Greg E. MacDonald

Factors affecting seed germination of tropical signalgrass (Urochloa subquadripara)

Abstract Tropical signalgrass is one of the dominant weeds in the Florida turfgrass industry and is potentially troublesome for the southeastern turfgrass industry. Tropical signalgrass is especially problematic for St. Augustinegrass sod producers because of lack of control options. The objectives of our research were to determine the effect of light, pH, temperature, water potential, and planting depth on tropical signalgrass germination and emergence. Tropical signalgrass germination does not require light and is optimum at pH 5 to 6, temperature 25 C, and water potentials greater than − 0.4 MPa. Tropical signalgrass shoots emerged from depths of 0 to 7 cm, with maximum germination when placed on the soil surface. Tropical signalgrass seedlings emerged in the field during the second week of March in Ft. Lonesome, FL. Weekly mean soil and ambient air temperatures at the time of emergence were 20 C. Tropical signalgrass emergence was first observed at 118 and 73 growing degree-days (GDD) (13 C base temperature), with a peak emergence period at 222 and 156 GDD for 2001 and 2002, respectively. Nomenclature:  Tropical signalgrass, Urochloa subquadripara (Trin.) R. D. Webster BRASU; St. Augustinegrass, Stenotaphrum secondatum (Wait.) Kuntz.

Response of Three Runner-Type Peanut Cultivars to Flumioxazin'

Field studies were conducted in 1999 and 2000 at Marianna and Gainesville, FL, to evaluate the response of three runner-type peanut cultivars, ‘Georgia Green’, ‘C-99R’, and ‘MDR-98’, to flumioxazin applied preemergence at 0, 71, 105, and 211 g/ha in a weed-free environment. Peanut exhibited excellent tolerance to flumioxazin, regardless of flumioxazin rate or peanut cultivar, at Gainesville in 1999 and both locations in 2000. In 1999, at Marianna, flumioxazin caused early-season stunting and season-long reduction in peanut canopy width. Peanut response was independent of cultivar and did not exceed 25%, with an increase in stunting with higher flumioxazin rates. Peanut stunting was associated with cool and extremely wet growing conditions during the first 2 mo after planting in 1999 at Marianna. Peanut yield and grade parameters, in both years, were not affected by flumioxazin treatment. Nomenclature: Flumioxazin; peanut, Arachis hypogaea L. ‘Georgia Green’, ‘C-99R’, ‘MDR-98’. Additional index words: Cultivar, extra large kernels, grade analysis, other kernels, sound mature kernels, sound splits, total sound mature kernels, yield. Abbreviations: DAP, days after planting; ELK, extra large kernels; OK, other kernels; PPI, preplant incorporated; PPO, protoporphyrinogen oxidase; PRE, preemergence; SMK, sound mature kernels; SS, sound splits; TSMK, total sound mature kernels.

Root diseases, weeds, and nematodes with poultry litter and conservation tillage in a sweet corn–snap bean double crop

Abstract The effect of continuous applications of poultry litter on root diseases, nematodes, and weeds with different tillage practices in vegetable production in Georgia was investigated. Litter treatments (noncomposted and composted broiler or breeder litter) applied 3–5 weeks before planting were compared with no litter. In sweet corn and snap bean there were differences among years, and tillage and litter treatments had a variable effect on yield, post-emergence damping-off and plant stand. Crown and brace root rot in corn induced by Rhizoctonia solani AG 2-2 increased after 3 years conservation tillage, but there were no differences among litter treatments. Population densities of R. solani AG-4 were increased by conservation tillage before planting snap bean the first year but not in later years, while litter treatments had no effect. After 3 years, populations of Pythium spp., Fusarium solani, and Fusarium spp. did not differ among treatments, but noncomposted broiler litter increased populations of saprophytic fungi in soil compared with no litter. Population densities of stylet-bearing nematodes were low and usually not different among treatments. After 3 years of sweet corn populations of Meloidogyne incognita increased with conventional tillage compared with conservation tillage, populations of Paratrichodorus christiei were reduced by litter treatments and populations of Helicotylenchus dihystera were reduced by noncomposted broiler litter compared with no litter. Weed infestation increased in both tillage systems with time, but in snap bean there was a greater infestation in conservation tillage than in conventional tillage. Litter treatments had little effect on weed infestation.

Tropical signalgrass (Urochloa subquadripara) control with preemergence- and postemergence-applied herbicides.

Tropical signalgrass is one of the most serious weed problems in the St. Augustinegrass sod production in Florida, and its presence increases production costs and lowers turfgrass quality. The objectives of our research were to: (1) evaluate herbicides preemergence and postemergence for control of tropical signalgrass and (2) compare control of tropical signalgrass and other problem weeds (torpedograss, blanket crabgrass, and India crabgrass) with postemergence herbicides. In preemergence herbicide field trials, only benefin + oryzalin, imazapic, imazapic + 2,4-D, and oryzalin provided ≥75% tropical signalgrass control 8 wk after application (WAA). By 11 WAA, only benefin + oryzalin and imazapic + 2,4-D provided ≥75% tropical signalgrass control. In greenhouse experiments, eight herbicide treatments were applied postemergence to tropical signalgrass seedlings at the two-, four-, six-, and eight-leaf stages. Asulam and CGA 362622 provided ≥89% tropical signalgrass control at all application timings. Imazaquin controlled tropical signalgrass ≥98% when applied before the eight-leaf stage. However, in field trials with mature tropical signalgrass (>20 cm stolons), none of the 20 herbicide treatments applied postemergence provided acceptable control. Nomenclature: Asulam; benefin; CGA 362622, N-[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]-3-(2,2,2-trifluoroethoxy)-2-pyridinesulfonamide; 2,4-D; imazapic; imazaquin; oryzalin; blanket crabgrass, Digitaria serotina (Walt.) Michx. #3 DIGSO; India crabgrass, Digitaria longiflora (Retz.) Pers. # DIGLO; St. Augustinegrass, Stenotaphrum secondatum (Walt.) Kuntz; torpedograss, Panicum repens L. # PANRE; tropical signalgrass, Urochloa subquadripara (Trin.) R. D. Webster # BRASU. Additional index words: BRASU, DIGLO, DIGSO, PANRE. Abbreviation: WAA, weeks after application.

Weed Management and Peanut Response from Applications of Saflufenacil

Abstract Saflufenacil is a new protoporphyrinogen oxidase–inhibiting herbicide registered for use before establishment of field corn and soybean. Generally, peanut plants are tolerant to other herbicides in this class, and no reports document the utility of saflufenacil for in-season weed control. Experiments were conducted to determine whether saflufenacil applied at 12, 25, and 50 g ha−1 could effectively control Benghal dayflower and Palmer amaranth. It was observed that saflufenacil, applied either PRE or POST, was ineffective for Benghal dayflower. The maximum control at 28 d after treatment (DAT) was 79% when 50 g ha−1 was applied to 5- to 10-cm plants. Control of Palmer amaranth from PRE applications was less effective than flumioxazin at 28 DAT. However, POST applications provided > 87% control at 28 DAT when applied to plants 5 to 10 cm in height. For plants 10 to 15 cm in height, > 90% Palmer amaranth control was only achieved by the 50 g ha−1 application rate. For plants 15 to 20 cm in height, no POST application provided > 70% control. Peanut response, in a weed-free environment, to saflufenacil rate and application timing were also evaluated. Peanut stunting ranged from 0 to 36%, relative to application timing. Applications made at 0 d after emergence (DAE) were least injurious, whereas those made at 15 DAE were most injurious. Application of 50 g ha−1 provided the greatest amount of stunting and foliar injury. However, stunting and saflufenacil application rate did not correspond to yield reduction. Saflufenacil application timing did influence peanut yield. Applications made between 0 and 30 DAE did not result in yield loss, whereas applications made at 45 and 60 DAE resulted in a 5 and 19% reduction, respectively. Though saflufenacil has many positive characteristics, higher application rates are required for optimum weed control. However, these higher use rates also resulted in unacceptable levels of injury. Nomenclature: Flumioxazin; saflufenacil; Benghal dayflower, Commelina benghalensis L.; Palmer amaranth, Amaranthus palmeri S. Watson; peanut, Arachis hypogaea L.

Spray Penetration into a Strawberry Canopy as Affected by Canopy Structure, Nozzle Type, and Application Volume

Strawberries, an important Florida crop, are grown on raised beds covered with plastic mulch. The plastic mulch provides good control of many weeds, but some problem species can emerge from the transplant hole during crop establishment. POST herbicide options for broadleaf weed control within the strawberry bed is limited to clopyralid, which only provides suppression. Strawberry canopy shielding may be responsible for the observed incomplete control with clopyralid application for problematic broadleaf weed species such as black medic and Carolina geranium. Two field experiments were established on mature strawberries to evaluate spray penetration through the canopy. The first examined spray penetration through the canopy of multiple strawberry cultivars at various distances from the crown. The second examined the effects of application volumes and nozzle selection on spray penetration. Cultivar selection had no effect on spray penetration through the canopy. In the first study, when applying at 281 L ha-1, the area around the planting hole (0 to 5 cm from the crown) had 8% coverage below the canopy while the area below the canopy edge (10 to 15 cm from the crown) had 27% coverage. In the second study, increasing the application volume from 187 to 375 L ha-1 increased coverage by 81%. Increasing the application volume from 375 to 740 L ha-1 increased coverage 33% with maximal coverage of 53% at 740 L ha-1. Nozzle type (standard even flat spray tip, Drift Guard, or TwinJet nozzles) did not affect coverage or deposition volume below the canopy. Overall, mature strawberry canopies demonstrated similar spray droplet penetration across cultivars with increased penetration with increased distance from the crown. Penetration increased with increasing application volume, but the nozzle types used in this experiment did not affect penetration. Additional research is needed to better define the effect of application volume on herbicide efficacy. Nomenclature: Strawberry, Fragaria × ananassa (Weston) Duchesne ex Rozier

Effect of Temperature on Clopyralid Safety in Strawberry

Abstract Strawberry is an important horticultural crop in Florida. The long growing season and escapes from fumigation and PRE herbicides necessitate POST weed management to maximize harvest potential and efficiency. Alternatives to hand-weeding are desirable, but clopyralid is the only broadleaf herbicide registered for use. Weed control may be improved by early-season clopyralid applications, but at risk of high temperature and increased strawberry injury. The effect of temperature on clopyralid safety on strawberry is unknown. We undertook a growth chamber experiment using a completely randomized design to determine crop safety under various temperature conditions across acclimation, herbicide application, and post-application periods. There was no effect of clopyralid on the number of strawberry leaves across all temperatures. Damage to the strawberry manifested as leaf malformations. Acclimation temperatures affected clopyralid-associated injury (p=0.0309), with increased leaf malformations at higher temperatures (27 C) compared to lower (18 C) temperatures. Pre-treatment temperatures did not affect clopyralid injury. Post-application temperature also affected clopyralid injury (p=0.0161), with increased leaf malformations at higher temperatures compared to lower ones. Clopyralid application did not reduce flowering or biomass production in the growth chamber. If leaf malformations are to be avoided, consideration to growing conditions prior to application is advisable, especially if applying clopyralid early in the season. Nomenclature: Clopyralid; strawberry, Fragaria × ananassa Duchesne

Control Recommendations for Black Medic (Medicago lupulina) Based on Growth and Development in Competition with Strawberry

Strawberries are an important horticultural crop in Florida. Black medic is among the most problematic weeds within the production system. To better coordinate control measures, black medic growth and development while in competition with strawberry was studied. Twelve plants were randomly selected at each of four field sites in Hillsborough County, FL, in 2014. Plants were repeatedly measured over the growing season for stem length and number of primary branches, flower buds, flowers, and seed clusters. Growing degree days (GDD) were calculated (Tbase=0 C) starting from the hole-punch application of the plastic mulch (October 8, 2014, to October 10, 2014) from weather station data generated from the Florida Automated Weather Network. Strawberry height and width increased consistently across all sites, but black medic growth and development varied considerably. Strawberry suppressed black medic growth up to 1,805 cumulative GDD at three of four sites where black medic remained beneath the strawberry canopy. After 1,805 GDD, the black medic stems still remained below but experienced exponential growth for total stem length and, in turn, flower buds, inflorescence, and immature seed clusters. Ideal clopyralid spray timing based on susceptible plant size was 890 to 1,152 GDD. Optimal hand-weeding time frames would likely occur as the plant stems expand beyond the strawberry canopy (to improve visibility) and before flower production to prevent seed return to the seedbank. First seed production was observed at 1,200 GDD at the earliest site and between 1,966 to 2,365 GDD across all the other sites. Overall, consistent trends were observed across sites, but betweensite variability was observed that could not be accounted for by differences in temperature. Nomenclature: Clopyralid; black medic, Medicago lupulina L. MEDLU; strawberry, Fragaria × ananassa Duchesne.

Impact of Irrigation Volume on PRE Herbicide Activity

The importance of PRE herbicide applications in cotton has increased since the evolution of glyphosate-resistant (GR) Palmer amaranth. Cotton producers are relying on residual herbicides for control of Palmer amaranth, as POST options are limited or ineffective. S-Metolachlor, acetochlor, fomesafen, and dicamba all provide PRE control of Palmer amaranth; however, little is known about the effect of irrigation rate on incorporation and herbicidal efficacy. In 2015, an experiment was conducted on fine sand and loamy sand soils to evaluate the influence of irrigation volume (0.0 to 12.7 mm ha−1) on Palmer amaranth control with PRE herbicides. Irrigation volume after herbicide application was significant for both S-metolachlor and acetochlor. Efficacy of S-metolachlor was greatest in plots receiving 6.4 and 12.7 mm of irrigation where Palmer amaranth biomass was reduced to 4 and 2% of a nontreated control (NTC), respectively, compared with 61% in plots with the 0-mm irrigation treatment. Palmer amaranth control by acetochlor incorporated at 3.2- to 12.7-mm irrigation did not differ but did reduce Palmer amaranth biomass compared with the 1.6-mm irrigation rate. Irrigation volume was not significant for the soil incorporation of fomesafen or dicamba. Across all herbicides, fomesafen-treated plots provided the most consistent control of Palmer amaranth, reducing its biomass to < 3% of NTC at all irrigation rates. Dicamba provided the least and most inconsistent control of Palmer amaranth, producing 17 to 51% of NTC biomass. Nomenclature: Acetochlor; dicamba; fomesafen; S-metolachlor; Palmer amaranth, Amaranthus palmeri S. Wats.; cotton, Gossypium hirsutum L. En algodón, la importancia de las aplicaciones de herbicidas PRE ha aumentado desde la evolución de Amaranthus palmeri resistente a glyphosate (GR). Los productores de algodón están dependiendo de herbicidas residuales para el control de A. palmeri, ya que las opciones POST son limitadas o inefectivas. S-metolachlor, acetochlor, fomesafen, y dicamba brindan control PRE de A. palmeri. Sin embargo, se conoce poco acerca del efecto de la dosis de riego sobre la incorporación en el suelo y la eficacia del herbicida. En 2015, se realizó un experimento en un suelo arenoso fino y en uno arenoso limoso para evaluar la influencia del volumen de riego (0.0 a 12.7 mm ha−1) sobre el control de A. palmeri con herbicidas PRE. El volumen de riego después de la aplicación del herbicida fue significativo para S-metolachlor y acetochlor. La eficacia de S-metolachlor fue mayor en parcelas que recibieron 6.4 y 12.7 mm de riego donde la biomasa de A. palmeri se redujo a 4 a 2% del testigo sin tratamiento (NTC), respectivamente, comparado con 61% en parcelas con 0 mm de riego. El control de A. palmeri con acetochlor incorporado con riego de 3.2 a 12.7 mm no difirió, pero redujo la biomasa al compararse con la dosis de riego de 1.6 mm. El volumen de riego no fue significativo para la incorporación en el suelo de fomesafen y dicamba. Entre todos los herbicidas, las parcelas tratadas con fomesafen brindaron el control más consistente de A. palmeri, reduciendo la biomasa a < 3% del NTC en todas las dosis de riego. Dicamba brindó el más inconsistente y el menor control A. palmeri, produciendo 17 a 51% de biomasa al compararse con el NTC.

Clopyralid Tolerance in Strawberry and Feasibility of Early Applications in Florida

Abstract Broadleaf species escape current integrated weed management strategies in strawberry [Fragaria×ananassa (Weston) Duchesne ex Rozier (pro sp.) [chiloensis×virginiana]] production. Clopyralid is a registered POST control option, but current application timings provide suppression of only some species. Earlier clopyralid application timings may increase spray coverage to weeds at the planting hole, but strawberry plant tolerance to applications shortly after transplant is unknown. The objectives of the study were to determine the degree of clopyralid tolerance when applied to mature strawberry plants according to current management strategies, whether clopyralid absorption and translocation were involved in the tolerance response demonstrated by mature strawberry plants, and whether clopyralid could be safely applied to immature strawberry plants shortly after transplant. Clopyralid caused no damage when applied to mature strawberry plants and did not affect crop height, number of crowns, flowers, immature berries, or yield. Maximal strawberry absorption of radiolabeled clopyralid was 82% of the recovered radioactivity and reached peak (90%) absorption at 15 h. Maximal total translocation of radioactivity from the treated leaf was 17% and reached peak translocation at 52 h. Translocation was primarily to the new leaves and reproductive structures. In the early-application experiment, damage induced by clopyralid for all application timings reached 0 by 8wk after treatment. Across all timings, maximal damage at 140 g ha-1 was 17% when applied 14 d after transplant (DATr) and 56% at 28 g ha-1 when applied at 21 DATr. Clopyralid dose did not affect the number of crowns, aboveground biomass, or yield. There was some stunting in plant height (3%) by the high labeled dose of clopyralid. Labeled dose clopyralid applications appear safe for application timings closer to strawberry transplant, though considerations of leaf cupping should be taken under consideration for label changes.