Julius-Kühn-Archiv (Feb 2014)
Reduction of selection pressure of herbicides - options and limits for blackgrass management by using clethodim in oilseed rape in the presence of the Leu1781 haplotype
Abstract
In field experiments the control of blackgrass (Alopecurus myosuroides, Huds.) in oilseed rape using clethodim (Select EC 240) and cycloxydim (Focus Ultra) with and without subsequent treatments with propyzamide (Kerb FLO) was tested at 6 locations in North and South Germany with assumed resistance problems. The field experiments were accompanied using molecular analysis leaf samples from the plots to seize the occurrence of black-grass with the Leu 1781 haplotype and to determine the frequency of the genotypes. The goal of the trials was to correlate the successes of blackgrass control with the occurrence of hetero- and homozygous resistant genotypes. It was shown in greenhouse trials that clethodim selects the haplotype Leu1781 more weakly (and it shows a higher partial efficacy) than cycloxydim and that heterozygous plants have a lower resistance factor than homozygous plants. The question raised whether the frequency of heterozygous plants has influence on increased efficacy of clethodim under field conditions. At 5 sites target-site resistance was detected. At one location the high proportion of heterozygous plants correlated positive with relative higher control using Select EC 240 (80%) compared to Focus Ultra (0%). At two locations with high proportion of homozygous resistant plants Select EC 240 and Focus Ultra treatments without subsequent treatments with Kerb FLO were not sufficient in solo variants. The subsequent treatments with Kerb FLO provided partly, but not sufficient control of black-grass. At one location no resistance was identified. The effect of Select EC 240 and Focus Ultra to control black-grass were comparable high in all variants with and without subsequent treatments of Kerb FLO. The investigations showed clearly a higher degree of control by plants with the haplotype Leu1781 by the active substance clethodim and pointed out the fact that the frequency of resistant genotypes (homo vs. heterozygous resistant plants) has a clear influence on the resistance selection. The use of DIMs at locations suspected to be resistant should be assured only with the help of propyzamide and nonchemical measurements (e. g. ploughing, crop rotation). The results also indicate that it is necessary to assess resistance mechanisms in resistance research individually depending on the active ingredients, inheritance and weed species.
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