Yield and optimum fungicide dose rates for winter wheat (Triticum aestivum L.) varieties with contrasting ratings for resistance to septoria tritici blotch

Abstract

This study evaluated the effect of varieties of winter wheat that differ in septoria tritici blotch (STB) resistance rating on yield and profit margin obtained when treated with varying application rates of an azole-only fungicide or an azole-plus-SDHI fungicide. Seven field trials were conducted over four growing seasons in the south-east of Ireland, 2012–2015. Each trial consisted of a factorial arrangement of two fungicide types (azole and azole+SDHI), five applications rates (0, 0.25, 0.50, 1.0 and 2.0 times the recommended dosage per application, applied twice during the growing season) and three varieties that differed in reported STB-resistance ratings (5, 7 and 8 on a scale of 0–9; SR5, SR7, SR8). The severity of STB and grain yield were determined and analysed by analysis of variance. The economic optimum rate of each fungicide product was also determined for each variety at each site-season. When untreated, the STB severity was lower for SR8 at GS71 than the other varieties at four of the five evaluated sites, while SR7 incurred lower STB severity than SR5 at only three of the five sites that comparisons were available. The STB severity for the SR8 variety was unaffected by the application rate of either fungicide at most sites. Increased rate of fungicide increased yield, and this effect was generally consistent among the SR5 and SR7 varieties at site-seasons with medium or high STB pressure. However, the yield benefit to increased rate of either fungicide type was minimal for SR8. The variation in the economic optimum rate of either fungicide type evaluated was greater between site-seasons than between the SR5 and SR7 varieties. However, the SR8 variety had a comparatively low economic optimum rate of an azole+SDHI in four of the five site-seasons. When averaged across all site-seasons, the margin above fungicide cost was greater for crops treated with the azole+SDHI treatment than an azole-only treatment for all varieties evaluated. These findings indicate that the range of STB resistance exhibited by the majority of varieties currently grown in Ireland (5–7 on the 0–9 scale) do not consistently confer differences in the optimum fungicide application rate. However the results also suggest that varieties exhibiting strong STB resistance may allow confident reductions to fungicide programs, should they become commonly available in the future.