CHANGES IN SENSITIVITY TO DMI FUNGICIDES IN RHYNCHOSPORIUM-SECALIS

Laboratory tests on 2000 isolates of Rhynchosporium secalis from throu ghout the UK over a 4-year period revealed a decline in sensitivity to triadimenol and propiconazole but not to prochloraz. Change occurred throughout the UK irrespective of disease pressure, and was not correl ated with fungicide use. Resistant isolates were no less pathogenic th an sensitive ones. Selection with triadimenol generated a bimodal popu lation distribution, whereas propiconazole produced a gradual shift of unimodal population towards a less sensitive mean. Some cross-resista nce occurred between triadimenol, propiconazole and a third triazole, tebuconazole, although the change in sensitivity to tebuconazole was a lways less than to the other two triazoles. No cross-resistance was ob served to the imidazole demethylation inhibitor (DMI), prochloraz. Fie ld-trial data collected over several years showed that the performance of triadimenol, and to a lesser extent, of propiconazole, had decline d. Control of Rhynchosporium with these fungicides could be improved b y using mixtures with carbendazim. Tebuconazole, either alone or in mi xtures with carbendazim or tridemorph, provided the best disease contr ol, and did not appear to select for lower sensitivity. The findings e mphasize that certain DMI fungicides may still be used in strategies w here performance of other DMIs has been altered because of resistance.