Steven D. Wright

Evolution and spread of glyphosate resistance in Conyza canadensis in California.

Recent increases in glyphosate use in perennial crops of California, USA, are hypothesized to have led to an increase in selection and evolution of resistance to the herbicide in Conyza canadensis populations. To gain insight into the evolutionary origins and spread of resistance and to inform glyphosate resistance management strategies, we investigated the geographical distribution of glyphosate resistance in C. canadensis across and surrounding the Central Valley, its spatial relationship to groundwater protection areas (GWPA), and the genetic diversity and population structure and history using microsatellite markers. Frequencies of resistant individuals in 42 sampled populations were positively correlated with the size of GWPA within counties. Analyses of population genetic structure also supported spread of resistance in these areas. Bayesian clustering and approximate Bayesian computation (ABC) analyses revealed multiple independent origins of resistance within the Central Valley. Based on parameter estimation in the ABC analyses, resistant genotypes underwent expansion after glyphosate use began in agriculture, but many years before it was detected. Thus, diversity in weed control practices prior to herbicide regulation in GWPA probably kept resistance frequencies low. Regionally coordinated efforts to reduce seed dispersal and selection pressure are needed to manage glyphosate resistance in C. canadensis.

Control of yellow starthistle (Centaurea solstitialis) and coast fiddleneck (Amsinckia menziesii) with aminopyralid.

Abstract Yellow starthistle is the most widespread broadleaf invasive plant in the western United States, and it is particularly prevalent in California. Prior to the registration of aminopyralid in 2005, the standard for chemical control of yellow starthistle was the herbicide clopyralid. We report on a compilation of several independent trials comparing the efficacy of aminopyralid and clopyralid on yellow starthistle. Treatments were applied at several rates and timings at 11 locations in four states between 2001 and 2007. Treatments were made pre-emergence and postemergence at the seedling and rosette stages of yellow starthistle. Results showed that aminopyralid, even at the low rate of 18 g ae ha−1, provided nearly complete control of yellow starthistle when treatments were made at the seedling stage. However, less consistent control (80 to 100%) resulted with applications made at the pre-emergence and rosette stages. At the seedling stage, aminopyralid is about four times more effective on yellow starthistle compared to clopyralid, based on the rate of acid equivalent. In the Central Valley of California, complete control was obtained at the lowest registered rate (53 g ae ha−1) when applications were made from December through February. At two locations we also evaluated control of the poisonous native plant coast fiddleneck. Although clopyralid does not adequately control coast fiddleneck, aminopyralid provided almost complete control when applied in the winter growing season. Applications of aminopyralid at the rosette stage resulted in a two-fold increase in annual forage grass biomass the following year. These results indicate that aminopyralid is a valuable tool for land managers and can play an important role in integrated management strategies for yellow starthistle and coast fiddleneck. Nomenclature: Aminopyralid; clopyralid; yellow starthistle; Centaurea solstitialis L.; coast fiddleneck; Amsinckia menziesii (Lehm.) A. Nels. & J. F. Macbr. var. intermedia (Fisch. & C. A. Mey.) Ganders. Interpretive Summary: Yellow starthistle is one of the most invasive species of rangelands and natural areas in the western United States. Several control options have been developed including mowing, grazing, burning, biological control, and herbicides. Until 2005, the herbicide clopyralid was considered the most effective chemical option for yellow starthistle control. However, clopyralid does not provide effective control of coast fiddleneck, which often co-occurs with yellow starthistle in rangelands of California. Coast fiddleneck is toxic to livestock and often increases in cover following clopyralid treatment. Aminopyralid was registered for use in noncrop areas in 2005. In this study we compared the efficacy of aminopyralid and clopyralid for yellow starthistle and coast fiddleneck control at several rates and timings at 11 locations in four states during the time period ranging from 2001 to 2007. Our results indicate that aminopyralid requires about one-fourth the active ingredient compared to clopyralid to achieve similar control of yellow starthistle at the seedling stage. Unlike clopyralid, aminopyralid also provides excellent control of coast fiddleneck. Although pre-emergence applications and late-season applications (rosette stage) also control yellow starthistle, those timings were less consistent at lower rates and therefore required higher rates as per label instructions. Late-stage applications were more effective given higher rainfall. We did find that late-season application resulted in increased annual forage grass production the following year. In the Central Valley of California, the optimal timing of application was from December through February. In cooler climatic regions the optimal application timing window might be somewhat later. Our results indicate that aminopyralid is a very effective tool for the management of yellow starthistle and coast fiddleneck and could be incorporated into an integrated management program.

Distribution of Conyza sp. in Orchards of California and Response to Glyphosate and Paraquat

Abstract 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. Nomenclature: Glyphosate; paraquat; hairy fleabane, Conyza bonariensis L. (Cronq.); horseweed, Conyza canadensis L. (Cronq.).

Glufosinate Safety in WideStrike® Acala Cotton

Abstract WideStrike® Acala cotton is a two-gene, in-plant trait that provides broad-spectrum and season-long control of lepidopteran insect pests, and the varieties available in California also have resistance to glyphosate. There have been indications that WideStrike cotton has some glufosinate tolerance as well, so the level of tolerance to glufosinate needed to be ascertained. A 2-yr (2008 and 2009) study was conducted in California to evaluate the potential crop injury caused by three different rates (0.59, 0.88, and 1.76 kg ai ha−1) of glufosinate–ammonium at four different growth stages (cotyledon, 2-node, 5- to 6-node, and 18- to 19-node stages) of WideStrike Acala cotton. The effects of these treatments on the cotton plants and yield were closely monitored. Glyphosate at 1.54 kg ae ha−1 was applied at all cotton growth stages as a standard application, and a nontreated control was included. The greatest level of injury (58%) was observed with the highest rate of glufosinate applied at both the cotyledon and the two-node stage of cotton. However, injury was less than 10% following glufosinate at 0.59 kg ha−1 applied at the 18- to 19-node stage. The level of injury increased with the higher application rate of glufosinate at all crop growth stages. In 2008 and 2009, the glufosinate treatments had no effect on cotton lint yield. Therefore, the study showed that glufosinate can be applied safely topically at 0.59 kg ha−1 at the cotyledon- to 2-node stage or as POST-directed spray between the 5- to 19-node stages. Although injury occurred at this rate, the plants recovered within 2 to 3 wk of the treatment. Increasing glufosinate rates beyond 0.59 kg ha−1 can increase the possibility of greater crop injury. Nomenclature: Glufosinate-ammonium; glyphosate; cotton; Gossypium hirsutum L. Resumen El algodón Acala WideStrike® posee dos genes que brindan control de amplio espectro de plagas insectiles-lepidóptera a lo largo de la temporada de crecimiento, y las variedades disponibles en California también tienen resistencia a glyphosate. Han habido indicaciones de que el algodón WideStrike también tiene algo de tolerancia a glufosinate, así que es necesario definir el nivel de tolerancia a este herbicida. Se realizó un estudio de dos años de duración (2008 y 2009) en California, para evaluar el potencial de daño al cultivo causado por tres dosis diferentes (0.59, 0.88, y 1.76 kg ai ha−1) de glufosinate ammonium en cuatro estadios de crecimiento (cotiledón, 2 nudos, 5 a 6 nudos, y 18 a 19 nudos) de algodón Acala WideStrike. Se le dio seguimiento detallado a los efectos de estos tratamientos en las plantas y el rendimiento del algodón. Se aplicó glyphosate a 1.54 kg ae ha−1 en todos los estadios de crecimiento como estándar de aplicación, y se incluyó un testigo sin tratamiento. El mayor nivel de daño (58%) se observó con la dosis mayor de glufosinate aplicada en los estadios de cotiledón y 2 nudos del algodón. Sin embargo, el daño fue menos de 10% después de aplicaciones de glufosinate a 0.59 kg ha−1 en el estadio de 18 a 19 nudos. El nivel de daño incrementó con la dosis mayor de glufosinate en todos los estadios de crecimiento del cultivo. En 2008 y 2009, los tratamientos de glufosinate no tuvieron ningún efecto en el rendimiento de fibra del algodón. Así, el estudio mostró que se puede aplicar glufosinate tópicamente en forma segura a 0.59 kg ha−1 en los estadios de cotiledón y de 2 nudos, o en forma POST-dirigida en los estadios de 5 a 19 nudos. Aunque hubo daños con esta dosis, las plantas se recuperaron 2 a 3 semanas después del tratamiento. Aumentar las dosis de glufosinate más allá de 0.59 kg ha−1 puede incrementar la posibilidad de observar un mayor daño en el cultivo.

Impact of Early Defoliation on California Pima Cotton Boll Opening, Lint Yield, and Quality

Chemical defoliation is a necessary pre-harvest practice in Pima cotton (Gossypium barbadense L.) production in California. Growers begin defoliating as early as possible but yield and quality loss can occur if the bolls are not fully mature. Harvest aids can advance harvest dates, avoid late-season pests, and adverse weather conditions in California. A study was conducted on Pima cotton, cv. ‘Phytogen-802’. Different rates of Ginstar (ai thidiazuron/diuron, Bayer CropScience) or Ginstar plus Finish 6-Pro (ai ethephon/cyclanilide, Bayer CropScience) were applied at 6 to 7 nodes above cracked boll (NACB) or 4 to 5 NACB at various rates. Results showed that these harvest aids could be applied at the tested rates at both timings without any adverse effects on percent open bolls, and lint yield and quality. Therefore, application of these harvest-aid materials starting at 6 to 7 NACB can benefit Pima cotton growers in California as early harvests can be achieved without compromising lint yield or quality.

Moisture and Salt Tolerance of a Forage and Grain Sorghum Hybrid during Germination and Establishment

ABSTRACT Sorghum [Sorghum bicolor (L.) Moench] is being explored in California as a multi-utility, water-use efficient crop, but water and salt stresses are common in the state’s San Joaquin Valley (SJV). Experiments were conducted to study the effect of water and salt stress on seed germination and the effect of salt stress on the early growth of a forage (SS405) and a grain (NK5418) sorghum hybrid. Solutions ranging from 0 to −5.56 MPa and 0 to 25 ds m−1 were prepared using polyethylene glycol and sodium chloride, respectively, and germination was evaluated in these solutions. Salinity tolerance at early growth stage was tested on plants irrigated with solutions ranging from 0 to 20 ds m−1. Plants were harvested 6 weeks after planting and their aboveground biomass was recorded. Chlorophyll content and stomatal conductance were also measured. Germination was reduced by 50% in SS405 and NK5418 at −2.5 MPa and −1.5 MPa, respectively, and by 50% in SS405 at 19.3 dS m−1, but NK5418 had 70% germination even at 25 dS m−1. Biomass of both hybrids was reduced by 50% at 16.8 dS m−1. Chlorophyll content and stomatal conductance of both hybrids was reduced by 50% at 10 dS m−1. Such studies may help in varietal selection of sorghum for cultivation in the SJV.

Growth and development of fall- and spring-planted populations of Conyza canadensis and C. bonariensis

ABSTRACT The growth and phenological development of spring- and fall-planted Conyza canadensis and C. bonariensis were studied in Fresno, CA, USA. A glyphosate-resistant (GR) and a glyphosate-susceptible (GS) population of each species were used. Time taken by each plant to reach the rosette, bolting, appearance of first bud, appearance of first open flower, and initial seed set were converted to growing degree days (GDDs). The fall- and spring-planted C. canadensis required similar GDDs to complete their life cycles but the GR type required fewer GDDs than the GS type. In C. bonariensis, the GDDs required to complete its life cycle differed between the fall- and spring-plantings but not between the GR and GS types. The total aboveground biomass of both fall- and spring-planted Conyzas at initial seed set was similar but the spring-planted GS C. canadensis produced more biomass than the GR type, whereas the fall-planted GS C. bonariensis produced more biomass than the GR type. Plant development based on GDD models can have global implications and help in planning for timely applications of post-emergence herbicides.

Response of Acala Cotton to Nitrogen Rates in the San Joaquin Valley of California

The responses of Acala cotton (Gossypium hirsutum L.) in California to a range of applied nitrogen (N) treatments were investigated in a 5-year, multisite experiment. The experiment’s goals were to identify crop growth and yield responses to applied N and provide information to better assess the utility of soil residual N estimates in improving fertilizer management. Baseline fertilizer application rates for the lowest applied N treatments were based on residual soil nitrate-N (NO3-N) levels determined on soil samples from the upper 0.6 m of the soil collected prior to spring N fertilization and within 1 week postplanting each year. Results have shown positive cotton lint yield responses to increases in applied N across the 56 to 224 kg N/ha range in only 41% (16 out of 39) of test sites. Soil NO3-N monitoring to a depth of 2.4 m in the spring (after planting) and fall (postharvest) indicate most changes in soil NO3 occur within the upper 1.2 m of soil. However, some sites (those most prone to leaching losses of soluble nutrients) also exhibited net increases in soil NO3-N in the 1.2- to 2.4-m depth zone when comparing planting time vs. postharvest data. The lack of yield responses and soil NO3-N accumulations at some sites indicate that more efforts should be put into identifying the amount of plant N requirements that can be met from residual soil N, rather than solely from fertilizer N applications.