Azole fungicide sensitivity and molecular mechanisms of reduced sensitivity in Irish Pyrenopeziza brassicae populations

Abstract

BACKGROUND: Light leaf spot, caused by Pyrenopeziza brassicae, is amongst the most damaging diseases of winter oilseed rape (Brassica napus), and currently the sterol 14⊍-demethylase (CYP51) inhibitors (azoles) represent the main class of fungicides used to control light leaf spot development. However, a shift in sensitivity to azole fungicides in P. brassicae populations has been observed in different European countries, including Ireland. RESULTS: To assess the sensitivity status of Irish P. brassicae populations to azole fungicides, three collections of P. brassicae from 2018-2020 were tested in vitro against tebuconazole and prothioconazole-desthio, and the PbCYP51 gene targeted by this class of fungicides was genotyped in different isolates. A change in sensitivity to azole fungicides was observed and differences in sensitivity to tebuconazole between Irish populations were present. There were two substitutions within PbCYP51 (G460S and S508T) and inserts of different sizes in its promoter region. The presence of the G460S/S508T double mutant was reported for the first time, and the diversity in insert size was greater than previously known. Compared to wild type isolates, those carrying G460S or S508T were less sensitive to both fungicides and, where inserts were also identified, they further reduced sensitivity to azole fungicides. CONCLUSIONS:The results of this study suggest that azole fungicides are still very effective in controlling light leaf spot in Ireland. However, using azole fungicides in mixtures of fungicides with different modes of action is recommended.

Integrated pest control in oilseed crops—new advances from the rapeseed research community

“…A shift in sensitivity observed in P. brassicae populations observed in different European countries suggests the farmers should start using mixtures of fungicides with different modes of action, to avoid further unfavorable changes in the pathogen populations. 32 The recent findings summarized here and described in this Special issue push forward the boundaries and open new gates leading towards biological and integrated protection of oilseed rape which will result in healthier and safe food products and a less polluted environment through reduced use of synthetic pesticides. We are happy to be a part of this process!…”

“…The disease is currently controlled mainly by azole fungicides. A shift in sensitivity observed in P. brassicae populations observed in different European countries suggests the farmers should start using mixtures of fungicides with different modes of action, to avoid further unfavorable changes in the pathogen populations 32 …”