Fungicide doses in sequences and mixtures for winter wheat

Abstract

The performance of individual fungicides applied as single sprays to control foliar diseases in winter wheat has been evaluated in HGCA-funded research since 1994. A new project was commissioned to add information on the effects of using a sequence of two sprays or a mixture of two or more products, with the aim of establishing principles by which farmers could develop effective programmes from HGCA fungicide performance data. The project focused on control of Septoria tritici ⁽¹⁾, and the impact of mixtures on the dose-response of the triazole fungicide Opus (epoxiconazole). Field experiments were conducted on three varieties representing a range of susceptibilities to septoria, and at four locations over a three year period, to give a range of disease pressures. The main treatments compared single sprays (at GS39) against 2-spray programmes (at GS32 and GS39), and Opus applied alone against mixtures with Bravo (chlorothalonil) or Bravo plus the strobilurin Vivid (pyraclostrobin).

In 2005 and 2006 septoria was the main disease present and the major influence on green leaf area and grain yield. Adding Bravo to a 2-spray Opus programme reduced the dose of triazole needed to obtain equal control of septoria, often by 50% or more, allowing optimum yields to be obtained at 15-30% lower doses of Opus and improving margins. Even when mixed with Bravo, optimum triazole (Opus) doses were 45-65% higher for the most compared to the least septoria-susceptible variety, and 40-60% higher under medium-high than under low septoria pressure. In 2007 yellow or brown rust were present as well as septoria at all locations, and in some cases these had an equal or greater influence on yield. The strobilurin Vivid was of more benefit than Bravo as a partner to control rusts. Adding Vivid, as well as Bravo, resulted in up to 70% lower optimum Opus doses whilst maintaining or improving margins.

Raising the grain price from £80 to £160/t increased the optimum dose of Opus by 25-30%. However, a range of doses would have given margins close to the optimum, and applying a dose slightly above the expected optimum would allow for application delays or higher than expected disease pressure. Early septoria progress, as indicated by within-season disease monitoring experiments, did not correlate well with end of season septoria levels or fungicide responsiveness. The Wheat Disease Manager computer programme, designed to predict yield response to fungicides from information available at the time of treatment, and a mixture calculator that was created to combine dose-response curves for disease for two individual fungicide active ingredients and produce a value for the mixture, both gave promising results. 

⁽¹⁾ Most recent scientific name is Mycosphaerella graminicola