Molecular genotyping of resistant grassweeds: Focussing on ACCase resistance

HESTIA NIENABER and SCOTT SYDENHAM, ARC-Small Grain Institute, an Institute of the Field Crops Division, Bethlehem

The concept of molecular genotyping of herbicide resistant weeds has been explained in a previous article (SA Graan/Grain, September 2013, page 93). Using molecular genotyping to identify resistance is a much shorter and time-efficient process, as opposed to damage characterisation in the glasshouse, which can take anywhere from three to six weeks. Since molecular genotyping makes use of the plant’s DNA, it is also very accurate.

While the ARC-Small Grain Institute (ARC-SGI) is in the process of fine-tuning the method, producers were asked to submit samples to be tested. Fresh green leaf material is crucial for extracting DNA, emphasising the importance of submitting such material to the ARC-SGI as soon as possible.

A few of the samples that were received could not be used due to the loss of chlorophyll or the development of fungal growth. Fortunately though, the majority of the samples were received in a useable state. Apart from the ryegrass samples, Bromus diandrus (brome grass) and Avena fatua (wild oats) were also received.

Results

Herbicides that inhibit lipid synthesis are known as ACCase inhibitors. In the Herbicide Resistance Action Committee chart, ACCase inhibitor herbicides are classified as Group A herbicides. This group includes Aryloxyphenoxypropionates (fops), Cyclohexanediones (dims) and Pinoxaden (den).

Currently three markers are utilised to identify mutations linked to the ACCase herbicides. The markers also indicate heterozygous/homozygous resistance in the identified samples. Heterozygous means that only one of the alleles is carrying the resistance mutations, whereas homozygous indicates that both alleles are carrying the resistance mutations.

The mutation which occurred most frequently in the samples received was for marker 2078 (Photo 1). This marker confers resistance to many fops and all dims, including clethodim. In some samples, marker 1999 and marker 2027 were also observed. Marker 1999 confers resistance to Fenoxaprop and marker 2027 (Photo 2) confers resistance to all fops.

A summary of the results of all tested samples to date can be seen in Table 1.

Sending your samples

Producers are welcome to send ryegrass seedlings or seeds to the ARC-SGI in Bethlehem. Please make sure that the sample was taken from plants distributed over the entire field, so as to constitute a representative sample of the field. Seedlings must be kept moist and preferably couriered, as this will assure that fresh seedlings arrive in Bethlehem.

Please indicate the GPS-coordinates where the sample was taken. Seeds must be stored in brown paper bags to prevent microbial contamination. GPS-coordinates must again be indicated. Note: An adequate number of seeds/seedlings must be submitted for the screening process to be conducted successfully.

Please courier samples to:
Hestia Nienaber/Scott Sydenham
ARC-Small Grain Institute
Blydskap Road, S191
Bethlehem
9700

Summary

From data obtained during this screening, it is evident that resistance to ACCase inhibitor herbicides is a reality, but by knowing which mutations are occurring in the field, informed choices can be made for better management of resistant ryegrass.

For further information regarding this topic or on how to sample your weeds, please contact the authors at 058 307 3420 or deweth@arc.agric.za or sydenhams@arc.agric.za