Control of Glyphosate-Resistant Giant Ragweed (<i>Ambrosia trifida</i>L.) with 2,4-D Followed by Pre-Emergence or Post-Emergence Herbicides in Glyphosate-Resistant Soybean (<i>Glycine max</i>L.)

Control of glyphosate-resistant giant ragweed is a challenge, particularly for soybean growers, because of limited effective post-emergence (POST) herbicide options. Many soybean growers in no-till production systems use 2,4-D in burndown application for control of broadleaf weeds, including giant ragweed. Field experiments were conducted at David City, NE, in 2012 and 2013 to evaluate 2,4-D followed by PRE or POST herbicide programs for control of glyphosate-resistant giant ragweed in glyphosate-resistant soybean. Results suggested that burndown application of 2,4-D or saflufenacil plus imazethapyr resulted in 89 to 99% control of giant ragweed at 21 days after treatment. Burndown-only treatments of S-metolachlor plus metribuzin or sulfentrazone plus cloransulam resulted in poor control (≤65%) of giant ragweed and reduced soybean yield (≤ 577 kg&middotha-1). Burndown application of 2,4-D followed by saflufenacil plus imazethapyr, S-metolachlor plus metribuzin, or sulfentrazone plus cloransulam applied pre-emergence (PRE) or cloransulam, chlorimuron, fomesafen, imazethapyr, or lactofen in tank-mixtures with acetochlor applied POST resulted in 87% to 99% giant ragweed control, reduced density to ≤7 plants m-2, and resulted in soybean yield from 2519 to 3823 kg&middotha-1. There was no difference among and between 2,4-D followed by PRE or POST herbicides for giant ragweed control, density, or soybean yield, indicating all the two pass herbicide programs were effective. It is concluded that glyphosate-resistant giant ragweed can be effectively controlled in soybean by including 2,4-D in burndown program followed by PRE or POST herbicides tested in this study.

Control of Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.) with Isoxaflutole and Metribuzin Tankmix

Five field trials were conducted over a two-year period (2013, 2014) to determine the control of glyphosate-resistant (GR) giant ragweed with isoxaflutole (IFT) and metribuzin (MTZ) applied alone and in combination. Treatments were designed to assess the dose response of an IFT plus MTZ tank-mix as well as each chemical applied alone to classify the response using Flint’s adaptation of Colby’s equation. Two factor factorial experiments were performed in the growth room to ascertain the response of IFT versus glyphosate, IFT versus MTZ, and IFT plus MTZ versus glyphosate on single plants. Field experiments evaluated the control of GR giant ragweed with IFT plus MTZ in tank-mix in a 1:4 ratio. The rate of IFT plus MTZ for 80% control of GR giant ragweed at 4 and 8 weeks after application (WAA) was 518 (104 g a.i. ha-1 IFT + 414 g a.i. ha-1 MTZ) and 631 g a.i. ha-1 (126 g a.i. ha-1 IFT + 505 g a.i. ha-1 MTZ), respectively. A rate of 668 and 467 g a.i. ha-1 was required to reduce GR giant ragweed density and biomass by 80%, respectively. Field experiments evaluating the control of GR giant ragweed with tank-mixes of IFT plus MTZ, where glyphosate was a constant tank-mix partner, were mostly synergistic. However, the low tank-mix rate (52.5 + 210 g a.i. ha-1) had an additive response for GR giant ragweed biomass reduction. When tested in the greenhouse and growth room, glyphosate susceptible (GS) giant ragweed showed some antagonism with glyphosate and isoxaflutole tank-mixes at rates less than commercial field rates. GR giant ragweed showed an additive response across all treatments in the growth room. Greenhouse experiments evaluating IFT versus MTZ and IFT plus MTZ versus glyphosate revealed all tank-mix treatments to be synergistic at 2 WAA.

Preplant and Postemergence Control of Glyphosate-Resistant Giant Ragweed in Corn

Glyphosate-resistant (GR) giant ragweed has recently been identified in southwestern Ontario and has the potential to be a significant problem for regional corn producers. Eight field trials [four with preplant (PP) and four with postemergence (POST) herbicides] were conducted from 2013 to 2014 on various Ontario farms infested with GR giant ragweed to determine the efficacy of PP and POST tank-mixes in corn. Glyphosate tank-mixed with atrazine, dicamba, dicamba/atrazine, mesotrione plus atrazine, flumetsulam, isoxaflutole plus atrazine, saflufenacil/dimethenamid-P, S-metolachlor/atrazine and rimsulfuron applied PP provided up to 54%, 95%, 93%, 95%, 40%, 89%, 91%, 50% and 93% control of GR giant ragweed and reduced dry weight 69%, 100%, 99%, 100%, 30%, 92%, 98%, 66% and 99%, respectively. POST application of glyphosate alone and tank-mixed with 2,4-D ester, atrazine, dicamba, dicamba/diflufenzopyr, dicamba/atrazine, bromoxynil plus atrazine, prosulfuron plus dicamba, mesotrione plus atrazine, topramezone plus atrazine, tembotrione/thiencarbazone-methyl and glufosinate provided up to 31%, 84%, 39%, 94%, 89%, 86%, 83%, 78%, 72%, 43%, 63% and 58% GR giant ragweed and reduced dry weight 55%, 99%, 72%, 99%, 99%, 98%, 96%, 96%, 93%, 89%, 91% and 95%, respectively. In general, PP control of GR giant ragweed was greater than POST applied herbicides evaluated. Based on these results, glyphosate tank-mixes containing dicamba or mesotrione plus atrazine applied PP, and dicamba applied POST will provide the most consistent control of GR giant ragweed in corn.

Distribution of glyphosate and cloransulam-methyl resistant giant ragweed(Ambrosia trifida L.)populations in southern Ontario

Giant ragweed is a very competitive weed in row crop production and has been found to drastically reduce soybean yield. In 2008, giant ragweed was the first weed species with confirmed resistance to glyphosate in Canada. As of 2010 there were 48 locations with confirmed glyphosate resistant giant ragweed in Essex, Kent and Lambton counties. In addition, there was suspected resistance to cloransulam-methyl. The objectives of this research were 1) to conduct an expanded field survey on the distribution of glyphosate resistant giant ragweed in Ontario, 2) to determine the distribution of cloransulam-methyl resistant giant ragweed in Ontario, and 3) to determine the distribution of multiple resistant (glyphosate and cloransulam-methyl) giant ragweed in Ontario. In 2011 and 2012 giant ragweed seed was collected from 85 field sites in Essex (16), Kent (34), Lambton (23), Elgin (3), Middlesex (6), Lennox & Addington (1), Huron (1) and Brant (1) counties. In total there are 34 additional locations confirmed with glyphosate resistant giant ragweed in Ontario. There are 11 locations confirmed with cloransulam-methyl resistant giant ragweed and 5 locations with multiple resistance to both glyphosate and cloransulam-methyl. Glyphosate resistant giant ragweed has been found in 4 additional counties.

Effect of False Ragweed (Iva Xanthifolia Nutt) Seed Extracts on Plants

Bioassay method was used to study the biological activity of different polar solvent extracts of False ragweed seed. Specially, water extracts of False ragweed has an effect on plant germination and root length. The study found that False ragweed seed extracts had different degrees of inhibition on many plant seed germinations, and inhibited the germination of cucumber seeds best, the maximum inhibition rate was 79.5%, for cabbage, sorghum and maize seed germination inhibition rate was more than 30%. False ragweed seed extracts also had strong inhibition on cucumber and many other plant root length, and inhibited the root length of cucumber best followed by red beans. The result showed that False ragweed seed extracts contained some materials that could inhibit germination and growth of some plants.

Glyphosate Resistance in Giant Ragweed (<i>Ambrosia trifida</i>L.) from Mississippi Is Partly Due to Reduced Translocation

A giant ragweed population from a glyphosate-resistant (GR) soybean field in Mississippi, USA was suspected to be resistant to glyphosate. Greenhouse and laboratory studies were conducted to confirm and quantify the magnitude of glyphosate resistance in a resistant biotype selected from this population and to elucidate possible physiological and molecular mechanisms of glyphosate resistance. Glyphosate dose response studies indicated that ED50 (effective dose required to reduce plant growth by 50%) values for glyphosate-resistant (GR-MS) and glyphosate-susceptible (GS-MS) biotypes, based on percent injury, were 0.52 and 0.34 kg ae/ha glyphosate, respectively, indicating a 1.5-fold level of resistance in GR-MS. The absorption pattern of 14C-glyphosate in the two giant ragweed biotypes was similar throughout the measured time course of 168 h after treatment (HAT). The amount of 14C-glyphosate that translocated out of treated leaves of the GR-MS and GS-MS plants was similar up to 24 HAT. However, the GS-MS biotype translocated more (71% and 76% of absorbed at 48 and 96 HAT, respectively) 14C-glyphosate than the GR-MS biotype (44% and 66% of absorbed at 48 and 96 HAT, respectively) out of the treated leaf. No target site mutation was identified at the Pro106 location of the EPSPS gene of the GR-MS biotype. The mechanism of resistance to glyphosate in giant ragweed from Mississippi, at least, is due to reduced glyphosate translocation.

Effects of NO_(2) on Inflorescence Length,Pollen/Seed Amount and Phenolic Metabolites of Common Ragweed(Ambrosia artemisiifolia L.)

Ambrosia artemisiifolia L. (common ragweed) is an annual ruderal plant that is native to Northern America but nowadays is also spreading across Europe, and its pollen is known to be highly allergenic. Air pollution, e.g. NOx and climate change may affect the plant growth, pollen production and duration of the entire pollen season. In this study, ragweed plants were grown over an entire vegetation period under 40 ppb NO2/clean air (control) and 80 ppb NO2 (treatment). The inflorescence length was not affected by this air pollutant. However, the pollen amount increased, while the seed production decreased in both populations upon elevated NO2 concentrations. Regarding phenolic metabolites elevated NO2 had no effect on the amount of total phenolic metabolites, while individual metabolites showed significant changes.

Linuron Biologically Effective Dose for Glyphosate-Resistant Giant Ragweed (<i>Ambrosia trifida</i>L.) Control in Soybean (<i>Glycine max</i>L.)

Glyphosate-resistant (GR) giant ragweed (Ambrosia trifida L.) was first identified in Canada in 2008 and has since been found throughout southwestern Ontario. Six field trials were conducted over a two-year period (2012, 2013) on Ontario farms with GR giant ragweed to evaluate the efficacy of linuron applied pre-plant (PP) in soybean (Glycine max (L.) Merr.). The dose required for 50%, 80%, and 95% GR giant ragweed control was 1238, 2959, and 6018 g·ai·ha-1 four weeks after application (WAA), respectively. The linuron dose needed for 50%, 80%, and 95% reduction in density was 1554, 3181, and 5643 g·ai·ha-1 and 1204, 2496, and 4452 g·ai·ha-1 for dry weight, respectively. Application of 7874 g·ai·ha-1 linuron was needed to obtain soybean yields that were 90% of the weed-free control;approximately 3.5 times the maximum field recommended dose. To achieve 95% and 98% yields, greater than 8640 g·ai·ha-1 linuron was required. Application of linuron plus glyphosate PP in soybean will help to control GR giant ragweed as well as reduce GR selection pressure.

Is intraspecific trait differentiation in Parthenium hysterophorus a consequence of hereditary factors and/or phenotypic plasticity?

Of the various strategies adopted by an invasive plant species for expanding its niche breadth,phenotypic differentiation(either due to plasticity and/or adaptive evolution) is proven to be the most successful.Lately,we studied the persistence of substantial morpho-functional variations within the individuals of alien invasive plant,Parthenium hysterophorus in Chandigarh,India,through field surveys.Based on observed differences,the individuals were categorized into two morphotypes,PAand PB.PAhad higher leaf area,leaf biomass,and chlorophyll content as compared with PB.However,PBhad a higher stem circumference,stem specific density,twig dry matter content,profuse branching,bigger canopy,and better reproductive output than PA.To substantiate the persistence of intraspecific variations in P. hysterophorus and to deduce the possible genesis of these variations,we propagated both the morphotypes under experimental conditions in winter and summer.Apart from the key morpho-functional differences observed during the field studies,protein and carbohydrate metabolism were studied in leaves and roots of the propagated plants.Differences in plant metabolism were observed only during the early growth period,whereas the morpho-functional traits varied in the mature flowering plants.The effect of growth season was highly significant on all the studied morpho-functional and biochemical parameters(p ≤0.05).Parent morphotypes(P) and interactions between morphotypes and seasons significantly affected several growth parameters(p ≤0.05).The analyses revealed that the contrasting growth conditions at the time of transplantation and early growth may regulate the phenotype of P. hysterophorus.The pattern of intraspecific variations observed during the study is justified to consider morphotype PAas winter biotype and morphotype PBas summer biotype of P. hysterophorus.The study points towards the role of plasticity or a combination of genetic and environmental(G×E) factors in producing the phenotypic variability observed in the population of P. hysterophorus.

Density and Seasonal Dynamics of Bemisia tabaci(Gennadius) Mediterranean on Common Crops and Weeds Around Cotton Fields in Northern China

The density seasonal dynamics of Bemisia tabaci MED were evaluated over two years in a cotton-growing area in Langfang, Hebei Province, northern China on cotton （Gossypium hirsutum L.） and six other co-occurring common plants, common ragweed （Ambrosia artemisiifolia L.）, piemarker （Abutilon theophrasti Medicus）, sunflower （Helianthus annuus L.）, sweet potato （Ipomoea batatas L.）, soybean （Glycine max L.）, and maize （Zea mays L.）. The whitefly species identity was repeatedly tested and confirmed; seasonal dynamics on the various host plants were standardized by the quartile method. B. tabaci MED appeared on weeds （the common ragweed and piemarker） about 10 days earlier than on cotton, or the other cultivated plants. The peak population densities were observed over a span of 2 to 3 weeks on cotton, starting in early （2010） or mid-August （2011）. The common ragweed growing adjacent to cotton supported the highest B. tabaci densities （no. on 100 cm2 leaf surface）, 12-22 fold higher than on cotton itself. Sunflower supported more B. tabaci than the other plants, and about 1.5-2 fold higher than cotton did, Our results indicate that weeds （esp. the common ragweed） around cotton fields could increase the population density of B. tabaci MED on cotton, while sunflower could act as a trap crop for decreasing pest pressure on cotton.