Successful weed management, particularly use of chemical control, is very important for commercial lettuce production on organic soils in the Everglades Agricultural Area in south Florida. Field experiments were conducted in 2016 and 2017 to determine the efficacy of herbicides (pronamide, bensulide, imazethapyr, or oxyfluorfen) applied preemergence (PRE) either alone or followed by a postemergence (POST) application of imazethapyr for weed control and lettuce (romaine and iceberg) yield. Preemergence-applied oxyfluorfen (0.56 kg ha−1) resulted in significant lettuce injury, including stand loss, while PRE applications of pronamide (4.44 kg ha−1), bensulide (5.6 and 10.1 kg ha−1), or imazethapyr (0.035 g ha−1) resulted in transient lettuce injury and no significant stand loss. Similarly, PRE-applied pronamide, bensulide, and imazethapyr followed by POST-applied imazethapyr did not result in significant lettuce stand loss or injury. When contrasted as a group, PRE-applied herbicides followed by a POST application of imazethapyr provided better spiny amaranth and common lambsquarters control compared with PRE-applied herbicides or POST-applied imazethapyr-only treatments. Lettuce yield was highest with PRE herbicides followed by POST imazethapyr compared with PRE herbicides or POST-applied imazethapyr-only treatments, indicating a yield benefit of having a PRE followed by POST herbicide weed control program in lettuce grown on organic soils. However, oxyfluorfen is not an option for lettuce on organic soils because of unacceptable stand reduction and crop injury. Whether to apply pronamide, bensulide, or imazethapyr PRE followed by a POST application of imazethapyr for broadleaf weed control in lettuce on organic soils depends on the species present, cost, and ease of application.

Increasing weed control costs and limited herbicide options threaten vegetable crop profitability. Traditional interrow mechanical cultivation is very effective at removing weeds between crop rows. However, weed control within the crop rows is necessary to establish the crop and prevent yield loss. Currently, many vegetable crops require hand weeding to remove weeds within the row that remain after traditional cultivation and herbicide use. Intelligent cultivators have come into commercial use to remove intrarow weeds and reduce cost of hand weeding. Intelligent cultivators currently on the market such as the Robovator, use pattern recognition to detect the crop row. These cultivators do not differentiate crops and weeds and do not work well among high weed populations. One approach to differentiate weeds is to place a machine-detectable mark or signal on the crop (i.e., the crop has the mark and the weed does not), thereby facilitating weed/crop differentiation. Lettuce and tomato plants were marked with labels and topical markers, then cultivated with an intelligent cultivator programmed to identify the markers. Results from field trials in marked tomato and lettuce found that the intelligent cultivator removed 90% more weeds from tomato and 66% more weeds from lettuce than standard cultivators without reducing yields. Accurate crop and weed differentiation described here resulted in a 45% to 48% reduction in hand-weeding time per hectare.

Weed management is a major constraint in organic cropping systems. In 2004, the Cornell Organic Vegetable Cropping Systems Experiment was established in central New York state using a split-plot randomized complete block design with two crop rotation entry points (split-plot factor). Four organic vegetable cropping systems that varied in cropping intensity and tillage (main plot factor) were compared: (1) intensive, (2) intermediate, (3) bio-extensive, and (4) ridge tillage. The basic crop rotation was cabbage, lettuce, potato, and winter squash, with additional short-season crops in the intensive system and with cover crops and fallow substituted for cabbage and potato in the bio-extensive system. In 2014, two uniformity trials were conducted in which oat and then a mixture of sorghum-sudangrass plus Japanese millet were grown uniformly over the entire experiment. Prior to sowing oat, soil samples were collected from each plot and an emergence bioassay was conducted to assess the soil weed seedbank. Crop biomass, weed density, and weed biomass were sampled in the uniformity crops. Soil weed seedbank density was three to four times greater in the intensive, intermediate, and ridge-tillage systems than in the bio-extensive system. The bio-extensive system also had lower weed density and weed biomass in the oat uniformity trial compared with the other three systems. Oat biomass did not differ between the cropping systems. Weed density and biomass in oat were also affected by the crop rotation entry point. Cropping system legacy effects on weed abundance and community composition were greater in the oat than in the sorghum-sudangrass plus Japanese millet uniformity trial. Our results illustrate the effects of different organic vegetable production practices on weed community structure and highlight the value of tilled fallow periods, cover crops, and prevention of weed seed rain for reducing weed populations.

Greenhouse studies were conducted to determine the effect of phosphorus (P) fertility on intra- and interspecific competition between lettuce and spiny amaranth for 4 wk after emergence. Total lettuce shoot biomass per pot and weight per plant increased 39 and 44% in response to increased P fertility, respectively. P fertility had no impact on growth of spiny amaranth. Total shoot biomass of spiny amaranth increased with increasing density from four to eight plants, however, lettuce did not. Total shoot biomass of both species increased as density increased from 4 to 16 plants. Spiny amaranth, but not lettuce, weight per plant decreased in response to intraspecific competition. Reciprocal yield analysis showed that spiny amaranth produced 2.4 times more biomass than lettuce when competing intraspecifically and four times more biomass under interspecific competition. Lettuce weight per plant was not affected. Relative yield analysis indicated that spiny amaranth was more competitive than lettuce regardless of P fertility. However, increased P fertility increased competitiveness of lettuce. Relative crowding coefficients indicated that spiny amaranth at the low density with low P fertility was 33 times more competitive than lettuce. Addition of P caused lettuce and spiny amaranth to be equally competitive at the lowest density; however, at the highest density, spiny amaranth was 4 tunes more competitive than lettuce regardless of additional P.

A modified nutrient solution, used to study lettuce cultivar response to thiobencarb, provided an excellent medium for lettuce growth and is an alternative to other nutrient solutions. Leaf dry weight was inhibited 50% by 3- and 5-μM thiobencarb in ‘Dark Green Boston’ and ‘Great Lakes 366’, respectively, while ‘Bibb’, ‘Slobolt’ and ‘Valmaine’ responded intermediately. Growth inhibitions and herbicide injury symptoms, which included abnormal growth and stunted secondary leaves, were similar in both nutrient solution and soil assays. Due to its ease of preparation and elimination of the variability associated with soil assays, the nutrient solution may be considered a superior alternative to soil assays.

Field studies were conducted in 1991 and 1992 at the Faculty Farm of the U.A.E. Univ., Al-Ain, to evaluate the effectiveness of soil solarization for weed management in naturally infested plots of lettuce, and on subsequent planting of garlic. Solarization for 2 mo during the hottest summer period effectively suppressed weeds during the growing season of head lettuce. In subsequently-planted garlic, no significant effect was observed between solarized and nonsolarized treatments.

Replacement series studies were conducted under controlled conditions to determine the effect of phosphorus (P) rates and population densities on the competitiveness of smooth pigweed and common purslane with lettuce. Densities were 2, 4, and 8 plants per 113 cm2, whereas P rates were 0, 0.4, and 0.8 g PL−1 soil. A P-deficient Histosol (0.3 mg water-extractable P L−1 soil) was used. High P fertility enhanced the competitive ability of lettuce in smooth pigweed-lettuce mixtures. Smooth pigweed was not responsive to P rates. However, luxurious P consumption by smooth pigweed occurred, reducing the amount of the nutrient available for lettuce absorption. In common purslane-lettuce mixtures, the weed was responsive to P rates, increasing its competitive ability, whereas no increase in lettuce competitive ability was observed. Both weed species were more competitive than lettuce. Competition for P appears to be the main mechanism of common purslane interference on lettuce grown in low-P organic soils. Alternative fertilization strategies (i.e., banded applications) may reduce the effect of smooth pigweed on lettuce.

Laboratory and greenhouse studies were conducted to determine the mobility and bioactivity of thiobencarb on a Pahokee muck, Everglades muck, and Immokalee sand. On all soils > 93% of thiobencarb remained in the upper 1 cm after leaching during unsaturated flow. During saturated flow the retardation factor (RT) of thiobencarb on an Immokalee sand was 68, 20, 2, and 1 with 0, 25, 50, and 75% by volume methanol in 0.01 N CaCl2, respectively. There was a significant log-linear relationship (r2 = 0.97) between the Freundlich adsorption values (Kf) estimated from RT values and the percentage by volume methanol of the leachate. In general, thiobencarb in combination with overhead irrigation reduced lettuce vigor and plant dry weight on an Immokalee sand more than with subsurface irrigation.

Imazethapyr was evaluated PRE and POST in five lettuce types and chicory under Florida field conditions. The relative sensitivity of leafy crop vigor (most sensitive to most tolerant) to imazethapyr PRE, based on 20% inhibition determined using regression analysis, was as follows: Boston > bibb > crisphead > romaine > leaf > escarole > endive. Leafy crop injury increased as the rate of imazethapyr applied POST increased, with all leafy crops responding in a similar manner. Surfactant addition increased imazethapyr phytotoxicity. Imazethapyr PRE treatments at 0.067 kg ai/ha provided greater than 80% control of livid amaranth, common purslane, flatsedge, and common lambsquarters. Imazethapyr POST at 0.067 kg/ha, with surfactant provided control greater than 85% of all weed species. Greater than 85% spiny amaranth control was provided by imazethapyr POST at 0.017 kg/ha. Use of surfactant with imazethapyr did not improve spiny amaranth control over imazethapyr with no surfactant. POST treatments did not decrease leafy crop yield compared with the hand-weeded check. Imazethapyr applied PRE reduced crop yield compared to the POST treatments and the hand-weeded control.

Chloramben effectively controlled weeds in lettuce and endive grown on muck soils better than thiobencarb and chlorpropham. Chloramben at 1.1 kg ai/ha controlled two weeds economically important on muck soils, livid amaranth and oakleaf goosefoot. Chloramben applied preemergence at 1.1 kg/ha and higher caused early stunting of lettuce which the lettuce outgrew by harvest. Field-seeded endive was stunted severely at these same rates, severely reducing yield. Transplants improved tolerance of all crops. Effective use rates on muck soils were 25% to 50% of those typically used on mineral soils.

The introduced exotic vines pale and black swallowwort rapidly have become invasive throughout regions of the northeastern United States and adjoining areas of Canada. Preliminary studies have reported that the species are allelopathic, possibly contributing to their competitive ability and invasiveness. Results from our laboratory assays indicated that swallowwort root exudates caused significant root length reductions (e.g., 40% for butterfly milkweed and 20% for large crabgrass) and reduced germination (e.g., 25% for lettuce) of indicator species. Additional bioassays with dried swallowwort tissues demonstrated that tissue leachates caused varied responses in indicators, with both significant stimulatory and inhibitory effects. In particular, significant congeneric interactions were noted between the two swallowwort species. Evidence from this study of swallowwort tissue phytotoxicity has important implications for developing effective management and habitat restoration strategies for the two invasive species.

Continuous planting of lettuce from December to September each year in the Salinas Valley of California requires preparation of raised fallow beds ready for planting. Application of soil residual herbicides during winter fallow periods controls weeds but must be used with caution because germinating lettuce seedlings are very sensitive to herbicide soil residues. Three herbicides were applied to fallow beds at about 90, 60, and 30 d before lettuce was seeded. Herbicides tested were flumioxazin at 71, 105, and 211 g ai ha−1, two formulations of oxyfluorfen each at 280 and 560 g ha−1, and carfentrazone at 36 g ha−1. Herbicide injury to lettuce was assessed by visual estimates, crop stand, and number and weight of marketable lettuce heads. The 71 g ha−1 rate of flumioxazin, both oxyfluorfen formulations, and carfentrazone did not injure seeded lettuce or reduce the number or weight of lettuce heads. Flumioxazin at 211 g ha−1 injured lettuce, reducing the number of marketable heads, and should not be used within 90 d of lettuce planting.

Coevolving interactions between a plant population and its microbiota can potentially yield a rhizosphere enriched in metagenomes containing the blueprints for a vast array of natural products. We describe a method of isolating those metabolites through activity-based screening of soil metagenomic libraries. The method allows for the isolation of small molecules produced in vector–host expression systems containing large-insert DNA fragments extracted from the target plant rhizospheres. Allelopathic activities derived from selected clones were screened against a series of controls. Nonmetric multidimensional scaling (NMS) showed similar effects of the set of controls on lettuce growth, whereas annual bluegrass had a broader range of growth responses. Methanol extracts from clones indicating activity showed distinct patterns in grass seedling growth from the empty vector control, but the same extracts showed no effect on lettuce. The results indicate that the metagenomics method and bioassay screen of clone extracts are tools that can be used for initial determination of allelopathic activity from noncultured soil microbiota.

Greenhouse studies were conducted to assess the intensity of smooth pigweed and common purslane aboveground interference (AI) and belowground interference (BI) with lettuce and to determine primary mechanisms of interference of each species as affected by P fertility rates. Lettuce was transplanted in mixtures with either smooth pigweed or common purslane according to four partitioning regimes: no interference, full interference, BI, and AI. Soil used was low in P for optimum lettuce yields, therefore P was added at rates of 0, 0.4, and 0.8 grams of P per liter of soil. Shoot and root biomass and plant height were measured for each species, as well as P tissue content. The data obtained indicated that smooth pigweed interfered with lettuce primarily through light interception by its taller canopy. A secondary mechanism of interference was the absorption of P from the soil through luxury consumption, increasing the P tissue content without enhancing smooth pigweed biomass accumulation. In contrast, common purslane competed aggressively with lettuce for P. Because the weed grew taller than lettuce, light interception was a secondary interference factor.

Area-of-influence experiments were conducted on high–organic matter soils to assess the effect of common lambsquarters distances from ‘South Bay’ lettuce on yield and quality under two phosphorus (P) regimens. P was applied either banded (125 kg P/ha) or broadcast (250 kg P/ha) before lettuce planting. Lettuce plants were harvested at 0, 25, 50, 75, 100, and 125 cm from each side of the common lambsquarters plant after season-long interference. The regression equations were y = 0.40 + 0.36/(1 + e−0.09(x−52.5)), r 2 = 0.92, for banded P; and y = 0.40 + 0.29/(1 + e−0.07(x−69.3)), r 2 = 0.94 for broadcast P. Banded P produced 9% more fresh weight per head than the broadcast P treatment under weed-free conditions. Within the broadcast P treatments, common lambsquarters growing at 125 and 100 cm did not affect fresh weight of harvested lettuce. Values for fresh weight per head declined 18, 34, 38, and 38% when lettuce grew at 75, 50, 25, and 0 cm distance from common lambsquarters, respectively. Fresh weight of lettuce in banded P treatments was reduced only at 50, 25, and 0 cm distance from common lambsquarters plants, representing 31, 44, and 44% less than its weed-free control, respectively. Banding P at 50% (125 kg/ha) of the recommended broadcast rate reduced the area of influence of common lambsquarters with lettuce.

The use of triclopyr for the removal of woody and broad-leaf vegetation in right-of-ways and agricultural settings has been proposed for Alaska. Triclopyr concentrations in soil after application are of concern because residual herbicide may affect growth of subsequent vegetation. In order to measure triclopyr residues in soil and determine the amount of herbicide taken up by the plant, soil bioassays were developed. Four agricultural species, turnip, lettuce, mustard, and radish, were tested to determine sensitivity to triclopyr in a 1-wk bioassay. The sensitivity (I 50) of turnip, lettuce, mustard, and radish was 0.33 ± 0.05 kg ai ha−1, 0.78 ± 0.11 kg ai ha−1, 0.78 ± 0.07 kg ai ha−1, and 0.85 ± 0.10 kg ai ha−1 (mean ± SE), respectively. Mustard was the most consistent crop in the bioassay with a midrange response to triclopyr and lowest standard deviation for germination as compared to the other species. Thus, it was used in a bioassay to determine triclopyr concentrations in a field trial. The bioassay of mustard closely matched residual amounts of triclopyr in a field trial determined by chemical extraction. Estimates of residual triclopyr concentrations using the bioassay method were sometimes less than the triclopyr concentration determined using a chemical extraction. These differences in concentrations were most evident after spring thaw when the chemical extraction determined there was enough triclopyr in the soil to reduce mustard growth over 60%, yet the bioassay measured only a 10% reduction. The chemical extraction method may have identified nonphototoxic metabolites of triclopyr to be the herbicidal triclopyr acid. These methods, when analyzed together with a dose–response curve, offer a more complete picture of triclopyr residues and the potential for carryover injury to other plant species.

Laboratory and greenhouse experiments were conducted to determine the allelopathic potential of centipedegrass. Germination and growth of indicator species were evaluated in soil leachates, leaf debris, and aqueous leaf extracts of centipedegrass. Centipedegrass soil leachates did not inhibit annual bluegrass, goosegrass, henbit, large crabgrass, or perennial ryegrass germination compared with the nonfertilized control. Incorporated centipedegrass leaf debris did not reduce lettuce germination, shoot weight, or root weight compared with the control. However, shoot and root dry weights of radish were reduced with increasing rates of centipedegrass leaf debris. Six and 9 mg debris g−1 soil reduced radish shoot weight by 49 and 64%, respectively, compared with the control. Aqueous leaf extracts of centipedegrass reduced lettuce germination; however, radicle and hypocotyl length were similar to the control. These data do not conclusively suggest centipedegrass has widespread allelopathic activity; however, significant reductions in shoot and root weight of radish with increasing centipedegrass leaf debris demonstrate a pattern of inhibition of one species against another, which fulfills a requirement of allelopathic interactions.

A sulfonylurea (SU) herbicide resistance allele discovered in prickly lettuce was previously transferred to domestic lettuce with the cultivar name ‘ID-BR1’. ID-BR1 was acquired, and the SU resistance allele was transferred through traditional breeding methods to five common commercial lettuce types: butterhead, crisphead, green leaf, red leaf, and romaine. Field trials were conducted at Salinas, CA during 2011 and 2012 to evaluate POST applications of tribenuron-methyl (tribenuron) on SU-susceptible and SU-resistant lettuce types. Treatments included a nontreated control, pronamide applied PRE at 1,340 g ai ha−1, and tribenuron at 4, 9, and 17 g ai ha−1 applied POST. Data collected were: weed control, crop injury estimates (0 = safe, 100 = dead), stand counts, and lettuce yields. Injury to lettuce from tribenuron was high in SU-susceptible lettuce types and low in SU-resistant accessions. With the exceptions of a romaine lettuce line that still may have some susceptible individuals, tribenuron did not reduce yield of SU-resistant lettuce, but did reduce the yield of SU-susceptible lettuce. Suppression of weeds such as common groundsel and annual sowthistle was higher with tribenuron than with pronamide. Tribenuron should be considered for registration as a lettuce herbicide for SU-resistant lettuce to improve current weed management options for that crop.

The objective of this study was to evaluate the feasibility of using 2-aminobutyric acid (2-aba) as a chemical marker for the detection of sulfonylurea (SU) herbicides in crop and soil environments. A bioassay, on the basis of the injury index to vegetable seedlings, showed that both field mustard and Chinese mustard were more sensitive to bensulfuron, imazosulfuron, and pyrazosulfuron than lettuce and cabbage in winter. In a similar study, field mustard was found to be the most sensitive species among six summer vegetables. After foliar application of bensulfuron at 0.487 μM, 2-aba accumulation reached the maximum in field mustard within 6 to 12 h, before any visible symptoms appeared, and this high level was maintained up to 72 h. Among all vegetables tested for differential SU sensitivities, maximum accumulation of 2-aba occurred, in coincidence with the onset of injury, in field mustard. The accumulation of 2-aba in field mustard showed a linear regression to the log-transformed concentrations, ranging from 10−2 to 102 μM, of each of the three SUs. However, no such relationship between the applied rate, especially at the lower rates, and the residue content of SUs in field mustard plant was found. In addition, with the extract of SU-treated paddy soil sprayed on field mustard, the accumulation of 2-aba in this plant was found to reflect indirectly the amount of SU residues in the soil.

The root exudates produced by sorghums contain a biologically active constituent known as sorgoleone. Seven sorghum accessions were evaluated for their exudate components. Except for johnsongrass, which yielded 14.8 mg root exudate/g fresh root wt, sorghum accessions consistently yielded approximately 2 mg root exudate/g fresh root wt. Exudates contained four to six major components, with sorgoleone being the major component (> 85%). Three-dimensional structure analysis was performed to further characterize sorgoleone's mode of action. These studies indicated that sorgoleone required about half the amount of free energy (493.8 kcal/mol) compared to plastoquinone (895.3 kcal/mol) to dock into the QB-binding site of the photosystem II complex of higher plants. Light, cryo-scanning, and transmission electron microscopy were utilized in an attempt to identify the cellular location of root exudate production. From the ultrastructure analysis, it is clear that exudate is being produced in the root hairs and being deposited between the plasmalemma and cell wall. The exact manufacturing and transport mechanism of the root exudate is still unclear. Studies were also conducted on sorgoleone's soil persistence and soil activity. Soil impregnated with sorgoleone had activity against a number of plant species. Recovery rates of sorgoleone-impregnated soil ranged from 85% after 1 h to 45% after 24 h. Growth reduction of 9 14-d-old weed species was observed with foliar applications of sorgoleone.