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).     

On cyst colour changes, bionomics and distribution of potato cyst-eelworm (Heterodera rostochiensis Woll.) pathotypes in the East Midlands

Pot tests and field trials in 1966 confirmed earlier observations that pathotype A potato cyst-eelworm (Heterodera rostochiensis Woll.) showed marked colour differences from pathotypes B and C during development on the roots of susceptible potatoes. In hatching tests started in July, newly formed brown cysts of pathotype A released four times as many larvae as those of pathotypes B and C, without entering an immediate diapause; the latter produced larvae after a rest period of 18–21 days. Subsequent second-generation cyst production in glass-tube culture was greater from pathotype A. Field observations suggested that all pathotypes mature at about the same time and give rise to only one flush of cysts on both early and maincrop potatoes in the growing season. Cyst colour surveys in commercial crops, and pot and field trials using resistant potato selections, confirmed that pathotypes B and C are both dominant and widespread in the East Midlands.     

Variations in the resistance to Heterodera rostochiensis of potatoes derived from Solarium tuberosum ssp. andigena and S. multidissectum

The proportion of larvae from ten populations of Heterodera rostochiensis Woll. that became female was determined on five potato clones containing genes for resistance derived from S. tuberosum ssp. andigena., on three with genes from S. multidissectum and on four with genes for resistance from both sources. Variations in the resistance of the clones bred from andigena, especially to two of the populations, suggest the presence of more than one gene for resistance.     

Observations on the relationship between morphological variation and host range in populations of cereal cyst-nematode

British pathotype 3 of cereal cyst-nematode differs morphologically from Heterodera avenae. Mature cysts have a distinct underbridge which is absent from pathotypes 1 and 2. A population of pathotype 3 produced cysts on cereal genotypes resistant to pathotypes 1 and 2. The relationship of this pathotype to other graminaceous cyst-nematodes is discussed.     

Genetic mapping of the novel <Emphasis Type="Italic">Turnip mosaic virus</Emphasis> resistance gene <Emphasis Type="Italic">TuRB03</Emphasis> in <Emphasis Type="Italic">Brassica napus</Emphasis>

A new source of resistance to the pathotype 4 isolate of Turnip mosaic virus (TuMV) CDN 1 has been identified in Brassica napus (oilseed rape). Analysis of segregation of resistance to TuMV isolate CDN 1 in a backcross generation following a cross between a resistant and a susceptible B. napus line showed that the resistance was dominant and monogenic. Molecular markers linked to this dominant resistance were identified using amplified fragment length polymorphism (AFLP) and microsatellite bulk segregant analysis. Bulks consisted of individuals from a BC₁ population with the resistant or the susceptible phenotype following challenge with CDN 1. One AFLP and six microsatellite markers were associated with the resistance locus, named TuRB03, and these mapped to the same region on chromosome N6 as a previously mapped TuMV resistance gene TuRB01. Further testing of TuRB03 with other TuMV isolates showed that it was not effective against all pathotype 4 isolates. It was effective against some, but not all pathotype 3 isolates tested. It provided further resolution of TuMV pathotypes by sub-dividing pathotypes 3 and 4. TuRB03 also provides a new source of resistance for combining with other resistances in our attempts to generate durable resistance to this virus.     

Pathotype-specific genetic factors in chickpea (Cicer arietinum L.) for quantitative resistance to ascochyta blight

Ascochyta blight in chickpea (Cicer arietinum L.) is a devastating fungal disease caused by the necrotrophic pathogen, Ascochyta rabiei (Pass.) Lab. To elucidate the genetic mechanism of pathotype-dependent blight resistance in chickpea, F₇-derived recombinant inbred lines (RILs) from the intraspecific cross of PI 359075(1) (blight susceptible) × FLIP84-92C(2) (blight resistant) were inoculated with pathotypes I and II of A. rabiei. The pattern of blight resistance in the RIL population varied depending on the pathotype of A. rabiei. Using the same RIL population, an intraspecific genetic linkage map comprising 53 sequence-tagged microsatellite site markers was constructed. A quantitative trait locus (QTL) for resistance to pathotype II of A. rabiei and two QTLs for resistance to pathotype I were identified on linkage group (LG)4A and LG2+6, respectively. A putative single gene designated as Ar19 (or Ar21d) could explain the majority of quantitative resistance to pathotype I. Ar19 (or Ar21d) appeared to be required for resistance to both pathotypes of A. rabiei, and the additional QTL on LG4A conferred resistance to pathotype II of A. rabiei. Further molecular genetic approach is needed to identify individual qualitative blight resistance genes and their interaction for pathotype-dependent blight resistance in chickpea.     

Control of potato cyst-nematode, Heterodera rostochiensis, in peaty loam soil by D-D, aldicarb and a resistant variety of potato

In peaty loam soil in Cambridgeshire, 5.2 or 10.3 kg aldicarb/ha incorporated in the top-soil before potatoes were planted controlled potato cystnematode (Heterodera rostochiensis Woll.) better than 384, 769 or 1153 kg D-D/ha injected 15 cm deep into the top-soil in the preceding autumn. 10.3 kg aldicarb/ha applied in 1968 and 1969 permitted King Edward potatoes (susceptible to H. rostochiensis) to grow well in infested soil and prevented multiplication of pathotypes of H. rostochiensis on Maris Piper potatoes (resistant to H. rostochiensis pathotype A). Although large amounts of D-D applied in 1968 and 1969 increased the yield of King Edward potatoes in both years they did not control potato cyst-nematode in the second year.     

Multiple alleles for resistance and susceptibility modulate the defense response in the interaction of tetraploid potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis>) with <Emphasis Type="Italic">Synchytrium endobioticum</Emphasis> pathotypes 1, 2, 6 and 18

The obligate biotrophic, soil-borne fungus Synchytrium endobioticum causes wart disease of potato (Solanum tuberosum), which is a serious problem for crop production in countries with moderate climates. S. endobioticum induces hypertrophic cell divisions in plant host tissues leading to the formation of tumor-like structures. Potato wart is a quarantine disease and chemical control is not possible. From 38 S. endobioticum pathotypes occurring in Europe, pathotypes 1, 2, 6 and 18 are the most relevant. Genetic resistance to wart is available but only few current potato varieties are resistant to all four pathotypes. The phenotypic evaluation of wart resistance is laborious, time-consuming and sometimes ambiguous, which makes breeding for resistance difficult. Molecular markers diagnostic for genes for resistance to S. endobioticum pathotypes 1, 2, 6 and 18 would greatly facilitate the selection of new, resistant cultivars. Two tetraploid half-sib families (266 individuals) segregating for resistance to S. endobioticum pathotypes 1, 2, 6 and 18 were produced by crossing a resistant genotype with two different susceptible ones. The families were scored for five different wart resistance phenotypes. The distribution of mean resistance scores was quantitative in both families. Resistance to pathotypes 2, 6 and 18 was correlated and independent from resistance to pathotype 1. DNA pools were constructed from the most resistant and most susceptible individuals and screened with genome wide simple sequence repeat (SSR), inverted simple sequence region (ISSR) and randomly amplified polymorphic DNA (RAPD) markers. Bulked segregant analysis identified three SSR markers that were linked to wart resistance loci (Sen). Sen1-XI on chromosome XI conferred partial resistance to pathotype 1, Sen18-IX on chromosome IX to pathotype 18 and Sen2/6/18-I on chromosome I to pathotypes 2,6 and 18. Additional genotyping with 191 single nucleotide polymorphism (SNP) markers confirmed the localization of the Sen loci. Thirty-three SNP markers linked to the Sen loci permitted the dissection of Sen alleles that increased or decreased resistance to wart. The alleles were inherited from both the resistant and susceptible parents.     

Genetic Analysis of Resistance to Rice Blast: A Study on the Inheritance of Resistance to the Blast Disease Pathogen in an F3 Population of Rice

Blast caused by the fungus Magnaporthae grisea (Herbert) Borr. (anamorphe Pyricularia oryza Cav.) is a serious disease of rice (Oryza sativa L.). One method to overcome this disease is to develop disease resistant cultivars. Due to the genetic plasticity in the pathogen genome, there is a continuous threat to the effectiveness of the developed cultivars. Additional studies of the genetics of resistance, virulence stability and functional genomics are required to accelerate research into understanding the molecular basis of blast disease resistance. In this study, individual plants of the F₃ population derived from Pongsu Seribu 2 and Mahsuri were used for pathogenesis assays and inheritance studies of blast resistance. The study was performed with two of the most virulent Malaysian M. grisea pathotypes: P7.2 and P5.0. For blast screening, plants were scored based on the IRRI Standard Evaluation System (SES). F₃ populations showed a segregation ratio of 3R:1S for pathotype P7.2, indicating that resistance to this pathotype is likely controlled by a single nuclear gene. Chi‐square analysis showed that the F₃ families segregated in a 15R:1S ratio for pathotype P5.0. Therefore, locus interactions or epitasis of blast resistance occur against pathotype P5.0 in the F₃ population derived from Pongsu Seribu 2 and Mahsuri. This can be explained by the presence of two independent dominant genes that when present simultaneously, provide resistance to the M. gresia pathotype P5.0. These results indicated that blast resistance in rice is due to the combined effects of multiple loci with major and minor effects. The genetic data generated here will be useful in the breeding of local cultivars for resistance to field blast. The methodology reported here will facilitate the mapping of genes and quantitative trait loci (QTLs) underlying the blast resistance trait.     

Resistance of roses to pathotypes of Diplocarpon rosae

Colonies of Diplocarpon rosae derived from single conidia were isolated on malt extract agar, multiplied (at 23°C) and stored (at ?20°C) on surface‐sterilised leaf discs of a universally susceptible rose, ‘Frensham’. The resistance of 16 species and cultivars of Rosa to different isolates of D. rosae was assessed using surface‐sterilised leaf discs. Four pathotypes of D. rosae were distinguished on the basis of host range. One species and one hybrid were resistant to all pathotypes. Two species and two cultivars were susceptible to all pathotypes. Four species and six cultivars were interpreted as having vertical resistance because they were strongly resistant to some but not all pathotypes. Only species and hybrids of the section Cinnamomeae were resistant to the pathotype identified as CW1 whereas only roses of other origins were resistant to the pathotype DA2.     

Mapping genes Lr53 and Yr35 on the short arm of chromosome 6B of common wheat with microsatellite markers and studies of their association with Lr36

The rust resistance genes Lr53 and Yr35, transferred to common wheat from Triticum dicoccoides, were reported previously to be completely linked on chromosome 6B. Four F ₃ families were produced from a cross between a line carrying Lr53 and Yr35 (98M71) and the leaf rust and stripe rust susceptible genotype Avocet “S” and were rust tested using Puccinina triticina pathotype 53-1,(6),(7),10,11 and Puccinia striiformis f. sp. tritici pathotype 110 E143 A+. The homozygous resistant lines produced infection types of “;1−” and “;N” to these pathotypes, respectively. The Chi-squared tests indicated goodness-of-fit of the data for one leaf rust gene and one stripe rust gene segregation. Linkage analysis using this population demonstrated recombination of 3% between the genes. Microsatellite markers located on the short arm of chromosome 6B were used to map the genes, with the markers cfd1 and gwm508 being mapped approximately 1.1 and 4.5 cM, respectively, proximal to Lr53. Additional studies of the relationship between Lr36, also located on the short arm of chromosome 6B, and Lr53 indicated that the two genes were independent.     

Effects of growing resistant potatoes with gene H1 from Solarium tuberosum ssp. andigena on populations of Heterodera rostochiensis British pathotype A

Pure pathotype A populations of Heterodera rostochiensis produce a few females on ex andigena hybrids with the H₁ gene for resistance. As the proportion of larvae able to become female on ex andigena hybrids was not increased by reproducing the nematodes on such hybrids for 3 years, these females seem not to be genetically different from the rest of the population. The proportion increased rapidly when the initial population contained a few pathotype (species) E nematodes but again no increase in the proportion of pathotype (species) A larvae able to become female on ex andigena was detected and pathotype E replaced pathotype A.     

Genotyping with RAPD and microsatellite markers resolves pathotype diversity in the ascochyta blight pathogen of chickpea

 The poor definition of variation in the ascochyta blight fungus (Ascochyta rabiei) has historically hindered breeding for resistance to the chickpea (Cicer arietinum L.) blight disease in West Asia and North Africa. We have employed 14 RAPD markers and an oligonucleotide probe complementary to the microsatellite sequence (GATA)₄ to construct a genotype-specific DNA fragment profile from periodically sampled Syrian field isolates of this fungus. By using conventional pathogenicity tests and genome analysis with RAPD and microsatellite markers, we demonstrated that the DNA markers distinguish variability within and among the major pathotypes of A. rabiei and resolved each pathotypes into several genotypes. The genetic diversity estimate based on DNA marker analysis within pathotypes was highest for the least-aggressive pathotype (pathotype I), followed by the aggressive (pathotype II) and the most-aggressive pathotype (pathotype III). The pair-wise genetic distance estimated for all the isolates varied from 0.00 to 0.39, indicating a range from a clonal to a diverse relationship. On the basis of genome analysis, and information on the spatial and temporal distribution of the pathogen, a general picture of A. rabiei evolution in Syria is proposed. Received: 10 January 1998 / Accepted: 23 January 1998     

Characterization of nematode resistance genes in the section Procumbentes genus Beta: response to two populations of Heterodera schachtii

Three species of the section Procumbentes genus Beta, nine monosomic additions, and five translocation lines were tested for resistance to two Heterodera schachtii populations. Nematode population 129-v (129-virulent) was selected for virulence to resistance gene(s) transferred from chromosome 1 of Beta procumbens to the diploid resistant sugar beet KWS-NR1. This population is considered to be a pathotype. The unselected sib population 129-av (129-avirulent) was reared continuously on fodder rape, Brassica napus cv Velox. Monosomic additions with chromosome 1 from the three species of the section Procumbentes were susceptible to population 129-v, regardless of the origin of the alien chromosome. Translocations with a gene(s) for resistance from chromosome 7 of B. procumbens and B. webbiana were also susceptible to the pathotype. However, a monosomic addition with chromosome 7 of B. webbiana was resistant to population 129-v. The three wild beets of the section Procumbentes, Beta procumbens, Beta webbiana and Beta patellaris, also were highly resistant to the two populations. The results indicate the existence of just two different major genes for resistance to H. schachtii in the entire Procumbentes section.     

Genetic and physical mapping of xa13, a recessive bacterial blight resistance gene in rice

 The recessive gene, xa13, confers resistance to Philippine race 6 (PXO99) of the bacterial blight pathogen Xanthomonas oryzae pv oryzae. Fine genetic mapping and physical mapping were conducted as initial steps in an effort to isolate the gene. Using nine selected DNA markers and two F₂ populations of 132 and 230 plants, xa13 was fine-mapped to a genomic region <4 cM on the long arm of rice chromosome 8, flanked by two RFLP markers, RG136 and R2027. Four DNA markers, RG136, R2027, S14003, and G1149, in the target region were used to identify bacterial artificial chromosome (BAC) clones potentially harboring the xa13 locus from a rice BAC library. A total of 11 BACs were identified, forming four separate contigs including a single-clone contig, 29I3, associated with the RG136 STS marker, the S14003 contig consisting of four clones (44F8, 41O2, 12A16, and 12F20), the G1149 contig with two clones, 23D11 and 21H18, and the R2027 contig consisting of four overlapping clones, 42C23, 30B5, 6B7 and 21H14. Genetic mapping indicated that the xa13 locus was contained in the R2027 contig. Chromosomal walking on the R2027 contig resulted in two more clones, 33C7 and 14L3. DNA fingerprinting showed that the six clones of the R2027 contig were overlapping. Clone 44F8 hybridized with a single fragment from the clone 14L3, integrating the R2027 and S14003 contigs into a single contig consisting of ten BAC clones with a total size of approximately 330 kb. The physical presence of the xa13 locus in the contig was determined by mapping the ends of the BAC inserts generated by TAIL-PCR. In an F₂ population of 230 plants, the BAC-end markers 42C23R and 6B7F flanked the xa13 locus. The probes 21H14F and 21H14R derived from BAC clone 21H14 were found to flank xa13 at a distance of 0.5 cM on either side, using a second F₂ population of 132 plants. Thus, genetic mapping indicated that the contig and the 96-kb clone, 21H14, contained the xa13 locus. Received: 15 August 1998 / Accepted: 29 September 1998     

Testing potatoes bred from Solarium vernei for resistance to the white potato cyst-nematode, Heterodera pallida

Three clones of potatoes bred from Solanum vernei were tested for their resistance to the white potato cyst-nematode, Heterodera pallida, both by root-ball counts and by estimates of total cysts produced. Two susceptible clones were used as controls. Root-ball counts suggested a higher degree of resistance than that based on total cysts produced. There were differences in pathogenicity between the two populations. The degree of resistance found was such that in practice the resistant clones would be best used in conjunction with nematicide treatment in a system of integrated control.     

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     

Development of sequence characterized DNA markers linked to leaf rust (<Emphasis Type="Italic">Hemileia vastatrix</Emphasis>) resistance in coffee (<Emphasis Type="Italic">Coffea arabica</Emphasis> L.)

Coffee leaf rust due to Hemileia vastatrix is one of the most serious diseases in Arabica coffee (Coffea arabica). A resistance gene (S_H3) has been transferred from C. liberica into C. arabica. The present work aimed at developing sequence-characterized genetic markers for leaf rust resistance. Linkage between markers and leaf rust resistance was tested by analysing two segregating populations, one F₂ population of 101 individuals and one backcross (BC₂) population of 43 individuals, derived from a cross between a susceptible and a SH3-introgressed resistant genotype. A total of ten sequence-characterized genetic markers closely associated with the S_H3 leaf rust resistance gene were generated. These included simple sequence repeats (SSR) markers, sequence-characterised amplified regions (SCAR) markers resulting from the conversion of amplified fragment length polymorphism (AFLP) markers previously identified and SCAR markers derived from end-sequences of bacterial artificial chromosome (BAC) clones. Those BAC clones were identified by screening of C. arabica genomic BAC library using a cloned AFLP-marker as probe. The markers we developed are easy and inexpensive to run, requiring one PCR step followed by gel separation. While three markers were linked in repulsion with the S_H3 gene, seven markers were clustered in coupling around the S_H3 gene. Notably, two markers appeared to co-segregate perfectly with the S_H3 gene in the two plant populations analyzed. These markers are suitable for marker-assisted selection for leaf rust resistance and to facilitate pyramiding of the S_H3 gene with other leaf rust resistance genes.     

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.