Long-term performance of fungicides applied to control eyespot in winter wheat

Fungicide treatments, repeated twice yearly at timings suitable for controlling eyespot, to plots of continuous winter wheat over 17 yr (except for a year in "set-aside"), resulted in altered population structures of the eyespot pathogens (Tapesia spp.) and performance of the fungicides. Carbendazim failed to control eyespot after 3 yr of the experiment because of rapid selection for resistance. Carbendazim often increased disease compared with that in untreated plots. This may have resulted from control of unknown antagonistic fungi or, possibly, from selection for increased fitness in carbendazim-resistant strains. In the final years of the experiment, prochloraz, applied with or without carbendazim, became ineffective. This gradual loss of efficacy is explained by selection in favour of T. acuformis rather than T. yallundae (which had a naturally narrower range of sensitivity levels in its populations) and for strains with least sensitivity within the T. acuformis population, although no resistance to prochloraz was found. Effects of treatments on yield were usually not statistically significant. This may be explained partly by the predominance of the more slowly developing T. acuformis in many of the plots.