Search

Weed management in cantaloupe and other melon crops, is important to maximize fruit yield; however, there are few registered herbicides available in California. Several independent herbicide trials were conducted at University of California field stations in Davis (Yolo County), Five Points (Fresno County), and Holtville (Imperial County) from 2013 to 2019 to evaluate both registered and unregistered herbicides and incorporation methods (sprinklers, cultivation, or none) for crop safety and weed control in melons. Although specific treatments varied among locations depending on local practice and research objectives, ethalfluralin and halosulfuron were used in all experiments and bensulide and S-metolachlor were evaluated in 4 of 6 site-years. Additional herbicides included clethodim, clomazone, DCPA, napropamide, pendimethalin, sethoxydim, and sulfentrazone. Among registered herbicides, halosulfuron, halosulfuron + ethalfluralin, and ethalfluralin + bensulide combinations provided consistently beneficial weed control across all site-years compared to the non-treated control. S-metolachlor performed as well as the best of the registered herbicides tested at each site-year; while moderate injury was noted at the Davis location this did not reduce melon yield. The method used to incorporate preplant herbicides had a significant impact on weed control efficacy but varied by location. Mechanical incorporation of preplant herbicides resulted in improved weed control and yield compared to sprinklers. Early season weed control, whether by herbicides or hand weeding, resulted in significant yield increase in most site-years.

Herbicide resistance has been studied extensively in agronomic crops across North America but is rarely examined in vegetables. It is widely assumed that the limited number of registered herbicides combined with the adoption of diverse weed management strategies in most vegetable crops effectively inhibits the development of resistance. It is difficult to determine whether resistance is truly less common in vegetable crops or whether the lack of reported cases is due to the lack of resources focused on detection. This review highlights incidences of resistance that are thought to have arisen within vegetable crops. It also includes situations in which herbicide-resistant weeds were likely selected for within agronomic crops but became a problem when vegetables were grown in sequence or in adjacent fields. Occurrence of herbicide resistance can have severe consequences for vegetable growers, and resistance management plans should be adopted to limit selection pressure. This review also highlights resistance management techniques that should slow the development and spread of herbicide resistance in vegetable crops.

Sicklepod [Senna obtusifolia (L.) H. S. Irwin & Barneby], also known as Cassia obtusifolia (L.), is an annual, herbaceous, dicotyledonous plant in the Fabaceae (Leguminosae) family, which is commonly known as the bean, legume, and pea family. The Fabaceae consist of herbs, shrubs, vines, or trees; the family has a cosmopolitan distribution with members numbering approximately 751 genera and 19,500 species (Christenhusz and Byng 2016). Characteristics of the Fabaceae include alternate, stipulate, and compound leaves. Leaflets often have pulvini (i.e., cushion-like swellings at the base of leaves that are subject to changes in turgor pressure), which are responsible for growth-independent or “sleep” movements. Another interesting anatomical feature exhibited by many species in the family is the formation of parenchymatous root nodules that are generated in association with nitrogen-fixing bacteria (Zomlefer 1994). The ovary of the Fabaceae usually develops into a dehiscent legume (e.g., pod). Although some Fabaceae may be weedy pests, others are important food crops [e.g., soybean, Glycine max (L.) Merr.] and fodder and forage plants (e.g., alfalfa, Medicago sativa L.). Some members of the Fabaceae produce valuable gums [e.g., gum arabic, Acacia senegal (L.) Britton] and dyes (e.g., indigo, Indigofera tinctoria L.), whereas others are prized as desirable ornamentals (e.g., eastern redbud, Cercis canadensis L.). Many species in the Fabaceae produce alkaloids or cyanogenic glycosides in different plant structures. Rotenone, an isoflavone insecticide, is derived from Derris eclipta (Wall.) Benth.

Dicamba is a synthetic auxin herbicide that is prone to off-target movement, including drift and volatilization. Due to the increased acreage of dicamba-resistant soybean to control glyphosate-resistant weeds, dicamba drift injury to neighboring vegetable crops is of concern. A method to quantify leaf deformation (often referred to as leaf cupping) caused by dicamba injury was developed and compared to visual rating techniques to determine its accuracy and suitability. A second objective was to determine the relative dicamba sensitivity of several economically important vegetable crops. Soybean, snap bean, tomato, and cucumber were grown in a greenhouse and exposed to dicamba at 0, 56, 112, 280, 560, 1,120, and 2,240 mg ae ha−1, which is, respectively, 0, 1/10,000, 1/5,000, 1/2,000, 1/1,000, 1/500, and 1/250 of the maximum recommended label rate for soybean application (560 g ae ha−1). Plants were evaluated visually and using an imaging analysis technique that measures the leaf deformation index (LDI) with a leaf area scanner. LDI is calculated by dividing the two-dimensional projection of the area of the leaf in its natural configuration by the area of the flattened leaf. Across all four crops, log-logistic regression analysis indicated the LDI method had lower I50 values with lower standard error, demonstrating that the LDI method gives more precise estimates of sensitivity. This novel method provides an objective, quantitative method for measuring dicamba drift injury and determining relative sensitivities of valuable specialty crops.

Pollen-mediated gene flow (PMGF) refers to the transfer of genetic information (alleles) from one plant to another compatible plant. With the evolution of herbicide-resistant (HR) weeds, PMGF plays an important role in the transfer of resistance alleles from HR to susceptible weeds; however, little attention is given to this topic. The objective of this work was to review reproductive biology, PMGF studies, and interspecific hybridization, as well as potential for herbicide resistance alleles to transfer in the economically important broadleaf weeds including common lambsquarters, giant ragweed, horseweed, kochia, Palmer amaranth, and waterhemp. The PMGF studies involving these species reveal that transfer of herbicide resistance alleles routinely occurs under field conditions and is influenced by several factors, such as reproductive biology, environment, and production practices. Interspecific hybridization studies within Amaranthus and Ambrosia spp. show that herbicide resistance allele transfer is possible between species of the same genus but at relatively low levels. The widespread occurrence of HR weed populations and high genetic diversity is at least partly due to PMGF, particularly in dioecious species such as Palmer amaranth and waterhemp compared with monoecious species such as common lambsquarters and horseweed. Prolific pollen production in giant ragweed contributes to PMGF. Kochia, a wind-pollinated species can efficiently disseminate herbicide resistance alleles via both PMGF and tumbleweed seed dispersal, resulting in widespread occurrence of multiple HR kochia populations. The findings from this review verify that intra- and interspecific gene flow can occur and, even at a low rate, could contribute to the rapid spread of herbicide resistance alleles. More research is needed to determine the role of PMGF in transferring multiple herbicide resistance alleles at the landscape level. Nomenclature: Common lambsquarters, Chenopodium album L.; giant ragweed, Ambrosia trifida L.; horseweed, Erigeron canadensis (L.) Cronquist; kochia, Bassia scoparia (L.) A.J. Scott; Palmer amaranth, Amaranthus palmeri S. Watson.; waterhemp, Amaranthus tuberculatus (Moq.) Sauer

But your snobbiness, unless you persistently root it out like the bindweed it is, sticks by you till your grave. – George Orwell The real danger in a garden came from the bindweed. That moved underground, then surfaced and took hold. Strangling plant after healthy plant. Killing them all, slowly. And for no apparent reason, except that it was nature. – Louise Penny

Hophornbeam copperleaf (Acalypha ostryifolia Riddell) is an erect, herbaceous, dicot species in the Euphorbiaceae, or spurge, family that constitutes more than 200 genera and some 6,000 species (Mayfield and Webster 2013). Although the euphorbs have a cosmopolitan distribution, none are found in the Arctic (Mabberley 1997). Members of the Euphorbiaceae may be trees, shrubs, herbs (occasionally aquatic), or vines; sometimes succulent and cactus-like; and often have glands on vegetative plant parts (Mabberley 1997; Zomlefer 1994). Genera in the spurge family include Croton, Euphorbia, Ricinus, and Acalypha. Acalypha consists of 450 species that are native to both the Eastern and Western hemispheres (Zomlefer 1994). Acalypha was the name used by Hippocrates because the leaves resemble those of nettles, whereas ostryifolia alludes to the resemblance of leaves to plants in the genus Ostrya (hophornbeam trees; Burrows and Tyrl 2013; Haddock 2014; Hilty 2018). As plants mature in the fall, the leaves can turn reddish-brown, which may indicate why “copperleaf” is included in the species’ common name (Hilty 2018). Hophornbeam copperleaf is native to North America; it occurs in the United States ranging from Arizona east to Florida, north to Pennsylvania, and west to Nebraska (Anonymous 2019). It occurs in a variety of habitats including agronomic fields, cultivated areas, landscapes, roadsides, river and stream banks, thickets, pastures, and waste sites (Bryson and DeFelice 2010; Haddock 2014; Hilty 2018). This plant’s other common names include copperleaf, pineland three-seed mercury, Virginia copperleaf, hornbeam mercury, hornbeam three-seed mercury, mercury, and rough-pod copperleaf (Bryson and DeFelice 2010; Haddock 2014; Hilty 2018; Steckel 2006).

Primroses, The spring may love them; Summer knows but little of them. Foresight. William Wordsworth, 1819Ring-ting! I wish I were a Primrose A bright yellow Primrose, blowing in the Spring! Wishing. William Allingham“The snowdrop and primrose our woodlands adorn, and violets bathe in the wet o' the morn.” Robert Burns

The commercial release of crops with engineered resistance to 2,4-D and dicamba will alter the spatial and temporal use of these herbicides. This, in turn, has elicited concerns about off-target injury to sensitive crops. In 2014 and 2015, studies were conducted in Tifton, GA, to describe how herbicide (2,4-D and dicamba), herbicide rate (1/75 and 1/250 field use), and application timing (20, 40, and 60 DAP) influence watermelon injury, vine development, yield, and the accumulation of herbicide residues in marketable fruit. In general, greater visual injury and reductions in vine growth, relative to the non-treated check, were observed when herbicide applications were made before watermelon plants had begun to flower. Although the main effects of herbicide and rate were less influential than the timing of applications with respect to plant development, the 1/75 rates were more injurious than the 1/250 rates; dicamba was more injurious than 2,4-D. In 2014, the 1/75 and 1/250 rates of each herbicide reduced marketable fruit numbers 13 to 20%, but only for the 20 DAP application. The 1/75 rate of each herbicide when applied at either 20 or 40 DAP reduced the number of fruit harvested per plot in 2015. Dicamba residues were detected in marketable fruit when the 1/75 rate in 2014 and 2015 and the 1/250 rate in 2015 was applied to plants at 40 or 60 DAP. Residues of 2,4-D were detected in 2015 when the 1/75 and 1/250 rates were applied at 60 DAP. Across both years, the maximum level of residue detected was 0.030 ppm. While early season injury may reduce watermelon yields, herbicide residue detection is more likely in marketable fruit when an off-target contact incident occurs closer to harvest.Nomenclature: 2,4-D; dicamba; watermelon; Citrullus lanatus, (Thunb.) Matsum. & Nakai

Much of the agricultural area in California’s southwestern San Joaquin Valley (SJV) is prone to moisture stress and high soil-salinity conditions. Increased prevalence of glyphosate-resistant (GR) biotypes of junglerice [Echinochloa colona (L.) Link] in these environments and their ecological implications need to be further explored. Studies were conducted on GR and glyphosate-susceptible (GS) biotypes of E. colona to compare the effects of moisture and salinity stress on seed germination and salinity stress alone on growth and seed production. Intraspecific competition between the GR and GS plants was also assessed in a replacement series design experiment. With respect to germination, both biotypes were tolerant to moisture and salinity stress at germination; however, the GR biotype was more tolerant than the GS biotype. Water potential and electrical conductivity (EC) levels that reduced germination by 50% were estimated as −1.5 and −2.3 MPa and 8.5 and 12 dS m−1 for the GS and GR biotypes, respectively. The EC levels that reduced aboveground biomass by 50% were estimated as 9 and 11.5 dS m−1 for the GS and GR biotypes, respectively. Seed production was generally greater in the GR than the GS plants below 10 dS m−1. All plants produced up to 140 seeds plant−1, even at 20 dS m−1. The GR plants were more competitive and produced more aboveground dry biomass and seeds than the GS plants when grown together or alone. In conclusion, differences between these particular GR and GS biotypes to environmental stresses and intraspecific competition were noted that could have ecological implications for their prevalence in the southwestern SJV. The results also suggested that there could be high genetic variability and phenotypic plasticity in E. colona populations in the SJV and further population shifts could occur due to selection pressure from glyphosate.

Field bindweed is a deep-rooted and drought-tolerant perennial that can be difficult to control once it has become established in specialty crops. Field studies were conducted in 2013 and 2014 to evaluate the efficacy of currently registered preplant (PP), PPI, PRE, and POST herbicides for field bindweed management in both early and late-planted processing tomatoes. Results show that bindweed cover in PPI/PRE programs (trifluralin, alone or in combination with rimsulfuron;S-metolachlor; or sulfentrazone) was reduced > 50% in early planted tomatoes, relative to the no PPI/PRE herbicide treatment (0 to 31% cover at up to 6 wk after transplanting [WAT]). Similar trends were observed with respect to field bindweed density. PP applications of glyphosate to emerged bindweed in late-planted tomatoes, coupled with PPI/PRE herbicide applications, reduced weed cover (1 to 13% at up to 6 WAT) by more than one-half when compared with plots treated with residual herbicides alone (1 to 43% at up to 6 WAT); perennial vine density was also reduced > 50%. PP herbicide burndown applications and the use of residual products can significantly improve the suppression of field bindweed in processing tomato systems. The emergence and vigor of bindweed vines may differ with respect to the timing of transplant operations and should be considered when developing management strategies

Resistance to glyphosate in hairy fleabane and horseweed is a problem in orchards and vineyards in California. Population genetic analyses suggest that glyphosate resistance evolved multiple times in both species, but it is unknown if resistance to other herbicides is also present. Two approaches of research were undertaken to further evaluate herbicide resistance in Conyza sp. in the perennial crop systems of California. In the initial study, the distribution of Conyza sp. in the Central Valley, using a semistructured field survey, was coupled with evaluation of the presence and level of glyphosate resistance in plants grown from field-collected seed. In a subsequent study, single-seed descendants representing distinct genetic groups were self-pollinated in the greenhouse and these accessions were evaluated for response to glyphosate and paraquat. Conyza sp. were commonly found throughout the Central Valley and glyphosate-resistant individuals were confirmed in all field collections of both species. The level of glyphosate resistance among field collections varied from 5- to 21-fold compared with 50% glyphosate resistance (GR50) of the susceptible, with exception of one region with a GR50 similar to the susceptible. When self-pollinated accessions from different genetic groups were screened, the level of glyphosate resistance, on the basis of GR50 values, ranged from 1.7- to 42.5-fold in hairy fleabane, and 5.9- to 40.3-fold in horseweed. Three accessions of hairy fleabane from different genetic groups were also resistant to paraquat (40.1- to 352.5-fold). One glyphosate-resistant horseweed accession was resistant to paraquat (322.8-fold), which is the first confirmed case in California. All paraquat-resistant accessions of Conyza sp. identified so far have also been resistant to glyphosate, probably because glyphosate resistance is already widespread in the state. Because glyphosate and paraquat resistances are found across a wide geographical range and in accessions from distinct genetic groups, multiple resistant Conyza sp. likely developed independently several times in California.

Glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Wats) is redefining row crop weed management in the southeast United States due to its widespread distribution, high competitive ability, copious seed production, and resilience against standard weed management programs. Herbicides alone are failing to provide adequate control of GR Palmer amaranth; thus, use of conventional tillage is increasing in the Southeast in order to control GR Palmer amaranth. Cultural practices consistent with maintenance of conservation tillage were evaluated to determine if they could suppress weeds in cotton (Gossypium hirsutum L.). An on-farm research and demonstration project was initiated in the fall of 2009 in Alabama, Georgia, South Carolina, and Tennessee to address well-founded concerns that conservation tillage systems were at risk because of GR Palmer amaranth. The research continued in certain states for two additional years. Cultural practices contrasted in the conservation tillage system were a no-till planted high-residue cover crop, one-time fall inversion tillage followed by the planting of a high-residue cover crop, and winter fallow only without a cover crop. Cover crop biomass yields varied from 570 to 6,790 kg ha−1 (509 to 6,063 lb ac−1) depending on location, type of cover crop, planting date, and environment. Fall inversion tillage increased cover crop biomass at all locations, likely due to preparation of a favorable seed bed for seedling establishment or alleviation of soil compaction or both. One-time fall inversion tillage reduced the number of GR Palmer amaranth escapes compared with the other treatments. Where GR Palmer amaranth densities were relatively low (approximately 1,000 plants ha−1 [405 plants ac−1] or fewer), there were few differences in the number of GR Palmer amaranth escapes among treatments. Where GR Palmer amaranth densities were relatively high (18,000 plants ha−1 [7,284 plants ac−1] or greater), winter fallow systems had higher Palmer amaranth densities escaping weed management programs compared to either cover crop system. The number of Palmer amaranth escapes declined exponentially as a function of cover crop biomass regardless of tillage. For sites with two years of data, no year by treatment interaction was detected, indicating that relative GR Palmer amaranth escape densities were sustained in each treatment for two seasons. Trends in cotton yields were the opposite of those for Palmer amaranth escapes. High-residue cover crops tended to suppress Palmer amaranth and increase cotton lint yields. However, no cultural management system consistently netted greater returns than other systems across locations and years.

Cotton genetically engineered to be resistant to topical applications of 2,4-D could provide growers with an additional tool for managing difficult-to-control broadleaf species. However, the successful adoption of this technology will be dependent on the ability of growers to manage off-target herbicide movement. Field experiments were conducted in Moultrie, GA, to evaluate cotton injury resulting from the volatilization of 2,4-D when formulated as an ester, an amine, or a choline salt. Each formulation of 2,4-D (2.24 kg ha−1) was applied in mixture with glyphosate (2.24 kg ha−1) directly to the soil surface (10 to 20% crop residue) in individual square blocks (750 m2). Following herbicide applications, replicate sets of four potted cotton plants (five- to seven-leaf stage) were placed at distances ranging from 1.5 to 48 m from the edge of each treatment. Plants were allowed to remain in-field for up to 48 h before being removed. Cotton exposed to 2,4-D ester for 48 h exhibited maximum injury ratings of 63, 57, 48, 29, 13, and 2% at distances of 1.5, 3, 6, 12, 24, and 48 m, respectively. Less than 5% injury was noted for the amine and choline formulations at any distance. Plant height was also affected by formulation and distance; plants that were located closest to the ester-treated block were smaller than their more distantly-positioned counterparts. Exposure to the amine and choline formulations did not affect plant heights. Additionally, two plastic tunnels were placed inside of each treated block to concentrate volatiles and maximize the potential for crop injury. Injury ratings of 76, 13, and 5% were noted for cotton exposed to the ester, amine, and choline formulations, respectively when under tunnels for 48 h. Results indicate that the choline formulation of 2,4-D was less volatile and injurious to cotton than the ester under the field conditions in this study.

Invasive and endemic weeds pose recurring challenges for California land managers. The evolution of herbicide resistance in several species has imposed new challenges in some cropping systems, and these issues are being addressed by UC Cooperative Extension farm advisors, specialists and faculty. There are currently 24 unique herbicide-resistant weed biotypes in the state, dominated by grasses and sedges in flooded rice systems and, more recently, glyphosate-resistant broadleaf and grass weeds in tree and vine systems, roadsides and glyphosate-tolerant field crops. Weed scientists address these complex issues using approaches ranging from basic physiology and genetics research to applied research and extension efforts in grower fields throughout the state. Although solutions to herbicide resistance are not simple and are affected by many biological, economic, regulatory and social factors, California stakeholders need information, training and solutions to address new weed management problems as they arise. Coordinated efforts conducted under the Endemic and Invasive Pests and Disease Strategic Initiative directly address weed management challenges in California's agricultural industries.

In 2005, the existence of glyphosate-resistance in Palmer amaranth was confirmed at a single 250 ha field site in Macon County, Georgia. Currently, all cotton producing counties in Georgia are infested, to some degree, with glyphosate-resistant Palmer amaranth. In 2010 and 2011, surveys were administered to Georgia growers and extension agents to determine how the development of glyphosate-resistance has affected weed management in cotton. According to respondents, the numbers of cotton acres that were treated with paraquat, glufosinate and residual herbicides effective against Palmer amaranth more than doubled between 2000 to 2005 and 2006 to 2010. Glyphosate use declined between 2000 to 2005 and 2006 to 2010 although, on average, the active ingredient was still applied to a majority of cotton acres. Although grower herbicide input costs have more than doubled following the evolution and spread of glyphosate resistance, chemically-based control of Palmer amaranth is still not adequate. As a consequence, Georgia cotton growers hand weeded 52% of the crop at an average cost of $57 per hand-weeded ha; this represents a cost increase of at least 475% as compared to the years prior to resistance. In addition to increased herbicide use and hand weeding, growers in Georgia are also using mechanical, in-crop cultivation (44% of acres), tillage for the incorporation of preplant herbicides (20% of the acres), and post-harvest deep-turning (19% of the acres every three years) for weed control. Current weed management systems are more diverse, complex and expensive than those employed only a decade ago, but are effective at controlling glyphosate-resistant Palmer amaranth in glyphosate-resistant cotton. The success of these programs may be related to producers improved knowledge about herbicide resistance, and the biological attributes that make Palmer amaranth so challenging, as well as their ability to implement their management programs in a timely manner. Nomenclature: 2,4-D; diuron; fomesafen; flumioxazin; fluometuron; glyphosate; glufosinate; MSMA; paraquat; pendimethalin; pyrithiobac; S-metolachlor; trifluralin; Palmer amaranth

Field studies were conducted to evaluate the capacity of Amaranthus palmeri to grow and reproduce following incomplete physical control in Gossypium hirsutum fields. A. palmeri plants that emerged simultaneously with a G. hirsutum crop were selected for use. Treatments included severing the main stem of flowering plants at heights of 0, 3 and 15 cm above the soil level. A non-cut/intact control, in which the apical meristem was not removed, was also included. Six weeks after treatment, intact A. palmeri plants had grown to a mean height of 210 cm (SE = 38) and produced 477 408 (SE = 81 250) seeds per plant. Thirty-five percent of the A. palmeri plants cut back to a height of 15 cm above the soil level did not recover from the treatment; survivors regrew to a mean height of 102 cm (51% reduction, compared to intact plants) and produced 116 000 seeds per plant (73% reduction). A. palmeri plants cut to 3 cm above the soil level had a mortality rate of 64%, an 82% reduction in final plant height, and produced 28 000 seeds per plant. When stems were severed at the soil surface, plant mortality was 95%; final plant height and seed production of survivors were reduced by 95 and 99%, respectively, relative to the control. G. hirsutum seeded yields exceeded 3 t ha−1 when A. palmeri plants were cut back to at least 15 cm, whereas yield was reduced 50% in the control treatment, where A. palmeri growth was not interrupted by cutting. In conclusion, while there is immediate benefit of removing A. palmeri plants in terms of G. hirsutum yield, incomplete stem removal can have multi-season implications. Results demonstrate that severely pruned A. palmeri plants can resume growth, reach reproductive maturity and produce viable seed, which have the potential to repopulate soil seedbanks.

Field studies were conducted to evaluate Italian ryegrass control and winter wheat tolerance to applications of diclofop, mesosulfuron plus methylated seed oil (MSO) alone or with 30% urea ammonium nitrate (UAN), mesosulfuron plus thifensulfuron plus tribenuron plus MSO, mesosulfuron plus MCPA plus MSO, or flufenacet plus metribuzin. Treatments were applied to wheat PRE, two- to three-leaf wheat (2–3 LF) at Feekes stage 1.0 or to one- to two-tiller wheat (TILL) at Feekes stage 3.0, depending on label recommendations. Studies were conducted in Williamson, GA, and Plains, GA, from autumn 2003 to spring 2005. Italian ryegrass control was variable, depending on location and year. Maximum and most-consistent Italian ryegrass control (> 90%) occurred with mesosulfuron plus MSO and UAN. Without UAN, control of Italian ryegrass with mesosulfuron varied from 44 to 97%. That variability was partially attributed to unfavorable environmental conditions associated with cold night time temperatures at or below 0 C, following applications. Wheat injury observed in response to herbicide treatments was minimal (< 15%) and transient; wheat recovered with no differences in yield.

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.

The continued phase-out of methyl bromide (MBr) challenges vegetable growers' abilities to control weeds in plasticulture production. Herbicides, such as EPTC (S-ethyl dipropylthiocarbamate), may be needed as part of a MBr alternative system. An experiment was conducted during the springs of 2008 and 2009 in Ty Ty, GA, to determine tomato, pepper, eggplant, and watermelon tolerance to EPTC applied under mulch. Treatments consisted of a factorial arrangement of four rates of EPTC (0, 2, 3, or 4 kg ai ha−1) and two plastic mulch types (low density polyethylene [LDPE] mulch or a high barrier mulch [HBM]). Each crop was planted 28 d after applying herbicides and laying mulch. EPTC, regardless of rate, applied under LDPE mulch did not impact plant growth, fruit number produced, or fruit weights for any crop. Conversely, pepper, tomato, and eggplant heights were reduced 65 to 72%, 30 to 75%, and 9 to 32%, respectively, by EPTC at 2 to 4 kg ai ha−1when applied under HBM. Similar trends were observed for crop yield; fruit number and weight were reduced by 71 to 84% for pepper, 36 to 76% for tomato, and 7 to 15% for eggplant when EPTC was applied at 2 to 4 kg ai ha−1as compared to the no EPTC HBM control. Watermelon stem lengths, fruit number, and fruit weights were not impacted by EPTC applied under HBM mulch. It appears as though HBMs reduce the loss of EPTC through volatilization, thereby increasing the dose present at time of planting. EPTC could be included as part of a MBr alternative system for tomato, pepper, eggplant, and watermelon when applied under LDPE mulch, and may also be applied at labeled rates with the HBM utilized in this experiment for watermelon.

In 2006, Palmer amaranth with confirmed resistance to glyphosate (GLY-R) was not controlled effectively in cotton with pyrithiobac, an acetolactate synthase (ALS)-inhibiting herbicide. Glyphosate at 870 g ae ha−1or pyrithiobac at 70 g ai ha−1applied postemergence provided 5 to 28% control of a putative GLY/ALS-R Palmer amaranth biotype in the field. Glyphosate at 6,930 g ha−1and pyrithiobac at 420 g ha−1applied alone provided no more than 89 and 65% control 1 to 8 wk after treatment (WAT), respectively. When applied as a tank mixture, glyphosate plus pyrithiobac at 870 + 70 g ha−1provided between 16 and 41% control; glyphosate plus pyrithiobac at 6,930 + 420 g ha−1controlled the Palmer amaranth in the field 89 to 95%. Dose-response analyses developed from greenhouse data indicated that the estimated glyphosate rates required to produce 50% injury and reduce plant fresh weights by 50% relative to the nontreated control in a suspected GLY/ALS-R Palmer amaranth biotype were 12 and 14 times greater, respectively, than the estimated values for the susceptible (S) biotype. The predicted pyrithiobac rates required to produce the same responses in the putative resistant population were 151 (50% injury) and 563 times (50% fresh weight reduction) greater than the estimated rates for the S biotype. Field and greenhouse analyses confirm that the Palmer amaranth biotype evaluated in both studies is resistant to glyphosate and an ALS-inhibiting herbicide.

Introduction of glyphosate resistance into crops through genetic modification has revolutionized crop protection. Glyphosate is a broad-spectrum herbicide with favorable environmental characteristics and effective broad-spectrum weed control that has greatly improved crop protection efficiency. However, in less than a decade, the utility of this technology is threatened by the occurrence of glyphosate-tolerant and glyphosate-resistant weed species. Factors that have contributed to this shift in weed species composition in Georgia cotton production are reviewed, along with the implications of continued overreliance on this technology. Potential scenarios for managing glyphosate-resistant populations, as well as implications on the role of various sectors for dealing with this purportedtragedy of the commons, are presented. Benghal dayflower, a glyphosate-tolerant species, continues to spread through Georgia and surrounding states, whereas glyphosate susceptibility in Palmer amaranth is endangered in Georgia and other cotton-producing states in the southern United States. Improved understanding of how glyphosate susceptibility in our weed species spectrum was compromised (either through occurrence of herbicide-tolerant or -resistant weed species) may allow us to avoid repeating these mistakes with the next herbicide-resistant technology.

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.

Palmer amaranth is resistant to several herbicides, including glyphosate, and there is concern that the resistance traits can be transferred between spatially segregated populations via pollen movement. The objective of this study was to describe the physical properties of Palmer amaranth pollen, specifically size, density, and settling velocity (Vs), that influence pollen flight. The mean diameter for Palmer amaranth pollen, as determined by light microscopy, was 31 µm (range of 21 to 38 µm); mean pollen diameter as measured with the use of an electronic particle sizer was 27 µm (range of 21 to 35 µm). The mean density of the solid portion of the pollen grain was 1,435 kg m−3. Accounting for the density of the aqueous fraction, the mean density of a fully hydrated pollen grain was 1,218 kg m−3. By Stokes's law, the estimated mean theoreticalVsfor individual Palmer amaranth pollen grains was 3.4 cm s−1for the range of pollen diameters with a mean of 31 µm and 2.6 cm s−1for the range of pollen diameters with a mean of 27 µm. Results from laboratory studies indicated the majority of single pollen grains settled at a rate of 5.0 cm s−1. The difference between the theoretical and empirical estimates ofVswas likely due to changes in pollen density and shape postanthesis, which are not accounted for using Stokes's law, as well as the presence pollen clusters.

The compositions of the germinable weed seedbank and aboveground weed communities in a long-term tillage and rotation study were characterized 4, 5, and 6 yr (2002 to 2004) after the adoption of glyphosate-tolerant corn and soybean. Averaged across rotation, mean germinable weed seed density and diversity were greatest in the no-tillage treatment as compared to the minimum- and conventional-tillage treatments. Averaged over tillage, density and diversity were greater in the corn–oat–hay (ryegrass + alfalfa) system as compared to the continuous corn and corn–soybean rotations. Similar trends in density and diversity were observed for the aboveground weed communities. Differences in community composition among treatments were quantified with the use of a multiresponse permutation procedure. Results indicated that the weed seedbank community in a corn–oat–hay rotational system differed from the communities associated with the continuous corn and corn–soybean rotational systems. Weed seedbank communities developing under a no-tillage operation differed from those in minimum- and conventional-tillage scenarios. Compositional differences among the aboveground weed communities were less pronounced in response to tillage and rotation. Indicator species analyses indicated that the number of significant indicator weed species was generally higher for no tillage than minimum or conventional tillage for both the seedbank and the aboveground weed communities. The number of significant indicator species for the seedbank and weed communities was generally greater in the three-crop rotation as compared to the continuous corn and corn–soybean rotations. The trends observed in density, diversity, and community composition after the adoption of glyphosate-tolerant corn and soybeans, and a glyphosate-dominated weed management program, were also observed when soil-applied herbicides were included in the study. We suggest that the switch to a POST-glyphosate protocol did not significantly alter weed communities in the short term in this study.

Garlic mustard seeds are dormant at maturity, and 90 to 105 d of cold-moist stratification at 4 C have been used to induce germination. We studied methods for breaking dormancy and inducing germination without cold stratification, for use in laboratory and greenhouse experiments with garlic mustard. Seeds were collected from large infestations, stored at room temperature, and subjected to chemical and mechanical scarification treatments. For chemical scarification, seeds were immersed in 3% (v/v) H2O2 for 12, 24, or 48 h with constant stirring, or immersed in concentrated (95 to 97%) H2SO4 for 1 or 5 min with stirring. For mechanical scarification, seeds were placed in a sandpaper-lined tumbler for 1 or 3 s. Scarified seeds, along with non-scarified seeds, were placed in petri dishes on germination blotters saturated with gibberellic acid (GA3, 10−3 M) or deionized water, and incubated for 35 d at either 20/10 C or 15/6 C (12 hr/12 hr). None of the non-scarified seeds germinated, regardless of germination solution or temperature. Seeds germinated only following scarification, and only when imbibed in GA3 solutions. Seeds immersed in H2SO4 for 5 min or mechanically scarified for 3 s had the highest level of germination in GA3. Cumulative percent germination after 35 d was greater for seeds stored 30 mo (44 to 83%), than for seeds stored 6 (2 to 60%) or 18 mo (35 to 79%), regardless of scarification treatment. The germination results, along with scanning electron micrographs of seed coats, suggest that the intact garlic mustard seed coat is permeable to water but not GA3; therefore, both scarification and GA3 are needed to break dormancy and induce germination without cold stratification.

Field experiments were conducted in Georgia in 2004 and 2005 to evaluate the effects of S-metolachlor on summer squash fruit yield. Main treatment effects included summer squash cultivar (yellow or zucchini), planting method (seeded or transplanted) and herbicide program (nontreated control, S-metolachlor applied at planting and prior to transplanting [PRE] at 0.5 and 1.0 kg ai/ha, S-metolachlor applied postemergence [POST] 3 wk after planting [WAP] at 0.5 and 1.0 kg/ha, and S-metolachlor applied PRE at 0.5 kg/ha followed by POST at 0.5 kg/ha [PRE fb POST]). Fruit number and weight were measured 12 times during each growing season and the harvests combined into early (harvests 1 to 4), middle (harvests 5 to 8), late (harvests 9 to 12), and cumulative (harvests 1 to 12) yield categories. Mixed-models analyses were used to evaluate the effects of herbicide rate and timing, squash cultivar, and planting method on squash yield for each harvest period. Summer squash cultivar and planting method did not affect ...

The importance of managing weeds in seminatural habitats that are adjacent to farm fields is unclear. Weedy-margin vegetation may harbor pests or pathogens and may ALSo serve as source populations for ongoing immigration of weeds into the field. It is ALSo possible, however, that margin vegetation provides habitat for organisms that consume weed seeds or suppress the likelihood of pest or pathogen outbreak. We examined the nature of margin habitat using spatial-scaling of Weed-Species richness as an ecological assay. In 2003, we recorded the occurrence of weedy species along the perimeters of 63 fields in Wisconsin. The fields were distributed within six counties that differed in topography, geological history, local climate, and soil type and which spanned the range of variability in the agricultural landscape. We identified seven habitats that differed in geology and land use. The relationship between species richness and margin class was estimated using an analog of the power law. Additionally, we investigated broadscale correlates of habitat heterogeneity at the field level, using a modeling strategy that included additional explanatory factors logically connected to plant diversity. Using a model-confrontation approach, the survey supported the inclusion of two topographical diversity indices, elevation gradient and a field-shape index, into our model. Our broadscale survey provides information on one of a suite of important considerations needed to make decisions about the importance of managing weeds in field margins.

Knowledge about how the type, timing, and arrangement of cultural practices influence weed species composition is important for understanding the ecological results of control strategies and designing alternative crop management systems. We evaluated weed seed density, diversity, and community composition from 1997 to 1999 in a 35-yr-old study comparing cropping sequences (continuous corn, corn–soybean, corn–oat–hay) and tillage systems (conventional, minimum, and no-tillage) in Wooster, OH. Weed seedbank diversity, as measured by species richness (S), evenness (J), and the Shannon–Weiner index (H′), was influenced by crop diversity; mean values for each of the indices were generally higher for all combinations of the three-crop sequence than for the corn monoculture or the corn–soybean rotation. Except for 1998, mean seed density (to a depth of 10 cm) was higher in continuous cornthan in corn and soybean rotationsSpecies richness and seed density were also affected by tillage. Mean values forand mean germinable seedswere greatest in the no-tillage system, where the soil was disturbed only by the coulter units of the planter. Differences in weed seedbank community composition among tillage and rotation treatments were examined using two multivariate analyses. Using a multiresponse permutation procedure and canonical discriminant analysis, results suggest that the weed seed community in a corn–oat–hay rotational system differs in structure and composition from communities associated with continuous corn and corn–soybean systems. Additionally, germinable weed seed communities in no-tillage differed in composition from those in conventional and minimum tillage. Crop sequence and tillage system influenced weed species density and diversity and therefore community structure. Manipulation of these factors could help reduce the negative impact of weeds on crop production.

Many orchards use fumigation to control soilborne pests prior to replanting. Controlling emissions is mandatory to reduce air pollution in California. This research evaluated the effects of plastic film type [polyethylene (PE) or totally impermeable film (TIF)], application rate of Telone C35 [full (610 kg ha(-1) ), 2/3 or 1/3 rates] and carbonation at 207 kPa on fumigant transport (emission and in soil) and efficacy.While increasing fumigant concentrations under the tarp, TIF reduced emissions95% (∼2% and1% of total applied 1,3-dichloropropene and chloropicrin respectively) relative to bare soil, compared with ∼30% reduction by PE. All fumigation treatments, regardless of film type, provided good nematode control above 100 cm soil depth; however, nematode survival was high at deeper depths. Weed emergence was mostly affected by tarping and fumigant rate, with no effects from the carbonation.TIF can effectively reduce fumigant emissions. Carbonation under the studied conditions did not improve fumigant dispersion and pest control. The 2/3 rate with TIF controlled nematodes as effectively as the full rate in bare soil or under the PE film to 100 cm soil depth. However, control of nematodes in deeper soil remains a challenge for perennial crops.

Opportunities for automated weed control vary widely among cropping systems of the Western Hemisphere. High-value conventional and organic horticultural crops may provide the best initial opportunity for automated weed control because of dependency on labor for hand weeding and the lack of effective herbicides. Fresh market vegetable crops are planted year-round in small successive plantings which make them an attractive target for weed control automation equipment. The most likely immediate role for weed control automation in agronomic crops would be in the area of sprayer control so that weed-infested patches could be treated selectively and weed-free patches not treated. If automated weed control solutions are developed and made cost effective and time efficient, then broader adoption of automated weed control technology is possible.

Community composition of the soil seedbank were characterized 35 years after the implementation of a long-term study involving cropping sequences (continuous corn, corn-soybean, corn-oat-hay) and tillage systems (conventional-, minimum- and no-tillage). Germinable seeds within the top 10 cm of soil in early spring were identified and enumerated in 1997, 1998 and 1999. Species diversity, which was characterized by richness (S), evenness (E) and the Shannon-Weiner index (H'), was significantly influenced by crop rotation rather than tillage. Generally, diversity measures were greatest in the corn-oat-hay sequences as compared to the corn-soybean rotations and the corn monoculture. Species richness and H' typically declined with increasing soil disturbance (no-tillage > minimum-tillage > conventional-tillage), whereas E increased with more intense tillage. A synthetic importance value (RI), incorporating both density and frequency measures, was generated for each species in each plot. Multiresponse permutation procedures (MRPP) were used to examine differences in weed community composition with respect to management system for all three years. Results suggest that the weed seed community in a corn-oat-hay rotational system differs substantially, in structure and composition, from communities associated with continuous corn and corn-soybean systems. No tillage systems were significantly different in composition as compared to conventional tillage and minimum tillage treatments. Crop sequence and tillage system are important cultural methods of shifting weed species number and diversity, and therefore, community structure. Manipulation of these factors could help to reduce the negative impact of weeds on crop production.

Our lab characterized the growth and development of 83 velvetleaf accessions, collected from locations in Asia, India, Europe, Eastern Africa and North America, to test the hypothesis that two biotypes (“crop” and “weedy”) exist and are easily differentiated. Measurements taken to gauge morphological and phenological variability include: initial seed weight, stem height at 3, 7, and 11 weeks, leaf size at 3, 7, and 11 weeks, stem and petiole color, time to flowering, time to first capsule maturity, stem height at flowering, height to first mature capsule, basal stem diameter, number of capsules, and capsule size and color. Analyses indicate that accessions producing yellow-colored seed capsules were taller, produced fewer nodes, and were longer-lived than their brown-colored counterparts. This finding supports previous assertions that the yellow-colored varieties were originally selected for use as a fiber crop: i.e., increased stem yield resulted in longer lengths of lignified tissue. The accessions producing brown-colored capsules exhibited greater reproductive output, as measured by the number of capsules and the number of seed-containing valves per capsule, a desirable trait for weedy species. Using capsule color as an independent variable, Discriminant Analysis was able to correctly classify 96% of the observations by the remaining characters, further affirming that the yellow- and brown-capsuled accessions varied, significantly, with respect to their morphology and phenology. Velvetleaf is believed to have originated in China, where it was eventually domesticated. Early records suggest that velvetleaf, a noxious weed in modern agricultural production, was introduced to colonial America to serve as a fiber source for the burgeoning rope-making industry.