Investigating the Resistance of Winter Wild Oat Biotypes to ACCase Inhibitor Herbicide in Wheat Fields of Ramian County

Weeds are wild plant species which grow in agricultural ecosystems and compete with crops for resources such as water, nutrients, light, and space. Wild oat is reported as one of the most troublesome autumn weeds in croplands and its dominant species in Iran is winter wild oat (Avena ludoviciana Dur.). Herbicide resistance is the inherent ability of a weed biotype in survival after being exposed to a rate of herbicide which would be lethal to the wild type species. In the recent years, there have been several reports on the occurrence of herbicide resistance in Golestan province. The following research was conducted to identify the ACCase- resistant winter wild oat biotypes and generation of distribution map for the resistant biotypes in wheat fields of Ramian Township.

The present study was conducted in 2017-2018 at the greenhouse and Weed Science laboratory of Gorgan University of Agricultural Sciences and Natural Resources. Plant material included 80 putatively resistant winter wild oat biotypes which were collected from 100 wheat fields of Ramian Township. The susceptible biotype was also collected from the regions with no history of herbicide spray. Three Acetyl coenzyme A carboxylase (ACCase) inhibiting herbicides including clodinafop propargyl, fenoxaprop-P ethyl, and diclofop methyl which are common graminicides applied in wheat fields of the country were used to confirm the occurrence of resistance in winter wild oat biotypes. Three herbicides including pinoxaden, clethodim, and haloxyfop-R methyl ester were also applied on the biotypes as the possible alternative herbicides. First the winter wild oat seeds were hulled by hand. To obtain a more uniform germination, the seeds were placed in 9 cm Petri dishes topped with a filter paper and 2.5 mL of distilled water was added to the Petri dishes. Then, the Petri dishes were transferred to a refrigerator with a temperature of 4-5°C under darkness conditions. After pre-chilling, the petri dishes were incubated at the room temperature to germinate. To perform the discriminating concentration, pre- screening and concentration- response assays, 10 pre- germinated seeds from each biotype were placed on the Petri dishes as described above, then were treated with various concentrations of the mentioned herbicides. Three Petri dishes were used for each concentration, with each Petri dish serving as a replicate. Also, the Petri dishes treated with distilled water were regarded as control. The Petri dishes were kept at the room temperature for seven days and then the length of their coleoptile was measured. Four parametered log-logistic function was fitted to the data using R software (drc package). ArcGis V.10.3 was used to generate the distribution map of the herbicide- resistant biotypes.

Results of the present study indicate the confirmation of resistance to clodinafop propargyl, fenoxaprop-P ethyl, and diclofop methyl herbicides in 58 out of 80 winter wild oat biotypes gathered from wheat fields of Ramian Township. Resistance factor to clodinafop propargyl, fenoxaprop-P ethyl, and diclofop methyl herbicides were 48.69 to >851.33, 52.57 to >752.57, and 310.27 to >1443.90, respectively. For further investigation on the response of these resistant biotypes, three herbicides including pinoxaden, clethodim, and haloxyfop-R methyl ester were applied on the biotypes as a Petri dish assay. Two biotypes with the highest resistance factors (ram1, ram18) and two with the lowest resistance factors (ram15, ram4) were selected for this assay. In the haloxyfop-R methyl ester treatment, the highest and lowest resistance factors were observed in ram1 with 423.35 and ram15 with 284.50, respectively. In the other two herbicide however, resistance factor had no significant difference with the value 1.  Application of herbicides possessing different modes of action may lead to elimination of both susceptible and resistant biotypes. However, this will serve as a new selective pressure which will eventually result in intensification of resistance and furthermore, evolution of cross and multiple- resistant species. Thus, meticulous application of integrated weed management methods are of great importance to prevent or delay the evolution of resistance to herbicides.

Implementation of cultural methods for weed management and preventing the distribution of herbicides to which the weeds have readily developed resistance will lead to reduction of selective pressure. Adoption of crop and herbicide rotation principles will serve as a tool to debilitate the weed in competition with the crop, which in long term may contribute to reduced frequency of resistant alleles.