5.5.27
STABILIZING SELECTION ON ERYSIPHE GRAMiNIS F.SP. TRITICI POPULATIONS FOR SENSITIVITY TO STEROL BIOSYNTHESIS INHIBITORS

KM CHINl, FG FELSENSTEIN2, U GISIl

lNovartis Crop Protection AG, CH-4108 Witterswil, Switzerland; 2EpiLogic GmbH and Technical University Manchen, D-85350 Freising Weihenstephan, Germany

Background
Sterol Biosynthesis Inhibitors have been used widely since the early 1980s and, despite gradual adaptation in the sensitivity of some pathogens, continue to play a vital role for the control of cereal diseases. Fungicide use strategies which have contributed to the durability of these compounds e.g. fenpropimorph (FPM), fenpropidin (FPD), propiconazole (PPZ) and cyproconazole (CCZ) are supported by an extensive monitoring program of pathogen sensitivity in different European regions.

Materials and methods
Sensitivity of E. graminis f.sp. tritici populations (as measured by resistance factors relative to sensitive reference isolates) has been monitored each year on airborne samples of the pathogen from up to 20 regions in Europe since 1987 (FPM), 1988 (PPZ), 1990 (CCZ) and 1995 (FPD). Dose response measurements are based on fungal growth on fungicide-treated detached leaf segments.

Results and conclusions
Comparison of mean resistance factors (MRFS) from different countries show that sensitivity to FPM has remained stable from 199314 to 1997 in most locations across north-western Europe [MRFs of 7-10 in Belgium (B), Germany (D), Denmark (DK), France (F) and Great Britain (GB)]; and was only slightly shifted in eastern and southern Europe [MRFs of 1-2 in Austria (A), Italy (1) and Spain (E)]. Sensitivity to FPD has also remained stable in all major cereal countries since 1995 (MRFs of 4-7 in B, DK, F, D and GB, and MRF of about 1 in i and E). Sensitivity to CCZ and PPZ was stable from 1990, and from 1993/4 to 1996 respectively, with MRFs of about 10 (CCZ) and 20 (PPZ) in DK, F, D and GB; and about 2/4 in Italy and Spain.
These observations are supported by frequency distributions of individual isolate sensitivities. Comparison of current EC50s in northern Europe with that from earlier years, and/or with current sensitivity in southern Europe beyond the Alps and Pyrenees (little adaptation), show that directional selection occurred initially for decreased sensitivity in northern Europe. In recent years, there was continued loss of the more sensitive isolates, but also truncation selection against the more resistant isolates.
These changes resulted in a stabilisation of population means and a decrease in variance, typical of the effects of stabilising selection. It is probable that the lack of further change is due in part to the polygenic control of resistance evolution, and annual occurrence of sexual reproduction of the pathogen, so that rare and complex resistance combinations cannot be maintained over extended periods of time. Further changes apparently confer no additional advantages in mean Witnesses. An additional factor is the increase in the number of active ingredients currently available for mildew control in Europe, which has resulted in decreased selection pressure for adaptation to any one compound.
Current use patterns of cereal fungicides in Europe, appear to have contributed to a largely stable situation of sensitivity of wheat powdery mildew to fenpropimorph, fenpropidin, cyproconazole and propiconazole. However, because under conditions of intensive selection by repeated application, isolates with decreased sensitivity may be recovered, FRAC use recommendations of SBis must continue to be followed to ensure that the current situation remains stable.