Light leaf spot (Pyrenopeziza brassicae) in oilseed rape: Extent of triazole fungicide resistance in Scotland; fungicide strategies

Abstract

In recent seasons the efficacy of triazole fungicides for light leaf spot control in oilseed rape has markedly declined in high disease pressure areas. The objective of this project was to determine the distribution of triazole resistance in light leaf spot (Pyrenopeziza brassicae) in Scotland and to establish how this problem could be managed in cost effective fungicide programmes. Isolates of P. brassicae were taken from commercial crops and trial sites throughout the main arable areas of Scotland and their sensitivity to Folicur (a.i. tebuconazole) and Punch C (a.i. flusilazole and carbenazim) determined. The sensitivity of isolates was very variable and  some were up to forty times less sensitive than the most sensitive isolates. These less sensitive isolates were found in all the Scottish arable areas surveyed. There was no link between the sensitivity of isolates and the previous fungicide history of the crop from which they had been sampled.

The occurrence of less sensitive isolates within the P. brassicae population was critical in the efficacy of triazole fungicides in field trials. Tebuconazole residue analysis a month after autumn fungicide sprays were applied showed that the fungicide was detectable at more than background levels following reduced dose rate treatments. Even after a full dose rate treatment levels one month after treatment were low enough that less sensitive isolates would still have been able to develop. Disease control data from the trials showed that full and three quarter rate Folicur (tebuconazole) treatments tended to give significantly better control than half dose rate treatments. However there were crop safety issues involved in increasing the dose rate and a full dose rate caused significant losses in crop vigour that were reflected in yield. Punch C (flusilazole plus carbendazim) did not have the same crop safety issues at higher dose rates. Full rate treatment of Punch C, however, yielded less than a three quarter dose rate treatment, implying the three quarter rate was optimal. P. brassicae isolates assayed were generally more sensitive to Folicur than to Punch C but this was seldom translated into reduced efficacy compared to Folicur in the field trials. The addition of partner fungicides to the Folicur treatments with alternative modes of action such as Dithane, Thiovit or Bravo did not significantly improve disease control and consequently did not offer an effective anti-resistance strategy. Punch C may still offer an effective anti-resistance strategy in areas where sensitivity to MBC fungicides like carbendazim is still present. Triazole dose rates of below half as currently commonly used in commercial practice in Scotland will almost certainly not control light leaf spot effectively.