Palmer Amaranth Research Articles

Global Metabolome of Palmer Amaranth (Amaranthus palmeri) Populations Highlights the Specificity and Inducibility of Phytochemical Responses to Abiotic Stress.

Commonalities in adaptive responses to abiotic stressors could contribute to the development of cross-resistance in weeds. The degree to which herbicide-induced changes in weeds parallel those induced by other abiotic stress remains unknown. We investigated the specificity of metabolic perturbations induced by glyphosate and drought across three glyphosate-resistant (GR) and two glyphosate-susceptible (GS) biotypes of Palmer amaranth (Amaranthus palmeri) using global metabolomics approaches. Compared to GS-biotypes, in the absence of stress, the GR-biotypes had a higher abundance of primary metabolites, including sugars, nonaromatic amino acids, and organic acids. However, despite having a higher 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene copy number that could upregulate the phenylpropanoid metabolism, the nonstressed GR-biotypes were less abundant in specialized (secondary) metabolites. Under glyphosate stress, 80% of metabolites, including shikimate, that accumulated in GS-biotypes also increased in the GR-biotypes. However, glyphosate triggered the preferential accumulation of glycosides of dihydroxylated and methoxylated flavanols with higher antioxidant potential, and ferulic acid derivatives, specifically in GR-biotypes. The disruption of the shikimate pathway and the accumulation of phenylpropanoids upon glyphosate exposure suggest that the stress response of GR-biotypes could be partly induced. This differential response was less evident in other phytochemical classes and under drought, highlighting that the phytochemical responses are stress-specific rather than biotype-specific.

Effects of elevated CO2 on the response of glyphosate resistant and susceptible Palmer amaranth (Amaranthus palmeri S. Wats.) to varying rates of glyphosate

ABSTRACT Repeated application of herbicides has led to herbicide-resistant weed populations. Elevated CO2 often increases weed growth, exacerbating problems including impacts on crop yield and weed control. How rising CO2 affects herbicide-resistant weeds remains largely unknown. Palmer amaranth (Amaranthus palmeri) populations resistant to or susceptible to glyphosate were exposed to ambient or elevated (ambient +200 μmol mol−1) concentrations of CO2 in open-top chambers. Two weeks after CO2 exposure, each population was sprayed with glyphosate at 0, 0.5×, 1.0×, or 1.5× the label rate. Afterwards, CO2 exposure continued for an additional two weeks when visual herbicide injury and leaf gas exchange were assessed. Plants were harvested for determination of treatment effects on growth. Elevated CO2 had little effect on most variables except for decreasing stomatal conductance (~25.5%) and increasing water use efficiency (~49.2%). Glyphosate resistant plants showed decreased height (9.8%) and damage (~56.4%) and increased root dry weight (12.1%), photosynthesis (~41.2%), stomatal conductance (~63.3%), and water use efficiency (~32.0%). In general, increasing rates of glyphosate decreased growth and gas exchange variables, which was frequently only seen in the susceptible ecotype. Data suggest that rising atmospheric CO2 is unlikely to affect Palmer amaranth, including control of glyphosate-resistant populations.

Evaluation of electrical and mechanical Palmer amaranth (Amaranthus palmeri) management in cucumber, peanut, and sweetpotato

AbstractField studies were conducted to assess the efficacy of physical weed management of Palmer amaranth management in cucumber, peanut, and sweetpotato. Treatments were arranged in a 3 × 4 factorial in which the first factor included a treatment method of electrical, mechanical, or hand-roguing Palmer amaranth control and the second factor consisted of treatments applied when Palmer amaranth was approximately 0.3, 0.6, 0.9, or 1.2 m above the crop canopy. Four wk after treatment (WAT), the electrical applications controlled Palmer amaranth at least 27 percentage points more than the mechanical applications when applied at the 0.3- and 0.6-m timings. At the 0.9- and 1.2-m application timings 4 WAT, electrical and mechanical applications controlled Palmer amaranth by at most 87%. Though hand removal generally resulted in the greatest peanut pod count and total sweetpotato yield, mechanical and electrical control resulted in similar yield to the hand-rogued plots, depending on the treatment timing. With additional research to provide insight into the optimal applications, there is potential for electrical control and mechanical control to be used as alternatives to hand removal. Additional studies were conducted to determine the effects of electrical treatments on Palmer amaranth seed production and viability. Treatments consisted of electricity applied to Palmer amaranth at first visible inflorescence, 2 wk after first visible inflorescence (WAI) or 4 WAI. Treatments at varying reproductive maturities did not reduce the seed production immediately after treatment. However, after treatment, plants primarily died and ceased maturation, reducing seed production assessed at 4 WAI by 93% and 70% when treated at 0 and 2 WAI, respectively. Treatments did not have a negative effect on germination or seedling length.

Role of oxidative stress in the physiology of sensitive and resistant Amaranthus palmeri populations treated with herbicides inhibiting acetolactate synthase.

The aim of the present study was to elucidate the role of oxidative stress in the mode of action of acetolactate synthase (ALS) inhibiting herbicides. Two populations of Amaranthus palmeri S. Watson from Spain (sensitive and resistant to nicosulfuron, due to mutated ALS) were grown hydroponically and treated with different rates of the ALS inhibitor nicosulfuron (one time and three times the field recommended rate). Seven days later, various oxidative stress markers were measured in the leaves: H2O2, MDA, ascorbate and glutathione contents, antioxidant enzyme activities and gene expression levels. Under control conditions, most of the analysed parameters were very similar between sensitive and resistant plants, meaning that resistance is not accompanied by a different basal oxidative metabolism. Nicosulfuron-treated sensitive plants died after a few weeks, while the resistant ones survived, independently of the rate. Seven days after herbicide application, the sensitive plants that had received the highest nicosulfuron rate showed an increase in H2O2 content, lipid peroxidation and antioxidant enzymatic activities, while resistant plants did not show these responses, meaning that oxidative stress is linked to ALS inhibition. A supralethal nicosulfuron rate was needed to induce a significant oxidative stress response in the sensitive population, providing evidence that the lethality elicited by ALS inhibitors is not entirely dependent on oxidative stress.

Amaranthus hybridus L. RESISTANT TO GLYPHOSATE AND CHLORIMURON IN PARAGUAY

Amaranthus species are extremely problematic weeds due to their aggressiveness and resistance, being Amaranthus palmeri S. Watson, Amaranthus retroflexus L., Amaranthus tuberculatus (Moq.) J. D. Sauer, and Amaranthus hybridus L. prominent worldwide. The present research aimed to study A. hybridus populations in Paraguay where the use of glyphosate and chlorimuron herbicides presented management challenges. This research was conducted in the municipalities of Corpus Christi and Hernandarias, Paraguay, in 2016, 2017, 2018, 2019, and 2020. Weed seeds with resistance indicators were collected, cultivated, and following a screening process, cultivated for seed production (heritability up to F2), and the seeds were collected to obtain the herbicide dose-response curve. The herbicides glyphosate and chlorimuron were tested in portions of 1-8, 1-4, 1-2, 1, 2, 4, and 8 times the recommended dose on the package inserts (720 g acid equivalent (ae) ha 1 and 20 g active ingredient (ai) ha1, respectively). Among the A. hybridus populations evaluated in Paraguay, biotype from Hernandarias with multiple resistance to ALS-inhibiting (HRAC 2) and EPSPs-inhibiting (HRAC 9) herbicides, and biotype from Corpus Christi with resistance to ALS-inhibiting herbicides (HRAC 2) were found. Therefore, rotation and combination of herbicides, as well as integration with non-chemical measures, are essential to control and prevent selection of resistant biotypes. Studies aimed to develop integrated weed management plans are essential for effective control. In countries like Paraguay, this becomes imperative because of the lack of research on this topic.

Metabolism of 2,4-D in resistant Amaranthus palmeri S. Wats. (Palmer amaranth)

Amaranthus palmeri S. Wats. is one of troublesome weed species in field crops in the mid-southern USA. In our recent results of screening for herbicide resistance of various weeds collected from agricultural areas of the mid-southern USA, three A. palmeri accessions resistant to 2,4-D (R1, R2, and R3) were verified. In this study, all three resistant (R) accessions, along with two 2,4-D-susceptible (S1 and S2) standards, were subjected to dose-response experiments to characterize their sensitivity to 2,4-D. Based on the dose-response results, resistance/susceptibility ratios of the tested A. palmeri accessions were greater in the order of R2 (7.1) ≥ R1 (5.7) = R3 (5.7) > S2 (1.9) ≥ S1 (1.0). Treatment of malathion (cytochrome P450 inhibitor) or 4-chloro-7-nitrobenzofurazan [glutathione S-transferase (GST) inhibitor] followed by 2,4-D choline increased the mortality of R1 and R2 accessions, which means that enhanced metabolism of 2,4-D contributed to evolution of the herbicide resistance in such R accessions. In contrast, the mortality of R3 accession was not affected by either of the metabolic inhibitors. Absorption and translocation of 2,4-D were similar between S1 and R accessions for 48 h after the herbicide treatment. However, the 48-h metabolism of 2,4-D in R1 (48%) and R2 (68%) accessions were 2- to 3-fold greater than those in S1 (21%) and R3 (19%) accessions. Thus, enhanced metabolism by activities of cytochrome P450s and GSTs can be one of multiple mechanisms conferring 2,4-D resistance to A. palmeri. Further genetic studies are needed to verify the presence of target-site resistance mechanisms in all three R accessions.

Inheritance of 2,4-dichlorophenoxyacetic acid (2,4-D) resistance in Amaranthus palmeri

In this study, the inheritance of 2,4-D resistance in a multiple herbicide-resistant Palmer amaranth (KCTR) was investigated. Direct and reciprocal crosses were performed using 2,4-D-resistant KCTR and susceptible KSS plants to generate F1 progenies. 2,4-D dose–response assays were conducted to evaluate the response of progenies from each F1 family along with KCTR and KSS plants in controlled environmental growth chambers. Additionally, 2,4-D-resistant male and female plants from each of the F1 families were used in pairwise crosses to generate pseudo-F2 families. Segregation (resistance or susceptibility) of progenies from the F2 families in response to a discriminatory rate of 2,4-D (i.e., 560 g ae ha−1) was evaluated. Dose–response analysis of F1 progenies derived from direct and reciprocal crosses suggested that the 2,4-D resistance in KCTR is a nuclear trait. Chi-square analyses of F2 segregation data implied that 2,4-D resistance in KCTR is controlled by multiple gene(s). Overall, our data suggest that the 2,4-D resistance in KCTR Palmer amaranth is a nuclear inherited trait controlled by multiple genes. Such resistance can spread both via pollen or seed-mediated gene flow. In future, efforts will be directed towards identifying genes mediating 2,4-D resistance in KCTR population.

Leaf Morphology and Ecological Stoichiometry of C, N and P in Dioecious Invasive <i>Amaranthus palmeri</i>

Leaf Morphology and Ecological Stoichiometry of C, N and P in Dioecious Invasive <i>Amaranthus palmeri</i>

Response of Soybean (Glycine max (L.) Merr.) and Weed Control with Postemergence Herbicides and Combinations of Cytokinin Mixtures

A field study was conducted in 2015 and 2016 in Stoneville, MS, to evaluate the influence of cytokinin products on soybean injury and weed control when combined with common POST soybean herbicide treatments. Cytokinin treatments included no cytokinin mixture and two formulated cytokinin mixtures (kinetin-1 and kinetin-2) applied at 0.000227 kg ai ha−1. Herbicide treatments were no herbicide, glyphosate at 1.37 kg ae ha−1 alone and in combination with S-metolachlor at 1.42 kg ai ha−1 or fomesafen 0.395 kg ai ha−1. The addition of cytokinin treatments had no impact on soybean injury, plant height, or yield. Glyphosate plus fomesafen provided the greatest level of Palmer amaranth control, between 84 and 67%., 7 days and 28 days after treatment, respectively. Barnyardgrass control with glyphosate plus fomesafen was antagonized by one of two cytokinin products. To prevent possible reductions in herbicide efficacy, tank mixtures with cytokinin products should not be applied to soybean in POST herbicide applications.

Impact of integrated weed management practices on cotton economics and Palmer amaranth (Amaranthus palmeri) populations

AbstractThe threat of herbicide-resistant weed species, such as Palmer amaranth, has driven the development of robust weed management programs that rely on more than chemicals for weed control. Previous research has shown that zero-tolerance weed thresholds, cover crops, deep tillage, and diverse herbicide programs are effective strategies for controlling Palmer amaranth. Unfortunately, research investigating the integration of all four of these weed management strategies in a system is lacking. To better leverage these integrated weed management strategies in cotton production systems, a long-term study was initiated in fall 2018 near Marianna, AR, with zero tolerance, deep tillage, a cereal rye cover crop, and either a dicamba or non-dicamba in-crop herbicide program as factors. Results found that total Palmer amaranth emergence was reduced 76% as the result of deep tillage in 2019 and, in the absence of a zero-tolerance strategy, 73% in 2020. In the absence of a zero-tolerance strategy, the combination of a non–cover crop strategy and dicamba herbicide program decreased total Palmer amaranth emergence by 73%, while the combination of a cover crop strategy and dicamba herbicide program decreased total Palmer amaranth emergence by 78% compared to the combination of a cover crop and non-dicamba herbicide program. Under a zero-tolerance strategy in 2019, tillage reduced cotton yield by 12% and partial returns by US$370 ha−1. In 2020, tillage reduced cotton yield by 14% and partial returns of US$371 ha−1 under a non-zero-tolerance strategy, while a 12% yield reduction and a US$260 ha−1 decrease in partial returns were observed under a zero-tolerance strategy. In 2019, the non-dicamba program resulted in greater partial returns than the dicamba in-crop program because of greater yield and lower program costs. However, in 2020, partial returns were greater for the dicamba in-crop herbicide program owing to greater yields achieved by this program.

Biomass Production of an Overwinter Cover Crop with Biofumigation Properties in New Mexico

Soil-borne diseases and weeds can be inhibited by mustard family (Brassicaceae) cover crops that are mowed and incorporated into the soil with tillage—a process referred to as biofumigation. To determine whether a fall-seeded mustard cover crop produces enough biomass to be a biofumigant in spring, this study measured the amount of biomass produced by a mixture of ‘Caliente Rojo’ brown mustard (Brassica juncea) and ‘Nemat’ arugula (Eruca sativa) grown in three commercial fields and a university research farm in southern New Mexico, USA. This study also determined whether the mustard biomass incorporated in the soil inhibits a weed [Palmer amaranth (Amaranthus palmeri)], but does not affect a cash crop adversely [chile pepper (Capsicum annuum)]. Results indicated that, if the mustard cover crop was seeded before the first frost in fall, mustard cover crops produced biomass in quantities sufficient for biofumigation in spring. Mustard biomass incorporated in the soil reduced the survival and germination of Palmer amaranth seeds. Under greenhouse conditions, chile pepper plants grown in soil with mustard cover crop biomass were larger than chile plants grown in soil without mustard biomass. Chile pepper plants in soil with mustard biomass did not show symptoms of Verticillium wilt (Verticillium dahliae), whereas such symptoms were found on about 33% of chile pepper plants in soil without mustard biomass. These results suggest that a fall-seeded mustard cover crop that is tilled into the soil in early spring is a potential pest management technique for chile pepper in New Mexico.

Potential distribution prediction of Amaranthus palmeri S.Watson in China under current and future climate scenarios.

The vicious invasive alien plant Amaranthus palmeri poses a serious threat to ecological security and food security due to its strong adaptability, competitiveness, and herbicide resistance. Predicting its potential habitats under current and future climate change is critical for monitoring and early warning. In this study, we used two sets of climate data, namely, WorldClim1.4 and RCPs (the historical climate data of WorldClim version 1.4 and future climate data of RCPs), WorldClim2.1 and SSPs (the historical climate data of WorldClim version 2.1 and future climate data of SSPs), to analyze the dominant environmental variables affecting the habitat suitability and predict the potential distribution of A.palmeri to climate change in China based on the MaxEnt model. The results show that (i) Temperature has a greater impact on the distribution of A.palmeri. The relative contributions of temperature-related variables count to 70% or more, and the annual mean temperature (bio1) reached more than 40%. (ii) At present, the potentially suitable area is widely distributed in the central-east and parts of southwest China, and the high suitable area is focused on the North China Plain. The potential suitable area predicted by WorldClim1.4 and WorldClim2.1 both accounts for about 31% of China's total land area. (iii) Future climate change will expand the suitable habitats to high latitudes and altitudes. The overall suitable area maximum increased to 44.93% under SSPs and 38.91% under RCPs. We conclude that climate change would increase the risk of A.palmeri expanding to high latitudes and altitudes, the results have practical implications for the effective long-term management in response to the global warming of A.palmeri.

Confirmation of dicamba-resistant Palmer amaranth in Tennessee

AbstractPalmer amaranth has a long history of evolving resistance to herbicides to the point at which it has become a significant obstacle to row crop production. A survey of Palmer amaranth escapes in dicamba-resistant cotton and soybean fields in Tennessee was conducted in fall 2021 with the objective of determining whether poor control was due to environmental phenomena or the development of dicamba resistance. A greenhouse dicamba dose-response screen was conducted on 15 Tennessee accessions. Three accessions were identified with a relative resistance factor between 1.85 and 2.49, and one accession from Lauderdale County was found with a relative resistance factor of 14.25. The Lauderdale County 1 accession developed a higher dicamba resistance level than all others evaluated and can no longer be effectively controlled using dicamba. The history of Palmer amaranth escaping dicamba in the Lauderdale County 1 location from 2019 to 2021 in the field and in preliminary greenhouse screens would suggest that the dicamba resistance has passed between generations. This research documents the first findings of Palmer amaranth control failures in cotton and soybean fields due to the evolution of dicamba resistance.

Impact of volatility reduction agents on dicamba and glyphosate spray solution pH, droplet dynamics, and weed control.

Regulations in 2021 required the addition of a volatility reduction agent (VRA) to dicamba spray mixtures for postemergence weed control. Understanding the impact of VRAs on weed control, droplet dynamics, and spray pH is essential. Adding glyphosate to dicamba decreased the solution pH by 0.63 to 1.85 units. Across locations, potassium carbonate increased the tank-mixture pH by 0.85 to 1.65 units while potassium acetate raised the pH by 0.46 to 0.53 units. Glyphosate and dicamba in tank-mixture reduced Palmer amaranth control by 14 percentage points compared to dicamba alone and decreased barnyardgrass control by 12 percentage points compared to glyphosate alone 4 weeks after application (WAA). VRAs resulted in a 5-percentage point reduction in barnyardgrass control 4 WAA. Common ragweed, common lambsquarters, and giant ragweed control were unaffected by herbicide solution 4 WAA. Dicamba alone produced a larger average droplet size and had the fewest driftable fines (% volume < 200 μm). Potassium acetate produced a larger droplet size than potassium carbonate for Dv0.1 and Dv0.5 . The addition of glyphosate to dicamba decreased droplet size from the entire spray droplet spectrum (Dv0.1 , Dv0.5 , Dv0.9 ). A reduction in spray pH, droplet size, and weed control was observed from mixing dicamba and glyphosate. It may be advisable to avoid tank-mixtures of these herbicides and instead, apply them sequentially to maximize effectiveness. VRAs differed in their impacts on spray solution pH and droplet dynamics, but resulted in a minimal negative to no impact on weed control. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Influence of gibberellic acid on vegetable crop and weed emergence

AbstractSmall-seeded vegetable crop production is challenged by poor emergence, stand establishment and canopy development, as well as a lack of effective weed control options. The potential use of plant growth regulators such as gibberellic acid (GA) could enhance early emergence and growth rates while potentially synchronizing weed germination with control tactics. In response, a controlled environment study was conducted to investigate the effects of GA on garden beet, cabbage, carrot, and onion. At 7 d after seeding (DAS) carrot emergence was greater when carrot seeds were treated with 2, 4, 8, 16, or 64 ppm GA compared with nontreated seeds. Total carrot emergence over the study period was 14% greater when seeds were treated with 4 ppm GA compared with nontreated seeds. Treatment of cabbage with as low as 2 to 4 ppm GA increased cabbage emergence rate and total plant emergence over the study period relative to nontreated seeds. Onion response to GA treatment was variable and unremarkable and was hypothesized to be influenced by seed dormancy because emergence was also low with the nontreated seeds. The GA rates that stimulated vegetable crop seed germination and emergence were then explored with three common weed species to determine whether a similar response was observed. If so, GA could be used to stimulate weed emergence in synchrony with management tactics. Palmer amaranth emergence was strongly affected by GA treatment, whereby the total number of emerged plants was 48% greater when 4 ppm GA was applied than in the nontreated check. Velvetleaf emergence at 3 DAS with the 4 and 8 ppm GA was 2.9 and 3.0 plants pot−1, respectively, compared to no emergence in the nontreated pots. Redroot pigweed emergence was not affected by GA treatment at any rate.

In-field assessment of EPSPS amplification on fitness cost in mixed glyphosate-resistant and glyphosate-sensitive populations of Palmer amaranth (Amaranthus palmeri)

AbstractComparing fitness of herbicide-resistant and herbicide-susceptible weed biotypes is important for managing herbicide resistance. Previous research suggests there is little to no fitness penalty from amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene (a mechanism of glyphosate resistance) in Palmer amaranth (Amaranthus palmeri S. Watson) in controlled studies in the greenhouse or growth chamber. A field study was conducted in North Carolina at three locations naturally infested with A. palmeri to determine vegetative, reproductive, and germination fitness of plants with and without EPSPS amplification grown season-long with cotton (Gossypium hirsutum L.). Seed number was not correlated with EPSPS copy number. However, when plants were binned into two groups, those having an EPSPS copy number ≥2 (relative to reference genes) and those having an EPSPS copy number &lt;2, plant fresh weight and seed number were 1.4 and 1.6 times greater, respectively, for plants with fewer than 2 EPSPS copies. Amaranthus palmeri height and seed germination, and yield of cotton, did not differ when comparing the two binned groups. These data suggest that A. palmeri plants with EPSPS amplification are relatively less fit in the absence of glyphosate, but this reduced fitness does not translate into differences in interference with cotton.

4-Hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides: past, present, and future

AbstractThe herbicides that inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) are primarily used for weed control in corn, barley, oat, rice, sorghum, sugarcane, and wheat production fields in the United States. The objectives of this review were to summarize 1) the history of HPPD-inhibitor herbicides and their use in the United States; 2) HPPD-inhibitor resistant weeds, their mechanism of resistance, and management; 3) interaction of HPPD-inhibitor herbicides with other herbicides; and 4) the future of HPPD-inhibitor-resistant crops. As of 2022, three broadleaf weeds (Palmer amaranth, waterhemp, and wild radish) have evolved resistance to the HPPD inhibitor. The predominance of metabolic resistance to HPPD inhibitor was found in aforementioned three weed species. Management of HPPD-inhibitor-resistant weeds can be accomplished using alternate herbicides such as glyphosate, glufosinate, 2,4-D, or dicamba; however, metabolic resistance poses a serious challenge, because the weeds may be cross-resistant to other herbicide sites of action, leading to limited herbicide options. An HPPD-inhibitor herbicide is commonly applied with a photosystem II (PS II) inhibitor to increase efficacy and weed control spectrum. The synergism with an HPPD inhibitor arises from depletion of plastoquinones, which allows increased binding of a PS II inhibitor to the D1 protein. New HPPD inhibitors from the azole carboxamides class are in development and expected to be available in the near future. HPPD-inhibitor-resistant crops have been developed through overexpression of a resistant bacterial HPPD enzyme in plants and the overexpression of transgenes for HPPD and a microbial gene that enhances the production of the HPPD substrate. Isoxaflutole-resistant soybean is commercially available, and it is expected that soybean resistant to other HPPD inhibitor herbicides such as mesotrione, stacked with resistance to other herbicides, will be available in the near future.

Soil Nematode Trophic Group Composition and Influence on Growth of Amaranthus palmeri and Parthenium hysterophorus

Weeds have a significant impact on agricultural systems. They not only cause a loss in crop yield by competing with them for resources, but they can also serve hosts for several pests and parasties such as plant parasitic nematodes casusing additional crop loss. The aim of this study was to analyze plant-nematode feedback in two major weeds, Amaranthus palmeri S Watson and Parthenium hysterophorus L. First, a field survey was conducted to determine the rhizosphere nematode trophic groups associated with these two plants in the summer of 2020 and 2021. Then a 6-week greenhouse study was conducted where the two weed species were treated with nematode communities extracted from their respective rhizospheres. Results from this study show that both weeds harbored a high number of herbivore nematodes, followed by fungivore and bacterivore nematodes. Total number of these nematodes were highly influenced by total sol carbon, pH and salinity. Under greenhouse conditions, the nematode treatment did not have any impact on the growth of P. hysterophorus but A. palmeri plants treated with nematodes had significantly higher above ground biomass. In conclusion, plant-nematode relationships are complex. Given the extent of direct damage caused by these weeds and plant parasitic nematodes in global crop production, the weeds-nematode feedback warrants further detailed studies.

Transpiration response of palmer amaranth (Amaranthus palmeri) to drying soil in greenhouse conditions

Palmer amaranth (Amaranthus palmeri S. Watson) is a troublesome weed that can result in substantial crop yield loss in irrigated and rainfed agricultural systems of the U.S. Great Plains. Knowledge of the transpiration response to soil drying is necessary to better understand the competitive nature of Palmer amaranth against crops in limiting soil moisture conditions. The objective was to quantify the soil matric potential at which the transpiration rate of Palmer amaranth starts to decline during soil drying conditions. A greenhouse experiment consisting of six replications of well-watered and stressed plants growing in columns of a packed silt loam soil on automated load cells was established in May 2021 in Manhattan, KS. Hourly transpiration rates were computed as the difference of two consecutive mass readings. Stomatal conductance was measured every other day on the topmost fully developed leaf using a leaf porometer. Soil matric potential was estimated using a measured soil water retention curve. The stomatal conductance of plants undergoing soil drying conditions started to decrease at a soil matric potential of -120 kPa and the plant-level transpiration rate started to decrease at -176 kPa. Palmer amaranth exhibited high (246 g h-1) day-time transpiration rates when soil water was available at low tension levels (10 to 33 kPa). In advanced stages of development, well-watered plants also exhibited nighttime transpiration that accounted for ~10% of the day-time transpiration total. Mean transpiration use efficiency of well-watered and stressed plants considering total dry biomass was similar (P = 0.247), with values of 10.9 and 11.6 g kg-1, respectively. Overall, this study substantiates the competitive nature of Palmer amaranth for soil water in sub-humid and semi-arid environments and highlights the importance of early control of Palmer amaranth to minimize competition for soil moisture.

Prospective Alternate Hosts of an Emerging Polerovirus in Cotton Landscapes in the Southeastern United States.

The identification of alternate hosts that can act as virus inoculum sources and vector reservoirs in the landscape is critical to understanding virus epidemics. Cotton leafroll dwarf virus (CLRDV) is a serious pathogen in cotton production and is transmitted by the cotton/melon aphid, Aphis gossypii, in a persistent, circulative, and non-propagative manner. CLRDV was first reported in the United States in Alabama in 2017, and thereafter in several cotton-producing states. CLRDV has since established itself in the southeastern United States. The role of alternate hosts in CLRDV establishment is not clear. Fourteen common plant species in the landscape, including crops, weeds, and ornamentals (cotton, hollyhock, marshmallow, country mallow, abutilon, arrowleaf sida, okra, hibiscus, squash, chickpea, evening primrose, henbit, Palmer amaranth, and prickly sida) were tested as potential alternate hosts of CLRDV along with an experimental host (Nicotiana benthamiana) via aphid-mediated transmission assays. CLRDV was detected following inoculation in hibiscus, okra, N. benthamiana, Palmer amaranth, and prickly sida by RT-PCR, but not in the others. CLRDV accumulation determined by RT-qPCR was the highest in N. benthamiana compared with cotton and other hosts. However, aphids feeding on CLRDV-infected prickly sida, hibiscus, and okra alone were able to acquire CLRDV and back-transmit it to non-infected cotton seedlings. Additionally, some of the alternate CLRDV hosts supported aphid development on par with cotton. However, in a few instances, aphid fitness was reduced when compared with cotton. Overall, this study demonstrated that plant hosts in the agricultural landscape can serve as CLRDV inoculum sources and as aphid reservoirs and could possibly play a role in the reoccurring epidemics of CLRDV in the southeastern United States.