Andrew W. MacRae

Pollen-Mediated Dispersal of Glyphosate-Resistance in Palmer Amaranth under Field Conditions

Abstract In addition to being a strong competitor with cotton and other row crops, Palmer amaranth has developed resistance to numerous important agricultural herbicides, including glyphosate. The objective of this study was to determine if the glyphosate-resistance trait can be transferred via pollen movement from a glyphosate-resistant Palmer amaranth source to a glyphosate-susceptible sink. In 2006 and 2007 glyphosate-resistant Palmer amaranth plants were transplanted in the center of a 30-ha cotton field. Susceptible Palmer amaranth plants were transplanted into plots located at distances up to 300 m from the edge of the resistant pollen source in each of the four cardinal and ordinal directions. Except for the study plots, the interior of the field and surrounding acreage were kept free of Palmer amaranth by chemical and physical means. Seed was harvested from 249 and 292 mature females in October 2006 and 2007, respectively. Offspring, 14,037 in 2006 and 13,685 in 2007, from glyphosate-susceptible mother plants were treated with glyphosate when the plants were 5 to 7 cm tall. The proportion of glyphosate-resistant progeny decreased with increased distance from the pollen source; approximately 50 to 60% of the offspring at the 1- and 5-m distances were resistant to glyphosate, whereas 20 to 40% of the offspring were resistant at the furthest distances. The development of resistance was not affected by direction; winds were variable with respect to both speed and direction during the peak pollination hours throughout the growing season. Results from this study indicate that the glyphosate-resistance trait can be transferred via pollen movement in Palmer amaranth. Nomenclature: Glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; cotton, Gossypium hirsutum L.

Aminopyralid soil residues affect rotational vegetable crops in Florida

BACKGROUND Bahiagrass (Paspalum notatum Flueggé) is a poor host of several soilborne pests of vegetable crops; therefore vegetable crops are commonly grown in a rotation with bahiagrass pastures in Florida. The herbicide aminopyralid provides foliar and soil residual weed control and increases forage production in bahiagrass pastures; however, the soil residual activity of aminopyralid makes carryover injury likely in subsequent sensitive vegetable crops. Field research was conducted to determine the sensitivity of five vegetable crops to soil residues of aminopyralid. RESULTS At an aminopyralid soil concentration of 0.2 µg kg(-1) (the limit of quantitation for aminopyralid in this research), crop injury ratings were 48% (bell pepper), 67% (eggplant), 71% (tomato), 3% (muskmelon) and 3% (watermelon), and fruit yield losses (relative to the untreated control) at that concentration were 61, 64, 95, 8 and 14% in those respective crops. CONCLUSIONS The crops included in this research were negatively affected by aminopyralid at soil concentrations less than the limit of quantitation (0.2 µg kg(-1) ). Therefore, it was concluded that a field bioassay must be used to determine whether carryover injury will occur when these crops are planted on a site where aminopyralid has been previously applied.

Response of Five Summer-Squash (Cucurbita pepo) Cultivars to Halosulfuron1

Response of ‘Dixie’, ‘Lemondrop’, ‘Multipik’, ‘Superpik’, and ‘Seneca Prolific’ summer squash to halosulfuron PRE or POST at 0.036, 0.053, and 0.072 kg ai/ha, or halosulfuron PRE fb halosulfuron POST at 0.018 fb 0.018, 0.027 fb 0.027, and 0.036 fb 0.036 kg/ha was field evaluated in 1997 and 1998. All halosulfuron treatments and rates reduced the height of cultivars 17–19% at 6 WAP (weeks after planting) and summer-squash injury (chlorosis and necrosis of crop foliage) was 6, 14, and 11% from halosulfuron PRE, POST, and PRE fb POST, respectively. Early summer-squash flowering was reduced 32–82% by halosulfuron, resulting in reduced early yields. Dixie was the cultivar most tolerant to halosulfuron. Early flowering of Dixie was reduced 32–36% compared to 32–82% for the other cultivars. Marketable yield of summer squash was reduced 20–30% by all rates of halosulfuron when averaged over all application timings. Marketable yield of Seneca Prolific, Superpik, Dixie, Multipik, and Lemondrop was reduced 0–17% by halosulfuron PRE. Halosulfuron POST or PRE fb POST reduced marketable yield of all summer-squash cultivars by 25–46%. Thus, summer squash was not tolerant of POST halosulfuron; however, Dixie, Multipik, Seneca Prolific, and Superpik exhibited tolerance to halosulfuron PRE. Nomenclature: Halosulfuron; summer squash, Cucurbita pepo L. ‘Dixie’, ‘Multipik’, ‘Superpik’, ‘Seneca Prolific’ and ‘Lemondrop’. Additional index words: Sulfonylurea, vegetable tolerance to herbicides. Abbreviations: WAP, weeks after planting.

Impact of Fallow Programs and Fumigants on Nutsedge (Cyperus spp.) Management in Plasticulture Tomato

Abstract With the loss of methyl bromide (MeBr) and high prices of alternatives, tomato growers are applying lower fumigant rates or adopting a reduced system. Without the broad-spectrum control provided by the complete fumigant system, a fallow weed program might be needed to avoid an increase in pest pressure with consecutive years of application of the reduced-fumigant system. Nutsedges are among the pests of interest due to their fast reproduction by underground structures and ability to spread and quickly infest a field. Field trials were conducted between February and December of 2011 in Balm, FL, to evaluate the impacts of fallow treatments, fumigants, and halosulfuron on nutsedge control. The trial design was a split–split plot with main, sub-, and subsubplots being fallow, fumigant, and herbicide treatment, respectively. Fallow treatments were spaced evenly throughout the fallow season and consisted of sequential combinations of cultivation (C) and/or glyphosate (G) applied at 2.24 kg ae ha−1; including: C, CC, G, GG, CG, GC, GCG, and NO (nontreated control). Fumigant treatments included a reduced-fumigant system of 1,3-dichloropropene plus chloropicrin 40:60 (1,3-D + pic) at 281 kg ha−1, a complete fumigant system of dimethyl disulfide plus chloropicrin 79:21 (DMDS + pic) at 545 kg ha−1, and no fumigant (NoFum). Herbicide treatments were either two POST applications of halosulfuron at 39 g ai ha−1 (Hal) or no halosulfuron (NoHal). In general, the fallow weed treatment GCG was the most effective in reducing nutsedge shoot emergence through the plastic mulch. When the reduced-fumigant system 1,3-D + pic was combined with GCG fallow treatment and halosulfuron (GCG:1,3-D + pic:Hal), no differences were found between this combination and the complete fumigant system DMDS + pic with halosulfuron and combined with CC, G, GG, CG, GC or GCG. This study shows the importance of adding a fallow weed program and halosulfuron to either a reduced or complete fumigation system to minimize the reproduction and growth of nutsedges. Nomenclature: Chloropicrin (pic); dimethyl disulfide (DMDS); glyphosate; halosulfuron; 1,3-dichloropropene (1,3-D); nutsedge, Cyperus spp.; tomato, Lycopersicon esculentum Mill. Resumen Con la pérdida de methyl bromide (MeBr) y los altos precios de las alternativas, los productores de tomate están aplicando dosis más bajas de fumigante o adoptando un sistema reducido. Sin el control de amplio espectro que se obtiene con un sistema de fumigación completo, un programa de manejo de malezas con barbecho limpio podría ser requerido para evitar el incremento en la presión de esta plaga en los años consecutivos a la aplicación del sistema de fumigación reducida. Cyperus spp. está entre las plagas de interés debido a su rápida reproducción por medio de estructuras subterráneas y su habilidad de dispersarse y rápidamente infestar un campo. Se realizaron experimentos de campo entre Febrero y Diciembre de 2011 en Balm, FL, para evaluar los impactos de tratamientos de barbecho, fumigantes, y halosulfuron sobre el control de Cyperus spp. El diseño del experimento fue parcelas divididas en dos niveles siendo el barbecho, el fumigante y el tratamiento del herbicida la parcela principal, la subparcela y la sub-subparcela, respectivamente. Los tratamientos de barbecho fueron distribuidos en forma uniforme a lo largo de la temporada de barbecho y consistieron en combinaciones secuenciales de cultivo con rastra de discos (C) y/o glyphosate (G) aplicado a 2.24 kg ae ha−1; incluyendo: C, CC, G, GG, CG, GC, GCG, y NO (testigo no tratado). Los tratamientos de fumigantes incluyeron un sistema de fumigación reducida de 1,3-dichloropropene más chloropicrin 40:60 (1,3-D + pic) a 281 kg ha−1, un sistema de fumigación completa de dimethyl disulfide más chloropicrin 79:21 (DMDS + pic) a 545 kg ha−1, y sin fumigante (NoFum). Los tratamientos de herbicidas fueron dos aplicaciones POST de halosulfuron a 39 g ai ha−1 (Hal) o sin halosulfuron (NoHal). En general, el tratamiento de barbecho GCG fue el más efectivo en reducir la emergencia de plantas de Cyperus spp. a través de la cobertura plástica. Cuando el sistema de fumigación reducida 1,3 + pic fue combinado con el tratamiento de barbecho GCG y halosulfuron (GCG:1,3-D + pic:Hal), no se encontraron diferencias entre esta combinación y el sistema de fumigación completa DMDS + pic con halosulfuron y combinado con CC, G, GG, CG, GC o GCG. Este estudio muestra la importancia de agregar un programa de barbecho y halosulfuron a sistemas de fumigación completa o reducida para minimizar la reproducción y crecimiento de Cyperus spp.

Sequential Applications for Mesosulfuron and Nitrogen Needed in Wheat

Abstract Mesosulfuron is often applied to wheat at a time of year when top-dress nitrogen is also applied. Current labeling for mesosulfuron cautions against applying nitrogen within 14 d of herbicide application. Soft red winter wheat response to mesosulfuron and urea ammonium nitrate (UAN) applied sequentially and in mixtures was determined at three locations in North Carolina and Georgia during 2005 and 2006. Mesosulfuron at 0, 15, and 30 g ai/ha was applied in water to wheat at Feekes growth stage (GS) 3 followed by UAN at 280 L/ha 2 h, 7 d, 14 d, and 21 d after mesosulfuron. Mesosulfuron applied in UAN was also evaluated in 2006. Mesosulfuron injured wheat 6 to 9% in 2005 and 12 to 23% in 2006 when UAN was applied 2 h or 7 d after the herbicide. Wheat injury did not exceed 8% when UAN was applied 14 or 21 d after the herbicide. Greatest injury, 35 to 40%, was noted when mesosulfuron and UAN were combined. Wheat yield was unaffected by mesosulfuron or time of UAN application in 2005. In 2006, yield was affected by the timing of UAN application relative to mesosulfuron; wheat yield increased as the interval, in days, between UAN and herbicide applications increased. To avoid crop injury and possible yield reduction, mesosulfuron and UAN applications should be separated by at least 7 to 14 d. These findings are consistent with precautions on the mesosulfuron label. Nomenclature: Mesosulfuron (proposed common name), 2-[[[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]-4-[[(methylsulfonyl)amino]methyl]benzoic acid; soft red winter wheat, Triticum aestivum L. ‘26R61’, ‘Coker 9184’, ‘SS 8308’.

Oat and Rye Tolerance to Mesosulfuron and Tribenuron

The affect of mesosulfuron and tribenuron applied POST to oat and rye was determined at three locations in Georgia during 2004 and 2005. Five herbicide treatments were applied to oat and rye in Feekes scale (FS) 1.2 or 2 developmental stage. Herbicide treatments consisted of tribenuron at 13 or 26 g ai/ha, tribenuron at 13 g/ha plus MCPA at 561 g ai/ha, MCPA at 561 g/ha, and mesosulfuron at 15 g ai/ha. Tribenuron, MCPA, and tribenuron plus MCPA injured both crops less than 4% and did not affect grain yield. Mesosulfuron applied to rye in FS 1.2 injured the crop 36% at 2 wk after treatment (WAT), 12% at 5 WAT, and 0% at harvest. Mesosulfuron applied at FS 2 injured rye less than 9%. Mesosulfuron did not influence rye-grain yield with either application timing. Mesosulfuron injured oat 65 to 86% at 5 WAT and reduced yield at least 70% regardless of application timing. Nomenclature: MCPA; mesosulfuron-methyl; tribenuron-methyl; oat, Avena sativa L. ‘Horizon 32’; rye, Secale cereale L. ‘Wrens 96’.

Tomato Root Uptake of Carfentrazone

Abstract Fresh market tomato is an important and valuable crop in Florida, accounting for 630 million dollars farm-gate value, which was 45% of the total value of the U.S. crop in 2010. In order to maintain or increase its productivity, labeled herbicide alternatives to methyl bromide are important to limiting seed production of weeds emerging between the raised plasticulture beds. A study was conducted inside a greenhouse where carfentrazone was applied as a drench at 0.03125×, 0.0625×, 0.125×, 0.25×, 0.5×, 1×, 2×, 4×, and 8× and as a subsurface irrigation at 0.0625×, 0.125×, 0.25×, 0.5×, 1×, 2×, 4×, 8×, and 16× rates. The 1× rate equaled the maximum labeled rate of carfentrazone (35.1 g ai ha−1) that would be applied to an area of 0.360 m2. Both the drench and subsurface trials showed an increase in plant injury and reduced growth as the rate of carfentrazone increased. The drench trial, however, was observed to have higher visible injury and greater growth reduction (based on plant measurement) than the subsurface trial, when comparing similar rates. For the 1× rate of carfentrazone in the drench trial vs. the subsurface trial, injury was 66 and 24.5%, respectively. For the 1× rate the tomato plants had estimated growth, based on the curves fit for the data, of 4.8% vs. 39.9% for the drench and subsurface trials, respectively. The subsurface trial better represents what happens in the field when carfentrazone root uptake injury is observed since it is normally observed to be around 10% or less. This still leaves a level of concern; once a 10% injury level in the subsurface trial was estimated to have reduced tomato growth, fruit weight, and total shoot dry weight by 33, 15, and 9.5%, respectively. Nomenclature: Carfentrazone; methyl bromide; tomato; Lycopersicon esculentum L. Resumen El tomate fresco es un cultivo importante y valioso en Florida, representando 630 millones de dólares de valor a las puertas de las fincas, lo cual a su vez representó 45% del total del valor del cultivo en Estados Unidos en 2010. Con el fin de mantener o incrementar su productividad, los herbicidas registrados para este cultivo como alternativas a methyl bromide son importantes para limitar la producción de semillas de malezas que emergen entre las camas con cobertura plástica. Se realizó un estudio dentro de un invernadero donde se aplicó carfentrazone como “drench” a dosis de 0.03125×, 0.0625×, 0.125×, 0.25×, 0.5×, 1×, 2×, 4× y 8× y mediante irrigación subterránea a dosis de 0.0625×, 0.125×, 0.25×, 0.5×, 1×, 2×, 4×, 8× y 16×. La dosis 1× fue igual a la dosis máxima en la etiqueta de carfentrazone (35.1 g ai ha−1) que sería aplicada a un área de 0.360 m2. Ambas formas de aplicación, drench y subterránea, mostraron un incremento en el daño de la planta y redujeron el crecimiento conforme se aumentó la dosis de carfentrazone. Sin embargo, en el estudio con drench, se observó un mayor daño visible y una mayor reducción en el crecimiento (basándose en medidas de plantas) que en el estudio con aplicación subterránea, cuando se compararon dosis similares. Para la dosis 1× de carfentrazone en el estudio con drench vs. el estudio con aplicación subterránea, el daño fue 66 y 24.5%, respectivamente. Basándose en curvas de mejor ajuste de los datos, para la dosis 1×, las plantas de tomate tuvieron un crecimiento estimado de 4.8% vs. 39.9% para aplicaciones drench y subterráneas, respectivamente. El estudio con aplicación subterránea representa mejor lo que pasa en el campo cuando se observa un daño causado por la absorción de carfentrazone por las raíces, el cual es normalmente 10% o menor. Esto aún es preocupante, ya que se estimó que un nivel de daño de 10% en el estudio de aplicación subterránea redujo el crecimiento del tomate, el peso del fruto y el peso seco total de la parte aérea en 33, 15 y 9.5%, respectivamente.

Transplanted Onion Response to Previously Applied Residual Herbicides

Field trials were conducted in 2003/2004 and 2005/2006 at Reidsville, Georgia, to evaluate the effects of previously applied residual herbicides on onion growth and bulb production. Before transplanting onion, preplant applications of imazapic at 18 and 36 g ai/ha, diclosulam at 7 and 14 g ai/ha, pyrithiobac at 27 and 54 g ai/ha, trifloxysulfuron at 6.6 and 13.2 g ai/ha, diuron at 224 and 448 g ai/ha, and cloransulam at 22 g ai/ha were made. An untreated control was included for comparison. Trifloxysulfuron at 13.2 g/ha, diclosulam at 14 g/ha, pyrithiobac at 54 g/ha, and cloransulam at 22 g/ha injured onion 26, 73, 86, and 86% in 2003/2004, respectively, and 13 to 44% injury in 2005/2006. These same herbicides also reduced yield. Imazapic and diuron injured transplanted onion 6% during both seasons but did not reduce yield. This research suggests imazapic and diuron restrictions could possibly be reduced. However, the onion rotational restrictions for diclosulam, pyrithiobac, trifloxysulfuron, and cloransulam are accurate. Nomenclature: Cloransulam, 3-chloro-2-[[(5-ethoxy-7-fluoro[1,2,4]triazolo[1,5-c]pyrimidin-2yl)sulfonyl]amino]benzoic acid; diclosulam; imazapic; trifloxysulfuron; onion, Allium cepa L