Dynamics and management of the white potato cyst nematode Globodera pallida in commercial potato crops

Eight trials were conducted in commercial potato fields infested with the white potato cyst nematode (wPCN, Globodera pallida) and one in a field infested with the yellow PCN (yPCN, Globodera rostochiensis). Our aims were to produce data to validate and refine a computer‐based program (The Model) for the long‐term management of PCN, to determine nematicide effectiveness and to assess rates of PCN population decline between potato crops. Prior to planting, each farmer applied an overall nematicide treatment to his field, except for ten untreated plots that were widely spaced to encompass a range of PCN population densities. Each untreated plot was paired with a similar plot in the adjacent treated area and all plots were intensively sampled for PCN population densities at planting (P_i) and again at harvest (P_f) when tuber yields were determined. Four trials were re‐sampled 2–4 years later to determine PCN population decline rates. Regressions that form the basis of ‘The Model’ and described the relationship between P_i and tuber yield and PCN population density at harvest were fitted to the results from both the untreated and nematicide treated plots. These regressions also enabled us to estimate the yield potential at each site in the absence of PCN and showed that nematicide treatment generally did not increase yield potential and that both tuber yield and PCN multiplication decreased with increasing P_i. However, there were major differences between sites and cultivars. When untreated, the yield of cv. Maris Piper was hardly affected in a highly organic soil with P_i > 200 eggs g^?1 whereas the yield of partially resistant cv. Santé was decreased from a potential of c. 60 t ha^?1 to c. 20 t ha^?1 in a light silt with P_i = 20 egg g^?1 soil. Similarly, untreated wPCN multiplication rates at a low P_i ranged from 46‐fold to >100‐fold. Nematicide effectiveness was estimated from the regressions and, at several sites, yield was decreased despite nematicide treatment. Control of wPCN multiplication was even poorer. In only two of seven trials planted with susceptible cultivars was more than 50% control achieved – maximum populations in treated plots usually exceeded 250 eggs g^?1. Partially resistant Santé decreased the multiplication rate of wPCN in the two trials where it was planted. An alternative analysis using Genstat indicated that The Model tended to underestimate the maximum multiplication rate and overestimate the maximum population density. When four sites were re‐sampled 2–4 years after harvest the populations of wPCN had declined by between 15% and 33.5% per annum with a mean of 26% per annum. Modelling indicated that rotations longer than 8 years were required to control wPCN unless other effective control measures, such as growing a partially resistant cultivar, were used.     

Origin, distribution and 3D-modeling of Gr-EXPB1, an expansin from the potato cyst nematode Globodera rostochiensis

Southern analysis showed that Gr-EXPB1, a functional expansin from the potato cyst nematode Globodera rostochiensis, is member of a multigene family, and EST data suggest expansins to be present in other plant parasitic nematodes as well. Homology modeling predicted that Gr-EXPB1 domain 1 (D1) has a flat beta-barrel structure with surface-exposed aromatic rings, whereas the 3D structure of Gr-EXPB1-D2 was remarkably similar to plant expansins. Gr-EXPB1 shows highest sequence similarity to two extracellular proteins from saprophytic soil-inhabiting Actinobacteria, and includes a bacterial type II carbohydrate-binding module. These results support the hypothesis that a number of pathogenicity factors of cyst nematodes is of procaryotic origin and were acquired by horizontal gene transfer.     

A multipartite mitochondrial genome in the potato cyst nematode Globodera pallida

The mitochondrial genome (mtDNA) of the plant parasitic nematode Globodera pallida exists as a population of small, circular DNAs that, taken individually, are of insufficient length to encode the typical metazoan mitochondrial gene complement. As far as we are aware, this unusual structural organization is unique among higher metazoans, although interesting comparisons can be made with the multipartite mitochondrial genome organizations of plants and fungi. The variation in frequency between populations displayed by some components of the mtDNA is likely to have major implications for the way in which mtDNA can be used in population and evolutionary genetic studies of G. pallida.     

Screening for quantitative resistance to the white potato cyst nematode Globodera pallida

The resistance to Globodera pallida of potatoes bred from Solanum uernei was assessed rapidly in pot tests by counting cysts on the exterior of the root ball. Assessments of resistance made in this way in three consecutive years were reproducible on a ranking basis. There was also a significant agreement in the ranked results of pot and field tests. A more accurate assessment of nematode multiplication rates in the field could be provided by laboratory tests in which temperature, moisture and initial nematode numbers were strictly controlled.     

Inbreeding and population structure of the potato cyst nematode (Globodera pallida) in its native area (Peru)

The dispersal abilities and the population genetic structure of nematodes living in the soil are poorly known. In the present study, we have pursued these issues in the potato cyst nematode Globodera pallida, which parasitizes potato roots and is indigenous to South America. A hierarchical sampling regime was conducted in Peru to investigate gene flow on regional, field and plant scales. Multilocus genotypes of single individuals were obtained using eight polymorphic microsatellites markers. Large heterozygote deficiencies were observed at most loci. The limited active dispersal of larvae from their cyst, which favours mating between (half) siblings, could be responsible for this pattern. Within fields, as well as among fields within regions (even 35 km apart), low F(ST) values suggest extensive gene flow. Among fields within regions, only 1.5-4.4% genetic variability was observed. Passive dispersal of cysts by natural means (wind, running water, or wild animals) or by anthropogenic means (tillage, movement of infected seed tubers) is probably responsible for the results observed. Among regions, high F(ST) values were observed. Thus long-range dispersal (more than 320 km apart) is probably limited by major biogeographical barriers such as the mountains found in the Andean Cordillera. These results provide useful information for the management of resistant varieties, to slow down the emergence and spread of resistance-breaking pathotypes.     

Management of the potato cyst nematode (Globodera pallida) with bio-fumigants/stimulants

Field trials evaluated the effect of four plant-based bio-fumigants/stimulants on population levels of G. pallida and the resulting potato yields and quality. Three formulations contained seaweed biostimulants (Algifol, Nutridip and Metastim) and one bio-fumigant containing mustard and chilli pepper extracts (Dazitol). These were compared with the fumigant nematicide Nemathorin and untreated control plots. The effect of G. pallida on growing potato crops was assessed by recording haulm characteristics which indicated that the nematicide treatment gave most protection. Levels of PCN juveniles and migratory nematodes were assessed during the trial. Plots treated with Nemathorin and Dazitol had fewest PCN, whilst the highest number of migratory nematodes occurred in fallow plots. Sixteen weeks after planting the nematicide treatment produced highest yield and tuber numbers. Dazitol treatment produced a lower yield but the largest tubers.     

Evidence of animal mtDNA recombination between divergent populations of the potato cyst nematode Globodera pallida

Recombination is typically assumed to be absent in animal mitochondrial genomes (mtDNA). However, the maternal mode of inheritance means that recombinant products are indistinguishable from their progenitor molecules. The majority of studies of mtDNA recombination assess past recombination events, where patterns of recombination are inferred by comparing the mtDNA of different individuals. Few studies assess contemporary mtDNA recombination, where recombinant molecules are observed as direct mosaics of known progenitor molecules. Here we use the potato cyst nematode, Globodera pallida, to investigate past and contemporary recombination. Past recombination was assessed within and between populations of G. pallida, and contemporary recombination was assessed in the progeny of experimental crosses of these populations. Breeding of genetically divergent organisms may cause paternal mtDNA leakage, resulting in heteroplasmy and facilitating the detection of recombination. To assess contemporary recombination we looked for evidence of recombination between the mtDNA of the parental populations within the mtDNA of progeny. Past recombination was detected between a South American population and several UK populations of G. pallida, as well as between two South American populations. This suggests that these populations may have interbred, paternal mtDNA leakage occurred, and the mtDNA of these populations subsequently recombined. This evidence challenges two dogmas of animal mtDNA evolution; no recombination and maternal inheritance. No contemporary recombination between the parental populations was detected in the progeny of the experimental crosses. This supports current arguments that mtDNA recombination events are rare. More sensitive detection methods may be required to adequately assess contemporary mtDNA recombination in animals.     

Mode of action of fosthiazate used for the control of the potato cyst nematode Globodera pallida

Fosthiazate (Nemathorin 10G Ishihara Sangyo Kaisha Ltd, Japan) is a new nematicide approved for use on potatoes Solanum tuberosum L. in die UK for die control of die potato cyst nematodes Globodera rostochiensis (Woll). Skarbilovich and G. pallida (Stone). Fosdiiazate delayed and suppressed hatch of die potato cyst nematode Globodera pallida in bom in vitro laboratory tests and a glasshouse pot experiment. In vitro hatch was temporarily inhibited by fosdiiazate concentrations above 0.09 μg ml^-1 and increasing me fosdiiazate concentration further prolonged the duration of hatch inhibition. Analysis of fosthiazate soil concentrations, using high-pressure liquid chromatography, during me glasshouse experiment showed mat hatch was suppressed in the soil at concentrations above 0.5 mg kg^-1. Other factors such as the paralysis of hatched nematodes in the soil solution are also involved.     

Resolution of natural hatching factors for golden potato cyst nematode, Globodera rostochiensis

The hatching responses of Globodera rostochiensis (golden potato cyst nematode) to purified and partially-purified preparations of natural (including the potato glycoalkaloids solanine and α-chaconine) and artificial hatching factors (HFs) were bimodal. At least 10 HFs, mostly anionic, were resolved from potato root leachate by a combination of gel permeation and ion-exchange chromatography. Whereas potato roots were the principal source of HFs, haulm leachate also contained such chemicals. Root leachate from aseptically-grown potato plants lacked several HFs which were present in conventionally-produced leachate.     

Estimation of genomic variation between British populations of the potato cyst nematode Globodera pallida using RAPD-PCR

Random amplified polymorphic DNA (RAPD) has been used to investigate the interrelationships of 20 populations of Globodera pallida collected originally from field soils around the UK. RAPD analysis revealed a high level of relative genomic diversity within British G. pallida but there was no general correlation of genomic similarity with geographic distribution. Two populations of pathotype Pa1 were clearly divergent from the bulk of G. pallida and might represent a distinct introduction. Two other populations, from Scotland and Wales, were also dissimilar from each other and from the rest of the G. pallida populations.     

Enhancement of the efficacy of a carbamate nematicide against the potato cyst nematode, Globodera pallida, through mycorrhization in commercial potato fields

Two experiments were conducted over 2 years in commercial potato fields in Shropshire, UK, to evaluate the compatibility of the nematicide aldicarb with commercial inocula of arbuscular mycorrhizal fungi (AMF) in the control of the potato cyst nematode Globodera pallida. The AMF used were Vaminoc (mixed-AMF inoculum), Glomus intraradices (BioRize BB-E) and G. mosseae (isolate BEG 12). In the absence of AMF, the in-soil hatch of G. pallida increased 30% (P < 0.01) from wk-2 to wk-4 after planting. Inoculation of physiologically-aged potato (cv. Golden Wonder) tubers with AMF eliminated this delay in G. pallida hatch by stimulating a mean increase of 32% (P < 0.01) in hatch within 2 wk after planting. In the aldicarb-treated plots in Experiment 1, G. pallida multiplication rate was 38% lower (P < 0.05) in roots of AMF-inoculated than noninoculated plants, but in Experiment 2, this effect was slightly lower (P = 0.07). In these plots, the single AMF inocula showed also a weak trend (P = 0.10) towards greater tuber yields relative to their noninoculated counterparts. Mycorrhization therefore appears to enhance the efficacy of carbamate nematicides against G. pallida and consequently more research is proposed to validate these findings and fully explore the potential of this model.     

Control of potato cyst nematode (Globodera pallida) by host plant resistance and nematicide

Seven trials conducted over four years on sites naturally infested with the white potato cyst nematode established that potato clones bred for resistance to Globodera pallida allowed significantly less nematode multiplication than conventional cultivars under field conditions. Nematode multiplication was inversely related to initial infestation level. The nematicide, aldicarb, significantly reduced nematode multiplication. However, nematode multiplication on nematicide treated susceptible cultivars was greater than on untreated partially resistant clones, indicating that resistance may offer more effective control of G. pallida than chemical treatment. Integration of host plant resistance and nematicide treatment is discussed.     

The response of eggs of the white potato cyst nematode Globodera pallida to diffusate from potato and mustard roots

Brief exposure of eggs of Globodera pallida to potato root diffusate not only initiated hatching but also caused the majority of unhatched juveniles to respond more rapidly to subsequent treatment with diffusate. Eggs previously exposed to diffusate had a peak hatch after 1 or 2 days compared with 4 days for untreated eggs. Mustard root diffusate prevented hatch, and further stimulation with potato root diffusate was necessary to re-initiate it. Eggs previously treated with potato root diffusate for 24 h were much more sensitive to drought than untreated eggs. These results are discussed in relation to the theory that potato root diffusate alters the permeability of the eggshell as an initial step in the hatching process.     

Paternal leakage of mitochondrial DNA in experimental crosses of populations of the potato cyst nematode Globodera pallida

Animal mtDNA is typically assumed to be maternally inherited. Paternal mtDNA has been shown to be excluded from entering the egg or eliminated post-fertilization in several animals. However, in the contact zones of hybridizing species and populations, the reproductive barriers between hybridizing organisms may not be as efficient at preventing paternal mtDNA inheritance, resulting in paternal leakage. We assessed paternal mtDNA leakage in experimental crosses of populations of a cyst-forming nematode, Globodera pallida. A UK population, Lindley, was crossed with two South American populations, P5A and P4A. Hybridization of these populations was supported by evidence of nuclear DNA from both the maternal and paternal populations in the progeny. To assess paternal mtDNA leakage, a ~3.4?kb non-coding mtDNA region was analyzed in the parental populations and in the progeny. Paternal mtDNA was evident in the progeny of both crosses involving populations P5A and P4A. Further, paternal mtDNA replaced the maternal mtDNA in 22 and 40?% of the hybrid cysts from these crosses, respectively. These results indicate that under appropriate conditions, paternal leakage occurs in the mtDNA of parasitic nematodes, and supports the hypothesis that hybrid zones facilitate paternal leakage. Thus, assumptions of strictly maternal mtDNA inheritance may be frequently violated, particularly when divergent populations interbreed.     

The effect of pyramiding two potato cyst nematode resistance loci to Globodera pallida Pa2/3 in potato

Globodera pallida is a major nematode pest causing severe constraints in many potato production regions worldwide. The most prevalent G. pallida pathotypes are Pa2 and Pa3, which exist in mixed populations referred to as Pa2/3. Due to heterogeneity for avirulence genes within these nematode populations, so far, breeders have failed to identify a single resistance gene source which offers complete resistance. Currently, there are two quantitative trait loci (QTL) available, $ GpaIV_{adg}^{s} $ and Gpa5, conferring partial levels of resistance to G. pallida pathotype Pa2/3. The objective of this research was to pyramid $ GpaIV_{adg}^{s} $ and Gpa5 through marker-assisted selection to investigate whether or not pyramiding provides increased resistance to G. pallida Pa2/3. We developed a population segregating for both resistance QTLs and, using diagnostic genetic markers (Contig237 and HC), we identified groups of individuals from this population containing each individual QTL, both QTLs simultaneously, and neither QTL. These individuals were assessed for their resistance levels against G. pallida Pa2/3 population Chavornay. We demonstrated that individuals carrying both QTLs showed a significant reduction in the number of cysts formed in comparison to genotypes carrying $ GpaIV_{adg}^{s} $ or Gpa5, indicating an additive effect. Overall these results show that MAS-based pyramiding of these QTLs is an effective strategy for breeding cultivars exhibiting very high levels of resistance to G. pallida pathotype Pa2/3.     

Mapping QTLs for resistance to the cyst nematode Globodera pallida derived from the wild potato species Solanum vernei

Resistance to the potato cyst nematode (PCN) species Globodera pallida, derived from the wild diploid potato species Solanum vernei, has been investigated. This source of resistance, which is effective against all of the major pathotypes of G. pallida and Globodera rostochiensis, has been assumed to be due to several genetic factors, but it has proved difficult to deploy effectively in breeding strategies for potato cultivars. Diploid and tetraploid potato populations segregating for 'vernei' resistance were analysed. At the tetraploid level, a bulk segregant analysis (BSA) approach was employed and detected AFLP markers linked to a resistance QTL on potato linkage group V. Conventional linkage analysis of a diploid population identified QTL on linkage groups V and IX. A marker linked to a QTL on linkage group V has been converted to a single-locus PCR-based marker, which can be used to detect the presence of the QTL in diploid and tetraploid potato germplasm. Moreover, there is evidence that one of the AFLPs detected by BSA appears to be specific to an introgressed segment of DNA from S. vernei. These results are compared with those obtained from other studies on resistance to the PCN species G. pallida.     

The effect of the potato cyst nematode Globodera pallida on in vitro root growth of potato genotypes, differing in tolerance

Roots of eighteen potato genotypes, differing in tolerance of G. pallida, were grown from tuberpieces on agar in Petri dishes. Juveniles of G. pallida were inoculated directly onto root tips. Root length was measured at various times after inoculation. Inoculation reduced root growth within one day. At later stages, genotypes differed strongly in growth of inoculated roots. Between four and seven days after inoculation, growth of inoculated roots was not significantly correlated with growth of untreated roots, and was only poorly correlated with tolerance assessed in the greenhouse or in the field. However, multiple regression analysis revealed that the tolerance of the tested genotypes was associated with both the rate at which they induced hatching and the growth of roots after inoculation. The combination of these two variables accounted for high percentages explained variance.     

The effects of desiccation and root diffusates from potato and mustard on eggs of the white potato cyst nematode Globodera pallida

Following exposures to potato root diftusate of between 6 and 12 h, desiccation killed a proportion of juveniles in eggs of G. pallida and affected the hatching behaviour of survivors. In hatching tests of 9 wk duration, more juveniles hatched in the final wk from cysts, which were soahed and dried alternately for 9 wk than from cysts soaked in tap water for the final 2 wk only. Mustard root diffusate prevented eggs previously stimulated by potato root diffusate from hatching, but it did not alleviate the effects of desiccation.