivity to tebuconazole and metconazole was determined based on the effective concentration that reduced 50% of the mycelial growth (EC50) on agar-based media amended with increasing fungicide dosages (0, 0.1, 0.5, 1.0, and 5.0 µg/ml). FGSC-like colonies were detected in 75% of samples at relatively low mean incidence (3.3%). Among 65 FGSC isolates, three species were found: Fusarium graminearum (Fgra) with the 15-acetyl(A)deoxnyvalenol(DON) genotype was the most common (48 isolates) followed by F. meridionale (Fmer) (16 isolates) with the nivalenol (NIV) genotype and one F. austroamericanum isolate with the 3-ADON genotype. Species composition appeared to be influenced by barley cultivar (P = 0.002), with F. meridinoale prevailing in MN743 and MN620 and F. graminearum in BRS Cauê and BRS Elis. Taken together, FGSC isolates were around 4 × less sensitive to tebuconazole (mean = 0.773 µg/ml) compared to metconazole (mean = 0.136 µg/ml). F. graminearum isolates were less sensitive than F. meridionale to tebuconazole, but both were similarly sensitive to metconazole. These results are important to both risk assessment of mycotoxins in barley and the deployment of effective management strategies.