Plant defence traits, such as herbicide resistance mutations, may incur a fitness cost to plants that become evident when the trait is not needed. However, individuals with multiple herbicide resistance traits may decrease fitness beyond that of plants with a single herbicide resistance mutation. Multiple herbicide-resistant (MHR) Amaranthus tuberculatus populations are becoming more prevalent in Midwest United States agroecosystems. The objective was to determine whether selected MHR A. tuberculatus populations express differential development when grown in a herbicide-free environment. The hypothesis was that MHR A. tuberculatus populations become increasingly less fit when additional herbicide resistances evolve. Multiple herbicide-resistant and herbicide-susceptible A. tuberculatus populations were grown in a herbicidefree field for 20 weeks for two seasons. Differences (P < 0.001) in apical growth were detected 5 and 7 weeks after transplanting for all populations in 2016 and 2017 respectively. Gender and population influenced (P < 0.001) flowering date, with males flowering up to 1.5 weeks earlier than females, but did not cause pollination asynchrony. Shoot biomass was not different (P = 0.84) across A. tuberculatus populations, but there were differences (P < 0.001) for gender and year. Seed production was different amongst A. tuberculatus populations (P = 0.001), but was not influenced by the number of MHR traits. Conversely, a negative quadratic relationship between seed mass and the number of MHR traits was observed (r 2 = 0.32; P < 0.001). The experiment results demonstrate that MHR in A. tuberculatus populations is not incurring a fitness penalty that will remove the populations in the immediate future.

Complaints of control failures with acetolactate synthase (ALS)-inhibiting and protoporphyrinogen oxidase (PPO)-inhibiting herbicides on redroot pigweed (Amaranthus retroflexus L.) were reported in conventional soybean [Glycine max (L.) Merr.] fields in North Carolina. Greenhouse dose-response assays confirmed that the Camden and Pasquotank County populations were less sensitive to ALS and PPO-inhibiting herbicides compared to susceptible A. retroflexus populations, suggesting the evolution of resistance to these herbicides. Sanger sequencing of target genes determined the Camden County population carried a Trp574Leu mutation in the ALS gene and an Arg98Gly mutation in the PPX2 gene, while the Pasquotank County population carried a His197Pro mutation in the ALS gene (first documentation of the mutation in the Amaranthus genus) but no mutation was detected in the PPX2 gene. Single nucleotide polymorphism (SNP) genotyping assays were developed to enable efficient screening of future control failures in order to limit the spread of these herbicide-resistant populations. In addition, preliminary testing of these assays revealed the three mutations were ubiquitous in the respective populations. These two populations represent the first confirmed cases of PPO-inhibiting herbicide-resistant A. retroflexus in the United States; as well as the first confirmed cases of this particular herbicide resistance profile in A. retroflexus inhabiting North America. While no mutation was found in the PPX2 gene of the Pasquotank County population, we suggest that this population has evolved resistance to PPO-inhibiting herbicides but the mechanism of resistance is to be determined.

Greenhouse studies were conducted to evaluate the effects of soil organic matter content and soil pH on initial and residual weed control with flumioxazin by planting selected weed species in various lab-made and field soils. Initial control was determined by planting weed seeds into various lab-made and field soils treated with flumioxazin (71 g ha−1). Seeds of Echinochloa crus-galli (barnyard grass), Setaria faberi (giant foxtail), Amaranthus retroflexus (redroot pigweed), and Abutilon theophrasti (velvetleaf) were incorporated into the top 1.3 cm of each soil at a density of 100 seeds per pot, respectively. Emerged plants were counted and removed in both treated and non-treated pots two weeks after planting and each following week for six weeks. Flumioxazin control was evaluated by calculating percent emergence of weeds in treated soils compared to the emergence of weeds in non-treated soils. Clay content was not found to affect initial flumioxazin control of any tested weed species. Control of A. theophrasti, E. crus-galli, and S. faberi was reduced as soil organic matter content increased. The control of A. retroflexus was not affected by organic matter. Soil pH below 6 reduced flumioxazin control of A. theophrasti, and S. faberi but did not affect the control of A. retroflexus and E. crus-galli. Flumioxazin residual control was determined by planting selected weed species in various lab-made and field soils 0, 2, 4, 6, and 8 weeks after treatment. Eight weeks after treatment, flumioxazin gave 0% control of A. theophrasti and S. faberi in all soils tested. Control of A. retroflexus and Chenopodium album (common lambsquarters) was 100% for the duration of the experiment, except when soil organic matter content was greater than 3% or the soil pH 7. Eight weeks after treatment, 0% control was only observed for common A. retroflexus and C. album in organic soil (soil organic matter > 80%) or when soil pH was above 7. Control of A. theophrasti and S. faberi decreased as soil organic matter content and soil pH increased. Similar results were observed when comparing lab-made soils to field soils; however, differences in control were observed between lab-made organic matter soils and field organic matter soils. Results indicate that flumioxazin can provide control ranging from 75–100% for two to six weeks on common weed species.

Glufosinate is among the few remaining effective herbicides for postemergence weed control in North Carolina crops. The evolution of glufosinate resistance in key weeds is currently not widespread in North Carolina but to better assess the current status of glufosinate effectiveness, surveys were distributed at extension meetings in 2019 and 2020. The surveys were designed to provide information about North Carolina farmers’ perception of and use of glufosinate. Many North Carolina farmers (≥26%) apply glufosinate at the correct timing (5-10 cm weeds). In addition to applying glufosinate at the correct timing, North Carolina farmers (≥22%) are applying glufosinate as a complementary herbicide to other efficacious herbicides and to control herbicide-resistant weeds, suggesting glufosinate is part of a diverse chemical weed management plan. Conversely, survey findings indicated some North Carolina farmers (13 to 17%) rely exclusively on glufosinate for weed control. Additionally, North Carolina farmers (28 to 30%) reported glufosinate control failures and control failures were observed on several weed species in corn, cotton, and soybean. The results of the survey suggest that most North Carolina farmers are currently stewarding glufosinate but also support the need for extension personnel to keep educating farmers on how to correctly use glufosinate to delay the evolution of glufosinate-resistant weeds. Yearly surveys should be distributed to monitor where glufosinate control failures occur and the weed species not being controlled.

No abstract

Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] is one of the most challenging weeds for winter wheat (Triticum aestivum L.) growers to manage. Italian ryegrass has evolved resistance to the majority of the herbicides labeled for use in wheat and the competitive ability of the species makes it a significant factor driving winter wheat production practices around the world. Previous research has utilized remotely sensed spectral imagery to detect Italian ryegrass in winter wheat to aid weed control decisions. Two studies from 2016 to 2017 were initiated with the intent of identifying the spectral reflectance properties of Italian ryegrass and winter wheat using an unmanned aerial vehicle (UAV) equipped with a 5-band multispectral sensor. Image analysis was conducted to determine the potential for species discrimination throughout the growing season. Supervised classification of the imagery was used to evaluate the ability of the UAV platform for further discrimination between Italian ryegrass and winter wheat. Species differentiation proved to be possible, however the data was not able to be referenced across dates. Due to light variability, the reflectance values changed to such a degree that unsupervised classifications were not possible using a database of values from previous flights. Supervised classification of the multispectral image resulted in &gt;70% classification accuracy between the species. However, near infrared light consistently differed enough for accurate classification between Italian ryegrass and winter wheat across different weed densities, flight altitudes, and imaging dates. On a single field basis, species differentiation was successful and resulted in classified maps of Italian ryegrass and winter wheat. This study also analyzed the exact accuracy of the species differentiation based on the quality and uniformity of light conditions and growth stage of plants.

Palmer amaranth (Amaranthus palmeri S. Watson) is a difficult weed to manage due to competitive growth, fecundity, and evolved herbicide resistance. Limited information exist on the fecundity of vegetative stage Palmer amaranth surviving glufosinate applied at different timings. In addition, research has not investigated the germination or glufosinate susceptibility of the offspring from these surviving plants. Field experiments were conducted across three locations in 2019 to determine (a) the fecundity of Palmer amaranth plants surviving glufosinate applied at different vegetative growth stages, and (b) if the offspring from the surviving plants exhibited differential germination and susceptibility to glufosinate. Palmer amaranth was treated at three different vegetative sizes (5 cm, 7-10 cm, > 10 cm) and orthogonal combinations of these application timings. The application timings corresponded to early-, mid-and late-postemergence applications. Palmer amaranth plants surviving the mid-, late-, and the mid postemergence followed by late postemergence glufosinate application were fecund. Palmer amaranth plants surviving the mid postemergence followed by late postemergence application produced less seed than the plants surviving the mid postemergence and late postemergence application. Palmer amaranth was controlled with other glufosinate applications resulting in no seed production. Germination was affected across location and glufosinate treatments, but no clear trend/pattern was observed. The offspring from Palmer amaranth plants surviving glufosinate applications were controlled by all glufosinate rates tested. These experiments provide evidence that Palmer amaranth surviving glufosinate in the vegetative stages may remain fecund, but fecundity can vary with application timing. No measurable effect on the offspring germination or susceptibility to glufosinate was apparent.

Field studies were conducted in 2016 and 2017 to determine if multispectral imagery collected from an unmanned aerial vehicle (UAV) equipped with a five-band sensor could successfully identify Palmer amaranth (Amaranthus palmeri) infestations of various densities growing among soybeans (Glycine max [L.] Merr.). The multispectral sensor captures imagery from five wavebands: 475 (blue), 560 (green), 668 (red), 840 (near infrared [NIR]), and 717 nm (red-edge). Image analysis was performed to examine the spectral properties of discrete Palmer amaranth and soybean plants at various weed densities using these wavebands. Additionally, imagery was subjected to supervised classification to evaluate the usefulness of classification as a tool to differentiate the two species in a field setting. Date was a significant factor influencing the spectral reflectance values of the Palmer amaranth densities. The effects of altitude on reflectance were less clear and were dependent on band and density being evaluated. The near infrared (NIR) waveband offered the best resolution in separating Palmer amaranth densities. Spectral separability in the other wavebands was less defined, although low weed densities were consistently able to be discriminated from high densities. Palmer amaranth and soybean were found to be spectrally distinct regardless of imaging date, weed density, or waveband. Soybean exhibited overall lower reflectance intensity than Palmer amaranth across all wavebands. The reflectance of both species within blue, green, red, and red-edge wavebands declined as the season progressed, while reflectance in NIR increased. Near infrared and red-edge wavebands were shown to be the most useful for species discrimination and maintained their utility at most weed densities. Palmer amaranth weed densities were found to be spectrally distinct from one another in all wavebands, with greatest distinction when using the red, NIR and red-edge wavebands. Supervised classification in a two-class system was consistently able to discriminate between Palmer amaranth and soybean with at least 80% overall accuracy. The incorporation of a weed density component into these classifications introduced an error of 65% or greater into these classifications. Reducing the number of classes in a supervised classification system could improve the accuracy of discriminating between Palmer amaranth and soybean.