SELECTION ON RESPONSES OF BARLEY POWDERY MILDEW TO MORPHOLINE AND PIPERIDINE FUNGICIDES

Plots of a mildew-susceptible barley variety were treated with the piperidine fungicide fenpropidin (FPD) or one of two morpholines, fenpropimorph (FPM) or tridemorph (TDM). FPD and FPM strongly inhibit DELTA14 reduction in the ergosterol synthesis pathway, and DELTA8 --> DELTA7 isomerization more weakly, whereas TDM inhibits the isomerase more strongly than the reductase. At Levington. England, FPD and FPM selected isolates of the barley mildew pathogen, Erysiphe graminis f.sp. hordei, that were resistant to low doses of both these two chemicals, but selected resistance to TDM less strongly. TDM did not select resistance to itself, and intermediate doses of TDM selected against FPD/FPM-resistance. All isolates sampled from sites in Scotland were resistant to low doses of both FPD and FPM and, in a trial at Banff, the level of resistance was not affected significantly by treatment of plots with FPD, FPM or TDM. Isolates from Banff were more resistant to FPM than the most resistant isolates from Levington, but the maximum levels of FPD-resistance at the two sites were similar. FPD/FPM-resistant isolates were genetically diverse, but no resistant isolate carried virulence corresponding to the barley Mla13 mildew resistance allele. The present levels of FPD/FPM-resistance are unlikely to cause a substantial loss of effectiveness of the chemicals immediately after spraying. TDM could be an effective partner fungicide for FPD or FPM in a mixture or in an alternating spray programme. Previous application of TDM may reduce the frequency of FPD/FPM-resistance and thus increase the effectiveness of a later application of FPD or FPM.