SA Graan Maart 2018

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Graph 1 indicates the number of samples that showed resistance to
the various ALS mutations. The mutation markers also distinguished
between plants that are heterozygous (+r) (one copy – resistant al-
lele) or homozygous (+R) (two copies – resistant allele) resistant for
each specific target-site mutation. It is important to note that one
sample could show heterozygous and homozygous resistant indi-
viduals, as at least four replicates were included per sample.
From the results, most samples showed heterozygous resistance to
ALS mutation marker 574 (74), followed by heterozygous resistance
to ALS mutation marker 197. ALS mutation markers 574 and 197 con-
fer target-site resistance to IMI’s, SU’s and TP’s. What is worrisome,
is that at least one replicate of 52 samples showed a homozygous
resistant ALS 197 mutant. Very low numbers of the other ALS muta-
tion markers were observed.
Graph 2 indicates the number of samples that showed resistance
to the various ACCase mutations. The mutation markers again dis-
tinguished between plants that are heterozygous (+r) (one copy-
resistant allele) or homozygous (+R) (two copies – resistant allele)
resistant for each specific target-site mutation. It is again important
to note that one sample could show heterozygous and homozygous
resistant individuals, as at least four replicates were included per
sample.
From the results, most samples showed heterozygous resistance to
ACCase mutation marker 2078, followed by homozygous resistance
to the same mutation marker. ACCase mutation marker 2078 confers
resistance to many fops and all dims, including clethodim.
When heterozygous/homozygous genotypes to ACCase herbicides
are detected, most ACCase herbicides including clethodim, clo-
dinafop, diclofop, fluazifop, haloxyfop, butroxydim, sethoxydim,
tralkoxydim and pinoxaden should be avoided.
Summary
From data obtained from screening 165 different samples over three
years, it is evident that target-site resistance to ACCase- and ALS
inhibitor herbicides is very common in South Africa. The most fre-
quently observed target-site mutation was for ALS marker 197 and
ACCase marker 2078.
These confer a broad cross-resistance in varying levels to most AC-
Case- and ALS herbicides. By knowing the mutations that occur in
South Africa, a true resistance profile can be established for target-
site resistant biotypes, allowing for more informed management
choices.
For further information regarding this topic, please
contact Hestia Nienaber at 058 307 3420 or deweth@
or Scott Sydenham at sydenhams@arc.
The ARC, Winter Cereal Trust and National Research
Foundation are acknowledged for funding this project.
Grain SA/Sasol photo competition
– Corlea Enslin 2018