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Maart 2018
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Target-site herbicide resistance in
ryegrass:
A three-year summary
R
yegrass is the predominant grass
weed species occurring in the
small grain production areas of the
Western Cape. Ryegrass is geneti-
cally highly variable and can cross-pollinate
over some distance. This weed can rapidly
develop herbicide resistance, especially to
herbicides from Group A (fops, dims and
den) and Group B (SU’s, IMI’s and TP’s).
The occurrence of herbicide resistance
has had a negative impact on the control
options of this weed. Making use of DNA
markers to identify resistance has proven to
be a quick method for in-season screening
for target-site resistance. Since molecular
genotyping makes use of the plants’ DNA, it
is also diagnostically very accurate.
What is target-site
herbicide resistance?
Herbicide resistance can be defined by
the inherent ability of a weed to survive an
herbicide application that would otherwise
kill a normal plant/biotype/population of
the same weed species. In any given weed
population, there will be a plant that is ge-
netically resistant to the applied herbicide.
There are two main ways that a weed can
develop resistance to herbicides.
Target-site resistance occurs when a
change takes place in the protein that binds
the herbicide. An example would be where
a mutation within the target protein reduces
or eliminates binding of the herbicide. It is
very common to get cross-resistance to
other herbicides of the same herbicide
mode of action. Target-site resistance com-
monly involves herbicides from Group A
and Group B.
Non-target site resistance most commonly
occurs due to an increased herbicide de-
toxification process. Enhanced metabolism
is also typically observed in herbicides that
are used selectively, such as Group A and
Group B. Reduced translocation also falls
under this resistance group.
Research results
2015 - 2017
Mutation markers can be used to iden-
tity target-site mutations in weed species.
These markers are then used to genotype
specific resistant biotypes for specific mu-
tations. Since 2013, ARC-Small Grain has
been working on optimising the markers for
South African ryegrass.
Through the screening of multiple samples,
two very distinct mutations have been iden-
tified in South African ryegrass populations
occurring in small grain producing areas.
These markers are the ALS mutation mark-
ers 197, which confers resistance to all
group B herbicides (SU’s, Triazolopyrimi-
dines and Imidazolinones) and ACCase
mutation 2078, which confers resistance
to many Aryloxyphenoxy-propionates, all
Cyclohexanediones (including clethodim)
and Phenylpyrazolines.
This means that all grass weed samples
that are received, are screened for these
mutations first, but if none of the mutations
are present in the sample, the sample is
screened with all other mutation markers.
This is done to make sure that no mutation
is missed in the screening process.
From 2015 to 2017, 165 samples from differ-
ent fields/farms were received. At least four
replicates were performed per sample, but
often six to eight replicates were performed
if sufficient sample material was received.
The samples were all screened for various
ACCase and ALS mutations. Resistance to
more than one mutation were often record-
ed and most samples showed resistance to
both ACCase and ALS mutations.
FOCUS
Weed control on winter cereals
Special
HESTIA NIENABER
and
DR SCOTT SYDENHAM
, ARC-Small Grain, Bethlehem
Graph 1: Number of samples showing different ALS mutations.
Graph 2: Number of samples showing different ACCase mutations.
Number of samples
Number of samples
Different ALS mutaƟons
Different ACCase mutaƟons