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

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 fungicide application, 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 species within FGSC 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 season. 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 a.i./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 with the 15-acetyl(A)deoxnyvalenol(DON) genotype was the most common (48 isolates) followed by F. meridionale (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. meridionale prevailing in MN743 and MN620 and F. graminearum in BRS Caue and BRS Elis. Overall, 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.