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A goosegrass biotype with suspected resistance to acetyl-CoA carboxylase
(ACCase) inhibitors was identified in Georgia. The objectives of this
research were to evaluate the resistance level of this biotype to ACCase
inhibitors, efficacy of various herbicide mechanisms of action for control,
and the physiological and molecular basis of resistance. In greenhouse
experiments, the rate of diclofop-methyl that reduced dry shoot biomass 50%
(SR50) from the nontreated for the resistant (R) and
susceptible (S) biotypes measured 4,100 and 221 g ai ha−1,
respectively. The SR50 for sethoxydim measured 615 and 143 g ai
ha−1 for the R and S biotype, respectively. The R biotype was
cross resistant to clethodim, fenoxaprop, and fluazifop. The R and S
biotypes were equally susceptible to foramsulfuron, glyphosate, monosodium
methylarsenate (MSMA), and topramezone. In laboratory experiments, the two
biotypes had similar foliar absorption of 14C-diclofop-methyl.
Both biotypes metabolized 14C-diclofop-methyl to diclofop acid
and a polar conjugate, but the R biotype averaged ∼2 times greater
metabolism than the S biotype. Gene sequencing revealed an Asp-2078-Gly
substitution in the ACCase of the R biotype that has previously conferred
resistance to ACCase inhibitors. A second mutation was identified in the R
biotype that yielded a Thr-1805-Ser substitution that has been previously
reported, but is not associated with ACCase resistance in other species.
Thus, the Asp-2078-Gly substitution is the basis for resistance to ACCase
inhibitors for the R biotype. This is the first report of ACCase-inhibitor
resistance in goosegrass from the United States and from a turfgrass
Bermudagrass and goosegrass are problematic weeds with limited herbicides
available for POST control in creeping bentgrass. Metamifop effectively
controls these weeds with greater selectivity in cool-season grasses than
other ACCase inhibitors. The objectives of this research were to determine
the physiological basis for metamifop selectivity in turfgrasses. In
greenhouse experiments, metamifop rate required to reduce shoot biomass 50%
from the nontreated (GR50) at 4 wk after treatment was >
6,400, 2,166, and 53 g ai ha−1 for creeping bentgrass, Kentucky
bluegrass, and goosegrass, respectively. The GR50 for
bermudagrass treated with diclofop-methyl or metamifop was 2,850 and 60 g
ha−1, respectively. In laboratory experiments, peak absorption
of 14C-metamifop was reached at 48, 72, and 96 h after treatment
(HAT) for goosegrass, creeping bentgrass and Kentucky bluegrass,
respectively. Grasses translocated < 10% of the absorbed radioactivity
out of the treated leaf at 96 HAT, but creeping bentgrass translocated three
times more radioactivity than goosegrass and Kentucky bluegrass. Creeping
bentgrass, Kentucky bluegrass, and goosegrass metabolized 16, 14, and 25% of
14C-metamifop after 96 h, respectively. Goosegrass had around
two times greater levels of a metabolite at retention factor 0.45 than
creeping bentgrass and Kentucky bluegrass. The concentration of metamifop
required to inhibit isolated ACCase enzymes 50% from the nontreated
(I50) measured > 100, > 100, and 38 μM for creeping
bentgrass, Kentucky bluegrass, and goosegrass, respectively. In other
experiments, foliar absorption of 14C-metamifop in bermudagrass
was similar to 14C-diclofop-methyl. Bermudagrass metabolized 23
and 60% of the absorbed 14C-diclofop-methyl to diclofop acid and
a polar conjugate after 96 h, respectively, but only 14% of
14C-metamifop was metabolized. Isolated ACCase was equally
susceptible to inhibition by diclofop acid and metamifop (I50 =
0.7 μM), suggesting degradation rate is associated with bermudagrass
tolerance levels to these herbicides. Overall, the physiological basis for
metamifop selectivity in turfgrass is differential levels of target site
Seashore paspalum has high salinity tolerance, suggesting sodium chloride might have potential as a selective grassy weed herbicide. The objective of this research was to investigate sodium chloride rate and application timing for smooth crabgrass control and seashore paspalum and common bermudagrass injury. Five rates of sodium chloride (244, 488, 976, 1,952, or 3,904 kg ha−1) were compared with quinclorac at 0.84 kg ai ha−1 for controlling multileaf or multitiller smooth crabgrass. Sodium chloride at ≥ 976 kg ha−1 provided excellent control (90 to 100%) of multitiller smooth crabgrass from 7 to 28 d after treatment, but ≥ 1,952 kg ha−1 was required to achieve excellent control of multileaf populations. Furthermore, 976 kg ha−1 of sodium chloride applied to multitiller smooth crabgrass caused minimal seashore paspalum injury (0 to 6%), comparable to quinclorac, but was more injurious when applied earlier in the spring for multileaf smooth crabgrass control. Common bermudagrass injury increased with sodium chloride rate and was > 20% from sodium chloride at 488 and 976 kg ha−1 at both application timings. Overall, sodium chloride was most effective and safe on seashore paspalum when applied for smooth crabgrass control at the multitiller growth stage, whereas bermudagrass injury might be excessive at minimum rates required for control.
Field studies were conducted over 2 yr to evaluate weed control, yield, and net returns of glyphosate-resistant soybean using total postemergence (5 wk) (POST) herbicide systems with glyphosate–isopropylamine (Ipa) or glyphosate–trimethylsulfonium (Tms) alone, tank mixed with fomesafen, or in sequential treatments with bentazon, fomesafen, Ipa, or Tms. Soybean early-season injury ranged from 0 to 28% across the test. Although Ipa did not injure soybean, glyphosate–Tms early postemergence (3 wk) (EPOST) injured soybean from 7 to 17% depending on the rate. Glyphosate–Tms mixed with fomesafen EPOST injured soybean from 20 to 28%. Red morningglory control by Ipa and Tms at 0.8 kg ae/ha was no more than 88%. Sequential applications of Tms or Ipa controlled red morningglory 78% or less. Fomesafen improved red morningglory control by Ipa and Tms. Bentazon did not affect the control of red morningglory by these herbicides. Sicklepod, smooth pigweed, and large crabgrass control was 81, 93, and 79%, respectively, or greater for all herbicide treatments. By midseason, narrow-row soybeans had canopied, and competition from weeds was minimal. Overall, the net returns were reflective of soybean yield, and maximum net returns were recorded for treatments with reduced herbicide inputs. Conversely, sequential application of herbicides as EPOST followed by POST treatments resulted in lower net returns because of increased herbicide and application costs.
Field studies were conducted to evaluate weed control in herbicide-resistant canola in Georgia. The resistant canola cultivars and respective herbicides were ‘Pioneer 45A76’ and imazamox, ‘Hyola 357RR’ and glyphosate, and ‘2573 Invigor’ and glufosinate. Weed seed of Italian ryegrass and wild radish were sown simultaneously in October with canola and control of these species was evaluated along with other naturally occurring weeds. Herbicide treatments for the respective herbicide-resistant canola cultivar were imazamox at 0.035 and 0.071 kg ai/ha, glyphosate at 0.84 and 1.64 kg ae/ha, and glufosinate at 0.5 and 1.0 kg ai/ha. Herbicides were applied at one– two-leaf (LF) and three–four-LF canola stages. There was no significant injury to any canola cultivar as a result of herbicide rate or timing of application. By midseason (February), imazamox effectively controlled wild radish, henbit, and shepherd's-purse at both rates and at both timings. When applied to three–four-LF canola, the higher rates of glyphosate and glufosinate were required to provide 75% or greater control of Italian ryegrass, wild garlic, and henbit. Glufosinate did not adequately control wild radish at either rate or application timing. Greenhouse experiments provided similar results.
Determining the frequency of crop-wild transgene flow under field conditions is a necessity for the development of regulatory strategies to manage transgenic hybrids. Gene flow of green fluorescent protein (GFP) and Bacillus thuringiensis (Bt) transgenes was quantified in three field experiments using eleven independent transformed Brassica napus L. lines and the wild relatives, B. rapa L. and Raphanus raphanistrum L. Under a high crop to wild relative ratio (600:1), hybridization frequency with B. rapa differed among the individual transformed B. napus lines (ranging from ca. 4% to 22%), however, this difference could be caused by the insertion events or other factors, e.g., differences in the hybridization frequencies among the B. rapa plants. The average hybridization frequency over all transformed lines was close to 10%. No hybridization with R. raphanistrum was detected. Under a lower crop to wild relative ratio (180:1), hybridization frequency with B. rapa was consistent among the transformed B. napus lines at ca. 2%. Interspecific hybridization was higher when B. rapa occurred within the B. napus plot (ca. 37.2%) compared with plot margins (ca. 5.2%). No significant differences were detected among marginal plants grown at 1, 2, and 3 m from the field plot. Transgene backcrossing frequency between B. rapa and transgenic hybrids was determined in two field experiments in which the wild relative to transgenic hybrid ratio was 5–15 plants of B. rapa to 1 transgenic hybrid. As expected, ca. 50% of the seeds produced were transgenic backcrosses when the transgenic hybrid plants served as the maternal parent. When B. rapa plants served as the maternal parent, transgene backcrossing frequencies were 0.088% and 0.060%. Results show that transgene flow from many independent transformed lines of B. napus to B. rapa can occur under a range of field conditions, and that transgenic hybrids have a high potential to produce transgenic seeds in backcrosses.
The movement of transgenes from crops to weeds and the resulting consequences are
concerns of modern agriculture. The possible generation of “superweeds” from the
escape of fitness-enhancing transgenes into wild populations is a risk that is
often discussed, but rarely studied. Oilseed rape, Brassica napus (L.), is a crop
with sexually compatible weedy relatives, such as birdseed rape (Brassica rapa (L.)).
Hybridization of this crop with weedy relatives is an extant risk and an excellent
interspecific gene flow model system. In laboratory crosses, T3 lines of seven
independent transformation events of Bacillus thuringiensis (Bt) oilseed rape were
hybridized with two weedy accessions of B. rapa. Transgenic hybrids were generated
from six of these oilseed rape lines, and the hybrids exhibited an intermediate
morphology between the parental species. The Bt transgene was present in the hybrids,
and the protein was synthesized at similar levels to the corresponding independent
oilseed rape lines. Insect bioassays were performed and confirmed that the hybrid
material was insecticidal. The hybrids were backcrossed with the weedy parent,
and only half the oilseed rape lines were able to produce transgenic backcrosses.
After two backcrosses, the ploidy level and morphology of the resultant plants were
indistinguishable from B. rapa. Hybridization was monitored under field conditions
(Tifton, GA, USA) with four independent lines of Bt oilseed rape with a crop to
wild relative ratio of 1200:1. When B. rapa was used as the female parent,
hybridization frequency varied among oilseed rape lines and ranged from 16.9%
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