<p>Acetolactate synthase (ALS)
inhibitors are a widely used class of selective herbicides used to control
grass and broadleaf weeds. The repeated use of ALS-inhibiting herbicides has
selected for biotypes of weeds resistant to ALS inhibitors, especially in the
weeds most problematic to growers in the Midwest. While ALS inhibitor use seems
futile, new mechanisms of herbicide action are not predicted to be
commercialized in the near future to solve this problem. This leads to the main
objective of this research, determining what value ALS inhibitors provide in
controlling populations of weeds with resistance to ALS inhibitors. </p>
<p>Field
experiments with soil-applied (PRE) applications of ALS inhibitors on horseweed
(<i>Erigeron canadensis</i>) and tall
waterhemp (<i>Amaranthus tuberculatus </i>var.
<i>rudis</i>) exhibited higher efficacy than
would be expected given the frequency of the ALS resistance trait in the
population. Whereas control of these species with POST-applied applications was
similar or less than the proportion of the population characterized as susceptible
using molecular techniques. Soil-applied
applications, therefore, resulted in relatively greater control than POST
applications in populations with known ALS-inhibitor-resistance mechanisms.</p>
<p>Greenhouse
experiments showed that overall resistance ratios were higher for PRE
applications of ALS inhibitors in horseweed, tall waterhemp, and Palmer
amaranth (<i>Amaranthus palmeri</i>).
However, GR<sub>50</sub> values decreased for both susceptible and resistant
biotypes for the PRE applications compared to POST, suggesting the biologically
effective dose of these herbicides is lower in soil residual applications. This
research found that PRE applications of ALS inhibitors resulted in some level
of control on horseweed and tall waterhemp classified as resistant to ALS inhibitors
due to the higher efficacy of PRE herbicide applications.</p>
<p>Genetic
analysis assessing the amino acid substitutions that confer resistance to ALS
inhibitors in tall waterhemp confirmed a difference in selection pressure
between PRE and POST applications and between ALS active ingredients in tall
waterhemp. Applications of chlorimuron PRE at 11 g ai ha<sup>-1 </sup>selected
for 35% homozygous W574L genotypes and at 44 g ha<sup>-1</sup> selected for 70%
homozygous W574L genotypes. An increase of homozygous W574L individuals along
with a decrease in heterozygous individuals from 65 (11 g ha<sup>-1</sup>) to
29% (44 g ha<sup>-1</sup>) suggests that W574L is semi-dominant in tall
waterhemp and that high labeled rates of chlorimuron applied PRE can partially overcome
the heterozygous W574L-resistance mechanism. In horseweed, no difference in
selection pressure was observed between application timing or between
chlorimuron or cloransulam. A new mutation conferring ALS-inhibitor resistance
in horseweed was discovered, a Pro197Leu amino acid substitution, with
resistance ratios of 21X to chlorimuron and 8.6X to cloransulam. These
resistance ratios are slightly less than those reported for the Pro197Ala and
Pro197Ser amino acid substitutions in conferring ALS-inhibitor resistance in
horseweed. </p>
<p>Finally,
a survey of 42 populations of tall waterhemp in Indiana counties with confirmed
ALS-inhibitor resistant populations of tall waterhemp found that all
populations contained at least 16% individuals with the W574L amino acid
substitution, 35 populations contained at least 1% individuals with the S653N
substitution, and 9 populations contained at least 1% individuals with the
S653T substitution. Taking into consideration the three mutations tested, 8 of
the 42 populations tested contained <50% ALS-inhibitor resistant individuals
within the population. Using the same tall waterhemp populations as collected
in the survey, Next-Generation Sequencing was used to determine if other amino
acid substitutions conferring resistance to ALS inhibitors existed. Results
from WideSeq revealed that 10 other amino acid substitutions in the ALS protein
may be conferring resistance in tall waterhemp in Indiana: A122T, A122N, A122S,
P197T, P197L, P197H, D376E, and G654F. Further research from this survey also
suggests that metabolic resistance to ALS inhibitors is likely a contributor to
resistance in tall waterhemp in Indiana.</p>
<p>This
research suggests that ALS-inhibiting herbicides, more specifically
chlorimuron, would provide the greatest contribution to management of tall
waterhemp. Chlorimuron would perform best when used in soil residual
applications and in populations of tall waterhemp containing either individuals
susceptible to chlorimuron or individuals heterozygous for ALS inhibitor
resistance conferred by the W574L mutation. This research also demonstrates the
specificity of the amino acid substitutions in the ALS protein and by weed
species to realize the benefit of these herbicides for management of weeds
resistant to ALS inhibitors. Molecular characterization of target site
resistance to ALS inhibitors has traditionally been considered relatively
simple. However, we found 11 new amino
acid substitutions that confer resistance to ALS inhibitors in horseweed and tall
waterhemp. The complexity of ALS inhibitor resistance calls for the use of
methods such as NGS to detect all potential resistance mutations in a timely
manner and for the use of tests detecting metabolic resistance. Overall, this
research demonstrates that ALS inhibitors still provide some utility for
management of weed populations classified as resistant to ALS inhibitors and
that the resistance mechanisms in horseweed and tall waterhemp are more
numerous than previously reported. </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8039003 |
| Date | 11 June 2019 |
| Creators | Jodi E Boe (6632369) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Establishing_the_Value_of_ALS-Inhibiting_Herbicides_in_Fields_with_Confirmed_Weed_Resistance_to_ALS-Inhibiting_Herbicides/8039003 |
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