Brazilian wheat farmers rely on fungicides to protect fields against several foliar and flowering diseases, including Fusarium head blight caused mainly by Fusarium graminearum. A range of active ingredients are used in isolation or dual premixes that include a demethyl (DMI) and a quinone outside inhibitor (QoI) fungicides. Comprehensive information on fungicide resistance of F. graminearum is available only for DMIs, while for QoI, data are scarce and results inconsistent. We studied 225 strains collected from two states in southern Brazil (RS and PR), in relation to their response to two QoIs. In vitro sensitivity of F. graminearum isolates was assessed through the conidia germination assay. The effective concentration leading to 50% inhibition (EC50) of conidial germination was obtained based on three-parameter Weibull function. Isolates, in which the EC50 was not previously determined, were screened using discriminatory doses (DD) for both fungicides. Molecular analysis of the cytochrome b (cytb) gene was performed and nine isolates were selected and sequenced. Results showed that the median EC50 value for azoxystrobin (n = 25) was 2.20 μg mL−1 in the PR collection and 4.04 μg mL−1 in the RS collection. For pyraclostrobin (n =50), the median EC50 was 0.28 μg mL−1 in the PR collection and 0.24 μg mL−1 in the RS collection. A comparison between the two fungicides showed significant differences between them and pyraclostrobin was more fungitoxic than azoxystrobin. Evidence of cross resistance when correlating the EC50 values of the two fungicides could not be detected. Screening using DD for azoxystrobin, from PR (n = 75) and RS (n = 100) allowed the detection of 50% and 28% as less sensitive strains, respectively. Using DD for pyraclostrobin, 33% and 18.8% were classified as less sensitive in the PR and RS collections. In RS, the frequency of less sensitive isolates increased over five years (2007-2011). Sequence alignments showed no point mutation in any target spot (F129L, G137R, G143A) even in isolates with highly EC50. These results represent an important step towards monitoring the resistance to two most used QoIs in commercial premixes targeting FHB control in Brazil. 

ACKNOWLEDGEMENT AND DISCLAIMER 

This work is the graduate research of the first author. We would like to acknowledge: CAPES/Brazil for the scholarship to the first author; FAPEMIG for the financial support; and CNPq for a research fellowship to the senior author.