Identification of AFLP and SSR markers associated with quantitative resistance to Globodera pallida (Stone) in tetraploid potato (Solanum tuberosum subsp. tuberosum) with a view to marker-assisted selection

 Seventy eight clones from the cross between SCRI clone 12601ab1 and cv Stirling were used to explore the possibility of genetical linkage analysis in tetraploid potato (Solanum tuberosum subsp. tuberosum). Clone 12601ab1 had quantitative resistance to Globodera pallida Pa2/3 derived from S. tuberosum subsp. andigena. The strategy adopted involved identifying single- (simplex) and double- (duplex) dose AFLP markers in the parents from segregation ratios that could be unambiguously identified in their offspring, detecting linkage between a marker and a putative quantitative trait locus (QTL) for resistance, and placing the QTL on the linkage map of markers. The numbers of scorable segregating markers were 162 simplex ones present only in 12601ab1, 87 present in Stirling, and 32 present in both; and 72 duplex markers present only in 12601ab1 and 45 present in Stirling. The total map length was 990.9 cM in 12601ab1 and 484.6 cM in Stirling. A QTL with a resistance allele present in double dose (QQqq) in 12601ab1 was inferred from the associations between resistance scores (square root of female counts) and two duplex markers linked in coupling, which, in turn, were linked in coupling to four simplex markers also associated with resistance, but to a lesser degree. The largest marker class difference was the one for the duplex marker P61M34=15. It accounted for 27.8% of the phenotypic variance in resistance scores, or approximately 30% of the genotypic variance. Subsequently, this duplex marker was found to be linked in coupling with a duplex SSR allele Stm3016=a, whose locus was shown to be on chromosome IV in a diploid reference mapping population. The other QTLs for resistance segregating in the progeny were not identified for one or more of the following reasons: the markers did not cover the whole of the genome, there were unfavourable repulsion linkages between the QTLs and markers, or the gene effects were not large enough to be detected in an experiment of the size conducted. It is concluded that prospects appear good for detecting QTLs and using marker-assisted selection in a tetraploid potato breeding programme, provided that, in future, the population size is increased to over 250 and more SSR markers are used to complement the AFLPs; the same is likely to be true for other autotetraploid crops. Received: 16 December 1997 / Accepted: 4 March 1998     

Genetical dissection of H3-mediated polygenic PCN resistance in a heterozygous autotetraploid potato population

Potato Cyst Nematodes (PCN) currently represent a serious threat to potato cultivation. However, many sources of resistance are known amongst primitive and wild relatives of cultivated potato, Solanum tuberosum ssp. tuberosum. Currently, in the UK, the major threat is due to Globodera pallida, resistance to which has not yet been effectively deployed in potato cultivars. We have performed linkage and QTL analysis of a tetraploid potato population segregating for the H3 source of resistance to G. pallida that was introgressed into cultivated potato from the primitive species, Solanum tuberosum ssp. andigena. This source is highly effective against the most common UK pathotype of G. pallida (Pa2/3) and its deployment in breeding material is a major goal. We adopted an approach involving bulked segregant analysis (BSA) as well as genome wide linkage analysis using AFLP and SSR markers. BSA provided a concentration of markers linked in coupling to a QTL on linkage group IV, and improved the accuracy of the QTL localisation. By performing an analysis on residual scores after removal of the effects due to the major QTL, we detected a second QTL on linkage group XI.     

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.     

Using SNP markers to dissect linkage disequilibrium at a major quantitative trait locus for resistance to the potato cyst nematode Globodera pallida on potato chromosome V

The damage caused by the parasitic root cyst nematode Globodera pallida is a major yield-limiting factor in potato cultivation . Breeding for resistance is facilitated by the PCR-based marker ‘HC’, which is diagnostic for an allele conferring high resistance against G. pallida pathotype Pa2/3 that has been introgressed from the wild potato species Solanum vernei into the Solanum tuberosum tetraploid breeding pool. The major quantitative trait locus (QTL) controlling this nematode resistance maps on potato chromosome V in a hot spot for resistance to various pathogens including nematodes and the oomycete Phytophthora infestans. An unstructured sample of 79 tetraploid, highly heterozygous varieties and breeding clones was selected based on presence (41 genotypes) or absence (38 genotypes) of the HC marker. Testing the clones for resistance to G. pallida confirmed the diagnostic power of the HC marker. The 79 individuals were genotyped for 100 single nucleotide polymorphisms (SNPs) at 10 loci distributed over 38 cM on chromosome V. Forty-five SNPs at six loci spanning 2 cM in the interval between markers GP21-GP179 were associated with resistance to G. pallida. Based on linkage disequilibrium (LD) between SNP markers, six LD groups comprising between 2 and 18 SNPs were identified. The LD groups indicated the existence of multiple alleles at a single resistance locus or at several, physically linked resistance loci. LD group C comprising 18 SNPs corresponded to the ‘HC’ marker. LD group E included 16 SNPs and showed an association peak, which positioned one nematode resistance locus physically close to the R1 gene family. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Single nucleotide polymorphism (SNP) genotyping as basis for developing a PCR-based marker highly diagnostic for potato varieties with high resistance to Globodera pallida pathotype Pa2/3

Globodera pallida is a parasitic root cyst nematode of potato, which causes reduction of crop yield and quality in infested fields. Field populations of G. pallida containing mixtures of pathotypes Pa2 and Pa3 (Pa2/3) are currently most relevant for potato cultivation in middle Europe. Genes for resistance to G. pallida have been introgressed into the cultivated potato gene pool from the wild, tuber bearing Solanum species S. spegazzinii and S. vernei. Selection of resistant genotypes in breeding programs is hampered by the fact that the phenotypic evaluation of resistance to G. pallida is time consuming, costly and often ambiguous. DNA-based markers diagnostic for resistance to G. pallida would facilitate the development of resistant varieties. A tetraploid F₁ hybrid family SR-Gpa segregating for quantitative resistance to G.␣pallida was developed and evaluated for resistance to G. pallida population ‘Chavornay’. Two subpopulations of 30 highly resistant and 30 susceptible individuals were selected and genotyped for 96 single nucleotide polymorphism (SNP) markers tagging 12 genomic regions on 10 potato chromosomes. Seven SNPs were found significantly linked to the nematode resistance, which were all located within a resistance ‘hotspot’ on potato chromosome V. A haplotype model for these seven SNPs was deduced from the SNP patterns observed in the SR-Gpa family. A PCR assay ‘HC’ was developed, which specifically detected the SNP haplotype c that was linked with high levels of nematode resistance. The HC marker was only found in accessions of S.␣vernei. Screening with the HC marker 34 potato varieties resistant to G. pallida pathotypes Pa2 and/or Pa3 and 22 susceptible varieties demonstrated that the HC marker was highly diagnostic for presence of high levels of resistance to G. pallida pathotype Pa2/Pa3.Amirali Sattarzadeh and Ute Achenbach contributed equally to the work     

Application of DNA markers linked to the potato <Emphasis Type="Italic">H1</Emphasis> gene conferring resistance to pathotype Ro1 of <Emphasis Type="Italic">Globodera rostochiensis</Emphasis>

Ninety-one potato genotypes (cultivars and breeding lines) selected as resistant or susceptible to pathotype Ro1 of Globodera rostochiensis were screened for the presence of two PCR markers, 0.14 and 0.76 kb in length. Both PCR markers were linked with the H1 gene, located at the distal end of the long arm of chromosome V, and were present in 88 to 100% of the resistant cultivars and breeding lines. The 0.76 kb PCR marker was detected in all resistant genotypes and in approximately 86% of susceptible breeding lines as well as in all susceptible cultivars. The 0.14 kb marker was detected in 88% of resistant breeding lines and in 94% of resistant cultivars. Most of the susceptible genotypes tested (91% of cultivars, but only 50% of breeding lines) did not show the presence of the 0.14 kb marker. We conclude that the 0.14 kb H1 marker is likely to be useful for the proper selection of potato genotypes resistant to the Ro1 pathotype of G. rostochiensis.     

Breeding for resistance to the white potato cyst-nematode, Heterodera pallida

Progenies bred from material derived from the wild potato, Solanum vernei and from the cultivated potato, S. tuberosum ssp. andigena, clone CPC 2775, were compared for their resistance to Heterodera pallida, pathotype E. The influence of additional resistance derived from the wild species, S. multidissectum, was also investigated. Both S. vernei and CPC 2775 gave progenies with variable levels of resistance and there was often no clear segregation into resistant and susceptible categories. Incorporation of gene H₂ derived from S. multidissectum increased resistance to pathotype E of H. pallida for resistant material bred from both S. vernei and clone CPC 2775. The results indicate that adequate resistance to all British populations of potato cyst-nematodes can best be obtained by combining the factors for resistance from the two Andigena clones, CPC 1673 (gene H₁) and CPC 2775 (gene H₃), and from S. multidissectum (gene H₂).     

Mapping and marker-assisted selection for a gene for extreme resistance to potato virus Y

The chromosomal location of the major gene Ry _adg controlling extreme resistance to potato virus Y (PVY) in Solanum tuberosum subsp. andigena was identified by RFLP analysis of a diploid potato population. A total of 64 tomato and potato RFLP markers were screened with the bulked segregant analysis (BSA) on segregants extremely resistant, hypersensitive or susceptible to PVY. Four markers TG508, GP125, CD17 and CT168 at the proximal end of chromosome XI showed close linkage with extremely resistant phenotypes. TG508 was identified as the closest marker linked with the Ry _adg locus with the maximum map distance estimated as 2.0 cM. The 4 markers linked with the Ry _adg locus were tested on independent tetraploid and diploid potato clones and were subsequently found useful for marker-assisted selection for plants containing Ry _adg . Received: 5 July 1996 / Accepted: 19 July 1996     

Molecular and genetic characterization of the <Emphasis Type="Italic">Ry</Emphasis><Subscript><Emphasis Type="Italic">adg</Emphasis></Subscript> locus on chromosome XI from Andigena potatoes conferring extreme resistance to potato virus Y

Key message

We have elucidated the Andigena origin of the potato Ry_adg gene on chromosome XI of CIP breeding lines and developed two marker assays to facilitate its introgression in potato by marker-assisted selection.

Abstract

Potato virus Y (PVY) is causing yield and quality losses forcing farmers to renew periodically their seeds from clean stocks. Two loci for extreme resistance to PVY, one on chromosome XI and the other on XII, have been identified and used in breeding. The latter corresponds to a well-known source of resistance (Solanum stoloniferum), whereas the one on chromosome XI was reported from S. stoloniferum and S. tuberosum group Andigena as well. To elucidate its taxonomic origin in our breeding lines, we analyzed the nucleotide sequences of tightly linked markers (M45, M6) and screened 251 landraces of S. tuberosum group Andigena for the presence of this gene. Our results indicate that the PVY resistance allele on chromosome XI in our breeding lines originated from S. tuberosum group Andigena. We have developed two marker assays to accelerate the introgression of Ry_adg gene into breeding lines by marker-assisted selection (MAS). First, we have multiplexed RYSC3, M6 and M45 DNA markers flanking the Ry_adg gene and validated it on potato varieties with known presence/absence of the Ry_adg gene and a progeny of 6,521 individuals. Secondly, we developed an allele-dosage assay particularly useful to identify multiplex Ry_adg progenitors. The assay based on high-resolution melting analysis at the M6 marker confirmed Ry_adg plex level as nulliplex, simplex and duplex progenitors and few triplex progenies. These marker assays have been validated and can be used to facilitate MAS in potato breeding.

     

Effect of the trap crop Solanum sisymbriifolium and two biocontrol fungi on reproduction of the potato cyst nematode,Globodera pallida

The potato cyst nematode, Globodera pallida, is one of the most important pests of potato worldwide. Owing to regulatory considerations and potential environmental impact, control options for this nematode are becoming increasingly limited. Solanum sisymbriifolium and biological control agents offer viable alternative options for controlling G. pallida. Therefore, experiments were conducted to determine the effect of the nematode trap crop S. sisymbriifolium, alone or in combination with the biocontrol agents Trichoderma harzianum or Plectosphaerella cucumerina, on population decline of G. pallida. Experiments were conducted for three different ‘cropping systems’: potato (Solanum tuberosum), S. sisymbriifolium, or soil only (fallow), each followed by a potato crop. Soil was amended with P. cucumerina, T. harzianum or left unamended, and then infested with nematodes at a rate of five eggs g^?1 of soil. After 16 weeks in the greenhouse, plants were removed and the soil containing cysts was refrigerated at 4°C for 8 weeks, and then planted to potato. Cysts of G. pallida were counted after an additional 16‐week period. The P_f/P_i of G. pallida was significantly reduced by 99% in potato following S. sisymbriifolium compared to both the potato‐following‐fallow and the potato‐following‐potato treatments. Amendment of soil with T. harzianum significantly reduced P_f/P_i of G. pallida by 42–47% in the potato‐following‐potato but not in either the potato‐after‐fallow nor in the potato‐after‐S. sisymbriifolium cycles which supports evidence that the plant species may play a role in the biocontrol activity of this fungus. Addition of the fungus P. cucumerina resulted in a 64% decrease in P_f/P_i in the potato‐following‐fallow in one experiment, and an 88% decrease in P_f/P_i in potato‐following‐potato but the decrease in P_f/P_i was not consistent over all experiments. However, both biocontrol fungi resulted in lower numbers of progeny cysts after an initial 16‐week incubation with potato. To look at the effect of varied population density of the nematode on efficacy of S. sisymbriifolium to reduce G. pallida populations, potato, S. sisymbriifolium, or barley were planted into soil infested with G. pallida at rates of 5, 20 or 40 eggs g^?1 soil applied as cysts (20, 80 or 160 cysts pot^?1). After 16 weeks, numbers of cysts produced in each treatment were determined for each infestation rate. No new cysts were recovered from either S. sisymbriifolium or barley treatments, confirming that neither plant is a host for G. pallida. High numbers of cysts were recovered with potato. Soil from each treatment (containing original cysts and newly‐formed cysts when present) were then planted with potato. After an additional 16 weeks, few cysts were found in the potato‐after‐ S. sisymbriifolium treatments regardless of initial infestation rate. When potato followed barley, numbers of cysts were similar to those found after a single cycle of potato, indicating that the barley crop had no effect on the survival of initial inoculum. Overall, these results suggest that S. sisymbriifolium has potential to significantly reduce G. pallida populations, and also that the cropping system (i.e. the sequence of non‐host and host plants) may play a significant role in the efficacy of fungal biological control agents.     

Genome scans on experimentally evolved populations reveal candidate regions for adaptation to plant resistance in the potato cyst nematode Globodera pallida

Improving resistance durability involves to be able to predict the adaptation speed of pathogen populations. Identifying the genetic bases of pathogen adaptation to plant resistances is a useful step to better understand and anticipate this phenomenon. Globodera pallida is a major pest of potato crop for which a resistance QTL, GpaV_vrn, has been identified in Solanum vernei. However, its durability is threatened as G. pallida populations are able to adapt to the resistance in few generations. The aim of this study was to investigate the genomic regions involved in the resistance breakdown by coupling experimental evolution and high‐density genome scan. We performed a whole‐genome resequencing of pools of individuals (Pool‐Seq) belonging to G. pallida lineages derived from two independent populations having experimentally evolved on susceptible and resistant potato cultivars. About 1.6 million SNPs were used to perform the genome scan using a recent model testing for adaptive differentiation and association to population‐specific covariables. We identified 275 outliers and 31 of them, which also showed a significant reduction in diversity in adapted lineages, were investigated for their genic environment. Some candidate genomic regions contained genes putatively encoding effectors and were enriched in SPRYSECs, known in cyst nematodes to be involved in pathogenicity and in (a)virulence. Validated candidate SNPs will provide a useful molecular tool to follow frequencies of virulence alleles in natural G. pallida populations and define efficient strategies of use of potato resistances maximizing their durability.     

A QTL for broad-spectrum resistance to cyst nematode species (Globodera spp.) maps to a resistance gene cluster in potato

 Broad-spectrum resistance in potato to the potato cyst nematode (PCN) species Globodera rostochiensis and G. pallida is commonly regarded as a polygenically inherited trait. Yet, by use of QTL analysis and a selected set of PCN populations, resistance to both PCN species could be ascribed to the action of locus Grp1. Grp1 confers major resistance to G. rostochiensis line Ro₅-22 and G. pallida population Pa₂-D383 and partial resistance to G. pallida population Pa₃-Rookmaker. Grp1 was mapped on chromosome 5 using previously characterized AFLP markers. Cleaved amplified polymorphic sequence (CAPS) markers available for RFLP loci GP21 and GP179 revealed that Grp1 maps on a genomic region harboring other resistance factors to viral, fungal and nematodal pathogens. The present data indicate that Grp1 is a compound locus which contains multiple genes involved in PCN resistance. Received: 10 September 1997 / Accepted: 6 October 1997     

An RGA-Derived SCAR Marker Linked to PLRV Resistance from Solanum tuberosum ssp. andigena

A resistance gene analog (RGA)-derived sequence-characterized amplified region (SCAR) marker was successfully developed based on sequence homology with disease resistance genes of an AFLP molecular marker tightly linked to the Rl _adg gene of Solanum tuberosum ssp. andigena. The new marker was designated as ‘RGASC850’ (RGA-derived SCAR) based on the size of the amplified fragment. ‘RGASC850’ could be efficiently used for monitoring introgression of Rl _adg against backgrounds of improved gene pools with low likelihood of identifying false positives due to recombination. This SCAR proved to be highly predictive of Rl _adg -based resistance, as it did not amplify potato leafroll virus (PLRV) resistance sources other than andigena, and thus would be useful in developing cultivars with complementary sources of resistance to PLRV. In addition, a cleaved amplified polymorphic sequence (CAPS) marker based on ‘RGASC850’ was developed capable of distinguishing genotypes carrying Rl _adg . This CAPS marker would be useful for screening breeding populations derived from wide crosses, and confirming presence of the Rl _adg gene in those parents amplifying the ‘RGASC850’ marker.     

Observations on the control of potato cyst-nematodes, Globodera rostochiensis and G. pallida in 35 English potato soils in pots by oxamyl and resistant potatoes

The increase of 35 English field populations of potato cyst-nematodes (Globodera rostochiensis and/or G. pallida) was measured on Désirée, Maris Piper, Caxton (A25/11), Cromwell (A27/20) and clone 11233 ab 22 in pots of sandy, silty or peaty loam soil. Désirée was susceptible to all populations tested and, as in field soils, the final population (Pf) was inversely related to the initial population of potato cyst-nematode eggs (Pi) in the soil. Maris Piper and Cromwell were resistant to all G. rostochiensis populations, with one possible exception. Maris Piper was susceptible to all G. pallida populations. Caxton was susceptible to some and fairly resistant to other populations of G. rostochiensis, indicating the existence either of two biotypes within the one pathotype (Rol) as yet encountered in Britain, or the existence of an additional pathotype. Caxton and Cromwell were fairly resistant to G. pallida. Clone 11233 ab 22 was only moderately resistant to both species. Resistance to potato cyst-nematode increase varied considerably, especially in Caxton (to G. rostochiensis) and in 11233 ab 22 (to both species). Oxamyl greatly reduced the increase of G. rostochiensis populations on Désirée potatoes, with the notable exception of one population but it generally had much less effect on G. pallida populations, regardless of soil type. The difference in effect on the two species may be due to a longer period of hatching in G. pallida than in G. rostochiensis and also perhaps to a second generation in G. pallida.     

Molecular examination of a chromosome region that controls resistance to potato Y and A potyviruses in potato

 The gene Ry _adg that confers resistance to potato Y potyvirus (PVY) in the cultivated potato [Solanum tuberosum subsp. andigena, line 2x(v-2)7] is located on chromosome XI in a segment that contains three other known resistance genes in other syntenic solanaceous species. One of them is the gene N that controls resistance to tobacco mosaic tobamovirus in tobacco and has previously been isolated and sequenced. Three sequence-related, resistance gene-like (RGL) DNA fragments (354–369 bp) highly homologous to the gene N were PCR-amplified from the potato line 2x(v-2)7. Two RGL fragments (79 and 81% homologous to the N gene) co-segregated with Ry _adg among the 77 F1 progeny tested. These RGLs may originate from a resistance gene family on chromosome XI. The potato line 2x(v-2)7 also expressed resistance to potato A potyvirus (PVA), which was controlled by another locus on chromosome XI mapped ca. 6.8 cM distal to Ry _adg . Received: 18 December 1997 / Accepted: 30 December 1997     

QTL mapping of yield, agronomic and quality traits in tetraploid potato (Solanum tuberosum subsp. tuberosum)

Interval mapping of quantitative trait loci (QTL) for 16 yield, agronomic and quality traits in potato was performed on a tetraploid full-sib family comprising 227 clones from a cross between processing clone 12601ab1 and table cultivar Stirling. Thirty-eight AFLP primer combinations and six SSRs provided 514 informative markers which formed a molecular marker map comprising 12 linkage groups (LGs) in 12601ab1 (nine with four homologous chromosomes) which were aligned with 12 in Stirling (11 with four homologous chromosomes), with four partial groups remaining in 12601ab1. Two LGs were identified unequivocally as chromosomes IV and V and eight others were tentatively assigned with chromosomes VII and X unidentified. All of the traits scored had moderately high heritabilities with 54–92% of the variation in clone means over 3 years and two replicates being due to genetic differences. A total of 39 QTLs were identified. A QTL for maturity was identified on chromosome V which explained 56% of the phenotypic variance, whereas the other QTLs individually explained between 5.4 and 16.5%. However, six QTLs were detected for after-cooking blackening and four for each of regularity of tuber shape, fry colour both after storage at 4 and 10°C and sprouting. Just two QTLs were found for each of yield, the two ‘overall’ scores, crop emergence, tuber size and common scab and just one QTL was detected for each of dry matter content, keeping quality, growth cracks and internal condition. The implications for practical potato breeding and for practical linkage and QTL analysis in autotetraploids are discussed.     

Glucose phosphate isomerase polymorphism in field populations of the potato cyst nematodes, Globodera rostochiensis and G. pallida

The isozymic variation of glucose phosphate isomerase (GPI) was compared in 20 field populations of potato cyst-nematode (PCN) and related to variation in reference pathotypes of PCN. Populations pathotyped as G. rostochiensis Ro 1 by conventional differential cultivar tests were found to be different from populations of G. pallida. Variation within populations pathoptyped as G. pallida was seen and three populations were found to be similar to the reference G. pallida Pal pathotype. This seemed to be confirmed for two populations using cysts formed on a cultivar resistant to the Pa 1 pathotype when an isoenzyme pattern characteristic of G. palida Pa2 and Pa3 was found. The feasibility of the use of isozyme identification of PCN pathotypes is discussed in the light of these results.     

Mapping of the cyst nematode resistance locus Gpa2 in potato using a strategy based on comigrating AFLP markers

 The nematode resistance locus Gpa2 was mapped on chromosome 12 of potato using information on the genomic positions of 733 known AFLP markers. The minimum number of AFLP primer combinations required to map Gpa2 was three. This demonstrates that a reference collection of potato AFLP markers may be a valuable tool for mapping studies in potato. By use of RFLP probes, Gpa2 was more precisely mapped at the distal end of chromosome 12. Gpa2 confers resistance to a distinct group of populations of the potato cyst nematode Globodera pallida and originates from the same potato accession as locus H1, conferring resistance to pathotype Ro₁ of G. rostochiensis. This study shows that these two nematode resistance loci are unlinked and that Gpa2 is linked to the Rx1 locus conferring resistance to potato virus X. The efficiency of AFLPs for genetic mapping of a highly heterozygous crop like potato is discussed and compared with the RFLP technique. Received: 24 February 1997/Accepted: 2 May 1997     

Comparative Responses of Globodera rostochiensis and G. pallida to Hatching Chemicals

Globodera rostochiensis and G. pallida responded similarly to hatch stimulation by potato root leachate, but proportionally more second-stage juveniles (J2s) of G. rostochiensis hatched than of G. pallida in response to picrolonic acid, sodium thiocyanate, alpha-solanine, and alpha-chaconine. Fractionation of the potato root leachate identified hatching factors with species-selective (active toward both species but stimulating greater hatch of one species than the other), -specific (active toward only one species), and -neutral (equally active toward both species) activities. In a comparison of two populations of each of the two potato cyst nematode (PCN) species, however, greater similarity in response to the individual hatching factors was observed among populations of different species produced under the same conditions than among different populations of the same PCN species. Smaller numbers of species-specific and species-selective hatching factor stimulants and hatching inhibitors than of hatching factors were resolved. In a study to determine whether the different hatching responses of the two species to the same root leachate were associated with different ratios of species-selective and species-specific hatching factors, G. rostochiensis pathotype Ro1 exhibited greater hatch than did G. pallida pathotype Pa2/3 in response to leachate from older plants (more than 38 days old), while G. pallida exhibited greater hatch in response to leachate from younger plants (less than 38 days old); the response of G. pallida pathotype Pal with respect to plant age was intermediate between the other two populations. Combined molecular exclusion-ion exchange chromatography of the root leachates from plants of different ages revealed an increase in the proportion of G. rostochiensis-specific and -selective hatching factors as the plants aged.     

Integrated control of potato cyst nematodes using small amounts of nematicide and potatoes with partial resistance

The control of potato cyst nematode (PCN) by less than approved amounts of nematicide combined with partially resistant potato clones was studied in a series of field experiments. On a site heavily infested with Globodera pallida only the most resistant clone (12380ac2) decreased the population density in untreated soil. With aldicarb at its full approved rate (3·36 kg ha^-1) numbers of PCN were decreased under all the genotypes, including the non-resistant Maris Piper. Aldicarb at 1·68 kg ha^-1significantly decreased populations on all clones except 12380ac2. Aldicarb at 0·84 kg ha^-1still significantly decreased population densities and multiplication rates of G. pallida on two clones with intermediate resistance (12243acl and 11233ab22). At two G. rostochiensis sites with light infestations nematode multiplication rates were greater and the control given by aldicarb and partially resistant genotypes of potato was not as great as that at the site with G. pallida. Tuber yields were not increased by the application of aldicarb at the G. rostochiensis infested sites. However, at the site heavily infested with G. pallida the yield of the most intolerant genotype (12380ac2) was increased seven-fold by the full rate of aldicarb (3·36 kg ha^-1) and four-fold by the quarter rate (0·84 kg ha^-1)-Yield of the most tolerant genotype (12243acl) was unaffected by the application of aldicarb.