Effect of long‐term field application of pendimethalin: enhanced degradation in soil

The effect of long‐term application of pendimethalin in a maize–wheat rotation on herbicide persistence was investigated. Pendimethalin was applied at 1.5 kg AI ha⁻¹ separately as one or two annual applications for five consecutive years in the same plots. Residues of pendimethalin were determined by gas chromatography. Harvest‐time residues of the herbicide decreased gradually over the years and at the end of five years less than 3% of applied pendimethalin was recovered from soil as against 18% in the first year. Residues were found distributed in the soil profile up to 90 cm depth at the end of the experiment with peak distribution of 0.03 µg g⁻¹ in the surface layer of soil treated with 10 herbicide applications. The minimum distribution was, however, in the deepest soil (75–90 cm) profile. Some of the metabolites of pendimethalin ie dealkylated pendimethalin derivative, partially reduced derivative and cyclized product were also traced in surface and sub‐surface soils up to 90 cm. A study of the rate of degradation of pendimethalin in field‐treated soils under laboratory conditions revealed faster degradation compared to control soils. Only the surface soil (0–15 cm) showed this enhanced degradation of the herbicide, which could be due to the adaptability of the aerobic micro‐organisms to degrade pendimethalin. Microbes capable of degrading herbicide were isolated, identified and pendimethalin degradation was confirmed in nutrient broth. © 2000 Society of Chemical Industry     

二甲戊灵和乙氧氟草醚在生姜上的残留分析及土壤中的消解动态

建立了气相色谱-质谱联用（GC-MS）同时测定二甲戊灵和乙氧氟草醚在生姜、茎秆和土壤中残留量的分析方法,并采用该方法研究了两种农药在土壤中的消解动态及在土壤和生姜中的最终残留。样品前处理采用QuEChERS法,经乙腈提取,姜块和茎秆提取液分别用N-丙基乙二胺（PSA）和石墨化碳黑（GCB）净化,土壤提取液用弗罗里硅土柱净化,选择离子监测模式（SIM）扫描,外标峰面积法定量。结果表明:当添加水平为0.01~0.5 mg/kg时,二甲戊灵在生姜、茎秆和土壤中的回收率分别为91%~100%、90%~98%和86%~100%,相对标准偏差（RSD）分别为3.2%~3.7%、2.7%~4.2%和3.2%~5.1%;乙氧氟草醚在生姜、茎秆和土壤中的回收率分别为90%~95%、86%~91%和85%~95%,RSD分别为2.4%~4.4%、3.9%~5.5%和2.3%~4.9%;样品中二甲戊灵和乙氧氟草醚的定量限（LOQ）均为0.01 mg/kg。二甲戊灵在土壤中的消解半衰期在12.5~20.5 d之间,乙氧氟草醚在18.8~25.6 d之间。采用33%二甲戊灵乳油和24%乙氧氟草醚乳油混剂分别按推荐剂量（有效成分693 g/hm2和72 g/hm2）和高剂量（有效成分1 039 g/hm2和108 g/hm2）于种植后施药1次,在生姜收获期的姜块中均未检出二甲戊灵和乙氧氟草醚残留。研究结果可为二甲戊灵和乙氧氟草醚在生姜上的合理使用及其最大允许残留限量（MRL）标准制定提供参考。     

Analysis of metsulfuron‐methyl residues in wheat field soil: a comparison of HPLC and bioassay techniques

BACKGROUND: Metsulfuron‐methyl is a low‐application‐rate sulfonylurea herbicide that is widely used to control broad‐leaved weeds in wheat. Owing to its persistent nature, its residues may be present at phytotoxic levels for the next crop in rotation. Therefore, a comparative evaluation of HPLC and bioassay techniques was made for the analysis of this herbicide in wheat field soil. RESULTS: Metsulfuron‐methyl was applied to wheat crop at different rates (4, 8 and 12 AI ha^?1) at 28 days after sowing as a post‐emergence application, and the soil was analysed for metsulfuron‐methyl residues by HPLC and lentil seed bioassay techniques. The bioassay was found to be the more sensitive technique. At the recommended rate of application, 4 g AI ha^?1, the bioassay technique could detect the residue up to 30 days in surface soil, while, with HPLC, residues were not detectable on the 15th day. The half‐lives of metsulfuron‐methyl by HPLC and bioassay were calculated as 6.3–7.8 and 17.5 days respectively. Under field conditions, residues of metsulfuron‐methyl were also detected in subsurface soil by the bioassay technique at trace levels, but were not detected by the solvent extraction/HPLC method. CONCLUSION: Lentil seed bioassay is a more sensitive technique than HPLC. Traces of residues detected in subsurface soil indicated the mobility of metsulfuron‐methyl into lower layers. Copyright © 2009 Society of Chemical Industry     

In search of new herbicidal inhibitors of the non‐mevalonate pathway

The first five steps of the non‐mevalonate pathway have been tested in high‐throughput screening (HTS) campaigns, using enzymes of plant origin. Hit rates were in general relatively low, which could be attributed to the high polarity and charged nature of substrates and active sites of these enzymes. Still, for all the enzymes, apart from IspF (2‐methylerythritol 2,4‐cyclodiphosphate synthase), inhibitors could be identified with activities below 100 μM, and these were followed up to identify structure–activity relationships (SARs). For the enzyme IspD (2C‐methyl‐D‐erythritol 4‐phosphate cytidyltransferase), inhibitors with IC₅₀ down to 35 nM were identified that also showed herbicidal activity.     

Quantification of Mirafiori lettuce big‐vein virus and its vector,Olpidium virulentus,from soil using real‐time PCR

Big vein disease of lettuce (Lactuca sativa) is an economically important disease transmitted through soil by Olpidium virulentus, and has occurred in most production areas worldwide. The disease is assumed to be caused by Mirafiori lettuce big‐vein virus (MiLBVV). To understand the dynamics of the virus and its vector, MiLBVV and O. virulentus were directly detected in soil. DNA and RNA were extracted from 5 g soil using a bead beating method, followed by purification using adsorption to a column. Detection and quantification were performed using real‐time PCR and a TaqMan probe that was prepared based on the CP region of MiLBVV and the rDNA‐ITS region of O. virulentus, respectively. Furthermore, using a visual assessment of the incidence rate of big vein disease on lettuce in agricultural fields, the C_t values of MiLBVV and O. virulentus from soil were also determined using real‐time PCR. The results showed that MiLBVV concentrations in the soil were high in the field, as also determined by a visual assessment of the incidence rate of big vein disease on lettuce. However, the amount of O. virulentus in soil was not directly correlated with the incidence of MiLBVV. From these results, it is suggested that the risk of lettuce crops developing big vein disease can be estimated using an index of the amount of MiLBVV in the soil.     

Dissipation and movement of flumioxazin in soil at four field sites in Chile

BACKGROUND: Flumioxazin is a soil-applied herbicide recommended for broadleaf weed control in soybeans and peanuts, and was recently introduced for vineyard weed management. Considering the limited information available in relation to flumioxazin field soil behaviour, the main objectives of this study were to determine the persistence, adsorption and movement of flumioxazin in soil in four Chilean vineyard production areas. RESULTS: DT(50) values ranged from 10.6 +/- 1.0 to 32.1 +/- 3.1 days between localities, being correlated with rain events, time between herbicide application and first heavy rain event, and soil pH. Flumioxazin soil residue found at 90 days after application (DAA) varied from 9.6 to 24.9% of the initial amount applied, and depended on the total rainfall amount that occurred during the first 90 DAA. Herbicide leaching below 15 cm was approximately 45% of the flumioxazin detected at 90 DAA in the whole soil profile. Flumioxazin maximum leaching soil depth was 45 cm at all locations. K(d) values varied from 2.54 to 6.51 mg L(-1), depending on localities and soil profile depth, and correlated positively with organic carbon and clay content. CONCLUSIONS: These results indicate that flumioxazin is a herbicide with low environmental risk owing to its short DT(50), reduced soil residues 3 months after application and low effective dose.     

Effect of cropping cycles and repeated herbicide applications on the degradation of diclofop-methyl, bentazone, diuron, isoproturon and pendimethalin in soil

A greenhouse study was conducted to investigate the ability of four crops (wheat, corn, oilseed rape and soybean) to influence the degradation of bentazone, diclofop-methyl, diuron, isoproturon and pendimethalin in soil. The present study showed that microbial biomass-carbon was significantly higher in planted soils than in bulk soil, especially with wheat and corn, after several cropping cycles. The biomass in corn and soybean planted soils was adversely affected by bentazone but recovered after three cropping cycles. In wheat-planted soils, diclofop-methyl application resulted in persistent increase of the amount of microbial biomass. Bentazone did not show accelerated degradation even after five successive treatments, differing from diclofop-methyl, for which two applications were sufficient to enhance significantly its rate of degradation. Enhanced degradation of diclofop-methyl was even more pronounced in wheat-planted soil. The rates of mineralisation of diuron, isoproturon and pendimethalin were not affected after the first cropping cycle, but were significantly increased in planted soils after five cropping cycles. The results confirm that plants may promote pesticide degradation in soil by stimulating biodegradation processes. In the case of diclofop-methyl, stimulation of accelerated degradation was observed.     

The effect of initial concentration of carbofuran on the development and stability of its enhanced biodegradation in top‐soil and sub‐soil

Carbofuran was incubated in top‐soil and sub‐soil samples from a pesticide‐free site at a range of initial concentrations from 0.1 to 10 mg kg⁻¹. Amounts of the incubated soils were removed at intervals over the subsequent 12 months, and the rate of degradation of a second carbofuran dose at 10 mg kg⁻¹ was assessed. An applied concentration as low as 0.1 mg kg⁻¹ to top‐soil resulted in more rapid degradation of the fresh addition of carbofuran for at least 12 months. The degree of enhancement was generally more pronounced with the higher initial concentrations. When the same study was conducted in sub‐soil samples from the same site, an initial dose of carbofuran at 0.1 mg kg⁻¹ resulted in only small increases in rates of degradation of a second carbofuran dose. However, degradation rates in the sub‐soil samples were, in many instances, considerably greater than in the corresponding top‐soil samples, irrespective of pre‐treatment concentration or pre‐incubation period. Initial doses of 0.5 mg kg⁻¹ and higher applied to sub‐soil successfully activated the sub‐soil microflora. Application of the VARLEACH model to simulate carbofuran movement through the soil profile indicated that approximately 0.01 mg kg⁻¹ of carbofuran may reach a depth of 70 cm 400 days after a standard field application. The results therefore imply that adaptation of the sub‐soil microflora (c 1 m depth) by normal field rate applications of carbofuran is unlikely to occur. In experiments to investigate this in soils exposed to carbofuran in the field, there was no apparent relationship between top‐soil exposure and degradation rates in the corresponding sub‐soils. The results further confirmed that some sub‐soil samples have an inherent capacity for rapid biodegradation of carbofuran. The high levels of variability observed between replicates in some of the sub‐soil samples were attributed to the uneven distribution of a low population of carbofuran‐degrading micro‐organisms in sub‐surface soil. There was no apparent relationship between soil microbial biomass and degradation rates within or between top‐soil and sub‐soil samples. © 2001 Society of Chemical Industry     

Behaviour of trifloxystrobin and tebuconazole on grapes under semi‐arid tropical climatic conditions

BACKGROUND: A mixture of trifloxystrobin and tebuconazole is excellent in controlling both powdery and downy mildew of grapes. The objective of the present work was to study the behaviour of trifloxystrobin and tebuconazole on grape berries and soil following treatment with Nativo 75 WG, a formulation containing both fungicides (trifloxystrobin 250 + tebuconazole 500 g kg^?1). This study was carried out for planned registration of this mixture for use on grapes in India. RESULTS: Initial residue deposits of trifloxystrobin and tebuconazole on grapes were below their maximum residue limit (MRL) of 0.5 and 2 mg kg^?1, respectively, when Nativo 75 WG was applied at the recommended dose of 175 g product ha^?1. The residues dissipated gradually to 0.02 and 0.05 mg kg^?1 by 30 days, and were below the quantifiable limit of 0.01 mg kg^?1 at the time of harvest (60 days after the last treatment). Trifloxystrobin and tebuconazole dissipated at a pre‐harvest interval (PHI) of 36 and 34 days, respectively, from the recommended treatment dose. The acid metabolite of trifloxystrobin, CGA 321 113, was not detected in grape berries at any point in time. Soil at harvest was free of any pesticide residues. CONCLUSION: Residue levels of both trifloxystrobin and tebuconazole were below MRLs when grapes were harvested 30 days after the last of four applications of 175 g product ha^?1 (trifloxystrobin 44 g AI ha^?1, tebuconazole 88 g AI ha^?1) under the semi‐arid tropical climatic conditions of India. Copyright © 2010 Society of Chemical Industry     

Effects of glyphosate on soil microbial communities and its mineralization in a Mississippi soil

Transgenic glyphosate-resistant (GR) soybean [Glycine max (L.) Merr.] has enabled highly effective and economical weed control. The concomitant increased application of glyphosate could lead to shifts in the soil microbial community. The objective of these experiments was to evaluate the effects of glyphosate on soil microbial community structure, function and activity. Field assessments on soil microbial communities were conducted on a silt loam soil near Stoneville, MS, USA. Surface soil was collected at time of planting, before initial glyphosate application and 14 days after two post-emergence glyphosate applications. Microbial community fatty acid methyl esters (FAMEs) were analyzed from these soil samples and soybean rhizospheres. Principal component analysis of the total FAME profile revealed no differentiation between field treatments, although the relative abundance of several individual fatty acids differed significantly. There was no significant herbicide effect in bulk soil or rhizosphere soils. Collectively, these findings indicate that glyphosate caused no meaningful whole microbial community shifts in this time period, even when applied at greater than label rates. Laboratory experiments, including up to threefold label rates of glyphosate, resulted in up to a 19% reduction in soil hydrolytic activity and small, brief (<7 days) changes in the soil microbial community. After incubation for 42 days, 32-37% of the applied glyphosate was mineralized when applied at threefold field rates, with about 9% forming bound residues. These results indicate that glyphosate has only small and transient effects on the soil microbial community, even when applied at greater than field rates.     

Occurrence of sulfonylurea resistance in Sagittaria trifolia,a basal monocot species,based on target‐site and non‐target‐site resistance

Sagittaria trifolia L. is one of the most serious weeds in paddy fields in Japan. Since the late 1990s, severe infestations of S. trifolia have occurred following applications of sulfonylurea herbicides in Akita prefecture. In this study, two accessions of S. trifolia, R1 and R2, were collected from paddy fields with severe infestations and their resistance profiles were determined in comparison to a susceptible accession, S1. R1 and R2 were highly resistant to bensulfuron‐methyl. R1 was also highly resistant to pyrazosulfuron‐ethyl, but R2 was susceptible. Relative to S1, R1 had an amino acid substitution at the Pro₁₉₇ residue of acetolactate synthase (ALS), a well‐known mutation that confers sulfonylurea resistance, suggesting that R1 has a target‐site‐based resistance (TSR) mechanism. The sequence of the ALS gene in R2 was identical to that in S1. A Southern blot analysis indicated that there was only one copy of the ALS gene in S1 and R2. These results suggest that R2 has a non‐target‐site‐based resistance (NTSR) mechanism. R2 was moderately resistant to imazosulfuron but susceptible to thifensulfuron‐methyl. R2 and S1 were susceptible to pretilachlor, benfuresate, MCPA‐ethyl and bentazon. The results reveal the occurrence of two sulfonylurea‐resistant biotypes of S. trifolia that show different mechanisms of cross‐resistance to sulfonylureas related to TSR in R1 and NTSR in R2.     

Dissipation of the herbicide dithiopyr in soil and residues in wheat (Triticum aestivum L) grain under Indian tropical conditions

Dissipation of dithiopyr in soil was monitored after application to wheat crop as pre- or post-emergence applications at two rates, viz 100 and 200 g AI ha(-1). The level of dithiopyr in the soil was assessed by gas chromatography, and its disappearence was found to follow a first-order decay curve irrespective of rate or method of application. The half-life in soil ranged between 17.3 and 25.0 days and residues at harvest (150 days after application) ranged between 4.0 and 8.8% of amounts applied. Investigation of microbial degradation of dithiopyr was conducted in minimal salt and Czapek Dox media in which 80% of the compound degraded within 15 days. Residues were not detected in wheat grain at harvest.     

Evidence for 2,4‐D mineralisation in Mediterranean soils: impact of moisture content and temperature

BACKGROUND: The 2,4‐D degradation ability of the microbiota of three arable Mediterranean soils was estimated. The impact of soil moisture and temperature on 2,4‐D degradation was investigated. RESULTS: The microbiota of the three soils regularly exposed to 2,4‐D were able rapidly to mineralise this herbicide. The half‐life of 2,4‐D ranged from 8 to 30 days, and maximum mineralisation of ¹⁴C‐2,4‐D ranged from 57 to 71%. Extractable ¹⁴C‐2,4‐D and ¹⁴C‐bound residues accounted for less than 1 and 15% respectively of the ¹⁴C‐2,4‐D initially added. The highest amounts of ¹⁴C‐2,4‐D bound residues were recorded in the soil with the lowest 2,4‐D‐mineralising ability. Although all three soils were able to mineralise 2,4‐D, multivariate analysis revealed that performance of this degrading microbial activity was dependent on clay content and magnesium oxide. Soil temperature affected the global structure of soil microbial community, but it had only a moderate effect on 2,4‐D‐mineralising ability. 2,4‐D‐mineralising ability was positively correlated with soil moisture content. Negligible 2,4‐D mineralisation occurred in all three soils when incubated at 10 or 15% soil moisture content, i.e. within the range naturally occurring under the Mediterranean climate of Algeria. CONCLUSION: This study shows that, although soil microbiota can adapt to rapid mineralisation of 2,4‐D, this microbial activity is strongly dependent on climatic parameters. It suggests that only limited pesticide biodegradation occurs under Mediterranean climate, and that arable Mediterranean soils are therefore fragile and likely to accumulate pesticide residues. Copyright © 2009 Society of Chemical Industry     

Contaminant seeds in imported crop seed lots: a non‐negligible human‐mediated pathway for introduction of plant species to islands

Contaminant seeds in crop seed lots constitute a pathway for plant species introduction into new areas, and these non‐native weeds may be an environmental problem if they become invasive. Seed certification is a process that regulates and guarantees the quality of seed lots, including their purity. In this study, we assessed weed contamination in certified and non‐certified seed lots (n = 116) from 12 crop species commonly cultivated in the Balearic Islands. Contaminant seeds were separated using sieves, and then manually under a stereomicroscope, before they were germinated to confirm taxonomic identity. Weight, number and diversity of the contaminant seeds per kilogram of seed lot, number released per hectare according to sowing rate, and taxonomic identity and biogeographical origin of contaminant species were recorded. Although certification reduced the number of contaminant seeds in the seed lots, it did not entirely eliminate contaminants, because we found up to 2000 contaminant seeds kg^?1 of certified ryegrass and sulla. Overall, contaminant seeds represented 118 taxa; of which, 82 were identified to species level, 70% of species were native, 19% were cultivated, and 11% were non‐native. Two of the identified taxa were first records for the Balearic Islands. In conclusion, contaminant seeds in imported crop seed lots represent a non‐negligible pathway for plant species introduction.     

Evidence for Lettuce big‐vein associated virus as the causal agent of a syndrome of necrotic rings and spots in lettuce

Lettuce big‐vein associated virus (LBVaV, genus Varicosavirus) was shown to be responsible for characteristic necrotic symptoms observed in combination with big‐vein symptoms in lettuce breeding lines when tested for their susceptibility to lettuce big‐vein disease (BVD) using viruliferous Olpidium virulentus spores in a nutrient film technique (NFT) system. Lettuce plants showing BVD are generally infected by two viruses: Mirafiori lettuce big‐vein virus (MiLBVV, genus Ophiovirus) and LBVaV. New mechanical inoculation methods were developed to separate the two viruses from each other and to transfer both viruses to indicator plants and lettuce. After mechanical inoculation onto lettuce plants MiLBVV induced vein‐band chlorosis, which is the characteristic symptom of BVD. LBVaV caused a syndrome of necrotic spots and rings which was also observed earlier in lettuce plants inoculated in the NFT system, resembling symptoms described for lettuce ring necrosis disease (RND). This observation is in contrast with the idea that LBVaV only causes latent infections in lettuce. De novo next‐generation sequencing demonstrated that LBVaV was the only pathogen present in a mechanically inoculated lettuce plant with symptoms, providing evidence that LBVaV was the causal agent of the observed necrotic syndrome and thus fulfilling Koch’s postulates for this virus. The necrotic syndrome caused by LBVaV in lettuce is referred to as LBVaV‐associated necrosis (LAN).     

Developmental changes in glutathione S‐transferase activity in herbicide‐resistant populations of Alopecurus myosuroides Huds (black‐grass) in the field

Herbicide‐resistant populations of Alopecurus myosuroides Huds (black‐grass) have become widespread throughout the UK since the early 1980s. Clear evidence suggests that more than one resistance mechanism exists, and glutathione S‐transferases (GSTs) have been implicated in resistance due to enhanced metabolism. This study reports the determination of GST activity in four UK black‐grass populations from field sites situated in the East Midlands. Data demonstrate that, as untreated plants in the field mature, there is an accompanying natural elevation of GST activity with natural environmental changes from winter to spring. We speculate that this endogenous change in enzyme activity with plant development in the field contributes to reduced efficacy of some graminicides applied in the spring. These observations are discussed in relation to predicting herbicide efficacy to achieve maximum control of this important grass weed. © 2001 Society of Chemical Industry