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NONRACE-SPECIFIC DISEASE RESISTANCE (NDR1) is a widely characterized gene that plays a key role in defense against multiple bacterial, fungal, oomycete and nematode plant pathogens. NDR1 is required for activation of resistance by multiple NB and LRR-containing (NLR) protein immune sensors and contributes to basal defense. The role of NDR1 in positively regulating salicylic acid (SA)-mediated plant defense responses is well documented. However, ndr1-1 plants flower earlier and show accelerated development in comparison to wild type (WT) Arabidopsis plants, indicating that NDR1 is a negative regulator of flowering and growth. Exogenous application of gibberellic acid (GA) further accelerates the early flowering phenotype in ndr1-1 plants, while the GA biosynthesis inhibitor paclobutrazol attenuated the early flowering phenotype of ndr1-1, but not to WT levels, suggesting partial resistance to paclobutrazol and enhanced GA response in ndr1-1 plants. Mass spectroscopy analyses confirmed that ndr1-1 plants have 30-40% higher levels of GA3 and GA4, while expression of various GA metabolic genes and major flowering regulatory genes is also altered in the ndr1-1 mutant. Taken together this study provides evidence of crosstalk between the ndr1-1-mediated defense and GA-regulated developmental programs in plants.

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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.

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BACKGROUND: The dissipation of pesticides in soil and the occurrence of accelerated degradation following repeated applications are well-known phenomena with many pesticides, but much less so with soil fumigants. The fate of various soil fumigants was studied in different agricultural soils following repeated applications of chloropicrin. RESULTS: Fumigant dissipation reflected by Σconcentration × time (ΣC × T) and half-life values varied widely among the tested soils. Methyl iodide (MI) had the slowest dissipation rate compared with other fumigants in all tested soils. Elimination of biotic agents by soil sterilization prior to MI application did not affect MI concentration in Oxnard soil. Clay content and fumigant dose (ΣC × T values) of chloropicrin, 1,3-dichloropropene and MI were significantly correlated. No significant correlations were found between soil properties and ΣC × T values following metam sodium and methyl bromide (MBr) application. CONCLUSION: The fate of the tested soil fumigants is highly dependent on and specific to the fumigant, previous fumigant application and soil type. This study suggests that biotic factors are more essential in the dissipation of metam sodium and MBr than abiotic factors. By contrast, MI dissipation from the tested soils is affected more by abiotic factors than by biotic activities.

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Methyl isothiocyanate (MITC) generators, such as metam sodium (Met-Na), are used for soil fumigation of agricultural land. The ban on the fumigant methyl bromide has resulted in greater use of MITC generators. To understand the efficacy of MITC, it is necessary to assess its generation and disappearance kinetics when Met-Na is applied to soil. This study evaluated the movement of water and distribution and dissipation of MITC in soil after application of Met-Na through surface drip irrigation systems. The effects of varying water application volume (25, 50, and 75 mm) and rate (1.9, 5.0, and 7.5 L h m) were evaluated in a sandy loam soil. Good fumigant distribution within the sandy loam soil was observed under medium water application amount (50 mm) with slow to intermediate drip application rates (1.9-5.0 L h m). Low water application amount (25 mm) or high application rate (7.5 L h m) did not provide adequate MITC distribution throughout the soil bed width and rooting depth. Dissipation patterns of MITC in soil in all water application amounts and rates followed first-order kinetics, with a rate constant of 0.025 ± 0.004 h and a half-life of 27 ± 3 h. Simulated water distribution through the soil profile using HYDRUS 2D/3D fitted measured field data well, and the model accurately simulated MITC fumigant distribution in the soil.

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Raised beds are used to produce some high-value annual fruit and vegetable crops such as strawberry in California (CA) and tomato in Florida (FL), USA. Pre-plant soil fumigation is an important tool to control soil-borne pests in the raised beds. However, fumigant emissions have detrimental environmental consequences. Field trials were conducted to evaluate emissions of 1,3-dichloropropene (1,3-D) and chloropicrin (CP) in two different production systems with raised beds covered by different tarps. In the CA trial, InLine (60.8% 1,3-D and 33.3% CP) was drip-applied at 340 kg ha(-1) to 5 cm deep in the beds (30 cm high and 107 cm wide) tarped with polyethylene (PE) or virtually impermeable film (VIF). In the FL trial, carbonated Telone C35 (63.4% 1,3-D and 34.7% CP) was shank-applied at 151 kg ha(-1) to 20 cm deep in the beds (22 cm high and 76 cm wide) tarped with totally impermeable film (TIF). Emissions from tarped beds relative to furrows were contrary between the two trials. For the CA trial, the emission was 47% of applied 1,3-D and 27% of applied CP from PE tarped beds and 31% of applied 1,3-D and 15% of applied CP from VIF tarped beds, while that from uncovered furrows was<0.4% for both chemicals in both fields. In the FL trial, only 0.1% 1,3-D was emitted from the TIF tarped beds, but 27% was measured from the uncovered furrows. Factors contributing to the differences in emissions were chiefly raised-bed configuration, tarp permeability, fumigant application method, soil properties, soil water content, and fumigant carbonation. The results indicate that strategies for emission reduction must consider the differences in agronomic production systems. Modifying raised bed configuration and fumigant application technique in coarse textured soils with TIF tarping can maximize fumigation efficiency and emission reduction.

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Regulatory initiatives in the United States have created the impetus to reassess application methods for metam sodium (sodium -methyldithiocarbamate), a methyl isothiocyanate (MITC) generator, to reduce flux to the atmosphere. This paper compares flux rates in the years 1990 through 2002 with flux rates based on four studies conducted during the period 2008 through 2010 in California, Michigan, Wisconsin, and Washington using current shank-injection/compaction methods. Up to a 100-fold reduction in peak flux rates and total loss of MITC have been observed. A combination of the following factors led to these reductions in flux: soil moisture goals set at 70% of the field water holding capacity; improved design of shank-injection systems to break up the voids after injection; effective shank compaction to further reduce volatilization; and the use of water sealing, where applicable. These refinements in the application methods for metam sodium provide a means to merge environmental and agricultural goals in the United States and in other countries that use metam sodium. This paper documents the reduced atmospheric emissions of MITC under commercial production conditions when applied using good agricultural practices. This research also shows that MITC flux can be effectively managed without the use of high barrier tarp material.

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Water application is a low-cost strategy to control emissions of soil fumigant to meet the requirements of the stringent environmental regulations and it is applicable for a wide range of commodity groups. Although it is known that an increase in soil moisture reduces emissions, the range of soil moisture for minimizing emissions without risking pest control, is not well defined for various types of soils. With two column studies, we determined the effect of different soil moisture levels on emission and distribution of 1,3-dichloropropene and chloropicrin in three different textured soils. Results on sandy loam and loam soils showed that by increasing soil moisture from 30% to 100% of field capacity (FC), peak fluxes were lowered by 77-88% and their occurrences were delayed 5-15 h, and cumulative emissions were reduced 24-49%. For the sandy soil, neither peak fluxes nor the cumulative emissions were significantly different when soil moisture increased from 30% to 100% FC. Compared to the drier soils, the wetter soils retained consistently higher fumigant concentrations in the gas-phase, suggesting efficacy may not be impacted in these soils. The air-filled porosity positively and linearly correlated with the cumulative emission loss across all soil types indicating that it may serve as a good indicator for estimating emissions. These laboratory findings can be further tested under field conditions to conclude what irrigation regime should be used for increasing soil water content before fumigant application that can achieve maximum emission reduction and uniform fumigant distribution with high exposure index values.

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Tarping fumigated fields with low permeability films such as commercial Totally Impermeable Film (TIF) can significantly reduce emissions, but it can also increase fumigant residence time in the soil such that extended tarp-covering durations may be required to address potential exposure risks during tarp-cutting and removal. In an effort to develop safe practices for using TIF, a large field study was conducted in the San Joaquin Valley of California. Comprehensive data on emissions (measured with dynamic flux chambers), fate, and transport of 1,3-dichloropropene and chloropicrin were collected in a 3.3 ha field fumigated with Pic-Clor 60 via broadcast shank application. Low emission flux (below 15 µg m(-2) s(-1)) was observed from the tarped field throughout the tarp-covering period of 16 days with total emission loss of <8% of total applied for both chemicals. Although substantially higher flux was measured at tarp edges (up to 440 µg m(-2) s(-1)), the flux was reduced to below 0.5 µg m(-2) s(-1) beyond 2 m of tarp edge where total mass loss was estimated to be ≤ 1% of total applied to the field. Emission flux increased following tarp-cutting, but was much lower compared to 5 or 6 d tarp-covering periods determined in other fields. This study demonstrated the ability of TIF to significantly reduce fumigant emissions with supporting data on fumigant movement in soil. Proper management on use of the tarp, such as extending tarp-covering period, can reduce negative impact on the environment and help maintain the beneficial use of soil fumigants for agricultural productions.

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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.

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Soil concentrations and degradation rates of methyl isothio-cyanate (MITC), chloropicrin (CP), 1,3-dichloropropene (1,3-D), and dimethyl disulfide (DMDS) were determined under fumigant application scenarios representative of commercial raised bed, plastic mulched vegetable production systems. Five days after application, 1,3-D, MITC, and CP were detected at concentrations up to 3.52, 0.72, and 2.45 µg cm, respectively, in the soil atmosphere when applications were made in uniformly compacted soils with a water content >200% of field capacity and covered by a virtually impermeable or metalized film. By contrast, DMDS, MITC, and CP concentrations in the soil atmosphere were 0.81, 0.02, and 0.05 µg cm, respectively, 5 d after application in soil containing undecomposed plant residue, numerous large (>3 mm) clods, and water content below field capacity and covered by low-density polyethylene. Ranked in order of impact on the persistence of fumigants in soil were soil water content (moisture), soil tilth (the physical condition of soil as related to its fitness as a planting bed), the type of plastic film used to cover fumigated beds, and soil texture. Fumigants were readily detected 13 d after application when applied in uniformly compacted soils with water contents >200% of capacity and covered by a virtually impermeable or metalized film. By contrast, 1,3-D and MITC had dissipated 5 d after application in soils with numerous large (>3 mm) clods and water contents below field capacity that were covered by low-density polyethylene. Soil degradation of CP, DMDS, and MITC were primarily attributed to biological mechanisms, whereas degradation of 1,3-D was attributed principally to abiotic factors. This study demonstrates improved soil retention of agricultural fumigants in application scenarios representative of good agricultural practices.

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BACKGROUND: Metam-sodium (MS, sodium methyldithiocarbamate) has been identified as a promising alternative chemical to replace methyl bromide (MeBr) in soil preplant fumigation. One degradation product of MS in soil is the volatile gas methyl isothiocyanate (MITC) which controls soilborne pests. Inconsistent results associated with MS usage indicate that there is a need to determine cultural practices that increase pest control efficacy. Sealing the soil surface with water after MS application may be a sound method to reduce volatilization loss of MITC from soils and increase the contact time necessary for MITC to control pests. The objective of this research was to develop a preliminary soil surface water application amount that would potentially inhibit the off-gassing rate of MITC. RESULTS: Off-gassing rate was consistently reduced with increasing water seal application. The application of a 2.5-3.8 cm water seal provided significantly lower (71-74% reduction in MITC volatilization) total fumigant loss compared with no water seal. The most favorable reduction in MITC off-gassing was observed in the 2.5 cm water seal. CONCLUSION: This suggests that volatilization of MITC-generating compounds can be highly suppressed using adequate surface irrigation following chemical application in this soil type (sandy clay loam), based on preliminary bench-scale soil column studies. .

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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.

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Plastic tarps are commonly used in raised bed strawberry production to minimize emissions of preplant soil fumigants and are left in place throughout the growing season as part of the standard cultural practices. Soil amendments with chemicals such as thiosulfate (S2O3(2-)) can reduce fumigant emissions. A field study was conducted near Santa Maria, CA to determine the effects of low density polyethylene (LDPE) and virtually impermeable film (VIF) over raised-beds and applying potassium thiosulfate (KTS) in furrows on reducing chloropicrin (CP) emissions from a strawberry field. Four fields (or treatments) were tested with 224 kg ha(-1) CP drip-applied threecm under the soil surface. The CP flux from bed tops and furrows and gas-phase concentrations under the tarps were monitored for five d. The CP emission flux and concentration under tarp were highest immediately following application. Diurnal temperature change affected CP concentration and emission fluxes (higher values during the day and lower at night). Slightly higher CP cumulative emission occurred using LDPE tarp (19%) compared to VIF (17%). Normalized flux (CP emission flux from the beds divided by CP concentration under the tarp) being estimated from field measurement was slightly higher for LDPE than VIF indicating different tarp permeability in the field. Because of extremely low emissions from the furrows (<0.2% of total emission loss), KTS application to furrow treatments did not show further emission reductions than non-KTS treatments. This indicates that emission reduction should focus on the tarp above raised-beds when fumigant was drip-applied near bed-surface.

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Metam sodium (sodium N-methyl dithiocarbamate, metam-Na) is widely used in agricultural and floricultural production for controlling soilborne plant pathogens, parasitic nematodes, and weeds. It undergoes rapid decomposition to the biocide methyl isothiocyanate (MITC) in moist soils. In this study, the efficacy of 12 concentrations of metam-Na (10 to 2,650 µmol kg-1 soil) to control seeds or tubers of five major weed species, three soilborne pathogens, and one parasitic nematode was evaluated in a sandy loam soil under controlled conditions. Soils were exposed to the fumigant in microcosms for 24 h at 10 and 20°C. Generation and dissipation curves of MITC in soil under controlled conditions showed that MITC concentrations in soils were highest 2 h after metam-Na application and decreased steadily over the 24-h incubation period. After 24 h, remaining MITC concentrations in soil microcosms at 10 and 20°C were 53 and 38% of the original amount applied, respectively, indicating a 20% reduction in MITC dissipation at the lower soil temperature. Logistic dose-response models were used to estimate the effective concentration necessary to reduce soil pest viability by 50 (LC50) or 90 (LC90) percent under both temperatures. Seed of Portulaca oleracea, with LC90 values of ≤1,242 µmol kg-1 soil, was the most sensitive to soil fumigation with metam-Na, followed by Polygonum arenastrum with LC90 values of ≤1,922 µmol kg-1 soil. At 10°C fumigation temperature, metam-Na at the highest dose tested in this study, 2,650 µmol kg-1 soil, was not sufficient to achieve adequate control of Stellaria media and Malva parviflora seed and Cyperus esculentus tubers. Weed control efficacy (average reduction in LC90 values) of metam-Na was between 25 and 60% higher if soils were fumigated at 20°C compared with 10°C, with the exception of M. parviflora. Phytophthora cactorum and Pythium ultimum were more sensitive to soil fumigation with metam-Na (LC90 ≤ 165 µmol kg-1 soil) than Verticillium dahliae (LC90 ≤ 737 µmol kg-1 soil). The nematode Tylenchulus semipenetrans was highly sensitive to soil fumigation with metam-Na (LC90 ≤ 98 µmol kg-1 soil), and the efficacy of control increased by 30% if soil was fumigated at 20°C compared with 10°C. In this sandy loam soil, metam-Na at a concentration of 850 µmol kg-1 reduced the viability of Portulaca oleracea and Polygonum arenastrum seeds, C. esculentus tubers, and all soilborne pathogens and parasitic nematodes tested by 90% at 20°C after 24 h exposure. These results indicate that metam-Na can provide effective pest and disease control at maximum label rate for the commercial formulation, but there was a reduction in efficacy at low temperature.

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Low permeability tarps can effectively minimize fumigant emissions while improving fumigation efficacy by retaining fumigants under the tarp. However, when planting holes are cut through the tarps, high-concentration fumigants may be released and result in environmental and worker safety hazards. In a 11-day column study, we explored the effect of drip irrigation application of ammonium thiosulfate (ATS) on 1,3-dichloropropene (1,3-D) and chloropicrin (CP) degradation in soil. Decrease of 1,3-D and CP concentrations in soil-gas phase followed a three-parameter logistic equation for all treatments. It was slowest in the control with a half-life ( t 1/2) of 86.0 h for 1,3-D and of 16.3 h for CP and most rapid when ATS was applied at 4:1 ATS/fumigant molar ratio with a half-life of 9.5 h for 1,3-D and of 5.5 h for CP. Our results indicate that applying ATS via the drip-irrigation systems to soil can accelerate fumigant degradation in soil and thus reduce emissions. This technical solution may be applicable in raised-bed strawberry production where drip-application of fumigants under tarps has become common.

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