Effects of soil fumigation on soil nitrogen and on disease incidence in winter wheat

Winter wheat was grown in soil fumigated with D-D, 85% dazomet dust or formalin, and top-dressed with o or 125 kg nitrogen/ha. Six weeks after fumigation, there was much more ammonium nitrogen in fumigated than in unfumigated soil. Nitrate was also more after fumigation with dazomet, but less after D-D and formalin. After 5 months, only D-D plots had less nitrate and more total mineral nitrogen than unfumigated plots. Take-all (Ophiobolus graminis) was less prevalent in the first crop after dazomet or formalin, but was more prevalent in the second crop than after other treatments. Eyespot (Cercosporella herpotrichoides) was decreased only by formalin and only in the first crop after applying it. Fumigation had little effect on the incidence of other diseases recorded or on grain yield, except that yields were light on D-D plots in the first crop after fumigation, when ears were severely deformed.     

Effects of D-D, Telone or dazomet applied to potato ridges in spring on potato cyst-nematode, Heterodera rostochiensis, in sandy loam and silt loam soils

Large amounts of dazomet (329, 439 kg/ha) applied to potato ridge soil in spring, before potatoes were planted, controlled potato cyst-nematode (Heterodera rostochiensis) in sandy loam and silt loam more effectively than large amounts of D-D (359, 448 kg/ha). In heavily infested sandy loam, 329 kg dazomet/ha or 857 kg methyl bromide/ha applied in spring 1969 or 439 kg dazomet/ha applied in autumn 1968, greatly decreased the number of larvae able to invade potato roots, so Majestic potatoes grew and yielded well without increasing the number of nematodes left in the soil after harvest. Large amounts of D-D or Telone applied to the topsoil in autumn or to the ridges in spring were less effective in controlling potato cyst-nematode or increasing potato yields. Applied in spring 1969 to silt loam ridges, 439 kg dazomet/ha had more effect than 448 kg D-D/ha on potato cyst-nematode and on the increase in yield of Majestic potato. The yield of Maris Piper potatoes (resistant to H. rostochiensis pathotype A) in infested silt loam was increased greatly by D-D, as much by 112 as by 224 or 448 kg/ha.     

Treating potato ridges in spring with aldicarb, D-D or dazomet to control potato cyst-nematode, Heterodera rostochiensis, in sandy clay and peat loam soils

Applied to potato ridge soil in spring, before potatoes were planted, small amounts of aldicarb (10-3 kg/ha or less) controlled potato cyst-nematodes (Heterodera rostochiensis Woll.) better than large amounts of dazomet (110–466 kg/ha) or D-D (102–439 kg/ha). Applied in spring 1968 and 1969 to heavily infested sandy clay soil 466 kg dazomet/ha allowed Majestic potatoes to grow and yield well in both years without increasing the number of nematodes in the soil after harvest, but in peaty loam dazomet was toxic to potato plants and, when applied in autumn, killed fewer nematodes. D-D in potato ridges in spring controlled nematodes less well than dazomet or aldicarb, but 896 kg D-D/ha injected in sandy clay soil in autumn increased potato yield the following year without increasing the number of nematodes after harvest.     

The effects of soil sterilants on the cereal cyst-nematode (Heterodera avenae Woll.), take-all (Ophiobolus graminis Sacc.) and yields of spring wheat and barley

In a 3-year field experiment on sandy loam at Woburn, methyl bromide, chloropicrin, D-D mixture, dazomet, formalin and mercury salts were applied in the first year only, or in the first and second years, before drilling spring wheat. In the third year, their residual effects on spring barley were measured. All sterilants except mercury decreased cereal cyst-nematode ( Heterodera avenae ) numbers and take-all ( Ophiobolus graminis ) incidence, and increased crop yields in the year they were applied. Dazomet gave the best control of H. avenae in the first year and controlled O. graminis best in both years of application. In the third year, O. graminis increased in all plots previously treated (except after two successive D-D treatments), but H. avenae increased only after formalin. Two formalin applications more than trebled the H. avenae egg count by the end of the third year and depressed the yield of barley. Two successive applications of chloropicrin gave the best nematode control.
Other fungus diseases, Fusarium foot rot, Pythium root rot, eye-spot ( Cercosporella herpotrichoides Fron.) and sharp eye spot ( Rhizoctonia solani Kühn), were uncommon and sterilants had no detectable effects on them. The largest aggregate straw yield (3-year total) was obtained from two applications of dazomet, but the best yields after treatment in the first year only were given by D-D and methyl bromide.     

Effects of soil fumigation on disease incidence, growth, and yield of spring wheat

Plots were fumigated with various amounts of D-D or 85% dazomet dust and sown with spring wheat given various amounts of nitrogenous fertilizer. Dazomet increased yield and decreased take-all disease in the first crop after application, but increased the disease in the second crop. Although D-D increased take-all slightly, it increased yield in 1966, but in 1967 it decreased yield and its use was associated with a severe ear deformity. Fumigation had little effect on eyespot, sharp eyespot, root browning (Fusarium spp.), or browning root rot (Pythium spp.), but decreased nematode damage where nematodes were numerous.     

Control of potato cyst-nematode, Globodera pallida, on tomatoes grown under glass, by applying steam or chemical nematicides to the soil

In a sandy loam under glass, potato cyst-nematode, Globodera pallida, was controlled in soil to 20 cm deep, and tomato yields were greatly increased by sheet steaming for 4 h, by 977 kg methyl bromide ha^-1 or by 448 kg Telone (1,3-dichloropropene mixture or Di-Trapex CP (20% methyl isothiocyanate: 15% chloropicrin:65% dichloropropene-dichloropropane mixture) ha^-1 followed by 441 kg dazomet or 11.2 kg oxamyl ha^-1. Sheet steaming, unlike the other treatments, had relatively little effect on the nematode in soil 20 to 40 cm deep. Until the apices of the plants were cut off production of fruit was curvilinear against time in all treated soil but linear against time in untreated soil.     

不同熏蒸剂对草莓连作土壤养分和微生物群落的影响

【背景】草莓连年栽培导致土传病害问题突出,施用熏蒸剂进行土壤消毒因效果显著得以广泛应用。但不同熏蒸剂对土壤病原微生物的影响存在较大差异,同时对非靶标微生物和土壤理化性质也会有不同程度的影响。【目的】明确不同熏蒸剂对草莓连作土壤养分和土壤细菌、真菌群落结构的影响,为合理选择熏蒸剂提供科学依据。【方法】以连作土壤为材料设置5个处理：未熏蒸、石灰氮熏蒸、石灰熏蒸、棉隆熏蒸、威百亩熏蒸,测定熏蒸处理后土壤养分含量;采用PacBio测序平台分析土壤微生物多样性的变化。【结果】石灰氮和威百亩处理均增加了碱解氮含量,降低了有机质、有效磷和速效钾含量;棉隆处理土壤中各养分含量均增加;石灰处理除有机质含量增加外,碱解氮、有效磷和速效钾含量均降低;棉隆、石灰和威百亩处理显著降低pH值。5个处理草莓连作土壤样本中获得了1 164个细菌OTU和444个真菌OTU。细菌多样性和丰富度分析发现,4种熏蒸剂处理均增加了土壤细菌群落的丰富度,石灰氮、石灰和威百亩处理增加了土壤细菌菌落的多样性。4种熏蒸剂处理真菌菌落的丰富度低于对照;石灰、棉隆处理真菌菌落的多样性高于对照和石灰氮、威百亩处理低于对照,但差异不显著。在物种组成分析中,从门水平看,变形菌门(Proteobacteria)和芽单胞菌门(Gemmatimonadetes)为优势细菌门;与对照相比,石灰氮、石灰、棉隆处理变形菌门相对丰度增高,威百亩处理相对丰度降低。4种处理均降低了芽单胞菌门的相对丰度。其他门类中,4种处理均增加了浮霉菌门(Planctomycetota)、疣微菌门(Verrucomicrobia)的相对丰度。优势细菌群落分析表明土壤熏蒸减少了芽单胞菌属(Gemmatimonas)、藤黄单胞菌属(Luteimonas)、中慢生根瘤菌属(Mesorhizobium)等细菌的相对丰度,增加了噬几丁质菌属(Chitinophaga)、苍白杆菌属(Ochrobactrum)的相对丰度。子囊菌门(Ascomycota)为优势真菌,石灰氮、石灰、棉隆、威百亩4种处理均增加了子囊菌门的相对丰度。另外还检测到引起草莓根部土传病害的枝孢属(Cladosporium)和镰刀菌属(Fusarium)病菌,熏蒸处理后均降低了枝孢属和镰刀菌属的相对丰度,其中枝孢属在石灰氮、石灰、棉隆、威百亩处理中分别降低了1.35%、1.11%、0.90%和1.31%,镰刀菌属分别降低了0.71%、0.85%、0.19%和0.65%,但差异不显著。4种土壤熏蒸剂均增加了有益真菌毛壳菌属(Chaetomium)的相对丰度。【结论】采用熏蒸剂处理连作土壤改变了微生物群落构成,减少或灭杀土壤中的大部分致病菌属,起到有效防治草莓土传病害的作用,但不能灭杀所有病菌,而且对有益菌和土壤理化性质有不同程度的影响,因此处理后补充有益微生物非常关键。根据对病原菌的灭杀效果,石灰氮、威百亩处理的效果优于其他处理,可作为轮换施用的熏蒸剂,本试验条件下,棉隆是一种弱的处理剂。     

Changes in microorganisms populations in the soil after fumigation

Soil fumigation with dazomet, metam sodium, chloropicrin and chloropicrin + 1.3 D resulted in significant decrease of fungi and increase of bacteria populations in trials carried out in four farms located in different areas. Depending on the farm and the active substance applied, the fungi population was decreased by 1.4- to 3500-fold in comparison to control. Metam sodium and chloropicrin showed the best efficacy, both of them almost totally eliminated the fungi from the soil environment.The total number of bacteria was increased by the chemical fumigation with all tested products. While the population of fluorescent Pseudomonads in all treated plots increased from 2- to 100-fold, depending on the farm, the number of Bacillus spp. was not changed or decreased compared with non fumigated soil.The nematology analysis of the soil indicated that any chemical fumigant significantly limited the population of plant parasitic nematodes, which number was, anyway, below the damage threshold. However, in most cases dazomet and metam sodium reduced the total number of all nematodes present in the soil.The fumigation with chloropicrin and 1,3 D at dose of 30 g/m2 resulted in an increase of the total number of all nematodes in soil.     

Preplant Fumigation for Citrus Nematode Control in Florida

Preplant soil fumigation experiments were conducted to control the citrus nematode, Tylenchulus semipenetrans. Generally, D-D (1,3-dichloropropene, 1,2-dichloroptopane and related chlorinated C₃-hydrocarbons), Telone (1,3-dichloropropene and related chlorinated C₃-hydrocarbons), Telone PBC (80% 1,3-dichloropropene, 15% chloropicrin, 5% propargyl bromide), and EDB (ethylene dibromide) controlled T. semipenetrans effectively for 4 years. The trials involved four scion varieties, two rootstock varieties and three soil types. Tree growth and yield were increased with application of D-D at 374 or 561 liters/ha (40 or 60 gal/acre) or Telone at 299 or 449 liters/ha (32 or 48 gal/acre) in broadcast and strip treatments.     

Soil properties and actinorhizal vegetation influence nodulation of Alnus glutinosa and Elaeagnus angustifolia by Frankia

Nodulation (mean number of nodules per seedling) was 5 times greater for Elaeagnus angustifolia than for Alnus glutinosa overall when seedlings were grown in pots containing either an upland or an alluvial soil from central Illinois, USA. However, the upland Alfisol had 1.3 times greater nodulation capacity for A. glutinosa than for E. angustifolia. The presence of A. glutinosa trees on either soil was associated with a two-fold increase in nodulation capacity for E. angustifolia. Nodulation increases for soils under A. glutinosa were obtained for A. glutinosa seedlings in the Alfisol, but decreased nodulation for A. glutinosa seedlings occurred in the Mollisol. Greatest nodulation of E. angustifolia seedlings occurred near pH 6.6 for soil pH values ranging from 4.9 to 7.1, while greatest nodulation of A. glutinosa occurred at pH 4.9 over the same pH range. Nodulation was not affected by total soil nitrogen concentrations ranging from 0.09 to 0.20%. Mollisol pH was significantly lower under A. glutinosa trees than under E. angustifolia trees. For 4- to 8-year-old field-grown trees, A. glutinosa nodule weights were negatively correlated with soil pH, while for similar aged E. angustifolia trees nodulation in the acidic Alfisol was not detected.     

Chemical control of potato cyst-nematode, Heterodera pallida, on tomatoes grown under glass

In sandy clay loam under glass 11.2 kg a.i. Du Pont 1410 alone or with 448 kg Di-Trapex CP/ha, 441 kg dazomet with 448 kg Telone or 448 kg Di-Trapex CP/ha, prevented potato cyst-nematodes, Heterodera pallida, increasing on KNVF tomato rootstocks. In the heated glasshouse where the soil was treated with 441 kg dazomet/ha alone, the nematodes increased in the top 10 cm of soil, whether the soil was sealed with Polythene sheeting or water and whether dazomet was applied as dust or prill (small granules). Methyl bromide at 1276 kg/ha did not control the nematodes in soil 20–30 cm deep in one of the treated plots. Tomatoes grew well after all treatments except Du Pont 1410 which had little, if any, effect on grey sterile fungus (Pyrenochaeta lycopersici). Dazomet with Telone or dazomet with Di-Trapex CP inhibited root damage by the fungus.     

Effects of soil treatments and cultivars on 'early dying' disease of potatoes caused by Globodera rostochiensis and Verticilliurn dahliae

Plots in a field infested with Globodera rostochiensis and Verticillium dahliae were treated in 1971 with methyl bromide, dazomet, aldicarb, benomyl or aldicarb + benomyl and potatoes (cv. Pentland Dell) were grown in four consecutive years. In 1971, all treatments delayed the development of haulm symptoms, decreased soil populations of G. rostochiensis and increased yields. In the second year yields were increased by all treatments except dazomet whereas in the third and fourth years only benomyl or aldicarb + benomyl increased yields. Soil populations of V. dahliae were decreased by methyl bromide in 1971 but not by other treatments or in later years. In 1976, Pentland Dell and Pentland Crown were grown on an infested field in plots fumigated with methyl bromide. Fumigation slightly delayed shoot emergence but increased plant height, ground cover and the size and persistence of the leaves; development of haulm symptoms was delayed and yield increased. Fumigating with methyl bromide at five times the normal rate was more beneficial to Pentland Dell than to Pentland Crown. In a comparison of 10 cultivars in 1975, symptoms developed sooner and yields were smaller on a field infested with G. rostochiensis and V. dahliae than on a non-infested field and on both sites Maris Peer plants died soonest and yielded least. Pentland Crown yielded most on the non-infested field and Maris Piper on the infested field.     

几种替代杀线剂对甘薯茎线虫的毒力与活性

采用熏蒸法和浸渍法测定了4种土壤熏蒸杀线剂和6种非熏蒸杀线剂对甘薯茎线虫的毒力,在此基础上以各药剂的致死中浓度剂量处理线虫,设置不同时间处理,观察了药剂对线虫的活性动态变化.结果表明： 熏蒸杀线剂棉隆、1,3-二氯丙烯、威百亩、氯化苦对甘薯茎线虫的LC₅₀依次为0.49、0.89、0.91、3.60 mg·L^-1,非熏蒸杀线剂甲维盐、阿维菌素、灭线磷、噻唑膦、涕灭威、丁硫克百威对甘薯茎线虫的LC₅₀依次为31.2、48.1、224.3、288.4、632.3、823.9 mg·L^-1.致死中浓度处理线虫后,各药剂校正死亡率随处理时间延长有不同幅度升高,棉隆、1,3-二氯丙烯、阿维菌素和甲维盐对线虫有较高的持续抑制效果,处理48 h脱离药剂处理后校正死亡率没有降低,而传统杀线剂品种灭线磷和涕灭威处理均出现线虫明显复苏的现象.表明棉隆、1,3-二氯丙烯、阿维菌素和甲维盐可以替代传统杀线剂应用于甘薯茎线虫的防治.     

Survival and symbiotic characteristics of rhizobium in saline-alkali soils

Summary Survival, growth and symbiotic performance of rhizobia isolated from normal, saline-sodic and mildly acidic soils were studied in original and amended saline-alkali soils. Rhizobia of 4 out of 9 legumes studied for nodulation were found to be present in a highly saline-sodic soil. Majority of the strains of these bacteria did not survive in the original saline-sodic soil of pH 10.5 but as the pH was amended to lower than 10.0, all the strains survived in the soil. Virtually no differences were noticed in the survival and symbiotic characteristics of native and exotic strains ofRhizobium leguminosarum andRhizobium trifolii in the saline-sodic soil, though wide variations were observed among individual strains irrespective of their ecological origin. Rhizobia were found to possess greater tolerance for alkalinity than their host legumes. However, delayed nodulation in lentil (Lens esculenta) and berseem (Trifolium alexandrinum) resulting in decreased yield of the plants at pH values higher than 9.0 was observed.     

Frankia inoculation,soil biota,and host tissue amendment influence Casuarina nodulation capacity of a tropical soil

The effects of soil biota, Frankia inoculation and tissue amendment on nodulation capacity of a soil was investigated in a factorial study using bulked soil from beneath a Casuarina cunninghamiana tree and bioassays with C. cunninghamiana seedlings as capture plants. Nodulation capacities were determined from soils incubated in sterile jars at 21 °C for 1, 7, and 28 days, after receiving all combinations of the following treatments: ± steam pasteurization, ± inoculation with Frankia isolate CjI82001, and ± amendment with different concentrations of Casuarina cladode extracts. Soil respiration within sealed containers was determined periodically during the incubation period as a measure of overall microbial activity. Soil respiration, and thus overall microbial activity, was positively correlated with increasing concentrations of Casuarina cladode extracts. The nodulation capacity of soils inoculated with Frankia strain Cj82001 decreased over time, while those of unpasteurized soils without inoculation either increased or remained unaffected. The mean nodulation capacity of unpasteurized soil inoculated with Frankia CjI82001 was two to three times greater than the sum of values for unpasteurized and inoculated pasteurized soils. Our results suggest a positive synergism between soil biota as a whole and Frankia inoculum with respect to host infection.     

The effect of fumigants on soil recolonization in the glasshouse by organisms colonizing the spermosphere

Germinating turnip seeds were used as baits to measure changes in colonizing floras of the spermosphere in glasshouse soil boxes following fumigation. The spermosphere mycoflora of the untreated soil was always dominated by Fusarium, Pythium, and Gliocladium spp. Methyl bromide, chloropicrin, and MBC₃₃ (a 67:33 mixture of methyl bromide and chloropicrin), applied in polyethylene bags at a rate of 2·5 ml/cubic foot of soil, greatly reduced colonization by Fusarium and Pythium spp. for 120 days. Fungi and actinomycetes recolonizing treated soils were more active than in untreated soils, and each fumigant induced a characteristic recolonization pattern. The spermosphere of methyl bromide-treated soil was initially dominated by actinomycetes and later by Penicillium spp. In chloropicrin-treated soil, Trichoderma and later Penicillium spp. dominated, and in soil treated with MBC ₃₃, Penicillium and Trichoderma spp. were co-dominant. Dilution plates, made at the same time from the same soil, indicated a close correlation between inoculum density of different fungi in the soil and their inoculum potentials towards the spermosphere.     

土壤熏蒸剂棉隆和生物菌肥对草莓连作土壤真菌多样性的影响

【背景】草莓是我国和世界上重要的园艺作物,主要采用一年一栽的栽培模式,导致连作现象普遍存在,其中真菌性病害引起的土传病害是草莓栽培中面临的一个主要问题。【目的】以草莓连作土壤为材料,探讨土壤熏蒸剂棉隆加生物菌肥对草莓连作土壤微生物真菌多样性的影响,以期为草莓连作障碍治理提供理论依据。【方法】分别采集棉隆消毒前(A)、棉隆消毒后(B)、棉隆消毒后未添加生物菌肥(C1)和添加生物菌肥(C2)初花期的草莓连作土壤(或根际土壤)样本,提取DNA,通过PCR扩增建立文库,利用Hi Seq 2500平台Illumina第二代高通量测序技术并结合相关生物信息学,分析土壤真菌ITS1区域的丰富度、多样性以及群落结构。【结果】从4个不同处理草莓连作土壤样本中获得了723个真菌OTU(Operational taxonomic unit),其中子囊菌门和担子菌门均为优势真菌。多样性和丰富度研究发现,棉隆处理降低草莓连作土壤微生物丰富度和多样性;添加生物菌肥增加根际土壤生物丰富度,并降低其多样性。从门水平看,棉隆处理担子菌门真菌比例减少,子囊菌门真菌比例增加;添加生物菌肥处理两者比例均有增加。优势真菌群落分析表明棉隆消毒减少了枝顶孢属、曲霉属、管柄囊霉属、镰刀菌属、踝节菌属、链格孢属等真菌比例,增加了马拉色氏霉菌属、线孢虫草菌属、侧耳属等真菌比例;添加生物菌肥减少了管柄囊霉属、镰刀菌属、被孢菌属、轮枝菌属、隔指孢属等真菌比例,增加了曲霉属、青霉菌属、踝节菌属、简单壳菌属等真菌比例。【结论】采用棉隆消毒和生物菌肥处理草莓连作土壤可降低微生物真菌群落的多样性,并减少或灭杀土壤中的大部分致病菌属,增加有益菌属,起到有效防治草莓土传病害的作用。     

Effect of juglone on growthin vitro ofFrankia isolates and nodulation ofAlnus glutinosa in soil

Summary In vitro growth (total protein content) of 5Frankia isolates was significantly inhibited at 10^–4 M juglone (5-hydroxy-1, 4-napthoquinone) concentration, but the degree of inhibition varied with theFrankia isolate. Isolates fromAlnus crispa [Alnus viridis ssp.crispa (Ait.) Turril] were most tolerant of 10^–4 M juglone relative to controls, while an isolate fromPurshia tridentata (Pursh.) D.C. was most inhibited, displaying a dramatic decrease in growth and greatly altered morphology.Nodulation of black alder [Alnus glutinosa L. (Gaertn.)] in an amended prairie soil inoculated with aFrankia isolate from red alder (Alnus rubra Bong.) was significantly decreased by the addition of aqueous suspensions of 10^–3 M and 10^–4 M juglone. This decrease was partially independent of decreased plant growth. The addition of an equal volume of sand to the soil mixture further decreased nodulation of black alder.Frankia inoculation of the soil mixtures significantly increased the total number of nodules formed per seedling, and the degree of differences in seedling nodulation owing to juglone and soil treatments.     

Establishment of Bradyrhizobium japonicum for soybean by inoculation of a preceding wheat crop

On fields with no history of soybean (Glycine max (L.) Merr.) production, inoculation alone is often inadequate to provide for adequate nodulation the first time this crop is grown. The objective of this study was to determine if inoculation of spring wheat (Triticum aestivum L.) seed with Bradyrhizobium japonicum would lead to an increase of B. japonicum numbers in the soil, and improve nodulation of a subsequent soybean crop. In the greenhouse, wheat seed inoculation increased B. japonicum numbers from undetectable numbers to >9000 g^–1 of soil, whereas the numbers of introduced B. japonicum declined in unseeded pots. In the field, inoculation of wheat seed increased B. japonicum numbers in the soil from undetectable levels to >4000 g^–1 the following year. When soybean seed was inoculated, but grown in soil devoid of B. japonicum, nodules formed only near the point of seed placement. The heaviest nodulation, and widest distribution of nodules in the topsoil were found whenB. japonicum was established the year before by wheat seed inoculation, plus soybean seed inoculation. Wheat seed inoculation the year before growing soybean, combined with proper soybean seed inoculation, should provide for abundant nodulation the first time soybean is grown on a field.     

Control of potato cyst-nematode, Heterodera rostochiensis, in sandy, peaty and silt loam soils by dazomet and Telone applied in different ways

Dazomet applied in the ridges in autumn or in spring, before potatoes were planted in them, controlled potato cyst-nematode (Heterodera rostochiensis), British pathotype A, better in sandy loam and peaty loam than Telone (1,3-dichloropropene mixture). In sandy loam dazomet controlled potato cyst-nematode as well when applied in spring as when applied in autumn and as well when the soil was ridged after treatment as when it was not. Telone was as effective when applied to ridges in autumn as when applied to ridges in spring. In peaty loam potato cyst-nematodes were least abundant after a crop of Maris Piper potatoes. The yields of King Edward potatoes were greatly increased and nematode multiplication was greatly reduced by dazomet incorporated in the ridges in autumn. Two equal doses of dazomet, one incorporated in the topsoil before, the other after ploughing, controlled potato cyst-nematode as well and increased the yield of King Edward potatoes more than an equivalent amount of dazomet applied after ploughing. Dazomet applied to silt loam soil in two dressings, one before, the other after, ploughing, controlled potato cyst-nematode better than an equal amount applied as a single dressing after ploughing. The nematode was controlled best by two large dressings of dazomet or by a combined treatment of dazomet and Telone.