Sensitivity to tebuconazole and carbendazim in Fusarium graminearum species complex populations causing wheat head blight in southern Brazil

Field trials where fungicides have been applied for Fusarium head blight (FHB) control were established and monitored in 17 municipalities in the Guarapuava region, Southern Paraná, Brazil, during four years (2011 to 2014). A total of 227 isolates belonging to the Fusarium graminearum species complex were obtained from symptomatic wheat heads. The majority of the 227 isolates (64%) were of the 15-(A)cetyl-deoxynivalenol (DON), 29% of the nivalenol (NIV) and 4% of the 3-ADON genotype. A random sample of 35 isolates was selected to determine the effective concentration of tebuconazole and carbendazim that reduces 50% of the mycelial growth (EC 50 ). The estimated means of the EC 50 ranged from 0.0004 to 3.0 and from 0.91 to 2.65 μg/ml for tebuconazole and carbendazim, respectively. Isolates of the 15-ADON genotype were less sensitive to tebuconazole compared to NIV + 3ADON. Evidence of lower sensitivity for both fungicides were detected. Discriminatory doses (carbendazim = 1.4 μg/ml and tebuconazole = 2.0 μg/ml) were used to screen the entire population (n = 227) for less-sensitive isolates. In the 2011–2014 population, four tebuconazole-less sensitive (2%) and nine carbendazim-less sensitive (4%) isolates were found, all originating from Guarapuava. A contemporary sample of isolates from 2018 to 2020 ( n = 42) obtained from the same region were screened using the discriminatory doses. The frequency of the less-sensitive isolates increased over time from 2 and 4% to 24 and 95% for tebuconazole and carbendazim, respectively. Control efficacy was generally lower in plants inoculated with less-sensitive isolates. Greater number of less-sensitive individuals were recovered from plants inoculated with a mixture with a sensitive isolate and sprayed with either fungicide. The presence of less sensitive isolates suggest that selection may be associated with the frequent reports of decline in control efficacy for both fungicides tested in this study.