Sensitivity assessment and SDHC-I86F mutation frequency of Phakopsora pachyrhizi populations to benzovindiflupyr and fluxapyroxad fungicides from 2015 to 2019 in Brazil.

Background: Fungicides of the succinate dehydrogenase inhibitors (SDHIs) group have been used in soybean to control Asian soybean rust (ASR) caused by Phakopsora pachyrhizi. Fungal populations with less sensitivity to SDHI fungicides have been reported since 2015.

Results: In this study, fungal sensitivity to benzovindiflupyr (BZV) and fluxapyroxad (FXD) was assessed using a total of 770 P. pachyrhizi populations sampled over four soybean growing seasons. Cross-resistance, intrinsic activity, and frequency of SDHC-I86F mutation were also analyzed. The average effective concentration to inhibit 50% (EC ) and SDHC-I86F frequency increased over the 2015/2016, 2016/2017, 2017/2018 and 2018/2019 soybean-seasons. Fourteen P. pachyrhizi populations had the EC value above 10 mg L for both carboxamides. No difference was found in intrinsic active to BZV and FXD fungicides for sensitive P. pachyrhizi populations. For P. pachyrhizi classified as less sensitive BZV showed the highest fungitoxicity effect. High frequency of the C-I86F mutation was observed in samples collected in volunteer soybean plants. The maximum frequency of SDHC-I86F mutation in the population was 50% and resulting in ASR populations with low sensitivity to SDHIs. A low correlation between bioassay and SDHC-I86F mutation was observed possible due to the dikaryotic nature of rust fungi or other mutations in the other succinate dehydrogenase subunits.

Conclusion: The present work provides an overview of a large sampling size of P. pachyrhizi populations and their performance over the four crop seasons. The high frequency of SDHC-I86F mutation and low sensitivity to SDHIs are widely distributed in the main soybean growing regions in Brazil and present in volunteer plants in the soybean-free period. Further detailed studies are needed to identify novel point mutations affecting the effectiveness of SDHIs. © 2021 Society of Chemical Industry.