Tracing Resistance to Tribenuron-Methyl in Populations of Wild Mustard (Sinapis arvensis L.) and Turnipweed (Rapistrum rugosum L.) collected from Wheat Fields of Golestan Province and Introduction a New Method in order to Detect their Resistant Population

Tribenuron methyl, an ALS inhibiting herbicide, labeled for post-emergence use in wheat and barley fields and is extremely effective on broadleaf weeds. This herbicide plays a critical role in post-emergence weed management options for cereals especially in Golestan province in Iran. Recently, failures to control wild mustard (Sinapis arvensis L.) and turnipweed (Rapistrum rugosum L.) with recommended dose of tribenuron methyl applications have been reported for wheat fields (6 and 20). It motivated us to trace resistant populations and evaluate their degree of resistance and find a quick and cost-effective method to identify resistant populations in wild mustard and turnipweed .
At first, seeds were collected from wheat fields in Golestan province. These fields had been treated annually with tribenuron-methyl for more than 5 years, so wild mustard and turnipweed control had become erratic with this herbicide in recent years. Seeds were also collected from a site that had not been applied to any herbicide. This biotype was considered as tribenuron-methyl-susceptible (S). Plants obtained from the seeds collected were screened with recommended dose of tribenuron-methyl to indicate of resistant populations (R). Then greenhouse and laboratory experiments were conducted to investigate resistance level to tribenuron-methyl. The broken dormancy seeds were sown in suitable pots and were put in the greenhouse under natural sunlight. Plants with three to four true leaves were sprayed by different tribenuron-methyl rates based on a logarithmic scale. Above-ground biomass from the plants in each pot was harvested 4 weeks after treatment. Dry biomass data were expressed as a percentage of the untreated control within each biotype. For seed bioassay, different solution of teribenuron methyl was prepared with technical and commercial herbicide. A non-treated control was included and in these petri dishes added water instead of spraying herbicide. Seven days after trial initiation, shoot and root lengths were measured from the crown to the tip of the root or shoot to discriminate between R and S biotypes. Analyses of dose-response data in whole plant and seed bioassay were performed using the R software by fitting the data to a nonlinear log-logistic regression model.
In our study, the responses of the collected wild mustard and turnipweed populations after treated with the field rate of Granstar (tribenuron-methyl) confirmed that some of these populations are resistant to this herbicide. In these experiment 14 and 10 resistant biotypes (R) for wild mustard and turnipweed respectively according to results of screening test, and one susceptible biotype (S) were used for investigation of R biotypes response to increase concentration of herbicide. The concentration of tribenuron-methyl led to 50% inhibition shoot growth in control treatment was 9.93 and 10.90 gr a.i ha-1, whereas for R biotypes it was 2.6 to 6.6 and 2.2 to 16.8 fold more resistant relative to the S biotype for wild mustard and turnipweed respectively. Quick resistance test in petri dishes showed ability of this method for easy and reliable assessment of resistance in these plants. The results obtained by using commercial herbicide as well as shoot length data were not suitable for bio-evaluation and there were extreme variability when assayed, resistance would be more reliably detected using assays conducted on seeds of suspected R populations by root length measurement. Response of resistant populations to increasing concentrations of tribenuron-methyl herbicide were quite different from that of control treatment. Assays to determine the dose required to reduce root lenght by 50% indicated a resistance factor (RF) of 17.47 to 2.30 for wild mustard and 9.56 to 2.76 for turnipweed. On the other hand, the RF using whole plant dose–response assays were lower than petri dish assay. There is high correlation between the degree of resistance to obtained results from greenhouse and laboratory (90% and 83% wild mustard and turnipweed respectively).
The continuous advent of resistant weeds is a concern. Wheat is an important crop in Iran, which is mainly grown in monoculture and, only the used methods to remove the weeds is applying the herbicides especially in Golestan province. Probably the lack of crop and herbicide rotations are the main reasons of resistance to tribenuron-methyl in this regions. Having a rapid and reliable seedling bioassay is a useful tool to plan the best herbicide strategy for using in the field. Petri dish assay can be more convenient, potentially cheaper and give quicker results than pot tests but are not suitable for all weeds and herbicides. For this case, this test was done in turnipweed and wild mustard successfully.