psbA mutation (Asn266 to Thr) in Senecio vulgaris L. confers resistance to several PS II-inhibiting herbicides

DNA sequence analysis of the psbA gene encoding the D1 protein of photosystem II (PS II), the target site of PS II-inhibiting herbicides, identified a point mutation (Asn266 to Thr) in a bromoxynil-resistant Senecio vulgaris L. population collected from peppermint fields in Oregon. Although this mutation has been previously reported in Synechocystis, this is the first report of this particular point mutation in a higher plant exhibiting resistance to PS II-inhibiting herbicides. The resistant population displayed high-level resistance to bromoxynil and terbacil (R/S ratio 10.1 and 9.3, respectively) and low-level resistance to metribuzin and hexazinone (R/S ratio 4.2 and 2.6, respectively) when compared with the susceptible population. However, the population was not resistant to the triazine herbicides atrazine and simazine or to the urea herbicide diuron. A chlorophyll fluorescence assay confirmed the resistance levels and patterns of cross-resistance of the whole-plant studies. The resistant S. vulgaris plants produced fewer seeds. Differences in cross-resistance patterns to PS II-inhibiting herbicides and the difference in fitness cost could be exploited in a weed management program.     

psbA Mutation (valine219 to isoleucine) in Poa annua resistant to metribuzin and diuron

The herbicide‐binding region of the chloroplast psbA gene from a total of 20 biotypes of Poa annua L resistant and susceptible to metribuzin and diuron was selectively amplified using PCR. Sequence analysis of the fragment from six herbicide‐resistant biotypes of P annua exhibited a substitution from valine to isoleucine at position 219 of the D1 protein encoded by the psbA gene. This is the same mutation as reported for Chlamydomonas and Synechococcus through site‐directed mutagenesis and in cell cultures of Chenopodium rubrum L. To our knowledge this is the first report of a higher plant exhibiting resistance in the field to photosystem II inhibitors due to a psbA mutation other than at position 264. The existence of additional biotypes of P annua resistant to diuron or metribuzin but lacking mutation in the herbicide‐binding region indicates that resistance to these herbicides can also be attained by other mechanisms. © 2000 Society of Chemical Industry     

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.     

地肤子提取物抑菌机理和超微结构观察

为探讨地肤子甲醇萃取物经柱层析后分离得到的有效成分对桃褐腐病菌的作用机理,以桃褐腐病菌为供试菌种,对作用方式、体内酶活性以及蛋白质含量进行了测定,并在扫描电镜下对菌体的超微结构变化进行了观察。结果表明地肤子提取液对桃褐腐病菌起到了抑制作用,且随着提取液浓度的增加,桃褐腐病菌体内酶活性降低,而蛋白质的含量成增长的趋势。扫描电镜观察可看出其影响了孢子与菌丝正常生长,使其形态发生较大变化,有大量溢出物,且菌丝粗细不均一,发生断裂。即地肤子提取液通过降低桃褐腐病菌体内酶活性、增加蛋白质的含量以及破坏菌丝和孢子的正常生长来起到抑菌的作用。     

Recovery of non-target plants affected by airborne bromoxynil-octanoate and metribuzin

The present study was conducted to evaluate the recovery potential of non‐target plants affected by two airborne herbicides. Sunflower at the two‐leaf stage was used as a test plant and exposed for 24 h in a wind tunnel to a range of concentrations of airborne bromoxynil‐octanoate and metribuzin. Quantum yield (φ_PSII) of exposed leaves and of the second leaf pair developed after exposure was determined at a particular time up to 16 days following exposure. Maximum depression in quantum yield of exposed leaves from which a complete recovery occurred within 16 days was 63% for bromoxynil‐octanoate and 60% for metribuzin respectively. The corresponding maximum concentrations were 1.310 and 0.390 μg m^?3 respectively. The second leaf pair was also affected and showed a similar recovery potential. From the results it can be concluded that the significance of airborne bromoxynil‐octanoate and metribuzin must not be overestimated, as sunflower and non‐target plants with a similar sensitivity are likely to recover from air concentrations of both herbicides reported under field conditions.     

Evaluation of triticale accessions for resistance to wheat bacterial leaf streak caused by Xanthomonas translucens pv. undulosa

The bacterium Xanthomonas translucens pv. undulosa (Xtu) causes bacterial leaf streak (BLS) on wheat and other small grains. Several triticale accessions were reported to possess high levels of resistance to wheat Xtu strains. In this study, a worldwide collection of triticale accessions as well as the major North Dakota hard red spring and durum wheat cultivars were evaluated for reaction to two local Xtu strains. All wheat cultivars showed a susceptible reaction but a wide range of reactions was observed among triticale accessions. Out of the 502 accessions tested, 45 and 10 accessions were resistant to the two virulent strains BLS‐LB10 and BLS‐P3, respectively, with five accessions, PI 428736, PI 428854, PI 428913, PI 542545 and PI 587229, being highly resistant to both strains. Statistical analysis showed significant difference among the accessions, strains, and the accession by strain interaction (P < 0.001). Bacterial population growth in resistant triticale was significantly slower than that in susceptible triticale. Molecular cytogenetic characterization in four representative triticale accessions confirmed the hexaploid level of the species and the presence of 12 or 14 rye chromosomes. The triticale accessions identified are valuable materials for developing wheat germplasm with high levels of BLS resistance.     

Soil persistence studies with bromoxynil, propanil and [14C]dicamba in herbicidal mixtures

The persistence of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), [¹⁴C]dicamba (3,6-dichloro-2-methoxybenzoic-7-¹⁴C acid) and propanil [N-(3,4-dichlorophenyl)propionamide] at rates equivalent to 1 kg ha^?1, were studied under laboratory conditions in a clay loam, a heavy clay and a sandy loam at 85% of field capacity and at 20±1°C, both singly and in the presence of herbicides normally applied with these chemicals as tank-mix or split-mix components. The degradation of bromoxynil was rapid with over 90% breakdown occurring within a week in the heavy clay and sandy-loam soils, while in the clay-loam approximately 80% of the bromoxynil had broken down after 7 days. In all three soils degradation was unaffected by the presence of asulam, diclofop-methyl, flamprop-methyl, MCPA, metribuzin or propanil. Propanil underwent rapid degradation in all soil treatments, with over 95% of the applied propanil being dissipated within 7 days. There were no noticeable effects on propanil degradation resulting from applications of asulam, barban, bromoxynil, dicamba, MCPA, MCPB, metribuzin or 2,4-D. The breakdown of [¹⁴C]dicamba in a particular soil was unaffected by being applied alone or in the presence of diclofop-methyl, flampropmethyl, MCPA, metribuzin, propanil or 2,4-D. The times for 50% of the applied dicamba to be degraded were approximately 16 days in both the clay loam and sandy loam, and about 50 days in the heavy clay.     

Levels of fire blight (<Emphasis Type="Italic">Erwinia amylovora</Emphasis>) susceptibility of native apple,pear and quince germplasm from Lake Van Basin,Turkey

Fire blight resistance of apple, pear and quince genetic resources from Lake Van Basin (eastern Turkey) was tested using Erwinia amylovora strain Ea Van. Shoot tips of 92 native accessions (48 accessions for apple, 38 accessions for pear and 6 accessions for quince) were wounded for inoculation, and artificially inoculated with pathogenic bacteria under greenhouse conditions. The levels of resistance of accessions were classified in comparison with control varieties according to the genotype susceptibility index (GSI%) scores based on the lesion length on shoots of each genotype. Fire blight resistance of accessions consisted of five classes: resistant (R), moderately resistant (MR), moderately susceptible (MS), susceptible (S) and highly susceptible (HS). GSI% scores differed significantly among accessions from each fruit species (p < 0.01). GSI values ranged from 12.4% to 64.1% for apple genotypes, from 17.2% to 55.1% for pear genotypes, and from 17.8% to 43.4% for quince genotypes. No resistant genotypes of apple, pear and quince were observed. Seven accessions of apple, two accessions of pear and one accession of quince were MR. 25 accessions of apple, 14 accessions of pear and one accession of quince were MS. These findings indicate a considerable variation in fire blight resistance and could contribute to breeding efforts regarding fire blight resistance in apple, pear and quince.     

Sulfonylurea herbicide-resistant Monochoria vaginalis in Korean rice culture

Nine Monochoria vaginalis Pres1 accessions from Chonnam province, Korea were tested for resistance to the sulfonylurea herbicide, imazosulfuron, in whole-plant response bioassay. All accessions were confirmed resistant (R) to imazosulfuron. The GR50 (imazosulfuron concentration that reduced shoot dry weight by 50%) values of R accessions were 1112-3172 (accession #9) times higher than that of the standard susceptible (S) accession. Accession #9 exhibited cross-resistance to other sulfonylurea herbicides, bensulfuron-methyl, cyclosulfamuron and pyrazosulfuron-ethyl, but not to the imidazolinone herbicides, imazapyr and imazaquin. The R biotype could be controlled by other herbicides with different modes of action, such as mefenacet and pyrazolate, applied to soil at recommended rates. Foliar-applied herbicides, 2,4-D and bentazone, also controlled both the R and S biotypes. Sulfonylurea-based mixtures, except ethoxysulfuron plus fentrazamide, did not control resistant M. vaginalis. Rice yield was reduced 70% by resistant M. vaginalis that escaped pyrazosulfuron-ethyl plus molinate, compared with hand weeding in direct-seeded rice culture. In contrast, rice yield was reduced 44% by resistant M. vaginalis that survived the pyrazosulfuron-ethyl plus molinate treatment, compared with pyrazolate plus butachlor in transplanted rice culture. In vitro acetolactate synthase (ALS) activity of the R biotype was 183, 35, 130 and 31 times more resistant to imazosulfuron, bensulfuron-methyl, cyclosulfamuron and pyrazosulfuron-ethyl, respectively, than the S biotype. Imidazolinone herbicides, imazapyr and imazaquin had similar effect on in vitro ALS activity of the R and S biotypes. The in vivo ALS activity of the R biotype was also less affected than the S biotype by the sulfonylurea herbicides imazosulfuron and pyrazosulfuron-ethyl. Results of in vitro and in vivo ALS assays indicate that the resistance mechanism of M. vaginalis to sulfonylurea herbicides may be due, in part, to an alteration in the target enzyme, ALS. Since the level of resistance in the enzyme assay was much lower than that in the whole-plant assay, other mechanisms of resistance, such as herbicide metabolism, may be involved.     

Characterization of acetolactate synthase from sulfonylurea herbicide‐resistant Schoenoplectus juncoides

Ten accessions of sulfonylurea‐resistant Schoenoplectus juncoides were collected from paddy fields in Japan. In order to characterize acetolactate synthase from sulfonylurea‐resistant S. juncoides, acetolactate synthase amino acid substitutions, whole‐plant growth inhibition and acetolactate synthase enzyme inhibition were examined. Schoenoplectus juncoides has two acetolactate synthase genes (ALS1 and ALS2). The sulfonylurea‐resistant accessions harbored amino acid substitutions at Pro197 or Trp574 in either ALS1 or ALS2 (the amino acid number is standardized to the Arabidopsis thaliana sequence). The whole plants of all the sulfonylurea‐resistant accessions showed resistance to imazosulfuron. The resistance level depended on the altered amino acid residues in acetolactate synthase. The acetolactate synthase enzyme that was partially purified from all the sulfonylurea‐resistant accessions was less sensitive to imazosulfuron, compared to the susceptible accession, suggesting that the resistance is related to the altered acetolactate synthase enzyme. In addition, the concentration–response inhibition of acetolactate synthase activity by imazosulfuron in the sulfonylurea‐resistant accessions was remarkably different with the presence of an amino acid substitution in either ALS1 or ALS2. Furthermore, the concentration–response inhibition of acetolactate synthase activity in the sulfonylurea‐resistant accessions with a P197S, P197T or W574L mutation showed a double‐sigmoid curve. The regression analysis of enzyme inhibition suggested that the abundance ratio of ALS1 to ALS2 enzymes was approximately 70:30%, with a range of ±15%. Taken together, these results suggest that the resistance of sulfonylurea‐resistant accessions of S. juncoides is related to altered acetolactate synthase in either ALS1 or ALS2, although the abundance of the altered acetolactate synthase in the plants is different among the sulfonylurea‐resistant accessions.     

Triazine resistance in Amaranthus tuberculatus (Moq) Sauer that is not site-of-action mediated

While surveying Illinois Amaranthus tuberculatus (Moq) Sauer (tall waterhemp) half-sib populations for herbicide response variability, several were observed to segregate for resistance to atrazine. Studies were conducted on greenhouse-grown A tuberculatus plants to compare atrazine responses among populations that were segregating for resistance (SegR), uniformly sensitive (UniS) or uniformly resistant (UniR). In chlorophyll fluorescence assays, leaves of plants from the SegR and UniS populations displayed changes in fluorescence after treatment with atrazine, indicating that atrazine was inhibiting electron transport of photosystem II in chloroplasts. Sequencing of a fragment of psbA, which encodes the D1 protein, revealed that the SegR population did not contain the amino acid substitution that is typically found in triazine-resistant plants. Whole-plant herbicide dose-response experiments revealed that, relative to the UniS population, atrazine resistances in the UniR and SegR populations were > 770-fold and 16-fold, respectively. The SegR population was also resistant to cyanazine (59-fold), but not to metribuzin, linuron or pyridate. Triazine resistance in the SegR population was shown to be a nuclear inherited trait, unlike maternal inheritance of site-of-action mediated triazine resistance found in the UniR population. Taken collectively, these findings confirm the existence of two distinct triazine resistance mechanisms in A tuberculatus.     

Resistance among accessions of the generalycopersicon andsolanum to four of the main insect pests of tomato in Israel

Laboratory, greenhouse and field experiments were conducted to identify plant accessions of the genusLycopersicon and ofSolarium pennellii resistant toSpodoptera littoralis (Boisd.),Plusia chalcites (Esp.),Heliothis armigera (Hbn.), andPhthorimaea operculella (Zeu.), four insect pests of the cultivated tomato. Percent survival, larval weight, duration of development, damage scores, etc., were the criteria used to determine the relative resistance of the examined accessions. TheL. hirsutum accession LA 1777 and theL. hirsutum f.glabratum accession LA 407 were found to be highly resistant to all four insect pests. TheS. pennellii accession LA 716 was found to be resistant to the first three insects, but only partially resistant toP. operculella. The possible mechanisms of resistance,i.e., the physical entrapment of larvae and the toxic action of phytochemicals in the various accessions, are discussed.     

地肤提取物杀螨活性的研究

采用多种溶剂和方法对地肤全株进行了提取,测定了其中杀螨活性最高的氯仿提取物的多种溶剂萃取物对二斑叶螨(Tetranychus urticae Koch)和朱砂叶螨[Tetranychus cinnabarinus(Boisduval)]的生物活性。结果表明,氯仿提取物的生物活性显著高于石油醚和甲醇提取物,提取物的氯仿萃取物对2种螨的生物活性最强。本试验还对该种植物的杀螨活性有效成分进行了初步研究。氯仿萃取物经柱层分离得到流分112个,经薄层层析(TLC)检验后,相同成分者合并,最终得流分11个。生物活性测定结果表明,F₉活性最高,其对二斑叶螨、朱砂叶螨和山楂叶螨的致死中浓度分别达到了0.390 3、0.298 1、0.256 8 mg /mL。     

Soil persistence studies with [14C]MCPA in combination with other herbicides and pesticides

The persistence of [¹⁴C]MCPA at a rate equivalent to 1 kg ha^?1 was studied under laboratory conditions in a clay loam, heavy clay and sandy loam at 85% of field capacity moisture and 20±1°C both alone and in the presence of tri-allate, trifluralin, tri-allate and trifluralin, malathion, Vitaflow DB, malathion and Vitaflow DB, bromoxynil, bromoxynil and asulam, bromoxynil and difenzoquat, dicamba, dicamba and mecoprop, linuron, MCPB, metribuzin, propanil, TCA, benzoylprop-ethyl, diclofop-methyl, and flamprop-methyl. Except in the soils treated with asulam, the half-lives of [¹⁴C]MCPA in all three soil types were similar, being approximately 13±1 days, thus indicating that none of the other chemicals studied adversely affected the soil degradation of MCPA. In the asulam treated soils, the half-lives of the MCPA were about 3 days longer than in non-asulam treated soils; the effect was most marked in the clay loam.     

Metribuzin degradation in soil: I—effects of soybean residue amendment,metribuzin level,and soil depth

Plant residue and soil depth effects on metribuzin degradation were investigated. Dundee silt loam soil collected at depth increments of 0–10 cm (SUR) and 10–35 cm (SUB) was treated with labeled [5^?14 C]metribuzin. Samples were assayed at several time points up to 140 days after treatment. Soybean residue was added to half of the SUR samples (RES), with remaining SUR unamended (NORES). None of the SUB samples were amended with soybean residue. Metribuzin mineralization to ¹⁴CO₂ proceeded more slowly in RES and SUB than in NORES and SUR, respectively. Extractable components in SUR samples included polar metabolites, plus deaminated metribuzin (DA) in the RES, and parent metribuzin in the NORES. Deaminated diketometribuzin (DADK) and metribuzin comprised major ¹⁴C components extracted from SUB, while in SUR, faster degradation of metabolites resulted in metrizubin as the primary identifiable compound. Unextractable ¹⁴C increased until day 35 for both RES and NORES, after which it remained constant for NORES. but declined for RES. A corresponding rise in RES polar ¹⁴C suggested that as soybean residue decomposed, ¹⁴C bound in the residue was released as extractable polar material. Soil with soybean residue accumulation may alter metabolite degradation patterns, but does not impede initial metribuzin degradation. Depth differences in metribuzin degradation were attributed to reductions in microbial activity with increasing soil depth.     

Preliminary characterization of race-specific elicitors from<Emphasis Type="Italic">Peronospora parasitica</Emphasis> and their ability to elicit phenolic accumulation in<Emphasis Type="Italic">Arabidopsis</Emphasis>

Intercellular washing fluid (IWF) obtained from the susceptibleArabidopsis accession Ws-eds1 inoculated withPeronospora parasitica isolate Emoy-2, contained an elicitor of necrosis with ecotype specificity towardsArabidopsis accessions with particular resistance genes. This elicitor caused necrosis on the highly resistant accessions La-er, Nd-1 and partly on Col-5, but not on the susceptible accessions Ws-eds1 and Oy-0. In resistant plants, injection of IWF caused hypersensitive reaction (HR)-like cell collapse which was associated with the accumulation of phenolics and lignin-like material in walls of cells undergoing cell death. The elicitor is sensitive to proteinase K and pronase enzymes, heating and autoclaving but insensitive to periodate oxidation, freezing and thawing, and is not dialyzable. Results suggest that the elicitor is a protein. Fractionation experiments using size-exclusion membranes revealed that elicitor activity has a molecular weight in excess of 100 kDa. http://www.phytoparasitica.org posting July 13, 2003.     

Rapid diagnosis of sulfonylurea‐resistant Schoenoplectus juncoides [Roxb.] Palla using polymerase chain reaction–restriction fragment length polymorphism and isogene‐specific direct sequencing

Rapid diagnostic methods to detect known mutations in acetolactate synthase (ALS) genes that confer sulfonylurea (SU) resistance to Schoenoplectus juncoides were developed in this study. By using 11 SU‐resistant accessions (nine accessions with a Pro₁₉₇ substitution in ALS1 or ALS2, one accession with an Asp₃₇₆Glu substitution in ALS2 and one accession with a Trp₅₇₄Leu substitution in ALS2), polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) analysis for DNA fragments that were amplified simultaneously from genomic ALS1 and ALS2 and PCR–RFLP analysis for DNA fragments that were amplified from either of the genomic ALS1 or ALS2 were carried out. In each of the two PCR–RFLP analyses, a common PCR product was digested separately with the restriction enzymes, BspLI, MboI and MunI, in order to detect Pro₁₉₇ substitutions, an Asp₃₇₆Glu substitution and a Trp₅₇₄Leu substitution, respectively. In each of the lanes where the detection of SU‐resistant substitutions was aimed, a specific band to suggest the existence of the said substitutions was observed in theoretically assumable ways. Separately, a direct sequencing method also was established, which was able to selectively sequence ALS1 or ALS2 from common templates containing both ALS1 and ALS2 by the isogene‐selective primers that were designed to anneal either of the ALS genes. It is expected that these methods could be used for the genetic analysis of SU‐resistant S. juncoides by providing rapid and accurate diagnosis.     

The Principles and Practice of Weed Control for Notillage Soyabean in Southern Brazil Using the Bipyridyl Herbicides

Abstract

An ICI survey of the arable area of southern Brazil in 1971 confirmed the urgent need for some form of conservation tillage system for soyabean wheat primarily to control soil erosion. A practical system of no-tillage soyabean was developed for N. Parana, with weed control based on careful crop residue management and sprays of paraquat mixed with either linuron or metribuzin. The marked superiority of these mixtures over paraquat alone for the control of vigorous annual grasses was noted. This effect is, in part attributed to the capacity of metribuzin and linuron (along with other herbicides) to inhibit photosynthesis, thereby facilitating the translocation of paraquat. Where residual herbicides are not used, diuron at low rates in suitable formulation proved the best additive for paraquat. Problem broadleaved weeds are controlled by adding 2, 4-D to paraquat. Temperature, rainfall, the presence of fallows and the interval between harvest and the next planting, all affect pre-plant weed control for no-till soyabean. A key issue is to prevent excessive stands of weeds. Control within the crop can be provided by shielded, directed sprays of paraquat for which tractor mounted sprayers were developed. Recent discoveries of pre- and post-emergence selective herbicides provide improved and alternative in-crop weed control options. Crop diversification and further general no-till research show promise of further rational, chemical and cultural methods of weed control.     

四川水稻抗稻曲病种质资源筛选和多态性分析

 稻曲病近年来逐步上升为世界多个水稻产区的主要病害之一。利用丰富的抗性种质资源进行抗性品种选育是控制稻曲病害的重要手段。通过在四川省进行三年病圃筛选试验,我们从843份水稻材料中鉴定出179份完全不感稻曲病的抗性材料和两份高感材料(病穗率高达50%以上),其余材料的病穗率介于0.1%~48.8%之间,55.1%的病穗只有一个稻曲球,而高感稻曲病的蒲江6号单穗稻曲球高达38粒。然后我们选取36份材料进行多点多期播种,进一步验证其稻曲病抗感性。结果表明,有3份材料在所有试验中都不感稻曲病,18份材料偶发稻曲病呈现高抗,蒲江6号仍然表现为高感。我们用450对SSR引物对其中35份材料与蒲江6号进行多态性分析,发现其中12份材料与蒲江6号的多态性达10%以上,其中最高为ITAT144(16.1%),其次是泸香90-2(14.8%)和Domsia-2(14.4%)。因此,我们选择这些材料与蒲江6号杂交构建基因定位与克隆群体。     

Effects of tillage, application time and rate on metribuzin dissipation

Metribuzin efficacy and dissipation were determined in two silty clay loam soils following preplant (PP), pre-emergence (PRE) and split (PP+PRE) application to tilled and no-till soybeans in rotation with corn at Clay Center and Lincoln, Nebraska. A similar experiment was conducted in tilled and no-till soybeans in rotation with wheat at Lincoln. Corn and wheat residue in no-till plots reduced the amount of metribuzin that reached the soil by approximately 54 and 89%, respectively. No differences in weed control or soybean yield were observed between tillage treatments or time of metribuzin application in the corn-soybean rotation. However, both weed control and yield were reduced in the wheat-soybean rotation. Most of the metribuzin remained at the 0–5 cm depth, and dissipation was exponential. The mean metribuzin half-life at the 0–5 cm depth across locations, tillage treatments, application time and rates was 11 days. The metribuzin half-life was 4–19 days following PP application and 3–17 days following PRE application. The metribuzin concentration did not exceed 65 μg kg_?1 at the 5–10 cm or 10–20 cm depths in any treatment, indicating that little metribuzin had leached from the surface soil after PP or PRE application. The finding of a higher metribuzin concentration at 5-20 cm depth in tilled plots than in no-till could be attributed to higher initial soil concentrations in the absence of crop residue.