Aspects of Applied Biology 127, 2015 Crop Production in Southern Britain: Precision Decisions for Proitable Cropping 77 Triazole vs SDHI based fungicides: the incidence of septoria leaf blotch (Mycosphaerella graminicola) in winter wheat varieties and its impact on grain yield By S SHAH, S HOOKWAY, S WILKINSON, C FLINT and J M FLETCHER Agrii Technology Centre (AgriiFocus), North Farm, Swindon, SN8 2JZ, UK Corresponding Author Email: syed.shah@agrii.co.uk Summary The results reported in the paper conirmed that triazole and non-SDHI (succinate dehydrogenase inhibitor) based fungicide programmes alone were not effective in controlling septoria (septoria leaf blotch caused by Mycosphaerella graminicola). However, their effectiveness in controlling disease and enhancing yield was signiicantly improved when used either in a mixture or in a sequence with SDHI based fungicide products. Signiicant yield differences among varieties in terms of their responses to different fungicide programmes and drilling date were noticed. Earlier drilling had a very negative effect on the level of disease control obtained from a range of fungicide programmes for the susceptible variety Dickens and consequently yield compared with moderately resistant variety Crusoe. A triazole + SDHI fungicide programme increased yield by 4.2 and 4.0 t ha -1 for Solstice and Dickens respectively compared with their corresponding untreated plots. The same fungicide programme gave a modest yield enhancement of 1.8 and 2.4 t ha -1 for Skyfall and Solace respectively. This tends to suggest that for moderately resistant varieties such as Crusoe, Skyfall and Solace there may be more scope to reduce doses of fungicide programmes. Further trials are being conducted to investigate the effect of drilling date and fungicide programmes on disease incidence and grain yield of varieties with different levels of septoria resistance at the Agrii Technology Centre-Swindon. Key words: Wheat, septoria. triazole, SDHI, drilling date, resistance, disease incidence, grain yield Introduction In the UK, fungal diseases cause, on average, a 6.1 % reduction in yield in susceptible varieties even when sprayed with fungicides (O’Sullivan, 2011). Disease can also reduce grain quality (Dimmock, & Gooding, 2002). It has been reported that disease development is inluenced by crop density (Bainbridge, 1974) and a dense crop is more conducive for pathogen spread and disease development than a thinly sown crop (Walters and Bingham, 2007). Similarly high N-application and higher seed rate can increase disease severity in the susceptible varieties (Howard et al., 1994). This is partly due to the changes in crop density and microclimate (Bainbridge, 1974). This is because a large canopy and high shoot density is more conducive for spore transfer and pathogen spread than a sparse canopy (Walters & Bingham, 2007).