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Sequences and mixtures - background

Fungicide performance: Sequences and mixtures in winter wheat according to disease risk (Wheat AFD 2)

Project no. RD-2004-3026  

Over the last 10 years HGCA-funded field trials (AFD 1) have assessed the performance of individual fungicide active ingredients in winter wheat, when applied as single sprays to highly-susceptible varieties at high disease pressure sites. These experiments have allowed comparative dose-response curves to be produced for the control of a number of foliar diseases for a range of current and new fungicides.

However, in practice the majority of wheat crops receive more than one fungicide spray, and each application usually consists of a mixture of at least two products. In addition, a wide range of varieties are present on farm, and not all of these are highly susceptible to disease or grown in high disease pressure areas.

A new HGCA-funded project, Wheat AFD 2, started last year, to complement the existing research. It will add important information on the effects of using a sequence of two sprays, or a mixture of two or more products, on the shape of the disease and yield dose-response curves on susceptible and resistant varieties. It focuses on the main foliar disease in wheat, Septoria tritici, and the key triazole fungicide group (in this case epoxiconazole or 'Opus'), used for controlling it.

The effects of using a sequence are being evaluated by comparing single GS39 ('T2') sprays of epoxiconazole at various doses with two-spray sequences where the GS39 treatment is preceded by 60% of that dose applied at GS32 ('T1'). For example, 1.0 l/ha Opus applied at T2 preceded by 0.6 l/ha at T1. Mixtures are being evaluated by looking at the effect of adding a fixed dose of chlorothalonil (1.0 l/ha 'Bravo') or chlorothalonil plus pyraclostrobin (0.5 l/ha 'Vivid') to epoxiconazole at both timings in the two-spray sequences.

The impact of varietal susceptibility is being investigated by evaluating the treatments on three varieties at each site: Consort (rated 4 for resistance to Septoria tritici on HGCA Recommended List), Einstein (rated 5) and Robigus (rated 7). The trials are being repeated at four locations: southern, eastern and western England, and southern Scotland, to give a range of disease pressures.

Initial results from 2005 have been encouraging. In particular, the addition of a chlorothalonil partner had a clear effect on the shape of the epoxiconazole dose-response, restoring disease control to the levels obtained several years ago with epoxiconazole on its own. Typically, the triazole dose required to achieve the same level of disease control or yield was halved when chlorothalonil was added, and this would have been a more cost-effective approach than using triazole alone. The addition of pyraclostrobin had less effect on disease, but sometimes had an impact on yield.

The biggest impact on disease levels was that of varietal resistance. There were differences in the dose-response curves between the varieties, and in the doses required to achieve the same level of disease control, or the highest margin. Once information is available from enough years, this will help to quantify the financial value (in terms of triazole expenditure) of differences in septoria resistance rating between varieties on the HGCA Recommended List.

For further results and findings from the first year of the project, please see the 2005 Annual Project Report (project 3026).

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