Tapesia yallundae and Tapesia acuformis, the causal agents of eyespot of cereals: reduced sensitivity to prochloraz in Belgian wheat fields
1996
Moreau, J.M. | Gerard, J.M. | Sandri, L. | Emmerik, N. van | Maraite, H. (Universite Catholique de Louvain, Louvain-la-Neuve (Belgium). Unite de Phytopathologie) | Meeus, P.
Since 1988, fungicides trials in four to eight winter wheat fields were surveyed each year at late milk stage. Eyespot was assessed in untreated plots and in plots treated first at the first node detectable stage with a mixture of prochloraz and fenpropimorph (Rival CS, 2 litres per ha) and a second time at inflorescence emergence with triazole fungicides. Strains associated with eyespot lesions were isolated and classified as Tapesia yallundae or Tapesia acuformis on base of colony morphology. Since 1993, their sensitivity to prochloraz was also assessed in vitro. Analysis of frequency distribution of degrees of mycelium growth inhibition observed after transfer of conidia on PDA amended with 0.1 mg per litre prochloraz revealed, for both species, two groups of isolates, one with an inhibition higher than 80 per cent and considered as sensitive and an other with an inhibition ranging between 10 and 70 per cent and considered less sensitive. This kind of isolates have been detected each year. Their proportion in untreated plots ranged from 0 to 30 per cent of the total isolates tested and seems not to have increased between 1993 and 1995. In 1993 and 1994, less sensitive isolates in untreated plots were exclusively of T. acuformis, a few of T. yallundae being detected only in treated plots. In 1995, less sensitive T. yallundae isolates were also obtained in untreated plots of three of the seven fields surveyed, but their proportion never exceeded 5 per cent of the analysed populations. Comparison of eyespot populations obtained from untreated plots and treated plots reveals a selection by the fungicides of T. acuformis on one hand, and of reduced sensitivity to prochloraz on the other hand. Efficacy of Rival CS treatments at the first node detectable stage varies greatly from field to field, however the correlation between Rivalefficacy on eyespot and proportion of isolates with reduced sensitivity to prochloraz remains low.
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