Jill Schroeder

Carrier Volume Affects Herbicide Activity in Simulated Spray Drift Studies

Abstract: Field studies were conducted to determine if varying carrier volume proportionally with herbicide dosage, thus maintaining constant herbicide concentration in the carrier, would change the response of sweet corn to glyphosate and of cotton to 2,4-D when compared with using a constant carrier volume where herbicide concentration would vary and be more dilute. For all the parameters measured, more sweet corn injury occurred if the concentration of glyphosate was constant in all volumes of spray. The glyphosate no-effect level for sweet corn was determined to be 0.046 kg/ha when using the variable carrier volume but was over four times greater (0.185 kg/ha) when applied at the constant carrier volume of 281 L/ha. Cotton response to 2,4-D was similar, with the constant herbicide concentration in the carrier at the lower volumes causing greater injury. The response of seed cotton yield was not different when comparing constant to variable carrier volume. The highly sensitive growth stage of cotton at the time of application (bud formation before blooming) may explain this result. These studies demonstrate the need to use carrier volumes that are proportional to the herbicide dosage, thus maintaining constant herbicide concentration in the carrier, when conducting simulated herbicide drift research. Failure to do so could underestimate the potential for injury. Nomenclature: Glyphosate; 2,4-D; cotton, Gossypium hirsutum L. ‘Acala 1517-95’; sweet corn, Zea mays L. ‘Sweetie 82’. Additional index words: Crop injury, nontarget crops, off-site movement. Abbreviations: DAT, days after treatment; OM, organic matter; NMSU-PSRC, New Mexico State University Plant Sciences Research Center.

Mechanism of Resistance and Inheritance in Glyphosate Resistant Palmer amaranth (Amaranthus palmeri) Populations from New Mexico, USA

Abstract Two populations of Palmer amaranth from New Mexico have been confirmed to be resistant to glyphosate. In the present study, the molecular basis of resistance and the mode of inheritance of resistance in those populations were investigated. Quantitative real-time polymerase chain reaction analysis indicated up to an eightfold increase in genomic EPSPS copy number in glyphosate resistant plants compared with susceptible plants. The relative genomic EPSPS copy number of resistant plants was positively correlated with the relative EPSPS cDNA expression levels. Eight hours after treatment with glyphosate, the shikimate accumulation levels in resistant plants were negatively correlated with the genomic EPSPS copy numbers. Multiple sequencing of the EPSPS cDNA of resistant plants did not reveal any glyphosate resistance-conferring mutations. The evaluation of F1, reciprocal F1, and F2 Palmer amaranth families indicated that resistance to glyphosate does not follow a single-gene segregation pattern. Results suggest that the EPSPS amplification is the primary molecular basis of resistance in glyphosate resistant populations of Palmer amaranth from New Mexico. Nomenclature: Glyphosate, Palmer amaranth, Amaranthus palmeri S. Watts.

The role of weeds in nematode management

Abstract Weeds are alternative hosts for plant-parasitic nematodes and have long been recognized for their ability to maintain nematode populations targeted for suppression by various management strategies. The impact of weeds as alternative hosts depends largely on nematode feeding behavior, which is determined by the level of host specialization required for the parasite to feed successfully. In general, the more specialized feeding adaptations are associated with greater crop damage, more diverse nematode management options, and greater negative impact from weeds. Besides serving as alternative hosts, certain weeds can protect nematodes from pesticides and the environment, provide nematode suppression through antagonism, contribute to changes in future nematode biotic potential, or exert indirect effects through competition with crops or by the effects of weed control strategies on nematode populations. Shrinking nematicide options and increasing environmental concerns are making integrated pest management (IPM) a necessity for nematode management in many crops. A prominent similarity between most major weeds and plant-parasitic nematodes is that both are place-bound organisms that are passively dispersed. Weed–nematode interactions in agricultural production systems may be more intricate and complex than the simple function of weeds as alternative hosts. Their relationship may represent a normal adaptation resulting from the limited mobility of both groups of organisms and the obligate parasitism of phytophagous nematodes. The challenge that faces weed scientists and nematologists is to identify effective, compatible IPM strategies that address weed and nematode management collectively.

Impacts of crop pests on weeds and weed–crop interactions

Abstract The literature relating to the impact of other pests on weeds of agroecosystems is minimal. A great deal of literature discusses the effect of organisms used for biological control of weeds; however, pest organisms used as biological control agents are not the subject of this paper. The objective of this review is to present what is known about the impact of insect, pathogen, and nematode pests on weeds; to outline some of the gaps in our knowledge; to present concepts from the ecological literature that might provide insight; and to discuss implications for integrated pest management. The limited data that are available suggest that weeds require fewer resources to survive in the presence of the pest complex than the crop and that weeds would potentially have a greater ability to survive, compete, and reproduce in a competitive environment compared to the planted crop. We suggest that three categories of weed response to polyphagous crop pests may occur in agricultural fields: susceptible weed species or biotypes that host the pest with severe effects on growth and fecundity and therefore are of limited concern in terms of competition for resources; tolerant weed species that host the pest without severe effects on growth and fecundity, resulting in effective competition with the crop and larger pest populations; and resistant weed species that do not host the pest but compete effectively with the crop. We propose the hypothesis that the weed community in many agricultural fields is dominated by plant species that are tolerant or resistant to the endemic pest complex, particularly the soil pest complex, because of constant selection pressure from these pests.

Moisture and temperature requirements for London rocket (Sisymbrium irio) emergence

Abstract London rocket is hypothesized to be the most important overwintering host of the beet leafhopper in southern New Mexico. Knowledge of the environmental factors affecting the emergence of this plant is essential to developing a predictive model for London rocket and leafhopper populations. The temperature and moisture optima for London rocket germination and emergence were assessed using growth chamber studies. Temperatures of 5 to 35 C and soil water tension treatments of − 0.01 to − 1.5 MPa were tested. Optimal seed burial depth also was assessed. Optimal London rocket germination occurred at approximately 15 C and maximum emergence was achieved at 15 to 20 C with soil moisture tensions of − 0.01 to − 1.2 MPa. London rocket emergence occurred best at a seed burial depth of 2 mm. Nomenclature: London rocket, Sisymbrium irio L.

Resistance to Glyphosate in Palmer Amaranth (Amaranthus palmeri) Populations from New Mexico Pecan Orchards

Abstract Two populations of Palmer amaranth suspected of being resistant to glyphosate have been reported since 2007 in pecan orchards in Doña Ana County, New Mexico. The objectives of the study were to confirm and evaluate the level of resistance, to evaluate the effectiveness of alternative herbicide mechanisms of action, and to compare the cost of effective alternative herbicides for weed management in pecan orchards. Greenhouse experiments indicated that the resistant populations were able to survive glyphosate at 736 g ae ha−1. Compared with a susceptible (S) population, one of the resistant (R) populations had sevenfold resistance to glyphosate. POST application of 12 herbicides, with five different mechanisms of action, all provided at least 88% control of both R and S populations when applied at their recommended field rates. PRE application of trifluralin and metolachlor also provided more than 99% control of R and S populations. The results of field studies indicated that the financial benefit of season-long weed management with glyphosate, in pecan orchards, was comparable with some of the tested alternative herbicides. Nomenclature: 2,4-D; atrazine; carfentrazone-ethyl; dicamba; flumioxazin; glyphosate; glufosinate; imazethapyr; imazamox; metolachlor; oxyfluorfen; primisulfuron; prosulfuron; pyrithiobac; trifluralin; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; pecan, Carya illinoinensis (Wangenh) K. Koch. Resumen Dos poblaciones de Amaranthus palmeri sospechosas de ser resistentes a glyphosate han sido reportadas desde 2007 en plantaciones de pacana en el condado Doña Ana en New Mexico. Los objetivos de este estudio fueron confirmar y evaluar los niveles de resistencia, evaluar la efectividad de herbicidas con mecanismos de acción alternativos, y comparar la eficacia y el costo de herbicidas alternativos efectivos para el manejo de malezas en plantaciones de pacana. Experimentos de invernadero indicaron que las poblaciones resistentes fueron capaces de sobrevivir a glyphosate a 736 g ae ha-1. Comparada con una población susceptible (S), una de las poblaciones resistentes (R) tuvo una resistencia a glyphosate siete veces mayor. Aplicaciones en pos emergencia (POST) de 12 herbicidas, con cinco mecanismos de acción diferentes, proporcionaron un control de al menos 88% para ambas poblaciones R y S cuando se aplicaron a sus respectivas dosis de campo recomendadas. La aplicación en preemergencia (PRE) de trifluralin y metolachlor también brindó mas de 99% de control de poblaciones R y S. Los resultados de estudios de campo indicaron que en plantaciones de pacana, el beneficio monetario de realizar el manejo de malezas a lo largo de todo el ciclo productivo con glyphosate, fue comparable con algunas de los herbicidas alternativos evaluados.

Root-knot nematodes affect annual and perennial weed interactions with chile pepper

Abstract A field microplot experiment was conducted in 1996 and 1997 to determine the influence of root-knot nematodes on intra- and interspecific interactions between chile pepper (chile) and spurred anoda and between chile and yellow or purple nutsedge (or both) using a substitution design. An additional objective was to determine the influence of London rocket, a winter annual and host plant for root-knot nematodes, on the inter- and intraspecific interactions between chile and spurred anoda. Twelve plant combinations were planted into paired 76-cm-diam microplots at a density of 24 plants per microplot each year. Each pair of microplots had one root-knot nematode–infested and one uninfested plot. One randomly selected plant pair or triplet from each plot was destructively sampled in June, July, August, and September each year. Data included leaf area, plant dry weights (leaf, stem, root or root plus rhizome, chile fruit, and nutsedge tuber), and nematode egg production from the belowground biomass of the different plant species within a 2,355-cm3 sampled soil volume. Chile hosted the highest population of root-knot nematodes, followed by spurred anoda, purple nutsedge, and yellow nutsedge. Few root-knot nematode eggs were recovered from London rocket before incorporation into the microplots each spring. Root-knot nematode populations were higher in 1997 and, as a result, more interactions between nematodes and plant combinations were observed for chile. Spurred anoda and root-knot nematodes reduced chile shoot and root weights to levels not significantly different from zero in 1997. Yellow and purple nutsedge shoots, except for those from the original tuber, were removed throughout the season, and these species interfered less with chile. Spurred anoda was not affected by interspecific interference. Few interactions were observed between the spurred anoda plant combinations and root-knot nematodes. In June 1996, low populations of root-knot nematodes (< 4,000 eggs per gram of root) stimulated spurred anoda growth, but higher populations in June 1997 (> 30,000 eggs per gram of root) reduced spurred anoda growth. Prior presence of London rocket had little consistent influence on spurred anoda or chile. Yellow and purple nutsedge growth variables were affected by interference from chile and the other nutsedge species. Tuber number and tuber weight were higher when plants were infected with root-knot nematodes, particularly early in the season. The enhanced tuber production may increase early-season interference from nutsedge species under production conditions. The results suggest that the presence of root-knot nematodes influences plant interference, but the effect is species specific. Annual plants are affected differently compared with perennial nutsedges, possibly because of the continuous association between the perennials and the parasite. Nomenclature: London rocket, Sisymbrium irio L. SISIR; purple nutsedge, Cyperus rotundus L. CYPRO; spurred anoda, Anoda cristata (L.) Schleckt. ANVCR; southern root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood; yellow nutsedge, Cyperus esculentus L. CYPES; chile pepper, Capsicum annum L.

Purple Nutsedge (Cyperus Rotundus) Management in Direct-seeded Chile Pepper Using Halosulfuron and Cultivation

Field research was conducted in southern New Mexico to determine the effect of halosulfuron rate, timing of application, and adjuvant on purple nutsedge suppression and chile pepper injury under cultivated conditions. POST-directed treatments of 36 or 53 g ai/ha were applied 2 or 4 wk after thinning direct-seeded chile pepper. Herbicide treatments included a nonionic surfactant or methylated seed oil plus ammonium sulfate plus polyacrylamide polymer. Herbicide rate, adjuvant, and timing of the application had little effect on overall nutsedge suppression; however, herbicide treatment proved to be an important component for managing purple nutsedge in this experiment. Yields were higher with halosulfuron treatment compared with the weedy control. Whereas halosulfuron treatment with 53 g/ha increased chile yield relative to the weedy control, injury and yield results comparing 36 and 53 g/ha rates consistently suggested that halosulfuron should be applied at rates ≤ 36 g/ha in a single application to avoid possible injury. Halosulfuron treatment combined with cultivation and effective crop rotation may reduce the impact of purple nutsedge interference in chile pepper. Nomenclature: Halosulfuron, purple nutsedge, Cyperus rotundus L. CYPRO, chile pepper, Capsicum annuum L. ‘Sandia’

Southern Root-Knot Nematode Effect on Purple Nutsedge (Cyperus rotundus) and Chile Pepper Response to Halosulfuron'

Greenhouse experiments were conducted in the summer of 2002 to determine the effect of root-knot nematodes on purple nutsedge suppression and chile pepper response after applications of halosulfuron. Purple nutsedge and chile pepper plants were grown together in 20-cm-diam pots, inoculated with root-knot nematodes, and treated with halosulfuron 2 or 4 wk after inoculation. Root-knot nematode infection had no effect on purple nutsedge control or chile pepper injury after treatment with halosulfuron, regardless of application timing. However, root-knot nematode reproduction was lower in pots that received a halosulfuron treatment, particularly halosulfuron applied 2 wk after inoculation. Controlling purple nutsedge using halosulfuron in a field infested with root-knot nematodes could also decrease the amount of nematode inoculum that is available for infecting chile plants later in the season. Nomenclature: Halosulfuron; purple nutsedge, Cyperus rotundus L. #3 CYPRO; chile pepper, Capsicum annuum L. ‘Joe E. Parker’; southern root-knot nematode, Meloidogyne incognita (Kofoid and White) Chitwood, host race 3. Additional index words: Pest interactions, ALS herbicide, Capsicum annuum, CYPRO. Abbreviations: GH1, first greenhouse experiment; GH2, second greenhouse experiment; GH3, third greenhouse experiment.

Multi-phase US spread and habitat switching of a post-columbian invasive, Sorghum halepense

Johnsongrass (Sorghum halepense) is a striking example of a post-Columbian founder event. This natural experiment within ecological time-scales provides a unique opportunity for understanding patterns of continent-wide genetic diversity following range expansion. Microsatellite markers were used for population genetic analyses including leaf-optimized Neighbor-Joining tree, pairwise FST, mismatch analysis, principle coordinate analysis, Tajima’s D, Fu’s F and Bayesian clusterings of population structure. Evidence indicates two geographically distant introductions of divergent genotypes, which spread across much of the US in <200 years. Based on geophylogeny, gene flow patterns can be inferred to have involved five phases. Centers of genetic diversity have shifted from two introduction sites separated by ~2000 miles toward the middle of the range, consistent with admixture between genotypes from the respective introductions. Genotyping provides evidence for a ‘habitat switch’ from agricultural to non-agricultural systems and may contribute to both Johnsongrass ubiquity and aggressiveness. Despite lower and more structured diversity at the invasion front, Johnsongrass continues to advance northward into cooler and drier habitats. Association genetic approaches may permit identification of alleles contributing to the habitat switch or other traits important to weed/invasive management and/or crop improvement.