Monitoring sensitivity to the DMI group of fungicides in populations of the leaf blotch pathogen and the glume blotch pathogen of winter wheat
About this project
Leaf blotch, caused by the pathogen Septoria tritici is the most damaging disease recorded in the CSL/ADAS National Winter Wheat Disease Survey of England and Wales. Products in the 'DMI' group (e.g. propiconazole, prochloraz, triadimenol, flutriafol, flusilazole, cyproconazole, tebuconazole) now dominate, almost entirely, fungicide use in winter wheat. Most fungicides that offer effective control of leaf blotch belong to the DMI group and further base-line data on current sensitivity are required in order to help rapidly detect changes in sensitivity that may lead to resistance problems and failure of disease control. Leaves showing symptoms of leaf blotch from the 1990, 1991 and 1992 CSL/ADAS National Winter Wheat Disease Surveys were used to establish isolates of the fungus from all the cereal growing areas of England and Wales. During the three years of the study a total of 413 survey samples provided 1889 lesions from which 693 isolates were obtained. Of these, 687 isolates were tested successfully for their in-vitro sensitivity to three DMI fungicides, flutriafol, prochloraz, propiconazole and an MBC fungicide, benomyl, in 1990 (278 isolates) and to four DMI fungicides, flutriafol, prochloraz, propiconazole and triadimenol in 1991 and 1992 (409 isolates). In addition to isolates obtained from the survey samples during 1991 and 1992, 189 isolates were obtained from leaf tissue, sent to CSL by ADAS and other organisations, taken from crops in which the degree of leaf blotch control was much less than expected, i.e. instances of apparent disease control failure or partial failure. These 189 isolates were also tested successfully. Benomyl was excluded in 1991 and 1992 because it was obvious from the 1990 results that populations of S. tritici remained resistant to this fungicide.
Whilst collecting leaf tissue for isolation of Septoria tritici, lesions caused by Septoria nodorum on leaves and ears were also collected for isolation. During 1990 83 leaf samples provided 189 lesions that yielded 40 isolates all of which were tested success-fully for their in-vitro sensitivity to the four fungicides used in 1990. Few lesions caused by Septoria nodorum were collected during 1991 and 1992 and no isolates were obtained for sensitivity- testing. Concentrations of 0.05, 0.5 and 5 ug ai/ml agar were used for flutriafol, prochloraz and propiconazole, 0.1, 1 and 10 ug ai/ml agar for triadimenol (1991 and 1992) and 10, 25 and 50 ug ai/ml for benomyl (1990). There were indications that S. tritici was less sensitive totriadimenol in-vitro compared to the other three DMI fungicides. Flutriafol appeared to be more inhibitory in-vitro during 1992 than to the first two years of the project. Sensitivity varied between isolates but showed no reduction over that found in earlier work for prochloraz and propiconazole. In-vitro testing using plant inoculations with pycnidiospores confirmed the in-vitro results with flutriafol, prochloraz and propiconazole providing 50, 60 and 80% control of the disease at 5 ug ai/ml whilst triadimenol gave only 22% control of the disease at - 10 ug ai/ml. Results for benomyl in 1990 confirmed that the high incidence of resistance to this fungicide had been maintained in populations of S. tritici. Results for S. nodorum at 0.05, 0.5, 5 ug ai/ml for all fungicides were similar to those for S. tritici; however, with the exception of benomyl, a greater percentage of S. nodorum isolates grew at a given concentration of each fungicide.
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