Charles H. Gilliam

Herbicide Combinations in Tomato to Prevent Nutsedge (Cyperus esulentus) Punctures in Plastic Mulch for Multi-Cropping Systems

Yellow nutsedge can readily puncture the plastic mulch used in plasticulture tomato production, compromising the benefits of the mulch and hastening its deterioration. Our objective was to identify a PRE-applied (i.e., under the plastic) treatment to minimize yellow nutsedge puncturing. In a greenhouse study a series of halosulfuron rates were PRE-applied to soil planted with yellow nutsedge tubers. These rates were also applied to established plants but with selective spray contact. Nonlinear regression revealed that the concentration of halosulfuron required to reduce dry weights by 90% (GR90) for PRE-applied halosulfuron was 11.6 g/ha. The GR90 for POST-applied halosulfuron was 17.1, 28.1, and 11.6 g/ha for foliar-only, soil-only and foliar plus soil spray contact, respectively. Thus halosulfuron was more effective as a POST-applied, foliar-contacting treatment. However, soil activity was deemed likely sufficient to suppress plastic puncturing. In a noncrop field study, suppression of puncturing was influenced (P < 0.05) by the rate of both PRE-applied halosulfuron and S-metolachlor. A field study with tomato was conducted to evaluate six selective treatments using plastic mulch, PRE-applied S-metolachlor, and the combination of PRE or PRE/POST-split applications of halosulfuron. Plastic alone increased tomato yield threefold compared with bare ground. The addition of various herbicide programs neither increased nor reduced yield compared with plastic alone. Selected herbicide treatments did reduce mulch puncturing but not to the extent or duration that would allow sequential crops to receive the full benefit of nonpunctured plastic. Nomenclature: Halosulfuron, S-metolachlor, yellow nutsedge, Cyperus esculentus L. CYPES, tomato, Lycopersicon esculentum Mill. ‘Florida 91’

Nitrogen Immobilization in Plant Growth Substrates: Clean Chip Residual, Pine Bark, and Peatmoss

Rising costs of potting substrates have caused horticultural growers to search for alternative, lower-cost materials. Objectives of this study were to determine the extent of nitrogen immobilization and microbial respiration in a high wood-fiber content substrate, clean chip residual. Microbial activity and nitrogen availability of two screen sizes (0.95 cm and 0.48 cm) of clean chip residual were compared to control treatments of pine bark and peatmoss in a 60-day incubation experiment. Four rates (0, 1, 2, or 3 mg) of supplemental nitrogen were assessed. Peatmoss displayed little microbial respiration over the course of the study, regardless of nitrogen rate; followed by pine bark, 0.95 cm clean chip residual, and 0.48 cm clean chip residual. Respiration increased with increasing nitrogen. Total inorganic nitrogen (plant available nitrogen) was greatest with peatmoss; inorganic nitrogen in other treatments were similar at the 0, 1, and 2 mg supplemental nitrogen rates, while an increase occurred with the highest rate (3 mg). Clean chip residual and pine bark were similar in available nitrogen compared to peatmoss. This study suggests that nitrogen immobilization in substrates composed of clean chip residual is similar to pine bark and can be treated with similar fertilizer amendments during nursery production.

Interaction of Glyphosate and Diquat in Ready-To-Use Weed Control Products

Glyphosate-based, ready-to-use weed control products frequently contain diquat (typically, 0.04 by weight relative to glyphosate) under the supposition that the diquat, “makes glyphosate work faster.” However, in light of the known modes of actions of glyphosate and diquat, we hypothesize that diquat may be antagonistic to glyphosate activity. Greenhouse experiments using longstalked phyllanthus were conducted to test this hypothesis. Glyphosate was applied at a series of rates, ranging from 0.11 to 1.12 kg ae/ha, either alone or tank-mixed with either 0, 0.02, 0.04, and 0.06 diquat. Onset of visual injury was more pronounced with the glyphosate + diquat tank mixtures compared with glyphosate alone. However, long-term control, as expressed by regrowth suppression, was greater with glyphosate alone. Regression analysis indicated that, at marginally effective glyphosate rates, the amount of glyphosate must be increased by approximately 60% to compensate for the diquat-based antagonism. Absorption and translocation studies using 14C-glyphosate revealed that the antagonism of diquat toward glyphosate can be attributed to reduced translocation of absorbed glyphosate. Nomenclature: Diquat; glyphosate; longstalked phyllanthus, Phyllanthus tenellus Roxb PYLTE

Yellow Nutsedge (Cyperus esculentus) Control and Tomato Response to Application Methods of Drip-Applied Herbicides in Polyethylene-Mulched Tomato

Abstract Drip-applied herbicides provide farmers with a more timely and cost-effective approach for applying PRE herbicides; however, herbicide movement is often limited. Field studies were conducted evaluating drip-application methods for applying PRE herbicides under polyethylene-mulched beds on yellow nutsedge punctures and the corresponding responses of a tomato crop (height and yield). The experiment was a factorial treatment arrangement of three drip application methods and three PRE-applied herbicides [halosulfuron (54 g ai ha−1), S-metolachlor (1.4 kg ha−1), and fomesafen (280 g ha−1)]. Herbicides were applied either immediately following saturation of the planting beds (method A), over an extended period while saturating the beds (method B), or prior to bed saturation (method C). Additional treatments included a commercial standard (S-metolachlor sprayed to the bed surface prior to mulch application) and a nontreated control (polyethylene mulch only). Drip-applied fomesafen, halosulfuron, and S-metolachlor provided similar control of yellow nutsedge, produced comparable yields, and failed to elicit any negative growth responses when compared to our commercial standard. With the exception of nutsedge punctures counted 56 DAT, application method did not influenced measurable outcomes. At 56 DAT nutsedge punctures were significantly lower in treatments applied by method B compared to those applied with method A. Nomenclature: Fomesafen; halosulfuron; S-metolachlor; yellow nutsedge, Cyperus esculentus L; tomato, Solanum lycopersicum L Resumen Los herbicidas aplicados en sistemas de riego por goteo brindan a los productores un sistema más barato y flexible para la aplicación de herbicidas PRE en diferentes momentos. Sin embargo, el movimiento del herbicida es a menudo limitado. Se realizaron estudios de campo para evaluar métodos de aplicación de herbicidas por goteo para aplicar herbicidas PRE bajo camas con coberturas de polyethylene para el control de la perforación causada por Cyperus esculentus y la correspondiente respuesta del cultivo de tomate (altura y rendimiento). El experimento fue un arreglo factorial de tratamientos de tres métodos de aplicación por goteo y tres herbicidas aplicados PRE [halosulfuron (54 g ai ha−1), S-metolachlor (1.4 kg ha−1), y fomesafen (280 g ha−1)]. Los herbicidas fueron aplicados ya fuera inmediatamente después de saturar las camas de siembra (método A), a lo largo de un período extendido durante la saturación de las camas (método B), o antes de la saturación de las camas (método C). Tratamientos adicionales incluyeron un estándar comercial (S-metolachlor asperjado sobre la cama de siembra antes de la colocación de la cobertura) y un testigo sin tratamiento (solamente cobertura de polyethylene). Fomesafen, halosulfuron, y S-metolachlor aplicados por goteo brindaron un control similar de C. esculentus, produjeron rendimientos comparables, y no generaron respuestas negativas en el crecimiento del cultivo cuando se compararon con nuestro estándar comercial. Con la excepción del número de perforaciones causadas por C. esculentus contadas 56 DAT, el método de aplicación no influenció ninguna de los factores medidos. A 56 DAT, las perforaciones de C. esculentus fueron significativamente menores en tratamientos aplicados con el método B al compararse con los del método A.

Cost-Effectiveness of Glyphosate, 2,4-D, and Triclopyr, Alone and in Select Mixtures for Poison Ivy Control

Abstract Dermatitis from poison ivy is a significant health problem. Considerable effort is devoted to the control of this invasive and virulent weed in urban areas. Glyphosate, triclopyr, 2,4-D, a 1 ∶ 1 mixture of glyphosate and 2,4-D, and a 9 ∶ 1 mixture of glyphosate and triclopyr were evaluated for poison ivy control. Each of these three herbicides and two mixtures were applied at nine or ten rates, which ranged in phytotoxicity from none to death. Poison ivy plants had been propagated and container-grown. Percent control, as determined from plant fresh weight reduction, was determined at 1 and 4 mo after treatment (MAT). Data were subjected to ANOVA followed by nonlinear regression. Rates required for 95% control at 1 and 4 MAT and the associated costs were determined for each of the three herbicides and two mixtures. Acceptable control (i.e., ≥ 95%) at 1 and 4 MAT could be obtained at a much lower cost with either triclopyr or 2,4-D than with either glyphosate alone or with the two glyphosate-containing mixtures. Nonlinear regression also was used to evaluate whether the two mixtures were interactive (i.e., synergistic or antagonistic) or not (i.e., additive). Glyphosate plus triclopyr was synergistic for control at both 1 and 4 MAT. Glyphosate plus 2,4-D was synergistic for control at 4 MAT only. However, for both mixtures, synergism was only evident at rates that controlled poison ivy ≤ 80%. Both mixtures were noninteractive at rates required for acceptable control. Nomenclature: 2,4-D amine; glyphosate; triclopyr; poison ivy Toxicodendron radican (L.) Kuntze.

Interaction of Prodiamine and Flumioxazin for Nursery Weed Control

Abstract Both prodiamine and flumioxazin are used in the nursery production and landscape maintenance industries in the southeastern United States for preemergence weed control. Research was conducted to determine whether a tank mixture of these two herbicides would be more effective than either component applied alone. Prodiamine alone, flumioxazin alone, and a 72 ∶ 28 (by weight) prodiamine–flumioxazin mixture were each applied at a series of rates to containers filled with a pine bark–sand substrate that is typical for nursery production in the southeastern United States. Our intent was to have a rate range that hopefully extended from ineffective to lethal for each treatment series. Subsequent to treatment, containers were overseeded with either large crabgrass, spotted spurge, or eclipta. Percent control was determined by comparing treated weed foliage fresh weight to that of the appropriate nontreated control at 6 and 12 wk after application. ANOVA followed by nonlinear regression was used to evaluate the interaction of prodiamine and flumioxazin when combined and to determine the rate of each treatment series required for 95% control (if applicable) for each of the three weed species. Results varied with weed species. The mixture was synergistic and more cost effective than either of the components applied alone in controlling spotted spurge. With respect to large crabgrass control, the mixture was additive and slightly more cost effective than the components. Eclipta could only be controlled with flumioxazin, and this control was antagonized by the addition of prodiamine. Nomenclature: flumioxazin; prodiamine; eclipta, Eclipta alba (L.) Hassk.; large crabgrass, Digitaria sanguinalis (L.) Scop.; spotted spurge, Chamaesyce maculata (L.) Small.

Early Postemergence Control of Yellow Woodsorrel (Oxalis stricta) with Residual Herbicides

Abstract Three experiments were conducted to evaluate early POST control of yellow woodsorrel using PRE-applied herbicides. In experiment 1, yellow woodsorrel was seeded at two dates in a commercial pine-bark substrate and grown until reaching either the cotyledon–one-leaf (C-1L) or two- to four-leaf (2-4L) growth stage. The herbicides isoxaben, indaziflam, and dimethenamid-p were applied at these growth stages. Two rates of isoxaben and indaziflam provided yellow woodsorrel control (≥ 80% reduction in fresh weight [FW]) when applied at the C-1L stage; however, once yellow woodsorrel reached the 2-4L stage, indaziflam was the only herbicide that provided effective control at both rates tested. Experiments 2 and 3 were similar to experiment 1, except two labeled rates of dithiopyr were also evaluated. In experiment 2, all herbicides evaluated provided ≥ 90% reduction in FW of yellow woodsorrel at the C-1L stage. Although no differences in FW were observed among any of the herbicide treatments when yellow woodsorrel were treated at the 2-4L stage, control ratings indicated that indaziflam provided the most effective yellow woodsorrel control. Experiment 3 results also indicated that isoxaben, indaziflam, and dithiopyr controlled yellow woodsorrel (≥ 95% reduction in FW) when treatments were applied at the C-1L stage, whereas dimethenamid-p reduced shoot FW 70%. When yellow woodsorrel was treated after reaching the 2-4L stage, indaziflam provided the greatest control of any herbicide evaluated. Nomenclature: Dithiopyr; isoxaben; indaziflam, N-[(1R,2S)-2,3-dihydro-2,6-dimethyl-1H-inden-1-yl]-6-[(1RS)-1-fluoroethyl]-1,3,5-triazine-2,4-diamine); yellow woodsorrel, Oxalis stricta L. Resumen Se realizaron tres experimentos para evaluar el control POST temprano de Oxalis stricta usando herbicidas aplicados PRE. En el experimento 1, O. stricta se sembró en dos fechas en un sustrato comercial de corteza de pino y se dejó crecer hasta alcanzar los estados de desarrollo de cotiledón-una hoja (C-1L) o dos a cuatro hojas (2-4L). Los herbicidas isoxaben, indaziflam, y dimethenamid-p fueron aplicados en estos estados de desarrollo. Ambas dosis de isoxaben e indaziflam proveyeron control de O. stricta (≥80% reducción del peso fresco [FW] cuando se aplicó en el estado C-1L); sin embargo, una vez que O. stricta alcanzó el estado 2-4L, indaziflam fue el único herbicida que brindó control efectivo con ambas de las dosis evaluadas. Los experimentos 2 y 3 fueron similares al experimento 1, excepto que dos dosis de etiqueta de dithiopyr fueron también evaluadas. En el experimento 2, todo los herbicidas evaluados causaron ≥90% reducción de FW de O. stricta en el estado C-1L. Aunque no se observaron diferencias en FW entre ninguno de los tratamientos de herbicidas cuando O. stricta se trató en el estado 2-4L, las evaluaciones de control indicaron que indaziflam brindó el control más efectivo de esta maleza. Los resultados del experimento 3 indicaron que isoxaben, indaziflam, y dithiopyr controlaron O. stricta (≥95% reducción de FW) cuando los tratamientos fueron aplicados en el estado C-1L, mientras dimethenamid-p redujo el FW de la parte aérea 70%. Cuando O. stricta se trató después de alcanzar el estado 2-4L, indaziflam brindó el mayor control entre los herbicidas evaluados.

Interaction of Glyphosate and Pelargonic Acid in Ready-to-Use Weed Control Products

Abstract Glyphosate-based, ready-to-use weed control products often contain pelargonic acid (PA) at a concentration equivalent to that of the glyphosate. It remains unclear what benefit, if any, this combination provides. Greenhouse experiments using large crabgrass, yellow nutsedge, longstalked phyllanthus, and prostrate spurge were conducted to determine whether the addition of PA improved weed control efficacy compared to glyphosate alone. Glyphosate was applied at a series of rates, ranging from 0.11 to 1.12 kg ae/ha, either alone or with an equal rate of PA. Addition of PA to glyphosate was synergistic only in longstalked phyllanthus and yellow nutsedge, and this synergism was manifested only as an increase in the amount of early (i.e., 5 to 7 d after treatment) visual injury. Conversely, longer-term control and control of regrowth was either not affected or reduced by the addition of PA. We conclude that the addition of PA to glyphosate in ready-to-use weed control products is neither warranted nor justified. However, we also note that the increase in early injury that was observed in only two of the four species evaluated could be an important attribute for the consumers for which these products are targeted. Nomenclature: Glyphosate; pelargonic acid; large crabgrass, Digitaria sanguinalis (L.) Scoop DIGSA; longstalked phyllanthus, Phyllanthus tenellus Roxb.; prostrate spurge, Chamaesyce masculata (L.) Small EPHPT; yellow nutsedge, Cyperus esculentus L. CYPES.

Foliar vs. Root Sensitivity of Hairy Bittercress (Cardamine hirsuta) to Isoxaben

It has been previously reported that POST-applied isoxaben can effectively control established hairy bittercress. Experiments were conducted to determine the relative importance of root vs. foliar entry of POST-applied isoxaben. At a common isoxaben rate of 0.56 kg/ha, foliar-only and foliar plus soil applications provided 10.5 and 23.3% control, respectively, as determined by fresh weight reduction. In contrast, soil-only application provided 47.0% control. Hairy bittercress foliar absorption of 14C–isoxaben did not exceed 15% of the amount applied after 72 h. Therefore, the comparatively less effectiveness of foliar-only applications may be attributed primarily to limited absorption. Minimal isoxaben concentration required to inhibit root growth of hydroponically grown hairy bittercress was 0.0025 mg/L. Higher concentrations were required to produce a response in the foliage. Sorption of isoxaben by pine bark rooting substrate, typical of what is used in container nursery production, exceeded 99% of amount applied after 36 h. Even with 99% sorption, the probable concentration within the aqueous phase remains sufficient to inhibit hairy bittercress root growth. Additional studies with 14C–isoxaben established that approximately 35% of the root-absorbed isoxaben was translocated into the foliage. Translocation from the roots into the foliage was reduced to 16% when the experiment was repeated during environmental conditions less favorable for vegetative growth (i.e., longer day length and higher temperature). Results indicate that the control of hairy bittercress with POST-applied isoxaben is likely the result of root absorption and root-growth inhibition. Expression of phytotoxicity within the foliage is also a component, but is dependent upon the root-absorbed isoxaben being translocated into the foliage. Extent of this translocation is dependent upon plant maturity and prevalent environmental conditions. Nomenclature: Isoxaben; hairy bittercress, Cardamine hirsuta L. #3 CARHI. Additional index words: Herbicide sorption, hydroponics, organic rooting substrates. Abbreviations: Kd, distribution coefficient.

Potential for halosulfuron to control Eclipta (Eclipta prostrata) in container-grown landscape plants and its sorption to container rooting substrate

Eclipta is a seed-borne summer annual that is problematic in the production of container-grown landscape plants. Halosulfuron at 70 g/ha is registered as a directed application to landscape areas but not to container-grown landscape plants. Halosulfuron was applied preemergence (PRE) to seeded eclipta and postemergence (POST) to progressively older eclipta seedlings at rates ranging from 0.18 to 100 g/ha. For halosulfuron PRE treatments, eclipta control was determined from the foliage weight of surviving seedlings. For halosulfuron POST treatments, control was determined from the weight of foliage regrowth following the removal of the treated foliage 2 wk after treatment. Nonlinear regression and log-logistic analysis indicated that the rate required for 90% control (I90) for halosulfuron PRE was 45 g/ha. For halosulfuron POST, the I90 was 60 g/ha for plants having five or fewer true leaves and 98 g/ha for plants that had lateral branching from the basal crown. Analysis estimated the I90 for flowering-sized eclipta exceeded 300 g/ha. Selective placement studies revealed that the phytotoxicity resulting from POST treatments occurs by foliar and root uptake, with foliar exposure having greater activity. For POST treatments that were limited to foliage-only contact, a split application increased control up to 25% compared with a single application of the same total dosage. However, control remained inadequate because the rate required for 75% control (I75) was 157 and 121 g/ha for single and split applications, respectively. Halosulfuron sorption by a pine bark–based rooting substrate, as used in container production, was 96% of the amount applied. The propensity for surface-applied halosulfuron to be leached in this substrate was evaluated by eclipta bioassay. After 2 wk, with 23 cm of cumulative irrigation and rainfall, halosulfuron was detected 12 cm below the substrate surface. The propensity for substrate-adsorbed halosulfuron to return to the water phase may also contribute to PRE activity for eclipta control. Nomenclature: Halosulfuron; eclipta, Eclipta prostrata L., #3 ECLAL. Additional index words: Herbicide sorption, log-logistic analysis, organic rooting substrates. Abbreviations: Ix, x% growth inhibition; POST, postemergence; PRE, preemergence; WAT, weeks after treatment.