This work was carried out to study the sensitivity of barley and wheat powdery mildew (Erysiphe graminis f.sp. hordei and E. graminis f.sp. tritici respectively) to morpholine fungicides. Morpholine fungicides are classified as compounds with a low risk of resistance development. On the other hand, barley and wheat powdery mildews belong to the group of high risk fungi as far as development of fungicide resistance is concerned
Firstly, mildew isolates collected in the UK were assessed, over the period 1992 to 1995, for their sensitivity to three morpholine fungicides (tridemorph, fenpropimorph and fenpropidin). Barley isolates tended to be more sensitive to the fungicides than wheat isolates. In general, the barley mildew isolates were less sensitive to tridemorph than to fenpropimorph and fenpropidin. Cross resistance was found between fenpropimorph and fenpropidin but there was no significant correlation between barley mildew sensitivity to tridemorph and either fenpropimorph or fenpropidin. Isolates from Scotland were significantly less sensitive to fenpropimorph and fenpropidin than isolates from England. The wheat isolates tended to be more sensitive to fenpropidin than to fenpropimorph. There was no evidence of cross resistance between fenpropimorph and fenpropidin with the wheat isolates tested. A shift towards insensitivity over the period of testing was found for fenpropimorph. Wheat isolates from Scotland were found to be significantly less sensitive to fenpropidin than isolates from England.
Although variation in sensitivity among mildew isolates was recorded, the results of the monitoring work confirmed the findings of field experience that morpholines are continuing to maintain an effective control over barley and wheat mildew. The variation in sensitivity appeared to be as a continuous distribution, probably related at least in part to the use of bulk isolates for most of the sampling. There, was however, no evidence of any part of the population showing a very high level of resistance.
Secondly, it is common practice for farmers to use doses of fungicides below the recommended rate to maximise gross margins, but the effect this has on fungicide sensitivity has not been determined conclusively. To test the selective effect of reduced doses on the fungicide sensitivity of populations of powdery mildew, wheat and barley field experiments were conducted between 1992 and 1995. Isolates recovered from plots receiving reduced and full dose treatments were compared to see whether selection for insensitivity varied with dose rate. No significant differences between the effects of the full commercial rate and reduced doses were found
Observations from the wheat experiment indicated that relative sensitivities of populations in field plots may change quickly and that account should be taken of the relative fitness of sensitive and less sensitive isolates and of interference from neighbouring population sources in assessing the effects of fungicide treatments.
Finally, preliminary studies on the variation in the early development of isolates of Erysiphe graminis, of different sensitivities, in response to exposure to fenpropimorph were carried out. Three different methods of studying the sensitivity of isolates to fenpropimorph were compared, with a view to developing a less time consuming and more accurate method. All three methods showed that exposure to fenpropimorph reduced spore germination. Further fungal development in response to fungicide treatment varied with the method employed. Incidence of spore germination after 24 hours and spore germination, appressorial formation and development of secondary hyphae after 24 and 48 hours, were compared with the standard bioassay used in this work. Results were not consistent and the standard bioassay remained the most consistent and accurate method of measuring sensitivity.