Disease control programmes using triazole and strobilurin fungicides on winter wheat

Abstract

The project was based upon eight experiments at various locations within Britain that were conducted to evaluate the timing of the addition of strobilurins to a three spray triazole programme in each growing season from 2002 to 2004. During this period, resistance of Septoria tritici to the strobilurins rose from a trace in 2002 to such an extent that in 2004 field control was severely compromised. All experiments were conducted on the winter wheat cultivar Consort which is relatively susceptible to Septoria tritici (HGCA Recommended List rating 4) and brown rust (Puccinia recondita) (HGCA Recommended List rating 4). At all sites and in all years Septoria tritici was the predominant foliar disease but its development between seasons and locations was diverse.

In 2002, and to a lesser extent in 2003, the optimum timing for the addition of two strobilurins was at T1 (third final leaf emerging) and T3 (mid-anthesis). This approach was identified in TAG trials in Norfolk in previous years but showed little advantage over an alternative strategy of adding strobilurins to the T2 (full flag leaf emergence) and T3 applications of triazoles. However, the T1 and T3 approach was not surpassed by any other combination of strobilurin addition to the programme in any of the national trials in 2002 and the approach was still the most robust in 2003. This strongly suggests that such an approach was appropriate nationally and there was no need for regional variations in fungicide strategy in terms of strobilurin timing.

However, the mutation that conferred resistance of S. tritici to the strobilurins was detected in almost 100% of isolates by the end of the 2004 season. In this year, the addition of the strobilurins did not significantly increase yields but trends suggested that they were best applied at T1 and T3 or T2 and T3. This was despite disease pressure only being significant early in the season. This suggested a change in role for any addition of the strobilurins from S. triticicontrol and ear disease control to ear disease control alone. In addition, late strobilurin application has long been recognised to prolong green leaf area in the apparent absence of disease control and such a property can sometimes lead to an enhanced yield. Hence, the possible role for strobilurins in the future may be largely restricted to an application late in the season to control ear diseases, possibly supplement the triazoles on brown rust control and to prolong green leaf area. In addition, the experiments in 2004 identified the potential enhanced role of chlorothalonil in programmes where there was concern over the resistance of S. tritici to fungicides.