Weak selection by field sprays for flutriafol resistance in Septoria tritici

Change in sensitivity of populations of Septoria tritici resulting from single sprays of the sterol demethylation-inhibiting fungicide flutriafol was measured. Field trials were conducted over 3 years at two sites separated by about 15 km, on cvs Mercia and Riband at each site, in plots at least 10 × 12 m. Treatments included the full recommended rate and a reduced dose of flutriafol and a mixture of flutriafol with chlorothalanil. Sprays were applied at GS37; samples were taken just before spraying and as soon as lesions appeared on leaf 2, which had had no visible disease at the time of spraying. Epidemiological evidence suggests that sprays acted both curatively and as protectants. Significant shifts in sensitivity occurred after spraying in both water-sprayed and fungicide-sprayed plots; all plots became less sensitive. Disease severity after spraying clearly decreased with increasing fungicide concentration and with the use of a mixture of flutriafol and chorothalanil. However, after correction for the shifts in the water-sprayed plots, no dose produced a significant change in population sensitivity level, although sample sizes were large. There was no trend in sensitivity in the population over the 3 years of the experiment.