The first case of evolved protoporphyrinogen oxidase (PPO)-inhibitor resistance was observed in 2001 in common waterhemp [Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif]. This resistance in A. tuberculatus is most commonly conferred by deletion of the amino acid glycine at the 210th position (ΔGly-210) of the PPO enzyme (PPO2) encoded by PPX2. In a field in Kentucky in 2015, inadequate control of Amaranthus plants was observed following application of a PPO inhibitor. Morphological observations indicated that survivors included both A. tuberculatus and Palmer amaranth (Amaranthus palmeri S. Watson). Research was conducted to confirm species identities and resistance and then to determine whether resistance evolved independently in the two species or via hybridization. Results from a quantitative PCR assay based on the ribosomal internal transcribed spacer confirmed that both A. tuberculatus and A. palmeri coexisted in the field. The mutation conferring ΔGly-210 in PPO2 was identified in both species; phylogenetic analysis of a region of PPX2, however, indicated that the mutation evolved independently in the two species. Genotyping of greenhouse-grown plants that survived lactofen indicated that all A. tuberculatus survivors, but only a third of A. palmeri survivors, contained the ΔGly-210 mutation. Consequently, A. palmeri plants were evaluated for the presence of an arginine to glycine or methionine substitution at position 128 of PPO2 (Arg-128-Gly and Arg-128-Met). The Arg-128-Gly substitution was found to account for resistance that was not accounted for by the ΔGly-210 mutation in plants from the A. palmeri population. Results from this study provide a modern-day example of both parallel and convergent evolution occurring within a single field.

Susceptibility of a system to colonization by a weed is in part a function of environmental resource availability. Doveweed [Murdannia nudiflora (L.) Brenan] can establish in a variety of environments; however, it is found mostly in wet or low-lying areas with reduced interspecies competition. Four studies evaluated the effect of mowing height, interspecies competition, and nitrogen, light, and soil moisture availability on M. nudiflora establishment and growth. A field study evaluated the effect of mowing height on M. nudiflora establishment. In comparison with unmowed plots, mowing at 2 and 4 cm reduced spread 46% and 30%, respectively, at 9 wk after planting. Effect of mowing height and nitrogen fertilization on ‘Tifway’ bermudagrass (Cynodon dactylon Burtt-Davy×C. transvaalensis L. Pers.) and M. nudiflora interspecies competition was evaluated in a greenhouse trial. Murdannia nudiflora coverage was 62% greater in flats maintained at 2.6 cm than flats maintained at 1.3 cm. Supplemental application of 49 kg N ha−1 mo−1 increased M. nudiflora coverage 75% in comparison with 24.5 kg N ha−1 mo−1. A difference in M. nudiflora coverage could not be detected between flats receiving 0 and 24.5 kg N ha−1 mo−1, suggesting moderate nitrogen fertilization does not encourage M. nudiflora colonization. Effect of light availability on M. nudiflora growth and development was evaluated in a greenhouse study. Growth in a 30%, 50%, or 70% reduced light environment (RLE) did not affect shoot growth on a dry weight basis in comparison with plants grown under full irradiance; however, internode length was 28% longer in a 30% RLE and 39% longer in a 50% and 70% RLE. Effect of soil moisture on M. nudiflora growth and development was evaluated in a greenhouse study. Plants maintained at 50%, 75%, and 100% field capacity (FC) increased biomass>200% compared with plants maintained at 12.5% or 25% FC.

Field and greenhouse experiments were conducted from 2013 to 2015 at the University of Wyoming to evaluate the response of Beta vulgaris (L.) to reflected-light quality. Large-pail field studies included a factorial arrangement of three varieties of B. vulgaris (sugar beet, table beet, and Swiss chard) and reflected-light treatments (using either colored plastic mulch, grass, or bare-soil controls). Greenhouse studies included sugar beet as influenced by either grass or soil surroundings. In all studies, grass was grown in separate containers from B. vulgaris, so there was no root interaction. Grass was clipped regularly to prevent shading and competition for sunlight. Reflected light from different-colored plastic mulches (red, blue, green, black, clear) did not affect B. vulgaris growth. However, reflected light from the grass reduced the number of leaves in all B. vulgaris varieties such that there were 10 to 14 fewer leaves in B. vulgaris surrounded by grass compared with the soil treatment at 90 d after planting in the field study. Shade avoidance cues from surrounding grass reduced B. vulgaris total leaf area by 49% to 66%, leaf biomass by 21% to 30%, and root biomass by 70% to 72%. Similar results were observed in greenhouse experiments, where the grass treatment reduced sugar beet leaf biomass by 48% to 57% and root biomass by 35% to 64%. Shade avoidance cues have the potential to significantly reduce B. vulgaris yield, even in the absence of direct resource competition from weeds.

Information about weed biology and weed population dynamics is critical for the development of efficient weed management programs. A field experiment was conducted in Fayetteville, AR, during 2014 and 2015 to examine the effects of Palmer amaranth (Amaranthus palmeri S. Watson) establishment time in relation to soybean [Glycine max (L.) Merr.] emergence and the effects of A. palmeri distance from the soybean row on the weed’s height, biomass, seed production, and flowering time and on soybean yield. The establishment time factor, in weeks after crop emergence (WAE), was composed of six treatment levels (0, 1, 2, 4, 6, and 8 WAE), whereas the distance from the crop consisted of three treatment levels (0, 24, and 48 cm). Differences in A. palmeri biomass and seed production averaged across distance from the crop were found at 0 and 1 WAE in both years. Establishment time had a significant effect on A. palmeri seed production through greater biomass production and height increases at earlier dates. Amaranthus palmeri that was established with the crop (0 WAE) overtopped soybean at about 7 and 10 WAE in 2014 and 2015, respectively. Distance from the crop affected A. palmeri height, biomass, and seed production. The greater the distance from the crop, the higher A. palmeri height, biomass, and seed production at 0 and 1 WAE compared with other dates (i.e., 2, 4, 6, and 8 WAE). Amaranthus palmeri establishment time had a significant impact on soybean yield, but distance from the crop did not. The earlier A. palmeri interfered with soybean (0 and 1 WAE), the greater the crop yield reduction; after that period no significant yield reductions were recorded compared with the rest of the weed establishment times. Knowledge of A. palmeri response, especially at early stages of its life cycle, is important for designing efficient weed management strategies and cropping systems that can enhance crop competitiveness. Control of A. palmeri within the first week after crop emergence or reduced distance between crop and weed are important factors for an effective implementation of weed management measures against A. palmeri and reduced soybean yield losses due to weed interference.

Weed interference during crop establishment is a serious concern for Florida strawberry [Fragaria×ananassa (Weston) Duchesne ex Rozier (pro sp.) [chiloensis×virginiana]] producers. In situ remote detection for precision herbicide application reduces both the risk of crop injury and herbicide inputs. Carolina geranium (Geranium carolinianum L.) is a widespread broadleaf weed within Florida strawberry production with sensitivity to clopyralid, the only available POST broadleaf herbicide. Geranium carolinianum leaf structure is distinct from that of the strawberry plant, which makes it an ideal candidate for pattern recognition in digital images via convolutional neural networks (CNNs). The study objective was to assess the precision of three CNNs in detecting G. carolinianum. Images of G. carolinianum growing in competition with strawberry were gathered at four sites in Hillsborough County, FL. Three CNNs were compared, including object detection–based DetectNet, image classification–based VGGNet, and GoogLeNet. Two DetectNet networks were trained to detect either leaves or canopies of G. carolinianum. Image classification using GoogLeNet and VGGNet was largely unsuccessful during validation with whole images (Fscore<0.02). CNN training using cropped images increased G. carolinianum detection during validation for VGGNet (Fscore=0.77) and GoogLeNet (Fscore=0.62). The G. carolinianum leaf–trained DetectNet achieved the highest Fscore (0.94) for plant detection during validation. Leaf-based detection led to more consistent detection of G. carolinianum within the strawberry canopy and reduced recall-related errors encountered in canopy-based training. The smaller target of leaf-based DetectNet did increase false positives, but such errors can be overcome with additional training images for network desensitization training. DetectNet was the most viable CNN tested for image-based remote sensing of G. carolinianum in competition with strawberry. Future research will identify the optimal approach for in situ detection and integrate the detection technology with a precision sprayer.

An annual bluegrass (Poa annua L.) biotype with limited susceptibility to POST flumioxazin applications was identified in Georgia. The objectives of this research were to quantify tolerance levels of this biotype (R-biotype) to protoporphyrinogen oxidase (PPO) inhibitors and characterize physiological responses to flumioxazin. In dose–response experiments on 3- to 5-tiller plants, flumioxazin and sulfentrazone rates required to reduce dry-shoot biomass 50% from the nontreated were >14.5 and 10.4 times greater for the R-biotype, as compared with a susceptible (S)-biotype, respectively. Establishment of the R-biotype from seed was completely controlled by PRE applications of flumioxazin and oxadiazon, similar to the S-biotype. Tank mixtures of chlorpyrifos with flumioxazin did not enhance biomass reductions of the R-biotype, suggesting that tolerance levels may not be related to cytochrome P450–associated metabolism. In laboratory experiments, the R-biotype averaged 27% less electrolyte leakage, as compared with the S-biotype, after flumioxazin treatments. Lipid peroxidation in the R-biotype, as measured by malondialdehyde levels, averaged 25% less than the S-biotype at 72 h after broadcast flumioxazin treatments at 280 and 560 g ha−1. The tolerance to POST applications of PPO inhibitors in this P. annua biotype is associated with less lipid peroxidation and electrolyte leakage as compared with the S-biotype. These biochemical differences in biotypes may contribute to erratic levels of POST control from flumioxazin and could contribute to PPO-inhibitor resistance.

In the rapid response (RR) biotype of glyphosate-resistant (GR) giant ragweed (Ambrosia trifida L.), exposure to glyphosate elicits H2O2 production in mature leaves, resulting in foliage loss and reduced glyphosate translocation. When glyphosate is applied with POST herbicides intended to improve control of A. trifida, the RR to glyphosate has the propensity to antagonize these herbicide combinations. This research documents how transient changes in air temperature, soil moisture, and light intensity during a 6-d period surrounding herbicide application regulate induction of the RR and the effect on POST herbicide interactions with glyphosate. Air temperature had the greatest influence on H2O2 accumulation in leaf disks following treatment with glyphosate, as plants at 30 C produced more than twice the amount of H2O2 at 2.5 h after treatment compared with 10 C. Plants under field capacity conditions accumulated nearly 50% more H2O2 than those at one-third field capacity, while those under no shade had only 18% more H2O2 compared with those in a shaded environment. Despite these initial results, dry weight reduction at 21 d after treatment never differed by more than 8% between levels of environmental factors, thus indicating a negligible influence on glyphosate efficacy. The magnitude of glyphosate-induced antagonism was generally greater at 30 C (12% to 21% less than expected control) versus 10 C (11% to 16%) on atrazine, cloransulam, dicamba, and topramezone and was greater at field capacity (20% to 24%) versus one-third field capacity (11% to 15%) on cloransulam and topramezone. These results indicate air temperatures and soil moisture levels conducive to optimal plant growth accelerate the RR to glyphosate, thereby increasing the likelihood of glyphosate-induced antagonism on several translocated herbicides.