WEAK SELECTION BY FIELD SPRAYS FOR FLUTRIAFOL RESISTANCE IN SEPTORIA-TRITICI

Change in sensitivity of populations of Septoria tritici resulting fro m single sprays of the sterol demethylation-inhibiting fungicide flutr iafol was measured. Field trials were conducted over 3 years at two si tes separated by about 15 km, on cvs Mercia and Riband at each site, i n plots at least 10 x 12 m. Treatments included the full recommended r ate and a reduced dose of flutriafol and a mixture of flutriafol with chlorothalanil. Sprays were applied at GS37; samples were taken just b efore spraying and as soon as lesions appeared on leaf 2, which had ha d no visible disease at the time of spraying. Epidemiological evidence suggests that sprays acted both curatively and as protectants. Signif icant shifts in sensitivity occurred after spraying in both water-spra yed and fungicide-sprayed plots; all plots became less sensitive. Dise ase severity after spraying clearly decreased with increasing fungicid e concentration and with the use of a mixture of flutriafol and chorot halanil. However, after correction for the shifts in the water-sprayed plots, no dose produced a significant change in population sensitivit y level, although sample sizes were large. There was no trend in sensi tivity in the population over the 3 years of the experiment.