INFLUENCE OF AGRICULTURAL PRACTICE ON MORPHOLINE SENSITIVITY IN BARLEY POWDERY MILDEW ERYSIPHE GRAMINIS F.SP. HORDEI IN SCOTLAND AND ENGLAND
MCN ZZIWA, FJ BURNETT and MJ HOCART
Scottish Agricultural College, West Mains Road, Edinburgh EH9 3JG, UK
Background and objectives
Morpholine fungicides are classified as compounds with a low risk of resistance development in the target organism because they act at several sites. In contrast, barley powdery mildew, being a wind dispersed, highly sporulating, polycyclic pathogen with many asexual generations in a single season, belongs to the group of high risk fungi as far as fungicide resistance is concerned . Morpholines have been the main source of control of barley powdery mildew in the UK since triazole resistance became widespread in the mid -1980s. With a high selection pressure on the mildew population, there is a risk of fungicide resistance developing in this pathogen. As part of a programme to assess this risk, the morpholine sensitivity of mildew isolates was measured to determine whether the use of morpholine fungicides on cereals was associated with any change in the sensitivity to morpholines in the pathogen isolated from these crops. The fungicide sensitivity of isolates was then related to the previous history of the crop which were grouped as those collected from crops with no history of fungicide use, isolates obtained from different geographical regions and isolates obtained over several seasons, 1991-1994.
Materials and methods
Barley leaves, infected with powdery mildew were collected from different parts of the UK between 1991 and 1994. Mildew isolates obtained from these samples were tested for sensitivity to fenpropimorph, fenpropidin and tridemorph using a standard detached leaf segment method as follows: leaf segments that had been treated with a range of concentrations of the fungicides were inoculated with the isolates. After fourteen days the segments were assessed for percentage mildew cover from which EC50 (concentration producing 50% response) values were calculated using a Genstat 5 program.
Results and conclusions
Previous fungicide treatment did not significantly affect the sensitivity of isolates to these compounds. Isolates from Scotland were significantly less sensitive to fenpropimorph and fenpropidin than those from England. This could be attributed to previous extensive use of morpholine fungicides in Scotland as a result of previous cultivation of highly mildew susceptible cultivars of barley . In contrast, there were no significant differences in sensitivity to tridemorph between 'Scottish' and 'English' isolates. This could be due to tridemorph having a different mode of action from fenpropimorph and fenpropidin or to different patterns of tridemorph usage. The mean sensitivity of mildew isolates to all three fungicides did not differ significantly between seasons over the four years of sampling. These results are described in relation to the possible appearance of resistance to these fungicides.
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