Effect of enhanced CaCl2, MgSO4, and KH2PO4 on improved in vitro growth of potato

Potato (Solanum tuberosum L.) is a major global food crop. Contemporary potato production largely utilizes micropropagation to produce healthy seed potatoes. The micropropagation of potatoes is widely achieved through nodal explants using the conventional Murashige and Skoog (MS) medium. Currently, effective culture media that can facilitate rapid propagation are increasingly required for new cultivars that have been developed to possess improved traits. In this study, we evaluated the effect of enhanced meso nutrients (CaCl₂.2H₂O, MgSO₄, and KH₂PO₄) in MS medium on the growth of S. tuberosum. The cultivars used in this study were representative of Japanese, European, and Peruvian lines. Enhanced meso nutrients improved the overall quality of all cultivars, as indicated by longer shoots and larger leaves with dark color, compared with MS medium only. Shoots grown on enhanced mesos were approximately 1.5 times longer than on MS medium. Quantitative ion analysis revealed that plantlets with improved shoot length and leaf quality in most cultivars had increased calcium, magnesium, potassium, and phosphorus uptake than plantlets on MS medium. The results suggest that the reduced iron uptake on 3.0×MS, compared with 2.0× or 2.5×MS mesos, reduced plant growth. This study revealed for the first time that mesos concentrations higher than MS medium concentrations, complemented by enhanced calcium, magnesium, potassium, phosphorus, and iron uptake, play a significant role in improving the in vitro growth of potato.     

Inbreeding and true seed in tetrasomic potato. I. Selfing and open pollination in Andean landraces (Solanum tuberosum Gp. Andigena)

 True potato seed (TPS) may be an alternative method of potato production in developing countries. A breeding method for the sexual propagation of this vegetatively propagated crop should consider the development of parental lines and the type of cultivar to be released. Open-pollinated (OP) cultivars seem to be an inexpensive procedure to produce potato from true seed. However, OP progenies are the result of selfing and outcrossing in male-fertile tetraploid potatoes. The aim of the present research was to establish the effect of inbreeding and open pollination in TPS. Ten Andigena clones were used as parental material to derive hybrid (S₀), inbred (S₁ and S₂), and open-pollinated (OP₁ and OP₂) generations. Significant differences among generations were found for pollen production, pollen viability (as determined by its stainability with aceto-carmine glycerol), number of flowers and berries plant^-1, number of seeds berry^-1, weight of 1000 seeds, and tuber yield plant^-1. The parental populations were significantly different for most of the traits, but not for flower production and berry weight. The interaction of population ×generation was significant for pollen and seed production as well as for weight for 1000 seeds. All the traits evaluated except seed weight showed a strong inbreeding depression, while the OP progenies had intermediate values between the S₀ and the S₁. This demonstrates that open pollination in potatoes is not exclusively the product of selfing; it also results from outcrossing. Received: 10 November 1997 / Accepted: 3 March 1998     

Reaction of Six Solanum tuberosum Cultivars to Pratylenchus penetrans

Six potato cultivars - Superior, Yukon Gold, Monona, Norchip, Kennebec, and Russet Burbank - were grown in Tioga loamy sand in tile microplots noninfested or infested with 9,800-11,500 P. penetrans/kg soil. At midseason, soil nematode population densities on Superior potatoes were 39,800/kg compared with 17,500-25,800/kg on the other cultivars. At harvest, 118 or 139 days after planting depending on maturity date, the final soil P. penetrans population density was 24,400/kg with Superior and 34,100-51,500/kg with the other cultivars. No differences occurred in the rate of nematode build-up in soil on the six potato cultivars. The nematode suppressed yield of marketable tubers of Superior by 73% and of Yukon Gold by 25%. Losses for Russet Burbank (61%), Kennebec (55%), Monona (46%), and Norchip (43%) were intermediate.     

Effects of Initial Nematode Density on Population Dynamics of Globodera rostochiensis on Resistant and Susceptible Potatoes

The influence of resistant and susceptible potato cultivars on Globodera rostochiensis population density changes was studied at different nematode inoculum levels (Pi) in the greenhouse and field. Soil in which one susceptible and two resistant cultivars were grown and fallow soil in pots was infested with cysts to result in densities of 0.04-75 eggs/cm³ soil. A resistant cultivar was grown in an infested field with Pi of 0.7-16.7 eggs/cm³ soil. Pi was positively correlated with decline of soil population densities due to hatch where resistant potatoes were grown in the greenhouse and in the field but not in fallow soil. However, Pi was not correlated with in vitro hatch of G. rostochiensis cysts in water or potato root diffusate. Under continuous culture o f a resistant cultivar, viable eggs per cyst declined 60-90% per plant growth cycle (4 weeks) and the number of cysts containing viable eggs had decreased by 77% after five cycles. The rate of G. rostochiensis reproduction on both resistant and susceptible cultivars was negatively correlated with Pi. These data were used to predict the effect of resistant and susceptible potato cultivars on G. rostochiensis soil population dynamics.     

The white potato cyst nematode (Globodera pallida) – a critical analysis of the threat in Britain

Over the last 30 years, there has been an epidemic of the white potato cyst nematode (wPCN, Globodera pallida). It has progressively replaced the yellow species (yPCN, G. rostochiensis) throughout most of England and Wales and is now a widespread problem. As damaging populations of wPCN are enormous (>10⁹ eggs ha^?1), several crops of potato cultivars resistant only to yPCN were required to produce this change. The threat it poses is reflected in an increase in the numbers of soil samples being tested and in nematicide use, which has increased to > 25 000 ha of potatoes being treated annually. Computer modelling shows that current management of wPCN is mostly ineffective and populations will continue to increase. The multiplication rate of wPCN is inversely related to its population density at planting and, because of this, modelling shows that sufficient eggs are likely to survive to enable large populations of wPCN to “rebound” following nematicide treatment. This is supported by recent trial results showing that wPCN population increase was almost as great in nematicides‐treated plots as in the untreated. Modelling also showed that current rotations (typically potatoes once every 5 or 6 years) are too short to prevent wPCN populations from progressively increasing, even when used in conjunction with a nematicide. Similarly, except with avirulent populations, the partially resistant cultivars currently available will not prevent wPCN from increasing. However, as the effectiveness of partially resistant cultivars is independent of population density, they can be very effective when integrated with a nematicide. Unfortunately, only c. 8% of the potato area is planted with partially resistant cultivars, and much of that is in land not known to be infested with wPCN. Consequently, the current epidemic of wPCN is likely to become progressively more serious. However, many farmers are failing to recognise and respond to this threat until it is too late because of the slow rate of increase of wPCN, the difficulties of detecting small populations and the costs of nematicides. To respond to the current epidemic of wPCN, the greatest priority is to have available an increased number of commercially‐attractive partially resistant cultivars.     

Normal Operating Range of Bacterial Communities in Soil Used for Potato Cropping

In this study, the impacts of six potato (Solanum tuberosum) cultivars with different tuber starch allocations (including one genetically modified [GM] line) on the bacterial communities in field soil were investigated across two growth seasons interspersed with 1 year of barley cultivation, using quantitative PCR, clone library, and PCR-denaturing gradient gel electrophoresis (DGGE) analyses. It was hypothesized that the modifications in the tuber starch contents of these plants, yielding changed root growth rates and exudation patterns, might have elicited altered bacterial communities in the soil. The data showed that bacterial abundances in the bulk soil varied over about 2 orders of magnitude across the 3 years. As expected, across all cultivars, positive potato rhizosphere effects on bacterial abundances were noted in the two potato years. The bulk soil bacterial community structures revealed progressive shifts across time, and moving-window analysis revealed a 60% change over the total experiment. Consistent with previous findings, the community structures in the potato rhizosphere compartments were mainly affected by the growth stage of the plants and, to a lesser extent, by plant cultivar type. The data from the soil under the non-GM potato lines were then taken to define the normal operating range (NOR) of the microbiota under potatoes. Interestingly, the bacterial communities under the GM potato line remained within this NOR. In regard to the bacterial community compositions, particular bacterial species in the soil appeared to be specific to (i) the plant species under investigation (barley versus potato) or, with respect to potatoes, (ii) the plant growth stage. Members of the genera Arthrobacter, Streptomyces, Rhodanobacter, and Dokdonella were consistently found only at the flowering potato plants in both seasons, whereas Rhodoplanes and Sporosarcina were observed only in the soil planted to barley.     

Suppression of Pratylenchus penetrans Populations in Potato and Tomato using African Marigolds

Current strategies for management of Pratylenchus penetrans in both white potato and tomato consist of the use of fumigant or non-fumigant nematicides or crop rotation. The objective of this study was to determine if double-cropping African marigolds (Tagetes erecta) with potatoes or tomatoes could reduce P. penetrans populations. Plots were 10 m × 3 m arranged in a randomized complete block design with four replications. Treatments included marigolds, potatoes or tomatoes, and natural weedy fallow followed by either potatoes or tomatoes. Nematode populations were sampled before spring planting, between crops in August and after harvest in November. During the 3 years of the study, P. penetrans soil population density declined by an average of 93% from the pre-plant level when marigold was grown in rotation with potato and by 98% when marigold was grown.in rotation with tomato. Weedy fallow preceding potato resulted in an average decline in P. penetrans soil population density of 38%, and a similar decrease (37%) was seen when fallow preceded tomato. There was a significant reduction in the number of P. penetrans found in both potato and tomato roots when the crops followed marigolds. These results suggest that P. penetrans population density may be significantly reduced when marigolds are double-cropped with potatoes or tomatoes.     

A method for estimating the contribution of seed potatoes, machinery and soil tare in field infestations with potato cyst nematodes on a national scale

In order to make a cost benefit analysis for the management of the potato cyst nematodes Globodera rostochiensis and G. pallida, we developed a method to estimate the relative importance of three basic distribution channels of potato cyst nematodes: seed potatoes, machinery and soil tare. The baseline is determined by the area planted with potatoes, the area infested with potato cysts, the proportion of resistant potato cultivars and the distribution of cysts trough different channels. This quantification forms a basis for the evaluation of the effects of different control measures for potato cyst nematode on a national scale. The method can be useful as an example for application in other countries.     

Construction of Plant Expression Vectors and Identification of Transgenic Potato Plants

Potato virus X (PVX), potato virus Y (PVY) and potato leaf roll virus (PLRV) infection in potato may result in the loss of centrification of seed potatoes and affect the quality and yield of potatoes in agricultural production. The authors cloned coat protein (cp) genes of PVX, PVY and PLRV and constructed two kinds of plant expression vector which contain PVX and PVY or PVY and PLRV cp genes. Three major commercial cultivars of potato and one cultivar of tobacco were transformed via Agrobacterium tumefaciens mediated procedure. Transgenic plants were confirmed by PCR analysis. Transgenic tobacco plants containing both PVX and PVY cp genes were significantly resistant to PVX and PVY infection via mechanical inoculation.     

双价外壳蛋白基因植物表达载体的构建及马铃薯转基因植株的鉴定

在克隆了马铃薯Ｘ病毒（ＰＶＸ）、马铃薯Ｙ 病毒（ＰＶＹ）和马铃薯卷叶病毒（ＰＬＲＶ）的外壳蛋白基因的基础上,构建同时包含ＰＶＸ和ＰＶＹ 与ＰＶＹ 和ＰＬＲＶ 两个外壳蛋白基因植物表达框架的表达载体,通过农杆菌（Ａｇｒｏｂａｃｔｅｒｉｕｍ ｔｕｍｅｆａｃｉｅｎｓ）介导转化烟草（Ｎｉｃｏｔｉａｎａｔａｂａｃｕｍ ）和生产上常用的几个马铃薯（Ｓｏｌａｎｕｍ ｔｕｂｅｒｏｓｕｍ ）优良品种：“Ｆａｖｏｒｉｔａ”、“虎头”、“克４”。经ＰＣＲ检测证明外源基因已整合到植物的染色体上,得到批量转基因植株。在转ＰＶＸ＋ＰＶＹ 外壳蛋白基因的烟草上接种ＰＶＸ （５ μｇ／ｍ Ｌ）、ＰＶＹ（２０ μｇ／ｍ Ｌ）病毒,得到有一定抗性的植株     

Evaluation of the sensitization rates and identification of IgE-binding components in wild and genetically modified potatoes in patients with allergic disorders

Background

The potato is one of the most common types of genetically modified (GM) food. However, there are no published data evaluating the impact of genetic manipulations on the allergenicity of GM potatoes. To compare the allergenicity of GM potatoes with that of wild-type potatoes using in vivo and in vitro methods in adult allergy patients sensitized to potatoes.

Methods

A total of 1886 patients with various allergic diseases and 38 healthy controls participated in the study. Skin-prick testing and IgE-ELISA were carried out with extracts prepared from wild-type and GM potatoes. An ELISA inhibition test was used to confirm the binding specificity. IgE-binding components in extracts from the two types of potato were identified by SDS-PAGE and IgE-immunoblotting. The effects of digestive enzymes and heat on the allergenicity of the extracts was evaluated by preincubating the potatoes with or without simulated gastric and intestinal fluids in the absence or presence of heat.

Results

Positive responses (ratio of the wheal size induced by the allergen to that induced by histamine (A/H) ≥ 2+) to wild-type or GM potato extracts, as demonstrated by the skin-prick test, were observed in 108 patients (5.7%). Serum-specific IgE was detected in 0–88% of subjects who tested positively. ELISA inhibition tests indicated significant inhibition when extract from each type of potato was added. IgE-immunoblot analysis demonstrated the presence of 14 IgE-binding components within the wild-type potato and 9 within the GM potato. Furthermore, a common 45-kDa binding component that yielded similar IgE-binding patterns was noted in more than 80% of the reactions using sera from patients sensitized to wild-type or GM potato. Exposure to simulated gastric fluid and heat treatment similarly inhibited IgE binding by extracts from wild-type and GM potatoes, whereas minimal changes were obtained following exposure of the extracts to simulated intestinal fluid.

Conclusion

Our results strongly suggest that genetic manipulation of potatoes does not increase their allergenic risk. The sensitization rate of adult allergy patients to both types of extract was 5.7%, and a common major allergen (45 kDa) was identified.     

Increase of isoflavones in soybean callus by Agrobacterium-mediated transformation

Plant secondary metabolites have always been a focus of study due to their important roles in human medicine and nutrition. We transferred the isoflavone synthase (IFS) gene into soybean [Glycine max (L.) Merr.] using the Agrobacterium-mediated transformation method in an attempt to produce transformed soybean plants which produced increased levels of the secondary metabolite, isoflavone. Although the trial to produce transgenic plant failed due to unestablished hygromycin selection, transformed callus cell lines were obtained. The induction rate and degree of callus were similar among the three cultivars tested, but light illumination positively influenced the frequency of callus formation, resulting in a callus induction rate of 74% for Kwangan, 67% for Sojin, and 73% for Duyou. Following seven to eight subcultures on selection media, the isoflavone content of the transformed callus lines were analyzed by high-performance liquid chromatography. The total amount of isoflavone in the transformed callus cell lines was three- to sixfold higher than that in control callus or seeds. Given the many positive effects of isoflavone on human health, it may be possible to adapt our transformed callus lines for industrialization through an alternative cell culture system to produce high concentrations of isoflavones.     

Ecogeography of ploidy variation in cultivated potato (Solanum sect. Petota)

? Premise of the study: The taxonomy of cultivated potatoes has been highly controversial, with estimates of species numbers ranging from 3 to 17. Ploidy level has been one of the most important taxonomic characters to recognize cultivated potato species, containing diploid (2n = 2x = 24), triploid (2n = 3x = 36), tetraploid (2n = 4x = 48), and pentaploid (2n = 5x = 60) cultivars. We tested the environmental associations of different ploidy levels in cultivated potato species that traditionally have been recognized as Linnaean taxa to see whether, in combination with prior morphological, molecular, and crossing data, some of the ploidy variants can be recognized as distinct taxa. ? Methods: We summarize 2780 chromosome counts of landrace cultivated potatoes, provide georeferences to 2048 of them, and analyze these data for 20 environmental variables at 10-min resolution using the randomForest algorithm to explore associations with taxa and ploidy variants. ? Key results: Except for the S. tuberosum Chilotanum Group and extreme northern and southern range extensions of the Andigenum Group, it is impossible to find distinct habitats for the ploidy variants of the S. tuberosum Andigenum Group. ? Conclusions: Our distributional and ecological data, in combination with prior results from morphology, microsatellites, and crossing data, provide yet additional data to support a major reclassification of cultivated potato species. A rational, stable, and universally accepted taxonomy of this major crop plant will greatly aid all users of wild and cultivated potatoes from breeders to gene bank managers to ecologists and evolutionary biologists.     

Occurrence of potato virus X strain group 1 in seed stocks of potato cultivars lacking resistance genes

Potato virus X (PVX) isolates were obtained from a simple seed potato production scheme or from ware potatoes produced by seed potatoes obtained from it. In this scheme, PVX infection is widespread in seed stocks and most of the potatoes grown lack PVX resistance genes. Thirteen PVX isolates were typed to strain group by inoculation to potato cultivars containing different combinations of hypersensitivity genes Nx and Nb. Six failed to overcome either gene and therefore belonged to strain group 1, four overcame Nb only and were placed in strain group 3 and three were mixtures of the two. All 13 isolates failed to overcome extreme resistance/immunity gene Rx. Naturally infected cultivars of genotype nx.nb contained strain group 1 alone or strain groups 1 and 3, while those of genotype nx:Nb contained only strain group 3. The widespread occurrence of strain group 1 contrasts with the predominant occurrence of strain group 3 in potatoes in the UK. However, it resembles the UK situation before sophisticated seed potato production schemes were introduced and before PVX hypersensitivity genes Nx and Nb were deliberately exploited in potato breeding. Prior infection with potato leafroll virus (PLRV) did not affect expression of hypersensitivity to PVX in inoculated leaves of an nx:Nb genotype.     

Improving breeding efficiency in potato using molecular and quantitative genetics

Key message

Potatoes are highly heterozygous and the conventional breeding of superior germplasm is challenging, but use of a combination of MAS and EBVs can accelerate genetic gain.

Abstract

Cultivated potatoes are highly heterozygous due to their outbreeding nature, and suffer acute inbreeding depression. Modern potato cultivars also exhibit tetrasomic inheritance. Due to this genetic heterogeneity, the large number of target traits and the specific requirements of commercial cultivars, potato breeding is challenging. A conventional breeding strategy applies phenotypic recurrent selection over a number of generations, a process which can take over 10 years. Recently, major advances in genetics and molecular biology have provided breeders with molecular tools to accelerate gains for some traits. Marker-assisted selection (MAS) can be effectively used for the identification of major genes and quantitative trait loci that exhibit large effects. There are also a number of complex traits of interest, such as yield, that are influenced by a large number of genes of individual small effect where MAS will be difficult to deploy. Progeny testing and the use of pedigree in the analysis can provide effective identification of the superior genetic factors that underpin these complex traits. Recently, it has been shown that estimated breeding values (EBVs) can be developed for complex potato traits. Using a combination of MAS and EBVs for simple and complex traits can lead to a significant reduction in the length of the breeding cycle for the identification of superior germplasm.     

Structural characteristics and plant-beneficial effects of bacteria colonizing the shoots of field grown conventional and genetically modified T4-lysozyme producing potatoes

Genetically modified potatoes expressing antibacterial protein T4 lysozyme may offer effective control strategies for bacterial pathogens causing severe potato diseases. Apart from this beneficial effect, it is very important to investigate such engineered potatoes carefully for potential adverse effects on potato-associated bacteria which frequently exhibit plant beneficial functions such as plant growth promotion and antagonism towards pathogens invading the plant. Two field experiments were carried out in Spain to analyze the potential effects of conventional and genetically modified T4-lysozyme producing potatoes on shoot-associated bacteria. The first baseline field trial 2002 was performed in Meliana in which three conventional potato lines, Achirana Inta, Desirée, and Merkur, were cultivated and sampled at flowering. The second field trial was conducted in Cella in 2003 in order to compare the effects of a senescent transgenic, T4 lysozyme expressing potato trait, Desirée DL 12, with its isogenic, non-transformed parental line Desirée. Structural characteristics of potato shoot-associated bacteria was assayed by 16S rRNA-based terminal restriction fragment length polymorphism (T-RFLP) analysis and dominant community members within T-RFLP profiles were identified by sequence analysis of generated 16S rRNA gene libraries. Cultivable bacteria isolated from shoots of potatoes grown in the Meliana field trial were monitored for antibiosis against Ralstonia solanacearum, whereas isolates derived from shoots of potatoes cultivated in the Cella trial were screened for antagonism against Ralstonia solanacearum and Rhizoctonia solani, and for 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase production. Determined antagonists were identified by 16S rRNA gene analysis. All potato traits hosted a cultivar-specific community of bacteria with antagonism against the pathogens and/or potential to produce ACC deaminase. Several antagonists obtained from the Cella field potatoes were also observed as ACC deaminase producers. Community profiling revealed a greater diversity differentiation between the senescent T4 lysozyme expressing and parental Desirée lines grown in the Cella field as compared to the variations between the three flowering conventional lines cultivated in the Meliana field trial. Effects of the two varying field sites and different vegetation stages were greater than those of T4 lysozyme when investigating the community composition of bacteria colonizing the shoots of the Desirée line cultivated in both field trials.     

Genetic diversity in european and Argentinian cultivated potatoes (Solanum tuberosum subsp. tuberosum) detected by inter-simple sequence repeats (ISSRs).

In this study, the use of inter-simple sequence repeats (ISSR) to assess genetic diversity between cultivated potatoes (Solanum tuberosum subsp. tuberosum) is reported. ISSR technology rapidly reveals high polymorphic fingerprints and thus determines the genetic diversity among potato cultivars. Nine primers were selected according to the number of amplified markers and the level of polymorphism detected. Three primers (GAG(CAA)5, CTG(AG)8, and (AG)8) were used to cluster the 28 potato accessions and 77 polymorphic markers were sufficient to identify all of the accessions. Among the 15 simple sequence repeat (SSR) motifs tested, the most abundant were CAA and AG. Argentinian- and European-grown potatoes were easily distinguished, with a higher level of genetic diversity among potatoes from Argentina. An ISSR study using a limited number of cultivars and very few primers clearly differentiated between all cultivars, thus ISSR was revealed to be a good tool for the genetic identification of potato and for future germplasm-management programs.     

Detecting pathogenic bacterial wilt disease of potato using biochemical markers and evaluate resistant in some cultivars

Bacterial wilt caused by Ralstonia solanacearum (Smith), is one of the chief severe diseases of potato in warm temperate regions, tropics and subtropics of the world. The study was conducted to isolate and identify bacterial pathogens and select the most resistant cultivars and avoid the decrease in the total value of Egyptian potato exports to the European Union (EU) due to the quarantine restrictions imposed by the EU on potato tubers exported from Egypt affected by bacterial wilt. The results of traditional identification through morphological and serological studies showed that the five isolates were isolated and identified as Ralstonia solanacearum. Furthermore, the results illustrated that RS5 isolate showed the lowest percentage of disease incidence reduction on the three tested potatoes cultivar Bellini, Spunta and Mondial recorded 9.64%, 15.41% and 34.12%, respectively. While, RS8 isolate exhibited the highest effective one the percentage of disease reduction on all tested potato cultivars. This isolate reduced disease incidence 60.60%, 63.21% and 71.66%, compering to the healthy control treatment. The result of molecular identification represent that the probe used in Taq-man (PCR) was of the type (B2) capable to detect only biovar 2 of R. solanacearum bacterial wilt. Furthermore, primer and probe are specific for detection of the race 3 biovar 2 strain. Positive results were obtained in all assays used including IFAS, protein content and SDS-PAGE with all five isolates. So the isolate (RS5) was the most virulence one, followed by RS1, RS3, RS2 and RS8, registered that the tested isolates were R. solanacearum race 3, biovar 2. Also, studies focused on the form of genetic distances and similarities based on pathogenic and plant growth parameters. The results illustrate that the highest genetic similarity (0.998) was found between Bellini and Spunta cultivars as the closest but the lowest value (0.946) was found between Mondial and Bellini as most distant. These results were similarity with genetic distances and SDS-PAGE profile of the three tested potato cultivars.     

Effects of potato genotype, oxamyl and the numbers of potato cyst-nematodes, Globodera rostochiensis or G. pallida on tuber yields and nematode increase

A new technique is described for establishing different numbers of the potato cyst-nematode Globodera rostochiensis in field soil, which leaves the soil homogeneous in nutrient status. Field plots established in this way were used to compare yield losses in four potato cultivars (Maris Piper, Pentland Crown, Pentland Dell and Désirée) associated with different numbers of G. rostochiensis. Over the range of 7.4 to 148.4 eggs g^-1 soil at planting, yield losses were 18.7% (Maris Piper), 53.2% (Désirée), 55.7% (Pentland Crown) and 63.5% (Pentland Dell). Similar results were obtained in another experiment on the same field in a different year using only lightly and heavily infested plots. Treating the seedbed soil with oxamyl before planting prevented significant injury to potatoes by G. rostochiensis but increased the yield of Pentland Dell and perhaps Désirée (but not Maris Piper or Pentland Crown) more than expected from nematode control alone. Treating heavily infested soil with such a nematicide cannot therefore be recommended as part of a valid procedure for establishing lightly and heavily infested plots for comparing tolerances of attack by potato cyst-nematodes in a range of potato genotypes. In peaty loam soils moderately or heavily infested with G. pallida, oxamyl at 5.6 kg a.i. ha^-1 incorporated into the seedbed before potatoes were planted generally increased tuber yields, though the effects varied considerably with the cultivar grown. Increase of G. pallida in these soils was controlled better by growing potatoes bred for resistance to it (ZB 35 – 29, Caxton, Santé, Morag, 11233 ab 22, Fingal, A27/23, Cromwell). Increase of G. pallida on susceptible cultivars varied greatly and Romano increased G. pallida no more than the resistant Morag. G. pallida is probably controlled best in peaty loam by growing a resistant cultivar in soil treated with a granular (non-fumigant) nematicide.