Bradley D. Hanson

Distribution of Glyphosate-Resistant Horseweed (Conyza canadensis) and Relationship to Cropping Systems in The Central Valley of California

Abstract Horseweed is an increasing problem in perennial crops and noncrop areas of the Central Valley of California. Similar to the situation in glyphosate-tolerant crops in other regions, glyphosate-based weed-management strategies in perennial crops and noncrop areas have resulted in selection of a glyphosate-resistant horseweed biotype in California. Research was conducted to determine the level of resistance to glyphosate in horseweed using an in vivo enzyme assay and to determine the distribution of the resistant horseweed biotype in central California. The resistant biotype was 4.8-fold more resistant to in vivo glyphosate exposure compared with the susceptible biotype, although enzyme function was inhibited in both biotypes at high glyphosate concentrations. An intermediate in vivo glyphosate dose was used to discriminate between glyphosate-resistant and glyphosate-susceptible individuals in a roadside survey conducted in 2006 to 2007. Overall, 62% of the individuals tested from the Central Valley were classified as resistant to glyphosate. Resistant individuals were found at most locations throughout the Central Valley, although the proportion of resistant individuals was slightly lower in the northern-most area. No correlation could be made between proportion of resistant or susceptible individuals and land use patterns likely because of long-distance seed dispersal or different selection pressure for resistant biotypes on field margins compared with that within fields. Horseweed with an economically significant level of resistance to glyphosate is already widely distributed in the Central Valley of California. Grower awareness of the problem and adoption of best management practices are needed to minimize the effects of horseweed in this highly productive and diverse agricultural region. Nomenclature: Glyphosate; horseweed, Conyza canadensis (L.) Cronq. ERICA

Growth, Phenology, and Intraspecific Competition between Glyphosate-Resistant and Glyphosate-Susceptible Horseweeds (Conyza canadensis) in the San Joaquin Valley of California

Abstract Experiments were conducted in 2006 to 2008 to study growth, phenology, and competitive ability of glyphosate-resistant (GR) and -susceptible (GS) biotypes of horseweeds from San Joaquin Valley (SJV), CA. When grown alone, in pots, the GR horseweeds consistently developed more rapidly than the GS weeds, as evidenced by their earlier bolting, flowering, and seed set; the GR horseweeds set seeds nearly 25 d (approximately 190 fewer growing degree days) sooner than the GS horseweed. At seed set, the relatively slow-developing GS horseweeds had amassed 40% more shoot dry matter than the GR weeds at the same phenological stage, but neither biotype was consistently more fecund than the other. Although the GR biotype had lower shoot dry mass than the GS biotype when grown alone, in mixed populations under increasing levels of competition (in a replacement series design) and limited resources (mainly moisture), the GR weeds were not only taller, but also accumulated more dry matter than the GS weeds. Thus, the GR biotype was more competitive than the GS biotype, particularly when grown at high densities and under moisture-deficit stress. Therefore, under California conditions there is no apparent fitness penalty for this particular GR horseweed biotype, and it is likely to persist in the environment and outcompete the GS biotypes regardless of further glyphosate selection pressure. If so, this biotype of GR horseweed is likely to become increasingly common in the SJV until effective management strategies are developed and adopted. Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. ERICA

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.

Effects of Manure and Water Applications on 1,3-Dichloropropene and Chloropicrin Emissions in a Field Trial

Minimizing fumigant emissions is required for meeting air-quality standards. Application of organic materials to surface soil has been effective in reducing fumigant emissions during laboratory tests, but the potential to reduce emissions in the field has not been adequately evaluated. The objective of this study was to determine the effect of incorporated composted manure with or without water applications on fumigant emissions and the potential impact on pest control efficacy under field conditions. Treatments included a bare-soil control, composted dairy manure at 12.4 and 24.7 Mg ha(-1), postfumigation intermittent water seals (11 mm water irrigated immediately following fumigation and 4 mm at 12, 24, and 48 h), and incorporation of manure at 12.4 Mg ha(-1) combined with the water seals or a high-density polyethylene (HDPE) tarp. Telone C35 was shank-applied at 553 kg ha(-1), and emissions of 1,3-dichloropropene (1,3-D) and chloropicrin (CP) were monitored for 10 days. The results indicate that there was no significant difference in emission peak flux and cumulative emission loss between the control and the 12.4 Mg ha(-1) manure treatment. The higher manure rate (24.7 Mg ha(-1)) resulted in lower emission flux and cumulative emission loss than 12.4 Mg ha(-1), although the differences were only significant for CP. In contrast, the water treatments with or without manure incorporation significantly reduced peak emission rates (80% reduction) and cumulative emission loss ( approximately 50% reduction). The manure + HDPE treatment resulted in the lowest CP emissions but slightly higher 1,3-D emissions than the water treatments. Reductions in peak emission from water treatments can be important in reducing the potential acute exposure risks to workers and bystanders. This research demonstrated that incorporation of composted manure alone did not reduce fumigant emissions and effective emission reduction with manure amendment may require higher application rates and/or more effective materials than those used in this study.

Effects of Imazethapyr and Pendimethalin on Lentil (Lens culinaris), Pea (Pisum sativum), and a Subsequent Winter Wheat (Triticum aestivum) Crop1

Abstract: Lentil and pea are two important crops grown in rotation with winter wheat in the Palouse region of Idaho and Washington. Imazethapyr plus pendimethalin often is used to control weeds in lentil and pea, but the effects of these herbicides on these crops and the subsequently planted winter wheat crop are not well known. The component and combined effects of several rates of imazethapyr and pendimethalin on growth and yield of lentil and pea and the subsequently planted winter wheat crop were measured in 1997 and 1998 field experiments. No herbicide treatment reduced lentil or pea biomass or seed yield compared with the untreated control. Wheat biomass was reduced 35 to 51%, and grain yield was reduced 11 to 17% in all plots treated with 2,240 g/ha pendimethalin at the lentil hilltop site. Imazethapyr at 106 g/ha plus 1,120 g/ha pendimethalin also reduced wheat biomass 24% at the lentil hilltop site. Wheat was not injured at other sites or by other treatments at the lentil hilltop site. Nomenclature: Imazethapyr; pendimethalin; lentil, Lens culinaris L.; pea, Pisum sativum L.; wheat, Triticum aestivum L. Additional index words: Herbicide persistence, rotational crop safety. Abbreviations: DAP, days after planting; DAT, days after treatment; PPI, preplant incorporated.

Field tests of surface seals and soil treatments to reduce fumigant emissions from shank injection of Telone C35

A mixture of 1,3-dichloropropene (1,3-D) and chloropicrin (CP) (Telone C35) is an increasingly used fumigant product for pre-plant soil fumigation in California, USA. Atmospheric emissions of volatile organic compounds, including these important pesticides, is more heavily regulated in an effort to improve air-quality. Research has identified various methods of reducing fumigant emissions but effective and economically feasible field methods are still needed. The objective of this field study was to determine the effectiveness of several surface seal and soil treatment methods on emissions of 1,3-D and CP from shank-injected Telone C35. Treatments included control (bare surface), pre-irrigation (irrigation prior to fumigation), post-fumigation water seals with or without potassium thiosulfate (KTS) amendment, and standard high density polyethylene (HDPE) tarp over soils amended with either KTS or composted manure. The two KTS treatments resulted in the lowest fumigant emissions; but the soil surface in the treatments developed a reddish-orange color and an unpleasant odor that lasted for a few months. The pre-irrigation reduced emissions more than post-application water seals. An application of composted manure at 12.4 Mg ha(- 1) spread over the soil surface followed by HDPE tarp did not reduce 1,3-D emissions compared to the bare soils in this trial, indicating that a better understanding of processes is required in order to effectively use organic amendments for minimizing fumigant emissions. Chloropicrin emissions were generally lower than 1,3-D for all treatments.

Glyphosate Resistance Is More Variable Than Paraquat Resistance in a Multiple-Resistant Hairy Fleabane (Conyza bonariensis) Population

Abstract Reduced control of some glyphosate-resistant hairy fleabane populations with paraquat has raised concerns about evolved multiple resistance to both glyphosate and paraquat. The objective of this study was to confirm the presence of multiple-resistant (glyphosate and paraquat) hairy fleabane populations in California. A series of dose-response experiments was conducted to evaluate the effect of glyphosate and paraquat in a known susceptible (S) and putative multiple-resistant (R) population of hairy fleabane. The greenhouse experiments were conducted during summer, fall, and winter under controlled temperature and natural light conditions. Multiple-resistant hairy fleabane was identified; however, the level of resistance to glyphosate varied substantially among seasons. During the summer, the glyphosate rate required to reduce growth by 50% (GR50) for the R population was 0.94 kg ae ha−1, 5.2-fold more than for the S population. In the fall and winter experiments, however, the R population response to glyphosate was similar to the S population with a GR50 of 0.22 kg ae ha−1 or less. Multiple-resistant plants were controlled in the fall and winter at rates that did not control the same population during summer. GR50 of paraquat varied among seasons (0.94, 0.24, and 0.07 kg ai ha−1 during summer, fall, and winter, respectively); however, plant mortality was more consistent. This is the first reported case of glyphosate–paraquat resistance in hairy fleabane and the multiple-resistant population could pose a significant challenge to annual no-till and perennial cropping systems in California. Further research on the mechanisms of resistance and the physiological factors underlying the seasonally variable response to glyphosate is needed. Nomenclature: Glyphosate; paraquat; hairy fleabane, Conyza bonariensis (L.) Cronq.

Field evaluation of a new plastic film (vapor safe) to reduce fumigant emissions and improve distribution in soil.

Preplant soil fumigation is an important pest management practice in coastal California strawberry production regions. Potential atmospheric emissions of fumigants from field treatment, however, have drawn intensive environmental and human health concerns; increasingly stringent regulations on fumigant use have spurred research on low-emission application techniques. The objectives of this research were to determine the effects of a new low-permeability film, commonly known as totally impermeable film (TIF), on fumigant emissions and on fumigant distribution in soil. A 50/50 mixture of 1,3-dichloropropene (1,3-D) and chloropicrin (CP) was shank-applied at 314 kg ha in two location-separate field plots (0.4 ha each) in Ventura County, California, in fall 2009. One plot was surface-covered with standard polyethylene (PE) film, and the other was covered with TIF immediately after fumigant application. Data collection included emissions, soil-gas phase concentration profile, air concentration under the film, and soil residuals of the applied fumigants. Peak emission flux of 1,3-D and CP from the TIF field was substantially lower than from the PE field. Total through-film emission loss was 2% for 1,3-D and <1% for CP from the TIF field during a 6-d film covering period, compared with 43% for 1,3-D and 12% for CP from the PE field. However, on film-cutting, greater retention of 1,3-D in the TIF field resulted in a much higher emission surge compared with the PE field, while CP emissions were fairly low in both fields. Higher concentrations and a more uniform distribution in the soil profile for 1,3-D and CP were observed under the TIF compared with the PE film, suggesting that the TIF may allow growers to achieve satisfactory pest control with lower fumigant rates. The surging 1,3-D emissions after film-cutting could result in high exposure risks to workers and bystanders and must be addressed with additional mitigation measures.

Interactive effect of organic amendment and environmental factors on degradation of 1,3-dichloropropene and chloropicrin in soil.

Soil organic matter is an important factor affecting the fate of soil fumigants; therefore, the addition of organic amendments to surface soils could reduce fumigant emissions by accelerating fumigant degradation. Experiments were conducted to determine the degradation of fumigants [a mixture of cis- and trans-1,3-dichloropropene (1,3-D) and chloropicrin (CP), a similar composition as in Telone C35] in soils with organic amendment under a range of soil moisture, temperature, sterilization, and texture conditions. Degradation of the fumigants followed availability-adjusted first-order or pseudo-first-order kinetics with slower degradation of 1,3-D than CP. Increasing soil water content from 5 to 17.5% (w/w) slightly increased the degradation of 1,3-D, but not that of CP. Five different organic amendments at 5% (w/w) increased fumigant degradation 1.4-6.3-fold in this study. The degradation of both fumigants was accelerated with increasing amount of organic material (OM). Little interaction between soil moisture and OM was observed. Autoclave sterilization of soils did not reduce degradation of either fumigant; however, increasing the incubation temperature from 10 to 45 degrees C accelerated fumigant degradation 5-14 times. Soil texture did not affect 1,3-D degradation, but CP degraded more rapidly in finer-textured soil. These results suggest that OM type and rate and soil temperature are the most important factors affecting the degradation of 1,3-D and CP.

Effects of reduced-rate methyl bromide applications under conventional and virtually impermeable plastic film in perennial crop field nurseries.

BACKGROUND Producers of perennial crop nursery stock in California use preplant soil fumigation to meet state phytosanitary requirements. Although methyl bromide (MB) has been phased out in many agricultural industries, it is still the preferred treatment in the perennial nursery industry and is used under Critical Use Exemptions and Quarantine/Preshipment provisions of the Montreal Protocol. The present research was conducted to evaluate reduced-rate MB applications sealed with conventional and low-permeability plastic films compared with the primary alternative material. RESULTS Reduced rates (100-260 kg ha(-1)) of MB applied in combination with chloropicrin (Pic) and sealed with a low-permeability plastic film provided weed and nematode control similar to the industry standard rate of 392 kg ha(-1) MB:Pic (98:2) sealed with high-density polyethylene (HDPE) film. However, the primary alternative chemical, 1,3-dichloropropene (1,3-D), tended to provide slightly lower pest control even on sites with relatively low plant parasitic nematode, soil-borne pathogen and weed pest pressure. CONCLUSION If California regulations change to allow the use of low-permeability films in broadcast fumigant applications, the results of this research suggest that reduced rates of MB in perennial crop nurseries could serve as a bridge strategy until more technically, economically and environmentally acceptable alternatives are developed.