Herbicide / Resistant / Weeds / Molecular genotyping
Maart 2014
100
Molecular genotyping of resistant grassweeds:
Focussing on ACCase resistance
T
he 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 clas-
sified 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
.
ON FARM LEVEL
Pest control
HESTIA NIENABER
and
SCOTT SYDENHAM,
ARC-Small Grain Institute, an Institute of the Field Crops Division, Bethlehem
MUTATION
MARKER
SENSITIVE
HETEROZYGOUS
RESISTANT
HOMOZYGOUS
RESISTANT
TOTAL SAMPLES
TESTED
1999
36
5
0
41
2027
40
1
0
41
2078
36
19
6
61
Total
112
25
6
TABLE 1: INDICATING THE LEVELS OF RESISTANCE OF ALL SAMPLES TESTED TO GROUP A HERBICIDES.
1: Marker 2078 (red – homozygous resistant, yellow – heterozygous
resistant, and blue – sensitive).
2: Marker 2027 (yellow – heterozygous resistant).
