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Tolerance of Amaranthus palmeri populations from California to postemergence herbicides at various growth stages
Crop Protection 87 (2016) 6e12
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Tolerance of Amaranthus palmeri populations from California to postemergence herbicides at various growth stages Sonia I. Rios a, Steven D. Wright b, Gerardo Banuelos c, Anil Shrestha c, * a
University of California Cooperative Extension, 21150 Box Springs Rd. Ste. 202, Moreno Valley, CA 92557, USA University of California Cooperative Extension, 4437 S. Laspina St., Tulare, CA 93274, USA c Department of Plant Science, California State University, 2415 E. San Ramon Ave., Fresno, CA 93740, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2016 Received in revised form 15 April 2016 Accepted 22 April 2016
In recent years, poor control of Amarathus palmeri S. Wats. plants with glyphosate in many agricultural and non-crop has been observed in the San Joaquin Valley (SJV), California, USA. Studies were conducted to assess if these were glyphosate-resistant (GR) populations. Populations from 23 different locations of the SJV were exposed to glyphosate application of 840 g ae ha1 at the 5 to 8 leaf stage of the plant and compared against a known GR and glyphosate-susceptible (GS) population from New Mexico, USA. None of the plants from the SJV survived the glyphosate application suggesting that they were GS. Plant mortality following application of glyphosate (840 g ae ha1), glufosinate (490 g ai ha1), paraquat dichloride (660 g ai ha1), saflufenacil (50 g ai ha1), rimsulfuron (70 g ai ha1), and a tank-mix of glyphosate (840 g ae ha1) þ saflufenacil (50 g ai ha1) applied at the 4 to 6, 8 to 10, and 12 to 16 leaf stages of A. palmeri was determined on potted plants grown outdoors. Complete control was obtained with all the treatments applied at the 4 to 6 leaf stage but control was reduced to less than 70% and 20% with glyphosate and glufosinate, respectively at the later stages. The other treatments provided 100% control at all growth stages. Combinations of saflufenacil þ glyphosate, saflufenacil þ glufosinate, saflufenacil þ dicamba, rimsulfuron þ glyphosate, tembotrione þ glyphosate, flumioxazin þ pyroxasulfone þ glyphosate, flumioxazin þ pyroxasulfone þ glyphosate, dicamba þ paraquat dichloride, and glyphosate þ glufosinate were also tested on 8 to 10 leaf stage A. palmeri plants and all the combinations provided 100% control. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Growth stage Glyphosate Dicamba Glufosinate Paraquat Pyroxasulfone Rimsulfuron Saflufenacil Tembotrione
1. Introduction Glyphosate has been a popular herbicide for two decades with glyphosate-tolerant annual crops (Powles et al., 1998), but heavy reliance on glyphosate has led to the evolution of more than 30 weed species with glyphosate-resistant (GR) biotypes (Heap, 2015). In recent years, one species that has drawn a lot of attention in the USA is Amaranthus palmeri S. Wats. Glyphosate-resistant populations of A. palmeri have been confirmed in 26 states of the USA and in Brazil (Heap, 2016) since its initial documentation in Georgia, USA in 2005 (Culpepper et al., 2006). As such, the sustainability of conservation tillage, glyphosate-tolerant cropping systems are being challenged in several parts of the US (Price et al., 2011). This
* Corresponding author. Department of Plant Science, 2415 E. San Ramon Ave. M/ S AS 72, California State University, Fresno, CA 93740, USA. E-mail address: [email protected] (A. Shrestha). http://dx.doi.org/10.1016/j.cropro.2016.04.015 0261-2194/© 2016 Elsevier Ltd. All rights reserved.
species has not only become problematic because of its resistance to glyphosate but also because of its competitive ability, rapid growth rate, high fecundity, and resistance to other herbicides (Sosnoskie et al., 2009; Ward et al., 2013). For example, a study in Georgia, USA demonstrated that two GR A. palmeri plants spaced every 7 m of row (0.9 m row spacing) reduced cotton (Gossypium hirsutum L.) yield by as much as 23% (MacRae et al., 2007). This has resulted in increased costs in cotton production because of an increase in herbicide use, tillage, and hand weeding (Sosnoskie and Culpepper, 2014). Since 2012, growers in California's San Joaquin Valley (SJV) have also reported poor control of A. palmeri in glyphosate-tolerant cotton and maize (Zea mays L.) and cotton production systems (Rios et al., 2014). However, it is not known if these are cases of GR populations or survivors of application of glyphosate at more tolerant stages of the weed. It has been reported that glyphosate application at the recommended label rate of 840 g ae ha1 to weeds larger than 0.5 m in height can lead to reduced control of A.
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palmeri (Nosworthy et al., 2008). A similar phenomenon was also observed by Shrestha et al. (2007) with late applications of glyphosate on GR and glyphosate-susceptible (GS) Conyza canadensis L. Cronq. VanGessel et al. (2009) also reported poor control of C. canadensis when applications were made after the rosette stage of this weed. Furthermore, Steckel et al. (1997) reported some level of tolerance to glufosinate when herbicide applications were made at later growth stages of Chenopodium album L., Setaria faberi Herrm., Polygonum pensylvanicum L., and Xanthium strumarium L. Although variability in susceptibility of A. palmeri plants to glyphosate based on plant size has been suggested (Nosworthy et al., 2008), no studies seem to have been conducted to evaluate the response of A. palmeri plants to glyphosate at various growth stages. Therefore, it needs to be determined if the A. palmeri populations of the SJV are GR or survivors due to application of glyphosate at more tolerant plant stages. Should the populations of A. palmeri be determined as GR, alternative herbicides should be identified for their control before it becomes more invasive. If the plants are not confirmed as GR, identification of some potential alternative herbicides and their appropriate application times relative to the growth stage of the weed will still be valuable information so that growers have options for rotation of modes of action. Therefore, the objectives of this research were to: i) compare the mortality of SJV populations of A. palmeri plants to a known GR and a GS A. palmeri populations to glyphosate; ii) evaluate the mortality of a selected GS A. palmeri plants from SJV sprayed at different growth stages (4- to 6-leaf, 8- to 10-leaf, and 12- to 16-leaf stage) with glyphosate and other postemergence herbicides-alone and glyphosate þ saflufenacil; and iii) evaluate the mortality of a selected GS A. palmeri plants from SJV sprayed at the 8 to 10 leaf stage with several tank-mix combinations of postemergence herbicides. 2. Materials and methods 2.1. Seed collection Seeds of A. palmeri were collected from 23 locations in Tulare, Kings, Kern, and Fresno Counties in the southern region of the SJV of California, USA (Fig. 1) where glyphosate applications areas the
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dominant method of weed control. Geographical coordinates of each of the seed collection locations and description of the site were recorded (Table 1). Since A. palmeri is an obligate outcrossing, dioecious species (Wise et al., 2009), 10 to 20 female plants were harvested for their seeds at each location. 2.2. Preliminary screenings for glyphosate resistance The plants grown from the seeds from these locations were subject to preliminary glyphosate screenings at the California State University, Fresno, CA greenhouses. Seeds were scarified manually with sandpaper to break the hard seed coat to enable germination and planted 0.6e1.3 cm deep (Keeley et al., 1987) in plastic trays. Trays contained a 50e50% mixture of sunshine number 3 (Sun Gro Horticulture Canada Ltd., Vancouver, BC, Canada) commercial growing media containing sphagnum peat moss, coarse grade perlite, gypsum, dolomitic lime, and Kellogg Organic Palm, Cactus and Citrus mix (Kellogg Garden Products, Lockeford, CA, USA), which contained sand and perlite. Seeds were planted at a density of 5e7 into Common Element Standard Vacuum Plug Trays (McConkey Grower Products, Garden Grove, CA, USA). Once the seedlings reached the first true-leaf stage they were transplanted into 7.6 cm press fit pots (McConkey Grower Products, Garden Grove, CA, USA) and covered with 2.5 cm layer of the soil mixture. When the plants reached the 5 to 8 leaf stage, they were sprayed with 840 g ae ha1 of glyphosate þ2% v/v solution of ammonium sulfate. The spray application was made with a CO2-pressurized backpack sprayer equipped with TeeJet 8002 flat fan nozzles (TeeJet Technologies, Wheaton, IL, USA) calibrated to deliver a spray volume of 280 l ha1. Spray height was maintained at 0.5 m above the plants with the help of a 2.5 m long, 0.45 m wide, and 0.5 m tall frame made with polyvinyl chloride (PVC) pipes. The first preliminary screening with glyphosate was made on greenhouse-grown plants, whereas all subsequent screenings were made on plants grown in a growth chamber (Conviron Model E15, Controlled Environments Ltd., Winnipeg, MB, Canada). The temperature in the greenhouse was 25 /18 C day/night with ambient lighting. The growth chamber was programmed for a photoperiod of 14 h and the temperature was maintained at 35 ± 3 C during the light-period and at 30 ± 3 C during the dark period. Relative
Fig. 1. Map of California showing the sites in the San Joaquin Valley from where the A. palmeri plants were sampled. The geographical coordinates of these sites are listed in Table 1.
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Table 1 Geographical coordinates, the environment, and county from which the Amaranthus palmeri seeds were collected from the San Joaquin Valley of California and their mortality to 840 g ae ha1 of glyphosate. Site
Environment
Geographical coordinates
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Citrus orchard Alfalfa field Vineyard Roadside Sorghum field Maize field Maize field Vineyard Pomegranate orchard Maize field Cotton field Cotton field Maize field Alfalfa field Alfalfa field Roadside Maize field Maize field Vineyard Cotton field Almond orchard Fallow field Roadside
36 36 36 36 36 36 36 36 36 36 36 35 35 36 35 36 36 36 36 35 36 36 36
34.071 44.343 31.944 36.270 36.249 02.469 29.326 69.497 05.798 44.086 17.950 07.290 07.290 17.701 07.290 21.530 22.300 34.071 01.200 56.550 01.340 12.910 09.113
N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N,
humidity was set at 30%. Photosynthetically active radiation (PAR) level in the chamber was maintained at 600 mmol m2 s1. All spray applications were made in the mid-morning (10a.m.). After the plants were sprayed, they were immediately placed back into the growth chamber. The plants were observed for mortality every week for up to three weeks after being sprayed. Plant mortality was rated on a 0e100% scale where 0 ¼ completely dead with no green tissue and 100 ¼ completely alive with no necrotic symptoms. The populations that had some plants that survived this initial screening process were considered as suspected GR plants and selected for further evaluations. Further evaluations were made with a whole plant assessment method (Beckie et al., 2000) in comparison with a known GR and a known GS population of A. palmeri obtained from New Mexico (Mohseni-Mogadham et al., 2013). Planting techniques and pot sizes were similar to those mentioned earlier. Each plant in an individual pot was considered as an experimental unit. Plants were fertilized every 10 days with 16 ml of a NPK (12:4:8) solution (Scotts Miracle Grow, The Scotts Company LLC, Marysville, OH, USA) per pot to ensure optimum plant growth. The plants were grown in the same growth chamber described earlier with the same environmental settings. The treatments were replicated four times and the experiment was repeated three times (runs). In each run, plants from different populations of SJV were compared with the known GR and GS plants from NM. However, after the first run, light intensity in the growth chamber was increased to 1200 mmol m2 s1 in the subsequent runs. When the plants reached the 4 to 7 leaf stage, they were removed from the growth chamber, moved outdoors, and treated with the following rates of glyphosate (Roundup WeatherMax®; Monsanto) 0x, 0.5x, 1x, 1.5x, 2x, 2.5x, 3x, 3.5x, and 4x (where x ¼ 840 g ae ha1, i.e. the label rate for glyphosate). A 2% v/v solution of ammonium sulfate was added to all the glyphosate application rates. The spray application methods was similar to that described earlier. The plants were immediately moved back into the chambers after spraying and were observed for mortality every week for up to three weeks. Plant mortality rating method was similar to that as described earlier. The experimental design was a randomized complete block arranged as a factorial with four
119 119 120 119 119 119 119 119 119 119 119 118 118 119 118 119 119 119 119 119 119 119 119
42.095 41.086 19.656 56.560 59.080 13.565 57.590 69.996 53.049 41.086 28.392 49.225 49.225 34.690 49.225 37.410 35.250 42.095 10.770 13.480 11.150 12.760 17.115
W W W W W W W W W W W W W W W W W W W W W W W
County
Plant Mortality (%)
Fresno Fresno Fresno Fresno Fresno Fresno Fresno Fresno Fresno Fresno Kern Kern Kern Kern Kern Kings Kings Kings Tulare Tulare Tulare Tulare Tulare
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 90 100 100 100 80 100
replications. The population type and herbicide rate were the factors. If any plant from these tested populations survived glyphosate rates greater than 0.5x, they were selected for further evaluation. Altogether two (one from Tulare County and one from Kings County, CA, USA) populations were selected for further evaluation. 2.3. Secondary screenings for glyphosate resistance The two populations were further evaluated along with the GR and GS populations from NM using similar protocols as described earlier. After plant mortality was assessed at 21 days after transplanting (DAT), they were clipped at the soil surface, placed in paper bags, dried in a forced-air oven set at 60 C for 72 h, and dry weights were recorded. The experimental design was a randomized complete block arranged as a factorial with two factors (population type and herbicide rate). Each treatment was replicated four times where each plant in an individual pot was considered an experimental unit. The populations and the glyphosate application rates were considered as fixed effects and the experimental run and replications were considered as random effects. 2.4. Tolerance of plants to various herbicides applied at different growth stages One of the local SJV A. palmeri population (from Tulare County) previously utilized in the secondary screening study was selected for this study. The experiment was conducted twice (Run 1 and 2) with potted plants grown outdoors using similar procedures as in the screening studies. However, in this study, the plants were grown in 3.8 l plastic pots. The seeds were planted on June 3 and June 27, for Run 1 and Run 2, respectively. When the seedlings emerged and reached the first true-leaf stage, they were thinned to a single plant per pot. Plants were fertilized every 10 days with 16 ml of liquid fertilizer NPK (12:4:8) solution per pot as described earlier, in some occasions they were watered twice a day as described later. The pots were placed in a sunny, undisturbed area at the Horticulture Unit of California State University, Fresno, CA, USA. Average ambient air temperature during the course of this study ranged from 32 to 39 C. On some days, the daily high
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temperature reached up to 43 C. During these days, the plants were watered twice a day. The PAR outdoors was measured close to solar noon with an Accupar LP80 Ceptometer (Decagon Devices, Pullman, WA, USA). The average PAR during the course of the study ranged from 1600 to 1650 mmol m2 s1. When the plants reached the following growth stages: 4 to 6 leaf stage, 8 to 10 leaf stage, and 12 to 16 leaf stage, they were treated with different postemergence herbicides as listed in Tables 2 and 3. A non-treated control was also included. A 2% solution of ammonium sulfate was added to the glyphosate treatment, a nonionic surfactant (NIS) at 0.25% was added to the glufosinate, rimsulfuron, and paraquat dichloride treatments and a methylated seed soil (MSO) surfactant at 1% was added to the saflufenacil treatments. The spray application details and mortality evaluations were similar to that described earlier. At 28 DAT, the plants were clipped at the soil surface, placed in paper bags, and dried in a forced-air oven at 60 C for 72 h, and the dry weights were recorded. The herbicides were applied on June 13, June 17, and June 23, 2014 in Run 1 and on July 7, July 10, and July 14, 2014 in Run 2 to the 4 to 6, 8 to 10, and 12 to 16 leaf stage plants, respectively. The experimental design was a randomized complete block arranged as a split-plot with six replications and conducted twice as mentioned earlier. Main plot was the A. palmeri growth stage and the sub-plot was the herbicide treatment. 2.5. Tolerance of plants to various herbicide tank-mixes applied at the 8 to 10 leaf stage Mortality of potted A. palmeri plants treated at the 8 to 10 leaf stage with different postemergence tank-mixes were evaluated in an experiment that was conducted twice (Run 1 and Run 2). Procedures used to produce seedlings were similar to that described earlier. The seeds were planted on June 3 and June 27, for Run 1 and Run 2, respectively and the plants were grown in 3.8 l plastic pots using similar establishment, fertilization, and watering protocols as described earlier. Once the plants reached the 8 to 10 leaf stage, they were treated with different herbicide tank-mixes. The herbicide treatments and their application rates are listed in Table 4. A non-treated control was also included. Ammonium sulfate was added at 2% for the treatments containing glyphosate, a NIS at 1% was added to the treatments containing glufosinate, rimsulfuron and paraquat dichloride, an MSO was added at 1% v/v to the treatments containing saflufenacil, dicamba, and tembotrione. Herbicide applications were made on June 13, 2014 and July 7, 2014 in Run 1 and Run 2, respectively using similar spraying protocols as described earlier. However, the sprayer was calibrated to deliver a spray volume of 381 l ha1. Plant mortality evaluations were also conducted using the same protocols as described earlier. At 28 DAT,
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the plants were clipped at the soil surface, placed in paper bags, and dried in a forced-air oven at 60 C for 72 h and the dry weights were recorded.
2.6. Susceptibility of natural infestations of A. palmeri in a fallow field Studies were conducted in a fallow field in Tulare, CA, USA (36120 N, 119120 W) that had a history of poor control of A. palmeri with glyphosate. The field was naturally infested with A. palmeri plants. When the plants reached the 4 to 10 leaf stage, they were treated with different herbicides. The names of herbicides and their application rates are shown in Table 5. A non-treated control was also included. A 2% solution of ammonium sulfate was added to the glyphosate treatments, a NIS at 0.25% was added to glufosinate, rimsulfuron, and paraquat dichloride treatments, and a MSO surfactant at 1% was added to the saflufenacil-alone and tembotrione treatments. Each treatment plot was 6.1 m long and 1.5 m wide. The experiment was conducted twice (Run 1 and Run 2) in the same field but in different plots. Herbicide applications were made on June 13, 2014 and on July 16, 2014 in Run 1 and Run 2, respectively. All the treatment applications and spray application procedures were similar as described earlier. Plant mortality was evaluated weekly starting at 7 DAT and continued up to 28 DAT on a 0e100% scale as described earlier. Care was taken to evaluate only the plants that had initially been sprayed to avoid the inclusion of newer plants emerging after the treatment. The protocol was to avoid any plants with less than 5e6 leaves during subsequent evaluations. No biomass data was collected in this experiment. The experimental design was a randomized complete block with four replications.
2.7. Data analysis In all the experiments, herbicide treatments were considered as the fixed effect and the experimental run and the replications as random effects. Data were tested if the assumptions of ANOVA were met using the Shapiro-Wilk's test for normality and Levene's test for homogeneity of variance at a 0.05 level of significance. Data for biomass was log transformed prior to further analysis to meet the assumptions of ANOVA. Data were analyzed using the general linear model (PROC GLM) procedures of SAS. Tukey's test was used to separate the means whenever the ANOVA indicated significant difference at 0.05 level. Interactions between the run and herbicide treatments were tested.
Table 2 Mortality (average of two runs) of Amaranthus palmeri plants treated with various herbicides at the 4 to 6, 8 to 10, and 12 to 16 leaf growth stages. Treatmenta
Rate
Growth stage and evaluation date 4 to 6 leaf stage
8 to 10 leaf stage
12 to 16 leaf stage
7 DAT 14 DAT 21 DAT 28 DAT 7 DAT 14 DAT 21 DAT 28 DAT 7 DAT 14 DAT 21 DAT 28 DAT SaflufenacilA GlyphosateB þ SaflufenacilA GlyphosateB ParaquatC GlufosinateD RimsulfuronE Control a b
500 g ai ha1 840 g ae ha1 þ 500 g ai ha1 840 g ae ha1 660 g ai ha1 490 g ai ha1 70 g ai ha1 e
Control 100a 100a 100a 100a 100a 100a 0b
(%)b 100a 100a 99a 100a 97a 96a 0b
100a 100a 98a 100a 100a 85a 0b
100a 100a 96a 100a 100a 84b 0c
100a 100a 80b 100a 66b 98a 0c
100a 100a 55b 100a 59b 98a 0c
100a 100a 50b 100a 47b 86a 0c
100a 100a 43b 100a 39b 96a 0b
100a 100a 89b 99ab 54c 98ab 0d
100a 100a 79a 100a 47b 100a 0c
100a 100a 71b 100a 26c 100a 0d
Trade names: ATreevix®; BASF Corp.; BRoundup WeatherMax®, Monsanto; CGramoxone®, Syngenta AG; DRely 280®, Bayer Crop Science; EMatrix®, Du Pont. means within the column followed by the same lowercase letters are not significantly different according to the Tukey's test at a ¼ 0.05.
100a 100a 68b 100a 20c 100a 0c
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Table 3 Average biomass of the potted Amaranthus palmeri plants grown outdoors and treated with different herbicides at three different growth stages. Treatmenta
Growth stageb
Rate
4 to 6 leaf stage
8 to 10 leaf stage
12 to 16 leaf stage
0.28c 0.25c 0.30c 0.23c 0.24c 1.90b 20.19a
0.74c 0.64c 0.71c 0.72c 1.68b 0.71c 24.75a
1
a b
g plant 0.19c 0.07c 0.06c 0.16c 0.13c 0.52b 15.77a
500 g ai ha1 840 g ae ha1 þ 500 g ai ha1 840 g ae ha1 660 g ai ha1 490 g ai ha1 70 g ai ha1 e
SaflufenacilA GlyphosateB þ SaflufenacilA GlyphosateB ParaquatC GlufosinateD RimsulfuronE Control
Trade names: ATreevix®; BASF Corp.; BRoundup WeatherMax®, Monsanto; CGramoxone®, Syngenta AG; DRely 280®, Bayer Crop Science; EMatrix®, Du Pont. means within the column followed by the same lowercase letters are not significantly different according to the Tukey's test at a ¼ 0.05.
Table 4 Mortality (average of two runs) of Amaranthus palmeri plants at 28 days after treatment at the 8 to 10 leaf stage with different herbicides. Treatmenta
Plant mortality (%)b
Rate A
B
Saflufenacil þ Glyphosate SaflufenacilA þ GlufosinateC SaflufenacilA þ DicambaD RimsulfuronE þ GlyphosateB TembotrioneF þ GlyphosateB (Flumioxazin þ Pyroxasulfone)G þ GlyphosateB (Flumioxazin þ Pyroxasulfone)G þ GlufosinateC (Flumioxazin þ Pyroxasulfone)G þ DicambaD DicambaD þ ParaquatH GlufosinateC þ GlyphosateB Control
1
1
50 g ai ha þ 840 g ae ha 50 g ai ha1 þ 490 g ai ha1 50 g ai ha1 þ 510 ml ai ha1 70 g ai ha1 þ 840 g ae ha1 80 ml ai ha1 þ 840 g ae ha1 160 g ai ha1 þ 840 g ae ha1 160 g ai ha1 þ 490 g ai ha1 160 g ai ha1 þ 510 ml ai ha1 510 ml ai ha1 þ 660 g ai ha1 490 g ai ha1 þ 840 g ae ha1 e
100a 100a 100a 100a 100a 100a 100a 100a 100a 100a 0b
a Trade names: ATreevix®; BASF Corp.; BRoundup WeatherMax®, Monsanto; CRely 280®, Bayer Crop Science; DClarity®; BASF Corp.; EMatrix®, Du Pont; Laudis®, Bayer Crop Science; GFierce® (regulatory approval pending in CA), Valent Biosciences Corp.; HGramoxone®, Syngenta AG. b Means within the column followed by the same lowercase letters are not significantly different according to the Tukey's test at a ¼ 0.05; ae glyphosate reported in active ingredient. F
Table 5 Mortality (average of two runs) of Amaranthus palmeri plants with different herbicides in a fallow field. Treatmenta SafluefenacilA GlyphosateB þ SaflufenacilA Paraquat dichlorideC GlufosinateD GlyphosateB DicambaE RimsulfuronF TembotrioneG Control
Rate
7 DAT
14 DAT
21 DAT
28 DAT
50 g ai ha1 840 g ae ha1 þ 50 g ai ha1 660 g ai ha1 490 g ai ha1 840 g ae ha1 510 ml ai ha1 70 g ai ha1 90 ml ai ha1 e
Plant Mortality (%)b 99.8a 99.0a 87.4b 86.1b 98.5a 41.3d 72.5c 84.4b 0e
99.8a 99.4a 88.8b 77.3cd 99.1a 45.6e 73.1d 83.1cb 0f
97.5a 99.4a 86.4a 65.6b 99.8a 46.9c 56.9cb 57.5cb 0d
100.0a 98.8a 77.0b 46.3c 99.8a 33.1c 36.9c 37.5c 0d
a Trade names: ATreevix®; BASF Corp.; BRoundup WeatherMax®, Monsanto; CGramoxone®, Syngenta AG; DRely 280®, Bayer Crop Science; EClarity®; BASF Corp.; FMatrix®, Du Pont; GLaudis®, Bayer Crop Science. b means within the column followed by the same lowercase letters are not significantly different according to the Tukey's test at a ¼ 0.05; ae glyphosate reported in active ingredient.
3. Results and discussion
3.2. Tolerance of plants to various postemergence herbicides applied at various growth stages
3.1. Preliminary and secondary screenings for glyphosate resistance All the plants from the SJV collections, except for one or two plants from site 18 and 22 (Table 1), were completely controlled at the 1x rate of glyphosate in the preliminary screening study (data not shown). The seeds from site 18 and 22 that had one or two survivors at the 1x rate of glyphosate were included in the secondary screening. However, the plants from these sites did not survive the secondary screenings. Therefore, from this study, we conclude that all of the SJV populations tested are GS.
No interaction occurred between the experimental run and plant mortality (P ¼ 0.1808) or biomass (P ¼ 0.1303); therefore, the data for the two runs were combined. However, interactions (P < 0.0001) occurred between the growth stage and the herbicide treatment for both plant mortality and plant biomass; therefore data were analyzed separately for each growth stage. At the 4 to 6 leaf stage all the herbicides tested, except rimsulfuron, provided complete control of the A. palmeri plants by 28 DAT (Table 2). Although the rimsulfuron-treated plants showed good control up to 14 DAT, some of the plants regrew and accumulated more biomass than the plants treated with the other herbicides (Table 3).
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At the 8 to 10 leaf stage, the control of the A. palmeri plants treated with glyphosate or glufosinate was diminished compared to the 4 to 6 leaf stage (Table 2). More than 50% of the plants treated with these two herbicides survived at 28 DAT. Although the plants were alive, their growth was considerably suppressed as observed by the biomass (Table 3). However, the other herbicide treatments provided complete control of the A. palmeri plants at this growth stage. Results at the 12 to 16 leaf stage were similar to the 8 to 10 leaf stage where again glyphosate killed about 68% of the plants but glufosinate killed only 20% of the plants by 28 DAT (Table 2). Again, saflufenacil-alone, tank-mix of saflufenacil þ glyphosate, rimsulfuron, and paraquat provided complete control of the A. palmeri plants. Similar to the 8 to 10 leaf growth stage, the rimsulfuron treated plants had accumulated greater biomass than the other treatments before it died (Table 3). Therefore, it can be concluded that saflufenacil-alone, saflufenacil þ glyphosate, and paraquat may provide complete control of A. palmeri up to the later growth stages evaluated in this study and could be the best postemergence herbicide treatments in several perennial crops and other situations where their use is labeled. Although rimsulfuron can provide good control of the A. palmeri plants, some regrowth can occur with plants treated at an earlier growth stage and the plants could take longer to die than those treated with the other herbicides. Glyphosate-alone and glufosinate-alone could still be used for effective control of the SJV A. palmeri populations provided that they are applied before the 8 to 10 leaf stage. Delaying the application of these two herbicides to the 8 to 10 leaf stage or beyond would result in inconsistent control of the plants and may even select for herbicide-resistant plants. 3.3. Tolerance of plants to various postemergence herbicide tankmixes applied at the 8 to 10 leaf stage No interaction occurred between the experimental run and plant mortality (P ¼ 0.3363) or biomass (P ¼ 0.0893); therefore, the data for the two runs were combined. All the herbicide tank-mix combination tested provided complete control of the selected SJV population at 28 DAT when the application was made at the 8 to 10 leaf stage of the plants (Table 4). It must be noted that this population was the same one that some of which escaped glyphosatealone and glufosinate-alone applications when applied at the 8 to 10 leaf stage. Since all the herbicide-treated plants died and the remnants were mixed with the soil, the biomass at 28 DAT data is not shown. This study showed that any of these tank-mixes can be used for effective control of A. palmeri plants in the SJV depending on the cropping system and if the herbicide is labeled for that particular system. 3.4. Susceptibility of natural infestations of A. palmeri in a fallow field Similar to the other studies, there was no interaction (P ¼ 0.1005) between the herbicide treatments and the experimental runs in the field study either; therefore, the data for the two runs were combined and analyzed. At 7 DAT, the greatest level of control, based on visual plant injury symptoms, occurred in the saflufenacil, saflufenacil þ glyphosate, and the glyphosate-alone treatments (Table 5). These three treatments also were observed to have the greatest level of plant mortality at 28 DAT. Contrary to the potted-plant studies, paraquat controlled only 77% of the plants at 28 DAT but this level of control was greater than with glufosinate, dicamba, rimsulfuorn, or tembotrione. These latter four herbicides provided less than 50% control which may not be an acceptable level of control for grower standards. Again, this result for
11
rimsulfuron was different than the potted-plant study where the level of control was greater. The plants recovered and regrew from the initial injury in all four of these herbicide treatments. Some new plants also emerged but the emergence was low because this was a fallow-field with no water applied. The decrease in level of control with paraquat, and glufosinate was probably because these two are primarily contact herbicides and the new emerging weeds would not have been controlled. Although tembotrione is a systemic herbicide, and provided almost 85% control at the first evaluation date (7 DAT), the level of control diminished over the evaluation period as the plants regrew. Dicamba, in general, was weak on A. palmeri and the control never exceeded 50% at any of the evaluation dates. These results supported the findings of the pot study in which saflufenacil and saflufenacil þ glyphosate provided the most consistent control of the A. palmeri plants. Similarly, like the pot studies, the glyphosate-alone treatment provided good control of the A. palmeri plants. This was because the plants were at the early (4e10 leaf stage) in the field. Also, other than saflufenacil-alone or the tank-mixture of saflufenacil þ glyphosate the other herbicides did not provide as much control. Therefore, from a broad-spectrum weed control perspective and use of different modes of action perspective, a tank-mixture of saflufenacil þ glyphosate may be a good strategy. Tank-mixture of different herbicide modes of action is also a good herbicide resistance management strategy. Tank-mixes of three different herbicides provided the same level of control as tank-mixes of two different herbicide modes of action. Therefore, economics and the types of weed species present in the field may be an important factor during the selection process of these herbicide mixtures. However, it should be emphasized that all these treatments were applied at the 8 to 10 leaf stage of A. palmeri and it is not known if these combinations can provide the same level of control at later growth stages of the species. Although in the potted-plants study saflufenacil, saflufenacil þ glyphosate, and paraquat controlled A. palmeri plants up to the 12 to 16 leaf stage, it cannot be concluded if the same level of control will be obtained in field studies. It should also be mentioned that there was no regrowth or green tissue remaining in any of the herbicide-treated plants. Control of the most frequently occurring and troublesome weed species with glyphosate is rapidly being lost, especially in the southeast because of overall lack of stewardship and continuous use of glyphosate (Webster and Sosnoskie, 2010). With the occurrence of GR A. palmeri in the southwest and several other states in the USA, it is likely that GR A. palmeri may eventually be documented in California if pro-active measurements are not taken. Glyphosate is considerably mobile in plants and the rapid and widespread translocation of glyphosate is important in achieving herbicide efficacy (Claus and Behrens, 1976). It is possible that changes in the pattern of translocation of glyphosate could endow resistance in plants. As mentioned earlier, a few plants from two sites (site 18 and 22) survived the glyphosate label rate in the growth chamber study but it could not be ascertained that these were GR as the plants died in subsequent experiments. Therefore, either this may be a case of a mixed or segregating population where some plants in this location have evolved low level of resistance to glyphosate or the plants were random survivors due to some experimental error. However, it should be emphasized the plants wilted and displayed typical chlorosis and then regained turgor and re-sprouted from secondary growing points. Similar symptoms were observed when glyphosate resistance first appeared in an Australian population of rigid ryegrass (Powles et al., 1998). In another case, extensive studies within one population of rigid ryegrass showed resistance was not due to a resistant
12
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EPSPS or to glyphosate degradation (Lorraine-Colwill et al., 2002). Further, there was no difference between GR and GS populations in glyphosate absorption into leaf tissue; however, patterns of glyphosate translocation were different. Glyphosate applied to the GS plants tended to accumulate in the lower part of the plant and to a lesser extent, in the roots, whereas in GR plants, glyphosate accumulated in the tip of the treated leaf, with little translocation to the roots (Lorraine-Colwill et al., 2002). Similar mechanisms may be involved in be some of the SJV populations. Therefore, it may be just a matter of time before GR A. palmeri is definitively documented in the SJV. In conclusion, this study showed that none of the SJV A. palmeri plants were GR and, in the current context, glyphosate-escapes observed in the field were probably because of the herbicide being applied later than the 4 to 6 leaf stage of the plants. A number of other researchers have observed reductions in weed control in several species when herbicides applications were made to larger, compared to smaller plants (Hoss et al., 2003; Knezevic et al., 2009). This study also showed that A. palmeri control with most of the herbicides tested was greater at the 4 to 6 leaf stage than at the 12 to 16 leaf stage, especially with glyphosate. Probably this may be a major reason why growers are observing poor control of A. palmeri with glyphosate. Most growers tend to wait for maximum emergence of the plants before applying glyphosate thus causing escapes of the plants that emerged earlier and were in the advanced growth stages. Nevertheless, this study identified several herbicides notably saflufenacil, mixture of saflufenacil þ glyphosate, and paraquat as alternatives to glyphosate-alone for control of A. palmeri. Further, several tankmixes for excellent control of A. palmeri were also identified. Among these the notable combinations were saflufenacil þ glyphosate, saflufenacil þ glufosinate, saflufenacil þ dicamba, rimsulfuron þ glyphosate, tembotrione þ glyphosate, dicamba þ paraquat, glufosinate þ glyphosate, or combinations of some of these herbicides with flumioxazin up to the 8 to 10 leaf stage. Hence, if there is an onset of GR A. palmeri in the SJV, there are several viable postemergence control options available, depending on registration of these herbicides in particular crops. Several, but not all, of these herbicides are registered in orchard and field crops in California, such as flumioxazin þ pyroxasulfone. However, reliance on herbicides alone, especially postemergence herbicides, for weed control may not be the best strategy as this can select for the evolution of resistant weeds. Future studies should also investigate integrated weed management options for A. palmeri control. Acknowledgments Would like to thank and acknowledge the University of California Cooperative Extension and Cotton Incorporated for funding of these projects. Would like acknowledge the assistance of Sarah Parry, Erica Emitte, Nikhil Shinde and the faculty and staff at
California State University, Fresno, CA, USA.
References Beckie, H.J., Heap, I.M., Smeda, R.J., Hall, L.M., 2000. Screening for herbcide resistance in weeds. Weed Tech. 14, 428e445. Claus, J., Behrens, R., 1976. Glyphosate translocation and quackgrass rhizome bud kill. Weed Sci. 24, 149e152. Culpepper, A.S., Grey, T.L., Vencill, W.K., Kichler, J.M., Webster, T.M., Brown, S.M., York, A.C., Davis, J.W., Hanna, W.W., 2006. Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) confirmed in Georgia. Weed Sci. 54, 620e626. Heap, I., 2015. International Survey of Herbicide Resistant Weeds. http://www. weedscience.org (accessed 23.04.16.). Heap, I.M., 2016. International Survey of Herbicide Weeds. Herbicide Resistance Action Committee and Weed Science Society of America. http://www. weedscience.org (accessed: 10.01.16.). Hoss, N.E., Al-Khatib, K., Petterson, D.E., Lougin, T.M., 2003. Efficacy of glyphosate, glufosinate, and imazrthapyr on selected weeds. Weed Sci. 51, 110e117. Keeley, P.E., Carter, C.H., Thullen, R.J., 1987. Influence of planting date on growth of palmer amaranth (Amaranthus palmeri). Weed Sci. 35, 199e204. Knezevic, S.Z., Datta, A., Scott, J., Kelin, R.N., Golus, J., 2009. Problem weed control in glyphosate-resistant soybean with glyphosate tank mixes and soil applied herbicides. Weed Technol. 23, 507e512. Lorraine-Colwill, D.F., Powles, S.B., Hawkes, T.R., Hollinshead, P.H., Warner, S.A.J., Preston, C., 2002. Investigations into the mechanism of glyphosate resistant in Lolium rigidum. Pestic. Biochem. Physiol. 74, 62e72. MacRae, A.W., Culpepper, A.S., Webster, T.M., Kichler, J.M., 2007. The effect of glyphosate-resistant Palmer amaranth density and time of establishment on yield of cotton. In: Proc. South Weed Sci Soc, Nashville, TN 22e24 Jan, p. 228. Mohseni-Mogadham, M., Schroeder, J., Heerema, R., Ashigh, J., 2013. Resistance to glyphosate in Palmer amaranth (Amaranthus palmeri) populations from New Mexico pecan orchards. Weed Technol. 27, 85e91. Nosworthy, J.K., Scott, R.C., Smith, K.L., Oliver, L.R., 2008. Response of northeastern Arkansas Palmer amaranth (Amaranthus palmeri) accessions to glyphosate. Weed Sci. 22, 408e413. Powles, S.B., Lorraine-Colwill, D.F., Dellow, J.J., Preston, C., 1998. Evolved resistance to glyphosate in rigid ryegrass (Lolium rigidun) in Australia. Weed Sci. 46, 604e607. Price, A.J., Balkcom, K.S., Culpepper, S.A., Kelton, J.A., Nichols, R.L., Schomberg, H., 2011. Glyphosate-resistant Palmer amaranth: a threat to conservation tillage. J. Soil Water Conserv. 66, 265e275. Rios, S.I., Wright, S.D., Shrestha, A., 2014. Screening for glyphosate-resistance in Palmer amaranth (Amaranthus palmeri) populations of California's San Joaquin Valley. In: Proc. WSSA Ann. Mtg., Feb. 3-6, 2014, Vancouver, BC, Canada. Abst. No. 100. Shrestha, A., Hembree, K.J., Va, N., 2007. Growth stage influences level of resistance in glyphosate-resistant horseweed. Calif. Agric. 61 (2), 67e70. Sosnoskie, L.M., Webster, T.M., Dales, D., Rains, G.C., Grey, T.L., Culpepper, S.A., 2009. Pollen grain size, density, and settling velocity for Palmer Amaranth (Amaranthus palmeri). Weed Sci. 57, 404e409. Sosnoskie, L.M., Culpepper, A.S., 2014. Glyphosate-resistant Palmer Amaranth (Amaranthus palmeri) increases herbicide use, tillage, and hand weeding in Georgia cotton. Weed Sci. 62, 393e402. Steckel, G.J., Wax, L.M., Simmons, F.W., Phillips II, W.H., 1997. Glufosinate efficacy on annual weeds by rate and growth stage. Weed Technol. 53, 484e488. VanGessel, M.J., Scott, B.A., Johnson, Q.R., White-Hansen, S.E., 2009. Influence of glyphosate-resistant horseweed (Conyza canadensis) growth stage on response to glyphosate applications. Weed Technol. 23, 49e53. Ward, S.M., Theodore, M.W., Steckel, L.E., 2013. Palmer amaranth (Amaranthus palmeri): a review. Weed Technol. 27, 12e27. Webster, T.M., Sosnoskie, L.M., 2010. Loss of glyphosate efficacy: a changing spectrum in Georgia cotton. Weed Sci. 58, 73e79. Wise, A.M., Grey, T.L., Prostko, E.P., Vencill, W.K., Webster, T.M., 2009. Establishing the geographic distribution level of acetolactate synthase resistance of Palmer amaranth (Amaranthus palmeri) accessions in Georgia. Weed Technol. 23, 214e220.
Contents lists available at ScienceDirect
Crop Protection journal homepage: www.elsevier.com/locate/cropro
Tolerance of Amaranthus palmeri populations from California to postemergence herbicides at various growth stages Sonia I. Rios a, Steven D. Wright b, Gerardo Banuelos c, Anil Shrestha c, * a
University of California Cooperative Extension, 21150 Box Springs Rd. Ste. 202, Moreno Valley, CA 92557, USA University of California Cooperative Extension, 4437 S. Laspina St., Tulare, CA 93274, USA c Department of Plant Science, California State University, 2415 E. San Ramon Ave., Fresno, CA 93740, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2016 Received in revised form 15 April 2016 Accepted 22 April 2016
In recent years, poor control of Amarathus palmeri S. Wats. plants with glyphosate in many agricultural and non-crop has been observed in the San Joaquin Valley (SJV), California, USA. Studies were conducted to assess if these were glyphosate-resistant (GR) populations. Populations from 23 different locations of the SJV were exposed to glyphosate application of 840 g ae ha1 at the 5 to 8 leaf stage of the plant and compared against a known GR and glyphosate-susceptible (GS) population from New Mexico, USA. None of the plants from the SJV survived the glyphosate application suggesting that they were GS. Plant mortality following application of glyphosate (840 g ae ha1), glufosinate (490 g ai ha1), paraquat dichloride (660 g ai ha1), saflufenacil (50 g ai ha1), rimsulfuron (70 g ai ha1), and a tank-mix of glyphosate (840 g ae ha1) þ saflufenacil (50 g ai ha1) applied at the 4 to 6, 8 to 10, and 12 to 16 leaf stages of A. palmeri was determined on potted plants grown outdoors. Complete control was obtained with all the treatments applied at the 4 to 6 leaf stage but control was reduced to less than 70% and 20% with glyphosate and glufosinate, respectively at the later stages. The other treatments provided 100% control at all growth stages. Combinations of saflufenacil þ glyphosate, saflufenacil þ glufosinate, saflufenacil þ dicamba, rimsulfuron þ glyphosate, tembotrione þ glyphosate, flumioxazin þ pyroxasulfone þ glyphosate, flumioxazin þ pyroxasulfone þ glyphosate, dicamba þ paraquat dichloride, and glyphosate þ glufosinate were also tested on 8 to 10 leaf stage A. palmeri plants and all the combinations provided 100% control. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Growth stage Glyphosate Dicamba Glufosinate Paraquat Pyroxasulfone Rimsulfuron Saflufenacil Tembotrione
1. Introduction Glyphosate has been a popular herbicide for two decades with glyphosate-tolerant annual crops (Powles et al., 1998), but heavy reliance on glyphosate has led to the evolution of more than 30 weed species with glyphosate-resistant (GR) biotypes (Heap, 2015). In recent years, one species that has drawn a lot of attention in the USA is Amaranthus palmeri S. Wats. Glyphosate-resistant populations of A. palmeri have been confirmed in 26 states of the USA and in Brazil (Heap, 2016) since its initial documentation in Georgia, USA in 2005 (Culpepper et al., 2006). As such, the sustainability of conservation tillage, glyphosate-tolerant cropping systems are being challenged in several parts of the US (Price et al., 2011). This
* Corresponding author. Department of Plant Science, 2415 E. San Ramon Ave. M/ S AS 72, California State University, Fresno, CA 93740, USA. E-mail address: [email protected] (A. Shrestha). http://dx.doi.org/10.1016/j.cropro.2016.04.015 0261-2194/© 2016 Elsevier Ltd. All rights reserved.
species has not only become problematic because of its resistance to glyphosate but also because of its competitive ability, rapid growth rate, high fecundity, and resistance to other herbicides (Sosnoskie et al., 2009; Ward et al., 2013). For example, a study in Georgia, USA demonstrated that two GR A. palmeri plants spaced every 7 m of row (0.9 m row spacing) reduced cotton (Gossypium hirsutum L.) yield by as much as 23% (MacRae et al., 2007). This has resulted in increased costs in cotton production because of an increase in herbicide use, tillage, and hand weeding (Sosnoskie and Culpepper, 2014). Since 2012, growers in California's San Joaquin Valley (SJV) have also reported poor control of A. palmeri in glyphosate-tolerant cotton and maize (Zea mays L.) and cotton production systems (Rios et al., 2014). However, it is not known if these are cases of GR populations or survivors of application of glyphosate at more tolerant stages of the weed. It has been reported that glyphosate application at the recommended label rate of 840 g ae ha1 to weeds larger than 0.5 m in height can lead to reduced control of A.
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palmeri (Nosworthy et al., 2008). A similar phenomenon was also observed by Shrestha et al. (2007) with late applications of glyphosate on GR and glyphosate-susceptible (GS) Conyza canadensis L. Cronq. VanGessel et al. (2009) also reported poor control of C. canadensis when applications were made after the rosette stage of this weed. Furthermore, Steckel et al. (1997) reported some level of tolerance to glufosinate when herbicide applications were made at later growth stages of Chenopodium album L., Setaria faberi Herrm., Polygonum pensylvanicum L., and Xanthium strumarium L. Although variability in susceptibility of A. palmeri plants to glyphosate based on plant size has been suggested (Nosworthy et al., 2008), no studies seem to have been conducted to evaluate the response of A. palmeri plants to glyphosate at various growth stages. Therefore, it needs to be determined if the A. palmeri populations of the SJV are GR or survivors due to application of glyphosate at more tolerant plant stages. Should the populations of A. palmeri be determined as GR, alternative herbicides should be identified for their control before it becomes more invasive. If the plants are not confirmed as GR, identification of some potential alternative herbicides and their appropriate application times relative to the growth stage of the weed will still be valuable information so that growers have options for rotation of modes of action. Therefore, the objectives of this research were to: i) compare the mortality of SJV populations of A. palmeri plants to a known GR and a GS A. palmeri populations to glyphosate; ii) evaluate the mortality of a selected GS A. palmeri plants from SJV sprayed at different growth stages (4- to 6-leaf, 8- to 10-leaf, and 12- to 16-leaf stage) with glyphosate and other postemergence herbicides-alone and glyphosate þ saflufenacil; and iii) evaluate the mortality of a selected GS A. palmeri plants from SJV sprayed at the 8 to 10 leaf stage with several tank-mix combinations of postemergence herbicides. 2. Materials and methods 2.1. Seed collection Seeds of A. palmeri were collected from 23 locations in Tulare, Kings, Kern, and Fresno Counties in the southern region of the SJV of California, USA (Fig. 1) where glyphosate applications areas the
7
dominant method of weed control. Geographical coordinates of each of the seed collection locations and description of the site were recorded (Table 1). Since A. palmeri is an obligate outcrossing, dioecious species (Wise et al., 2009), 10 to 20 female plants were harvested for their seeds at each location. 2.2. Preliminary screenings for glyphosate resistance The plants grown from the seeds from these locations were subject to preliminary glyphosate screenings at the California State University, Fresno, CA greenhouses. Seeds were scarified manually with sandpaper to break the hard seed coat to enable germination and planted 0.6e1.3 cm deep (Keeley et al., 1987) in plastic trays. Trays contained a 50e50% mixture of sunshine number 3 (Sun Gro Horticulture Canada Ltd., Vancouver, BC, Canada) commercial growing media containing sphagnum peat moss, coarse grade perlite, gypsum, dolomitic lime, and Kellogg Organic Palm, Cactus and Citrus mix (Kellogg Garden Products, Lockeford, CA, USA), which contained sand and perlite. Seeds were planted at a density of 5e7 into Common Element Standard Vacuum Plug Trays (McConkey Grower Products, Garden Grove, CA, USA). Once the seedlings reached the first true-leaf stage they were transplanted into 7.6 cm press fit pots (McConkey Grower Products, Garden Grove, CA, USA) and covered with 2.5 cm layer of the soil mixture. When the plants reached the 5 to 8 leaf stage, they were sprayed with 840 g ae ha1 of glyphosate þ2% v/v solution of ammonium sulfate. The spray application was made with a CO2-pressurized backpack sprayer equipped with TeeJet 8002 flat fan nozzles (TeeJet Technologies, Wheaton, IL, USA) calibrated to deliver a spray volume of 280 l ha1. Spray height was maintained at 0.5 m above the plants with the help of a 2.5 m long, 0.45 m wide, and 0.5 m tall frame made with polyvinyl chloride (PVC) pipes. The first preliminary screening with glyphosate was made on greenhouse-grown plants, whereas all subsequent screenings were made on plants grown in a growth chamber (Conviron Model E15, Controlled Environments Ltd., Winnipeg, MB, Canada). The temperature in the greenhouse was 25 /18 C day/night with ambient lighting. The growth chamber was programmed for a photoperiod of 14 h and the temperature was maintained at 35 ± 3 C during the light-period and at 30 ± 3 C during the dark period. Relative
Fig. 1. Map of California showing the sites in the San Joaquin Valley from where the A. palmeri plants were sampled. The geographical coordinates of these sites are listed in Table 1.
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Table 1 Geographical coordinates, the environment, and county from which the Amaranthus palmeri seeds were collected from the San Joaquin Valley of California and their mortality to 840 g ae ha1 of glyphosate. Site
Environment
Geographical coordinates
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Citrus orchard Alfalfa field Vineyard Roadside Sorghum field Maize field Maize field Vineyard Pomegranate orchard Maize field Cotton field Cotton field Maize field Alfalfa field Alfalfa field Roadside Maize field Maize field Vineyard Cotton field Almond orchard Fallow field Roadside
36 36 36 36 36 36 36 36 36 36 36 35 35 36 35 36 36 36 36 35 36 36 36
34.071 44.343 31.944 36.270 36.249 02.469 29.326 69.497 05.798 44.086 17.950 07.290 07.290 17.701 07.290 21.530 22.300 34.071 01.200 56.550 01.340 12.910 09.113
N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N, N,
humidity was set at 30%. Photosynthetically active radiation (PAR) level in the chamber was maintained at 600 mmol m2 s1. All spray applications were made in the mid-morning (10a.m.). After the plants were sprayed, they were immediately placed back into the growth chamber. The plants were observed for mortality every week for up to three weeks after being sprayed. Plant mortality was rated on a 0e100% scale where 0 ¼ completely dead with no green tissue and 100 ¼ completely alive with no necrotic symptoms. The populations that had some plants that survived this initial screening process were considered as suspected GR plants and selected for further evaluations. Further evaluations were made with a whole plant assessment method (Beckie et al., 2000) in comparison with a known GR and a known GS population of A. palmeri obtained from New Mexico (Mohseni-Mogadham et al., 2013). Planting techniques and pot sizes were similar to those mentioned earlier. Each plant in an individual pot was considered as an experimental unit. Plants were fertilized every 10 days with 16 ml of a NPK (12:4:8) solution (Scotts Miracle Grow, The Scotts Company LLC, Marysville, OH, USA) per pot to ensure optimum plant growth. The plants were grown in the same growth chamber described earlier with the same environmental settings. The treatments were replicated four times and the experiment was repeated three times (runs). In each run, plants from different populations of SJV were compared with the known GR and GS plants from NM. However, after the first run, light intensity in the growth chamber was increased to 1200 mmol m2 s1 in the subsequent runs. When the plants reached the 4 to 7 leaf stage, they were removed from the growth chamber, moved outdoors, and treated with the following rates of glyphosate (Roundup WeatherMax®; Monsanto) 0x, 0.5x, 1x, 1.5x, 2x, 2.5x, 3x, 3.5x, and 4x (where x ¼ 840 g ae ha1, i.e. the label rate for glyphosate). A 2% v/v solution of ammonium sulfate was added to all the glyphosate application rates. The spray application methods was similar to that described earlier. The plants were immediately moved back into the chambers after spraying and were observed for mortality every week for up to three weeks. Plant mortality rating method was similar to that as described earlier. The experimental design was a randomized complete block arranged as a factorial with four
119 119 120 119 119 119 119 119 119 119 119 118 118 119 118 119 119 119 119 119 119 119 119
42.095 41.086 19.656 56.560 59.080 13.565 57.590 69.996 53.049 41.086 28.392 49.225 49.225 34.690 49.225 37.410 35.250 42.095 10.770 13.480 11.150 12.760 17.115
W W W W W W W W W W W W W W W W W W W W W W W
County
Plant Mortality (%)
Fresno Fresno Fresno Fresno Fresno Fresno Fresno Fresno Fresno Fresno Kern Kern Kern Kern Kern Kings Kings Kings Tulare Tulare Tulare Tulare Tulare
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 90 100 100 100 80 100
replications. The population type and herbicide rate were the factors. If any plant from these tested populations survived glyphosate rates greater than 0.5x, they were selected for further evaluation. Altogether two (one from Tulare County and one from Kings County, CA, USA) populations were selected for further evaluation. 2.3. Secondary screenings for glyphosate resistance The two populations were further evaluated along with the GR and GS populations from NM using similar protocols as described earlier. After plant mortality was assessed at 21 days after transplanting (DAT), they were clipped at the soil surface, placed in paper bags, dried in a forced-air oven set at 60 C for 72 h, and dry weights were recorded. The experimental design was a randomized complete block arranged as a factorial with two factors (population type and herbicide rate). Each treatment was replicated four times where each plant in an individual pot was considered an experimental unit. The populations and the glyphosate application rates were considered as fixed effects and the experimental run and replications were considered as random effects. 2.4. Tolerance of plants to various herbicides applied at different growth stages One of the local SJV A. palmeri population (from Tulare County) previously utilized in the secondary screening study was selected for this study. The experiment was conducted twice (Run 1 and 2) with potted plants grown outdoors using similar procedures as in the screening studies. However, in this study, the plants were grown in 3.8 l plastic pots. The seeds were planted on June 3 and June 27, for Run 1 and Run 2, respectively. When the seedlings emerged and reached the first true-leaf stage, they were thinned to a single plant per pot. Plants were fertilized every 10 days with 16 ml of liquid fertilizer NPK (12:4:8) solution per pot as described earlier, in some occasions they were watered twice a day as described later. The pots were placed in a sunny, undisturbed area at the Horticulture Unit of California State University, Fresno, CA, USA. Average ambient air temperature during the course of this study ranged from 32 to 39 C. On some days, the daily high
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temperature reached up to 43 C. During these days, the plants were watered twice a day. The PAR outdoors was measured close to solar noon with an Accupar LP80 Ceptometer (Decagon Devices, Pullman, WA, USA). The average PAR during the course of the study ranged from 1600 to 1650 mmol m2 s1. When the plants reached the following growth stages: 4 to 6 leaf stage, 8 to 10 leaf stage, and 12 to 16 leaf stage, they were treated with different postemergence herbicides as listed in Tables 2 and 3. A non-treated control was also included. A 2% solution of ammonium sulfate was added to the glyphosate treatment, a nonionic surfactant (NIS) at 0.25% was added to the glufosinate, rimsulfuron, and paraquat dichloride treatments and a methylated seed soil (MSO) surfactant at 1% was added to the saflufenacil treatments. The spray application details and mortality evaluations were similar to that described earlier. At 28 DAT, the plants were clipped at the soil surface, placed in paper bags, and dried in a forced-air oven at 60 C for 72 h, and the dry weights were recorded. The herbicides were applied on June 13, June 17, and June 23, 2014 in Run 1 and on July 7, July 10, and July 14, 2014 in Run 2 to the 4 to 6, 8 to 10, and 12 to 16 leaf stage plants, respectively. The experimental design was a randomized complete block arranged as a split-plot with six replications and conducted twice as mentioned earlier. Main plot was the A. palmeri growth stage and the sub-plot was the herbicide treatment. 2.5. Tolerance of plants to various herbicide tank-mixes applied at the 8 to 10 leaf stage Mortality of potted A. palmeri plants treated at the 8 to 10 leaf stage with different postemergence tank-mixes were evaluated in an experiment that was conducted twice (Run 1 and Run 2). Procedures used to produce seedlings were similar to that described earlier. The seeds were planted on June 3 and June 27, for Run 1 and Run 2, respectively and the plants were grown in 3.8 l plastic pots using similar establishment, fertilization, and watering protocols as described earlier. Once the plants reached the 8 to 10 leaf stage, they were treated with different herbicide tank-mixes. The herbicide treatments and their application rates are listed in Table 4. A non-treated control was also included. Ammonium sulfate was added at 2% for the treatments containing glyphosate, a NIS at 1% was added to the treatments containing glufosinate, rimsulfuron and paraquat dichloride, an MSO was added at 1% v/v to the treatments containing saflufenacil, dicamba, and tembotrione. Herbicide applications were made on June 13, 2014 and July 7, 2014 in Run 1 and Run 2, respectively using similar spraying protocols as described earlier. However, the sprayer was calibrated to deliver a spray volume of 381 l ha1. Plant mortality evaluations were also conducted using the same protocols as described earlier. At 28 DAT,
9
the plants were clipped at the soil surface, placed in paper bags, and dried in a forced-air oven at 60 C for 72 h and the dry weights were recorded.
2.6. Susceptibility of natural infestations of A. palmeri in a fallow field Studies were conducted in a fallow field in Tulare, CA, USA (36120 N, 119120 W) that had a history of poor control of A. palmeri with glyphosate. The field was naturally infested with A. palmeri plants. When the plants reached the 4 to 10 leaf stage, they were treated with different herbicides. The names of herbicides and their application rates are shown in Table 5. A non-treated control was also included. A 2% solution of ammonium sulfate was added to the glyphosate treatments, a NIS at 0.25% was added to glufosinate, rimsulfuron, and paraquat dichloride treatments, and a MSO surfactant at 1% was added to the saflufenacil-alone and tembotrione treatments. Each treatment plot was 6.1 m long and 1.5 m wide. The experiment was conducted twice (Run 1 and Run 2) in the same field but in different plots. Herbicide applications were made on June 13, 2014 and on July 16, 2014 in Run 1 and Run 2, respectively. All the treatment applications and spray application procedures were similar as described earlier. Plant mortality was evaluated weekly starting at 7 DAT and continued up to 28 DAT on a 0e100% scale as described earlier. Care was taken to evaluate only the plants that had initially been sprayed to avoid the inclusion of newer plants emerging after the treatment. The protocol was to avoid any plants with less than 5e6 leaves during subsequent evaluations. No biomass data was collected in this experiment. The experimental design was a randomized complete block with four replications.
2.7. Data analysis In all the experiments, herbicide treatments were considered as the fixed effect and the experimental run and the replications as random effects. Data were tested if the assumptions of ANOVA were met using the Shapiro-Wilk's test for normality and Levene's test for homogeneity of variance at a 0.05 level of significance. Data for biomass was log transformed prior to further analysis to meet the assumptions of ANOVA. Data were analyzed using the general linear model (PROC GLM) procedures of SAS. Tukey's test was used to separate the means whenever the ANOVA indicated significant difference at 0.05 level. Interactions between the run and herbicide treatments were tested.
Table 2 Mortality (average of two runs) of Amaranthus palmeri plants treated with various herbicides at the 4 to 6, 8 to 10, and 12 to 16 leaf growth stages. Treatmenta
Rate
Growth stage and evaluation date 4 to 6 leaf stage
8 to 10 leaf stage
12 to 16 leaf stage
7 DAT 14 DAT 21 DAT 28 DAT 7 DAT 14 DAT 21 DAT 28 DAT 7 DAT 14 DAT 21 DAT 28 DAT SaflufenacilA GlyphosateB þ SaflufenacilA GlyphosateB ParaquatC GlufosinateD RimsulfuronE Control a b
500 g ai ha1 840 g ae ha1 þ 500 g ai ha1 840 g ae ha1 660 g ai ha1 490 g ai ha1 70 g ai ha1 e
Control 100a 100a 100a 100a 100a 100a 0b
(%)b 100a 100a 99a 100a 97a 96a 0b
100a 100a 98a 100a 100a 85a 0b
100a 100a 96a 100a 100a 84b 0c
100a 100a 80b 100a 66b 98a 0c
100a 100a 55b 100a 59b 98a 0c
100a 100a 50b 100a 47b 86a 0c
100a 100a 43b 100a 39b 96a 0b
100a 100a 89b 99ab 54c 98ab 0d
100a 100a 79a 100a 47b 100a 0c
100a 100a 71b 100a 26c 100a 0d
Trade names: ATreevix®; BASF Corp.; BRoundup WeatherMax®, Monsanto; CGramoxone®, Syngenta AG; DRely 280®, Bayer Crop Science; EMatrix®, Du Pont. means within the column followed by the same lowercase letters are not significantly different according to the Tukey's test at a ¼ 0.05.
100a 100a 68b 100a 20c 100a 0c
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Table 3 Average biomass of the potted Amaranthus palmeri plants grown outdoors and treated with different herbicides at three different growth stages. Treatmenta
Growth stageb
Rate
4 to 6 leaf stage
8 to 10 leaf stage
12 to 16 leaf stage
0.28c 0.25c 0.30c 0.23c 0.24c 1.90b 20.19a
0.74c 0.64c 0.71c 0.72c 1.68b 0.71c 24.75a
1
a b
g plant 0.19c 0.07c 0.06c 0.16c 0.13c 0.52b 15.77a
500 g ai ha1 840 g ae ha1 þ 500 g ai ha1 840 g ae ha1 660 g ai ha1 490 g ai ha1 70 g ai ha1 e
SaflufenacilA GlyphosateB þ SaflufenacilA GlyphosateB ParaquatC GlufosinateD RimsulfuronE Control
Trade names: ATreevix®; BASF Corp.; BRoundup WeatherMax®, Monsanto; CGramoxone®, Syngenta AG; DRely 280®, Bayer Crop Science; EMatrix®, Du Pont. means within the column followed by the same lowercase letters are not significantly different according to the Tukey's test at a ¼ 0.05.
Table 4 Mortality (average of two runs) of Amaranthus palmeri plants at 28 days after treatment at the 8 to 10 leaf stage with different herbicides. Treatmenta
Plant mortality (%)b
Rate A
B
Saflufenacil þ Glyphosate SaflufenacilA þ GlufosinateC SaflufenacilA þ DicambaD RimsulfuronE þ GlyphosateB TembotrioneF þ GlyphosateB (Flumioxazin þ Pyroxasulfone)G þ GlyphosateB (Flumioxazin þ Pyroxasulfone)G þ GlufosinateC (Flumioxazin þ Pyroxasulfone)G þ DicambaD DicambaD þ ParaquatH GlufosinateC þ GlyphosateB Control
1
1
50 g ai ha þ 840 g ae ha 50 g ai ha1 þ 490 g ai ha1 50 g ai ha1 þ 510 ml ai ha1 70 g ai ha1 þ 840 g ae ha1 80 ml ai ha1 þ 840 g ae ha1 160 g ai ha1 þ 840 g ae ha1 160 g ai ha1 þ 490 g ai ha1 160 g ai ha1 þ 510 ml ai ha1 510 ml ai ha1 þ 660 g ai ha1 490 g ai ha1 þ 840 g ae ha1 e
100a 100a 100a 100a 100a 100a 100a 100a 100a 100a 0b
a Trade names: ATreevix®; BASF Corp.; BRoundup WeatherMax®, Monsanto; CRely 280®, Bayer Crop Science; DClarity®; BASF Corp.; EMatrix®, Du Pont; Laudis®, Bayer Crop Science; GFierce® (regulatory approval pending in CA), Valent Biosciences Corp.; HGramoxone®, Syngenta AG. b Means within the column followed by the same lowercase letters are not significantly different according to the Tukey's test at a ¼ 0.05; ae glyphosate reported in active ingredient. F
Table 5 Mortality (average of two runs) of Amaranthus palmeri plants with different herbicides in a fallow field. Treatmenta SafluefenacilA GlyphosateB þ SaflufenacilA Paraquat dichlorideC GlufosinateD GlyphosateB DicambaE RimsulfuronF TembotrioneG Control
Rate
7 DAT
14 DAT
21 DAT
28 DAT
50 g ai ha1 840 g ae ha1 þ 50 g ai ha1 660 g ai ha1 490 g ai ha1 840 g ae ha1 510 ml ai ha1 70 g ai ha1 90 ml ai ha1 e
Plant Mortality (%)b 99.8a 99.0a 87.4b 86.1b 98.5a 41.3d 72.5c 84.4b 0e
99.8a 99.4a 88.8b 77.3cd 99.1a 45.6e 73.1d 83.1cb 0f
97.5a 99.4a 86.4a 65.6b 99.8a 46.9c 56.9cb 57.5cb 0d
100.0a 98.8a 77.0b 46.3c 99.8a 33.1c 36.9c 37.5c 0d
a Trade names: ATreevix®; BASF Corp.; BRoundup WeatherMax®, Monsanto; CGramoxone®, Syngenta AG; DRely 280®, Bayer Crop Science; EClarity®; BASF Corp.; FMatrix®, Du Pont; GLaudis®, Bayer Crop Science. b means within the column followed by the same lowercase letters are not significantly different according to the Tukey's test at a ¼ 0.05; ae glyphosate reported in active ingredient.
3. Results and discussion
3.2. Tolerance of plants to various postemergence herbicides applied at various growth stages
3.1. Preliminary and secondary screenings for glyphosate resistance All the plants from the SJV collections, except for one or two plants from site 18 and 22 (Table 1), were completely controlled at the 1x rate of glyphosate in the preliminary screening study (data not shown). The seeds from site 18 and 22 that had one or two survivors at the 1x rate of glyphosate were included in the secondary screening. However, the plants from these sites did not survive the secondary screenings. Therefore, from this study, we conclude that all of the SJV populations tested are GS.
No interaction occurred between the experimental run and plant mortality (P ¼ 0.1808) or biomass (P ¼ 0.1303); therefore, the data for the two runs were combined. However, interactions (P < 0.0001) occurred between the growth stage and the herbicide treatment for both plant mortality and plant biomass; therefore data were analyzed separately for each growth stage. At the 4 to 6 leaf stage all the herbicides tested, except rimsulfuron, provided complete control of the A. palmeri plants by 28 DAT (Table 2). Although the rimsulfuron-treated plants showed good control up to 14 DAT, some of the plants regrew and accumulated more biomass than the plants treated with the other herbicides (Table 3).
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At the 8 to 10 leaf stage, the control of the A. palmeri plants treated with glyphosate or glufosinate was diminished compared to the 4 to 6 leaf stage (Table 2). More than 50% of the plants treated with these two herbicides survived at 28 DAT. Although the plants were alive, their growth was considerably suppressed as observed by the biomass (Table 3). However, the other herbicide treatments provided complete control of the A. palmeri plants at this growth stage. Results at the 12 to 16 leaf stage were similar to the 8 to 10 leaf stage where again glyphosate killed about 68% of the plants but glufosinate killed only 20% of the plants by 28 DAT (Table 2). Again, saflufenacil-alone, tank-mix of saflufenacil þ glyphosate, rimsulfuron, and paraquat provided complete control of the A. palmeri plants. Similar to the 8 to 10 leaf growth stage, the rimsulfuron treated plants had accumulated greater biomass than the other treatments before it died (Table 3). Therefore, it can be concluded that saflufenacil-alone, saflufenacil þ glyphosate, and paraquat may provide complete control of A. palmeri up to the later growth stages evaluated in this study and could be the best postemergence herbicide treatments in several perennial crops and other situations where their use is labeled. Although rimsulfuron can provide good control of the A. palmeri plants, some regrowth can occur with plants treated at an earlier growth stage and the plants could take longer to die than those treated with the other herbicides. Glyphosate-alone and glufosinate-alone could still be used for effective control of the SJV A. palmeri populations provided that they are applied before the 8 to 10 leaf stage. Delaying the application of these two herbicides to the 8 to 10 leaf stage or beyond would result in inconsistent control of the plants and may even select for herbicide-resistant plants. 3.3. Tolerance of plants to various postemergence herbicide tankmixes applied at the 8 to 10 leaf stage No interaction occurred between the experimental run and plant mortality (P ¼ 0.3363) or biomass (P ¼ 0.0893); therefore, the data for the two runs were combined. All the herbicide tank-mix combination tested provided complete control of the selected SJV population at 28 DAT when the application was made at the 8 to 10 leaf stage of the plants (Table 4). It must be noted that this population was the same one that some of which escaped glyphosatealone and glufosinate-alone applications when applied at the 8 to 10 leaf stage. Since all the herbicide-treated plants died and the remnants were mixed with the soil, the biomass at 28 DAT data is not shown. This study showed that any of these tank-mixes can be used for effective control of A. palmeri plants in the SJV depending on the cropping system and if the herbicide is labeled for that particular system. 3.4. Susceptibility of natural infestations of A. palmeri in a fallow field Similar to the other studies, there was no interaction (P ¼ 0.1005) between the herbicide treatments and the experimental runs in the field study either; therefore, the data for the two runs were combined and analyzed. At 7 DAT, the greatest level of control, based on visual plant injury symptoms, occurred in the saflufenacil, saflufenacil þ glyphosate, and the glyphosate-alone treatments (Table 5). These three treatments also were observed to have the greatest level of plant mortality at 28 DAT. Contrary to the potted-plant studies, paraquat controlled only 77% of the plants at 28 DAT but this level of control was greater than with glufosinate, dicamba, rimsulfuorn, or tembotrione. These latter four herbicides provided less than 50% control which may not be an acceptable level of control for grower standards. Again, this result for
11
rimsulfuron was different than the potted-plant study where the level of control was greater. The plants recovered and regrew from the initial injury in all four of these herbicide treatments. Some new plants also emerged but the emergence was low because this was a fallow-field with no water applied. The decrease in level of control with paraquat, and glufosinate was probably because these two are primarily contact herbicides and the new emerging weeds would not have been controlled. Although tembotrione is a systemic herbicide, and provided almost 85% control at the first evaluation date (7 DAT), the level of control diminished over the evaluation period as the plants regrew. Dicamba, in general, was weak on A. palmeri and the control never exceeded 50% at any of the evaluation dates. These results supported the findings of the pot study in which saflufenacil and saflufenacil þ glyphosate provided the most consistent control of the A. palmeri plants. Similarly, like the pot studies, the glyphosate-alone treatment provided good control of the A. palmeri plants. This was because the plants were at the early (4e10 leaf stage) in the field. Also, other than saflufenacil-alone or the tank-mixture of saflufenacil þ glyphosate the other herbicides did not provide as much control. Therefore, from a broad-spectrum weed control perspective and use of different modes of action perspective, a tank-mixture of saflufenacil þ glyphosate may be a good strategy. Tank-mixture of different herbicide modes of action is also a good herbicide resistance management strategy. Tank-mixes of three different herbicides provided the same level of control as tank-mixes of two different herbicide modes of action. Therefore, economics and the types of weed species present in the field may be an important factor during the selection process of these herbicide mixtures. However, it should be emphasized that all these treatments were applied at the 8 to 10 leaf stage of A. palmeri and it is not known if these combinations can provide the same level of control at later growth stages of the species. Although in the potted-plants study saflufenacil, saflufenacil þ glyphosate, and paraquat controlled A. palmeri plants up to the 12 to 16 leaf stage, it cannot be concluded if the same level of control will be obtained in field studies. It should also be mentioned that there was no regrowth or green tissue remaining in any of the herbicide-treated plants. Control of the most frequently occurring and troublesome weed species with glyphosate is rapidly being lost, especially in the southeast because of overall lack of stewardship and continuous use of glyphosate (Webster and Sosnoskie, 2010). With the occurrence of GR A. palmeri in the southwest and several other states in the USA, it is likely that GR A. palmeri may eventually be documented in California if pro-active measurements are not taken. Glyphosate is considerably mobile in plants and the rapid and widespread translocation of glyphosate is important in achieving herbicide efficacy (Claus and Behrens, 1976). It is possible that changes in the pattern of translocation of glyphosate could endow resistance in plants. As mentioned earlier, a few plants from two sites (site 18 and 22) survived the glyphosate label rate in the growth chamber study but it could not be ascertained that these were GR as the plants died in subsequent experiments. Therefore, either this may be a case of a mixed or segregating population where some plants in this location have evolved low level of resistance to glyphosate or the plants were random survivors due to some experimental error. However, it should be emphasized the plants wilted and displayed typical chlorosis and then regained turgor and re-sprouted from secondary growing points. Similar symptoms were observed when glyphosate resistance first appeared in an Australian population of rigid ryegrass (Powles et al., 1998). In another case, extensive studies within one population of rigid ryegrass showed resistance was not due to a resistant
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EPSPS or to glyphosate degradation (Lorraine-Colwill et al., 2002). Further, there was no difference between GR and GS populations in glyphosate absorption into leaf tissue; however, patterns of glyphosate translocation were different. Glyphosate applied to the GS plants tended to accumulate in the lower part of the plant and to a lesser extent, in the roots, whereas in GR plants, glyphosate accumulated in the tip of the treated leaf, with little translocation to the roots (Lorraine-Colwill et al., 2002). Similar mechanisms may be involved in be some of the SJV populations. Therefore, it may be just a matter of time before GR A. palmeri is definitively documented in the SJV. In conclusion, this study showed that none of the SJV A. palmeri plants were GR and, in the current context, glyphosate-escapes observed in the field were probably because of the herbicide being applied later than the 4 to 6 leaf stage of the plants. A number of other researchers have observed reductions in weed control in several species when herbicides applications were made to larger, compared to smaller plants (Hoss et al., 2003; Knezevic et al., 2009). This study also showed that A. palmeri control with most of the herbicides tested was greater at the 4 to 6 leaf stage than at the 12 to 16 leaf stage, especially with glyphosate. Probably this may be a major reason why growers are observing poor control of A. palmeri with glyphosate. Most growers tend to wait for maximum emergence of the plants before applying glyphosate thus causing escapes of the plants that emerged earlier and were in the advanced growth stages. Nevertheless, this study identified several herbicides notably saflufenacil, mixture of saflufenacil þ glyphosate, and paraquat as alternatives to glyphosate-alone for control of A. palmeri. Further, several tankmixes for excellent control of A. palmeri were also identified. Among these the notable combinations were saflufenacil þ glyphosate, saflufenacil þ glufosinate, saflufenacil þ dicamba, rimsulfuron þ glyphosate, tembotrione þ glyphosate, dicamba þ paraquat, glufosinate þ glyphosate, or combinations of some of these herbicides with flumioxazin up to the 8 to 10 leaf stage. Hence, if there is an onset of GR A. palmeri in the SJV, there are several viable postemergence control options available, depending on registration of these herbicides in particular crops. Several, but not all, of these herbicides are registered in orchard and field crops in California, such as flumioxazin þ pyroxasulfone. However, reliance on herbicides alone, especially postemergence herbicides, for weed control may not be the best strategy as this can select for the evolution of resistant weeds. Future studies should also investigate integrated weed management options for A. palmeri control. Acknowledgments Would like to thank and acknowledge the University of California Cooperative Extension and Cotton Incorporated for funding of these projects. Would like acknowledge the assistance of Sarah Parry, Erica Emitte, Nikhil Shinde and the faculty and staff at
California State University, Fresno, CA, USA.
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