Evaluating Branched Broomrape (Phelipanche ramosa) Management Strategies in California Processing Tomato (Solanum lycopersicum).

Detections of the regulated noxious parasitic weed branched broomrape (Phelipanche ramosa) in California tomato fields have led to interest in eradication, sanitation, and management practices. Researchers in Israel developed a decision-support system and herbicide treatment regime for management of Egyptian broomrape (P. aegyptiaca) in tomato. Research was conducted in 2019 and 2020 to evaluate whether similar treatments could be used to manage branched broomrape in California processing tomatoes and to provide registration support data for the herbicide use pattern. Treatment programs based on preplant incorporated (PPI) sulfosulfuron and chemigated imazapic were evaluated in 2019 and 2020 to determine safety on the processing tomato crop and on common rotational crops. Three single-season tomato safety experiments were conducted and a single rotational crop study was conducted in which a tomato crop received herbicide treatments in 2019 and several common rotational crops were planted and evaluated in 2020 in a site without branched broomrape. In 2020, an efficacy study was conducted in a commercial tomato field known to be infested with branched broomrape to evaluate the efficacy of PPI sulfosulfuron and chemigated imazapic, imazapyr, imazethapyr, and imazamox. After two field seasons, sulfosulfuron and imazapic appeared to have reasonable crop safety on tomato in California; however, rotational crop restrictions will need to be considered if sulfosulfuron is used to manage branched broomrape. In the efficacy study, there was a trend in which the sulfosulfuron and imidazolinone treatments had fewer broomrape shoots per plot than the grower standard treatments, however, none were fully effective and there were no significant differences among the various sulfosulfuron and imidazolinone treatment combinations. Additional research is needed to optimize the treatment timing for management of branched broomrape in this cropping system. Because of registration barriers with imazapic in the California market, future research will focus on treatment combinations of PPI sulfosulfuron and chemigated imazamox rather than imazapic.

Role of Glutamine Synthetase Isogenes and Herbicide Metabolism in the Mechanism of Resistance to Glufosinate in Lolium perenne L. spp. multiflorum Biotypes from Oregon.

Glufosinate-resistant Lolium perenne L. spp. multiflorum biotypes from Oregon exhibited resistance levels up to 2.8-fold the field rate. One resistant biotype (MG) had an amino acid substitution in glutamine synthetase 2 (GS2), whereas the other (OR) exhibited the wild-type genotype. We hypothesized that the amino acid substitution in GS2 is involved in the resistance mechanism in MG and that non-target site resistance mechanisms are present in OR. OR metabolized glufosinate faster than the other two biotypes, with >75% of the herbicide metabolized in comparison to 50% in MG and the susceptible biotype. A mutation in GS2 co-segregating with resistance in MG did not reduce the enzyme activity, with results further supported by our enzyme homology models. This research supports the conclusion that a metabolism mechanism of glufosinate resistance is present in OR and that glufosinate resistance in MG is not due to an altered target site.

Increased temperatures and elevated CO2 levels reduce the sensitivity of Conyza canadensis and Chenopodium album to glyphosate.

Herbicides are the most commonly used means of controlling weeds. Recently, there has been growing concern over the potential impacts of global climate change, specifically, increasing temperatures and elevated carbon dioxide (CO2) concentrations, on the sensitivity of weeds to herbicides. Here, glyphosate response of both Conyza canadensis and Chenopodium album was evaluated under different environmental conditions. Reduced glyphosate sensitivity was observed in both species in response to increased temperature, elevated CO2 level, and the combination of both factors. Increased temperature had greater effect on plant survival than elevated CO2 level. In combination, high temperature and elevated CO2 level resulted in loss of apical dominance and rapid necrosis in glyphosate-treated plants. To investigate the mechanistic basis of reduced glyphosate sensitivity, translocation was examined using 14C-glyphosate. In plants that were subjected to high temperatures and elevated CO2 level, glyphosate was more rapidly translocated out of the treated leaf to shoot meristems and roots than in plants grown under control conditions. These results suggest that altered glyphosate translocation and tissue-specific sequestration may be the basis of reduced plant sensitivity. Therefore, overreliance on glyphosate for weed control under changing climatic conditions may result in more weed control failures.

EPSPS duplication and mutation involved in glyphosate resistance in the allotetraploid weed species Poa annua L.

BACKGROUND: Poa annua is a widespread winter annual weed species in California. Recently, poor control of this species with glyphosate was reported by growers in an almond orchard in California with a history of repetitive glyphosate use. The objectives of this research were to evaluate the level of glyphosate resistance in a developed S4 P. annua line (R) and identify the mechanisms of resistance involved. RESULTS: Whole-plant dose-response experiments confirmed glyphosate resistance in R, which required 18-fold more glyphosate to achieve a 50% growth reduction compared with a susceptible line (S), results that were supported by the lower shikimate accumulation observed in R compared with S. No differences in glyphosate absorption, translocation, or metabolism were observed, suggesting that non-target-site mechanisms of resistance are not involved in the resistance phenotype. A missense single nucleotide polymorphism was observed in EPSPS coding position 106 in R, resulting in a leucine to proline substitution. This polymorphism was observed exclusively in P. supina EPSPS homeologs. A seven-fold increase in the number of copies of EPSPS alleles was observed in R compared with S. CONCLUSIONS: We report the first case of glyphosate resistance associated with both EPSPS duplication and target-site mutation at position 106, leading to high levels of glyphosate resistance in the allotetraploid weed species Poa annua L. Data obtained in this research will be useful for the development of diagnostic tools for rapid glyphosate resistance identification, monitoring and containment. © 2018 Society of Chemical Industry.

Deep injection and the potential of biochar to reduce fumigant emissions and effects on nematode control.

Reducing fumigant emissions is essential for minimizing the environmental impacts of pre-plant soil fumigation. Low permeability plastic films are effective at reducing emissions but have high initial purchase, installation, and disposal costs. The objective of this study was to evaluate if deep fumigant injection and biochar soil amendments can reduce emissions, improve fumigant distribution in soil, and provide acceptable control of plant parasitic nematodes. A pre-plant soil fumigation trial was conducted in a commercial orchard in the San Joaquin Valley, CA, USA. Treatments included two rates of Telone® C-35 (a mixture of 1,3-dichloropropene and chloropicrin) under totally impermeable film or with no surface seal, two injection depths (45 or 65 cm), and two biochar rates (20 or 40 ton ha-1). Emission rates were generally low due to rain events encountered during the trial, but data clearly showed that the deep injection enhanced fumigant delivery to depths below 60 cm and resulted in significantly lower peak emission compared to the standard injection depth. Biochar applied at 40 ton ha-1 had the lowest emission rates during 1-month monitoring period. Although variability in nematode survival was high, tarped, deep injection, and biochar treatment showed lower survival of nematodes at various depths. Increase in fumigant persistence, especially chloropicrin, was observed in this study, likely due to the high soil moisture and low temperature. All data indicate that biochar amendments can help reduce fumigant emissions without reducing nematode control; however, additional research is needed to optimize treatments, determine the affordability of various biochar materials, and validate results under a range of field conditions.

Multiple target site resistance to glyphosate in junglerice (Echinochloa colona) lines from California orchards.

BACKGROUND: In California specialty cropping systems such as vineyards and orchards, Echinochloa colona is present as a summer annual weed. It is able to germinate throughout the growing season whenever favorable conditions are present, and management relies heavily on glyphosate applications. Glyphosate-resistant (GR) E. colona biotypes are present in the state, but the levels of resistance observed suggest that there may be differences in mechanisms of resistance among populations. RESULTS: Echinochloa colona lines collected from different regions of California's Central Valley presented resistance levels ranging from 1.4 to 4.3-fold compared to susceptible lines. No differences in the absorption and translocation of [14 C]-glyphosate were observed among lines. Resistant lines accumulated eight-fold less shikimic acid after treatment with 435 and 870 g a.e. ha-1 glyphosate compared to the most susceptible line. Sequencing of a region of the EPSPS gene revealed three single nucleotide changes leading to amino acid substitutions at Proline 106, including Pro106Leu, Pro106Thr and Pro106Ser. CONCLUSION: These results indicate that an altered target site in EPSPS is contributing to resistance in these lines and resistance has evolved independently, multiple times in the Central Valley of California. Additional research is needed to further understand the genomic contributions of resistance loci in this polyploid weed species. © 2018 Society of Chemical Industry.

Evaluation of potassium thiosulfate as a nitrification inhibitor to reduce nitrous oxide emissions.

Potassium thiosulfate (KTS, K2S2O3) has been shown to function as a nitrification inhibitor, thus has the potential to reduce nitrous oxide (N2O) emissions and play an important role in effective N management. The objective of this research was to determine the potential effects of KTS on N2O emissions and N transformation processes in comparison with commercial N transformation inhibitors (stabilizers). A laboratory incubation experiment was conducted using urea and ammonium nitrate (UAN) applied at 150mgNkg-1 in a Hanford sandy loam soil (coarse-loamy, mixed, superactive, nonacid, thermic Typic Xerorthents). Treatments included three rates of KTS (26, 51, and 102mgS2O32--Skg-1), a urease and nitrification inhibitor (Agrotain® Plus), a nitrification inhibitor (N-Serve® 24), and an untreated control. Nitrous oxide emission, soil pH, and mineral N species were monitored for 35days. Total N2O emissions were reduced significantly by all KTS treatments as a function of KTS rate. At 102mgS2O32--Skg-1, KTS reduced N2O emissions by 48% (0.18% of total inorganic N), which was statistically similar to the N-Serve® 24 treatment (60% reduction) although lower than Agrotain® Plus (78% reduction). The KTS resulted in significantly less unaccounted (total N) loss compared to the commercial inhibitors. If the N2O emissions reductions observed in this laboratory study are validated in the field, using KTS for this purpose can also provide a fertility benefit and may reduce total chemical inputs into agronomic systems. Future research needs to determine the effectiveness of thiosulfate for improving overall nutrient management while reducing N2O emissions under field conditions.

Vacuolar Sequestration of Paraquat Is Involved in the Resistance Mechanism in Lolium perenne L. spp. multiflorum.

Lolium perenne L. spp. multiflorum (Lam.) Husnot (LOLMU) is a winter annual weed, common to row crops, orchards and roadsides. Glyphosate-resistant populations of LOLMU are widespread in California. In many situations, growers have switched to paraquat or other postemergence herbicides to manage glyphosate-resistant LOLMU populations. Recently, poor control of LOLMU with paraquat was reported in a prune orchard in California where paraquat has been used several times. We hypothesize that the low efficacy observed is due to the selection of a paraquat-resistant biotype of LOLMU. Greenhouse dose-response experiments conducted with a susceptible (S) and the putative paraquat-resistant biotype (PRHC) confirmed paraquat resistance in PRHC. Herbicide absorption studies indicated that paraquat is absorbed faster in S than PRHC, although the maximum absorption estimates were similar for the two biotypes. Conversely, translocation of 14C-paraquat under light-manipulated conditions was restricted to the treated leaf of PRHC, whereas herbicide translocation out of the treated leaf was nearly 20 times greater in S. To determine whether paraquat was active within the plant cells, the photosynthetic performance was assessed after paraquat application using the parameter maximum quantum yield of photosystem II (Fv/Fm). Paraquat reaches the chloroplasts of PRHC, since there was a transitory inhibition of photosynthetic activity in PRHC leaves. However, PRHC Fv/Fm recovered to initial levels by 48 h after paraquat treatment. No paraquat metabolites were found, indicating that resistance is not due to paraquat degradation. LOLMU leaf segments were exposed to paraquat following pretreatments with inhibitors of plasma membrane- and tonoplast-localized transporter systems to selectively block paraquat intracellular movement. Subsequent evaluation of membrane integrity indicated that pre-exposure to putrescine resulted in the resistant biotype responding to paraquat similarly to S. These results strongly indicate that vacuolar sequestration is involved in the resistance to paraquat in this population of LOLMU.

Transcription of putative tonoplast transporters in response to glyphosate and paraquat stress in Conyza bonariensis and Conyza canadensis and selection of reference genes for qRT-PCR.

Herbicide resistance is a challenge for modern agriculture further complicated by cases of resistance to multiple herbicides. Conyza bonariensis and Conyza canadensis are invasive weeds of field crops, orchards, and non-cropped areas in many parts of the world. In California, USA, Conyza populations resistant to the herbicides glyphosate and paraquat have recently been described. Although the mechanism conferring resistance to glyphosate and paraquat in these species was not elucidated, reduced translocation of these herbicides was observed under experimental conditions in both species. Glyphosate and paraquat resistance associated with reduced translocation are hypothesized to be a result of sequestration of herbicides into the vacuole, with the possible involvement of over-expression of genes encoding tonoplast transporters of ABC-transporter families in cases of glyphosate resistance or cationic amino acid transporters (CAT) in cases of paraquat resistance. However, gene expression in response to herbicide treatment has not been studied in glyphosate and paraquat resistant populations. In the current study, we evaluated the transcript levels of genes possibly involved in resistance using real-time PCR. First, we evaluated eight candidate reference genes following herbicide treatment and selected three genes that exhibited stable expression profiles; ACTIN, HEAT-SHOCK-PROTEIN-70, and CYCLOPHILIN. The reference genes identified here can be used for further studies related to plant-herbicide interactions. We used these reference genes to assay the transcript levels of EPSPS, ABC transporters, and CAT in response to herbicide treatment in susceptible and resistant Conyza spp. lines. No transcription changes were observed in EPSPS or CAT genes after glyphosate or paraquat treatment, suggesting that these genes are not involved in the resistance mechanism. Transcription of the two ABC transporter genes increased following glyphosate treatment in all Conyza spp. lines. Transcription of ABC transporters also increased after paraquat treatment in all three lines of C. bonariensis. However, in C. canadensis, paraquat treatment increased transcription of only one ABC transporter gene in the susceptible line. The increase in transcription of ABC transporters after herbicide treatment is likely a stress response based on similar response observed across all Conyza lines regardless of resistance or sensitivity to glyphosate or paraquat, thus these genes do not appear to be directly involved in the mechanism of resistance in Conyza spp.

Off-tarp emissions, distribution, and efficacy of carbonated fumigants in a low permeability film tarped field.

Carbonated fumigants have been shown to distribute quickly and uniformly in sandy soils and improve pest control efficacy for annual crops. Low permeability films, such as VaporSafe® (TIF), could further improve fumigant dispersion by effectively retaining the fumigant in soil; however, there is a concern that the TIF can lead to higher off-tarp edge emissions. An orchard field trial was conducted to determine the off-tarp emissions, distribution, efficacy, and fate of carbonated Telone® C35 [63.4% 1,3-dichloropropene (1,3-D), 34.7% chloropicrin (CP)] that was shank-injected at 46cm soil depth. Treatments included carbonated fumigants at full- or 2/3 rates and a full rate of regular (nitrogen-pressurized) fumigants covered with standard polyethylene (PE) film, TIF, or no surface seal. Fumigant emissions at the regular tarp edge (25cm from the shank line) peaked at 3.98µgm-2s-1 for 1,3-D and 0.05µgm-2s-1 for CP. The addition of a TIF tarp extension (to 85cm from the shank line) reduce peak off-tarp emissions to <1 and <0.03µgm-2s-1 for 1,3-D and CP, respectively. Fumigant concentration under TIF was usually at least twice that under PE regardless of carbonation. Carbonation at 345KPa with 1.5% of dissolved CO2 did not significantly improve fumigant dispersion in soil profile compared to the conventional nitrogen pressurization. In a citrus nematode bioassay, only the 2/3 rate of carbonated fumigation treatment sealed with PE failed to control nematodes at 25cm away from shank line. This research indicates that a 60-cm TIF extension can effectively reduce off-tarp edge emissions in strip fumigation treatments. While the adaptability of carbonation of fumigants is still questionable, further research efforts are needed in finding effective solutions to control plant parasitic nematodes, which remain a challenge in orchard fumigation.

A high-throughput, modified ALS activity assay for Cyperus difformis and Schoenoplectus mucronatus seedlings.

Cyperus difformis L. (CYPDI) and Schoenoplectus mucronatus (L.) Palla (SCHMU) are major weeds of California (CA) rice, where resistance to acetolactate synthase (ALS)-inhibitors was identified in several CYPDI and SCHMU populations that have also evolved resistance to photosystem II (PSII)-inhibiting herbicides. The mechanism of ALS resistance in these populations remains to be clarified but this information is crucial in a weed management program, especially in a scenario where resistance to multiple herbicides has been identified. ALS activity assays are commonly used to diagnose resistance to ALS-inhibitors, but protocols currently available are burdensome for the study of CYPDI and SCHMU, as they require large amounts of plant material from young seedlings and have low yields. Our objective was to investigate the ALS resistance mechanism in suspected ALS-resistant (R) CYPDI and SCHMU biotypes using a modified ALS activity assay that requires less plant material. ALS enzymes from suspected R biotypes were at least 10,000-fold less sensitive to bensulfuron-methyl than susceptible (S) cohorts, indicating ALS resistance that is likely due to an altered target-site. Protein concentration (mgg-1 tissue) did not differ between R and S biotypes within each species, suggesting that R biotypes do not over produce ALS enzymes. CYPDI biotypes had up to 4-fold more protein per mg of tissue than SCHMU biotypes, but up to 7-fold more acetoin per mg-1 protein was quantified in SCHMU, suggesting greater ALS catalytic ability in SCHMU biotypes, regardless of their herbicide resistance status. Our optimized protocol to measure ALS activity allowed for up to a 3-fold increase in the number of assays performed per g of leaf tissue. The modified assay may be useful for measuring ALS activity in other weed species that also produce small amount of foliage in early growth stages when protein in tissue is most abundant.

Fumigation efficacy and emission reduction using low-permeability film in orchard soil fumigation.

BACKGROUND: 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. RESULTS: While increasing fumigant concentrations under the tarp, TIF reduced emissions >95% (∼2% and <1% 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. CONCLUSION: 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.

Effect of application rate on fumigant degradation in five agricultural soils.

Soil fumigation is an important pest management tool for many high value crops. To address the knowledge gap of how fumigant concentration in soil impacts dissipation, and thereby efficacy, this research determined the degradation characteristics of four fumigants as affected by application rate. Laboratory incubation experiments were conducted to determine degradation rates of 1,3-dichloropropene (both cis- and trans isomers), chloropicrin (CP), dimethyl disulfide (DMDS), and methyl iodide (MeI) in five agricultural soils. Fitted to pseudo first-order kinetics, the degradation rate constant (k) of CP, DMDS, and MeI decreased significantly as application rate increased while the 1,3-D isomers were the least affected by rate. Half-lives increased 12, 17, and 6-fold for CP, DMDS, and MeI, respectively, from the lowest to the highest application rate. At low application rates, the degradation rate of all fumigants in the Hueneme sandy loam soil was reduced by 50-95% in sterilized soil compared to the biologically active controls. However, this difference became much smaller or disappeared at high application rates indicating that biodegradation dominates at low concentrations but chemical degradation is more important at high concentrations. When co-applied, CP degradation was enhanced with biodegradation remained above 50%, while 1,3-D degradation was either reduced or not changed. Among the fumigants tested, the relative importance of biodegradation was DMDS>CP>MeI>1,3-D. These results are useful for determining effective fumigation rates and for informing regulatory decisions on emission controls under different fumigation scenarios.

Automated Mobile System for Accurate Outdoor Tree Crop Enumeration Using an Uncalibrated Camera.

This paper demonstrates an automated computer vision system for outdoor tree crop enumeration in a seedling nursery. The complete system incorporates both hardware components (including an embedded microcontroller, an odometry encoder, and an uncalibrated digital color camera) and software algorithms (including microcontroller algorithms and the proposed algorithm for tree crop enumeration) required to obtain robust performance in a natural outdoor environment. The enumeration system uses a three-step image analysis process based upon: (1) an orthographic plant projection method integrating a perspective transform with automatic parameter estimation; (2) a plant counting method based on projection histograms; and (3) a double-counting avoidance method based on a homography transform. Experimental results demonstrate the ability to count large numbers of plants automatically with no human effort. Results show that, for tree seedlings having a height up to 40 cm and a within-row tree spacing of approximately 10 cm, the algorithms successfully estimated the number of plants with an average accuracy of 95.2% for trees within a single image and 98% for counting of the whole plant population in a large sequence of images.

Active optical sensors for tree stem detection and classification in nurseries.

Active optical sensing (LIDAR and light curtain transmission) devices mounted on a mobile platform can correctly detect, localize, and classify trees. To conduct an evaluation and comparison of the different sensors, an optical encoder wheel was used for vehicle odometry and provided a measurement of the linear displacement of the prototype vehicle along a row of tree seedlings as a reference for each recorded sensor measurement. The field trials were conducted in a juvenile tree nursery with one-year-old grafted almond trees at Sierra Gold Nurseries, Yuba City, CA, United States. Through these tests and subsequent data processing, each sensor was individually evaluated to characterize their reliability, as well as their advantages and disadvantages for the proposed task. Test results indicated that 95.7% and 99.48% of the trees were successfully detected with the LIDAR and light curtain sensors, respectively. LIDAR correctly classified, between alive or dead tree states at a 93.75% success rate compared to 94.16% for the light curtain sensor. These results can help system designers select the most reliable sensor for the accurate detection and localization of each tree in a nursery, which might allow labor-intensive tasks, such as weeding, to be automated without damaging crops.

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.

Subsurface drip application of alternative fumigants to methyl bromide for controlling nematodes in replanted grapevines.

BACKGROUND: Many California grape growers use preplant fumigation to ensure uniform and healthy grapevine establishment in replant situations. A field study was conducted to evaluate the performance of subsurface drip-applied chemical alternatives to methyl bromide on plant-parasitic nematodes, plant vigor and fruit yield during the 6 year period following replanting. RESULTS: Subsurface drip fumigation with 1,3-dichloropropene plus chloropicrin and with iodomethane plus chloropicrin had generally similar nematicide activity as methyl bromide in three grape types, while sodium azide was less effective. The combination of 1,3-dichloropropene plus chloropicrin enhanced vine vigor similarly to methyl bromide. However, all plots treated with alternative fumigants produced less fruit yield than methyl bromide over the 4 years of evaluation. CONCLUSION: Subsurface drip fumigation with alternative chemicals to methyl bromide generally provided adequate management of plant-parasitic nematodes during the vine establishment period. However, further research is required to increase the performance of alternative chemicals against other components of the replant problem, as grape yield in vines grown in the alternative treatments was lower than in methyl bromide.

Effects of surface treatments and application shanks on nematode, pathogen and weed control with 1,3-dichloropropene.

BACKGROUND: Preplant fumigation with methyl bromide (MeBr) has been used for control of soilborne pests in high-value annual, perennial and nursery crops, but is being phased out. In 2007 and 2008, research trials were conducted to evaluate the effects of surface treatments and two application shanks on pest control with 1,3-dicloropropene (1,3-D) in perennial crop nurseries. RESULTS: All 1,3-D treatments controlled nematodes similarly to MeBr. Application of 1,3-D with virtually impermeable film (VIF) reduced Fusarium oxysporum compared with unfumigated plots, but was not as effective as MeBr. Applications of 1,3-D with VIF or 1,3-D followed by metam sodium reduced Pythium spp., but 1,3-D followed by intermittent water seals was comparable with the untreated plots. When sealed with high-density polyethylene (HDPE) film or VIF, 1,3-D generally was as effective as MeBr for reducing weed density and total weed biomass, but weed control was reduced by intermittent water seals and in unsealed plots subsequently re-treated with additional 1,3-D or metam sodium. CONCLUSION: Applications of 1,3-D sealed with HDPE or VIF film or with intermittent water seals can control nematodes similarly to MeBr. However, additional management practices may be needed for effective pathogen and weed control if plastic film is not used.

Comparisons of soil surface sealing methods to reduce fumigant emission loss.

State and federal regulatory agencies depend on quality field data for determining the effects of agricultural management practices on fumigant emissions to develop sound, science-based policies and regulations on preplant soil fumigants. Field plot tests, using growers' standard field operation procedures, were used to simultaneously determine the effectiveness of several commonly proposed emission reduction methods, in a trial involving shank injection of Telone II [a.i. 1,3-dichloropropnene (1,3-D)] to a sandy loam soil to a target rate of 372 kg ha(-1). The experiment was conducted in late September 2008 in the San Joaquin Valley of California. Fumigant emissions were captured using dynamic flux chambers. The results showed that virtually impermeable film (VIF) reduced emissions >95% when compared to bare soil, and the glue joints in the film did not significantly affect the tarp performance. The VIF also created a more uniform distribution of gaseous fumigant in the soil profile, which would likely benefit pest control efficacy. Standard high-density polyethylene (HDPE) tarp reduced total 1,3-D emissions about 50% (higher than most reported values) in this trial, whereas postfumigation intermittent water treatments (seals) reduced cumulative emission losses by approximately 20%. Adding 49.4 Mg ha (equivalent to 20 tons per acre) of composted dairy manure to surface soils did not reduce 1,3-D emissions during this experiment. Use of VIF was the most promising technique in reducing emissions and has the potential to allow lower application rates while providing satisfactory pest control.

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.