Effects of Preconditioning Through Mycorrhizal Inoculation on the Control of Melon Root Rot and Vine Decline Caused by Monosporascus cannonballus

The use of inoculum of arbuscular mycorrhizal fungi (AMF) in nursery represents a promising field in horticulture because of its known benefits in terms of plant growth and bioprotection. The present work was undertaken to determine the effect of mycorrhizal inoculation with Rhizophagus irregularis in a nursery medium on the containment of melon root rot and vine decline (MRRVD) caused by the soil‐borne pathogen Monosporascus cannonballus. The percentage of mycorrhization, biomass and yield following mycorrhizal inoculation were also evaluated. Biocontrol activity was assessed in greenhouse pot experiments upon artificial inoculation of M. cannonballus and in a two‐season field experiment under production conditions in an unheated greenhouse with a history of MRRVD. On the basis of the mycorrhization parameters, the interaction appeared to be established within 30 days after inoculation. The total shoot growth in the mycorrhized plants was significantly higher when compared to the control, while the root growth was unaffected. Upon artificial inoculation of M. cannonballus, mycorrhization provided complete protection against the pathogen. Greenhouse experiments under production conditions during spring cropping season showed that pretransplanting inoculation with R. irregularis significantly decreased the severity of the disease. Also, the average fruit weight of mycorrhized plants was significantly higher than the untreated control. Nevertheless, in summer crop, the bioprotection activity of AMF failed. Present results indicate that the use of AMF in a nursery setting can contribute to the prevention of the onset of this problematic soil‐borne disease within a sustainable and integrated soil‐borne disease management.     

Direct Delivery of Inoculum to Shoot Tissue Interferes with Genotypic Resistance to Ralstonia solanacearum in Tomato Seedlings

Employing known susceptible and resistant genotypes and pure bacterial inoculum (0.1 OD; 10⁸ CFU/ml^?1), five different inoculation methods were tried to assess the response of tomato genotypes to Ralstonia solanacearum. This included seed‐soaking inoculation, seed‐sowing followed by inoculum drenching, or at 2‐week stage through petiole‐excision inoculation, soaking of planting medium with inoculum either directly or after imparting seedling root‐injury. Seed‐based inoculations or mere inoculum drenching at 2 weeks did not induce much disease in seedlings. Petiole inoculation induced 90–100% mortality in susceptible checks but also 50–60% mortality in normally resistant genotypes within 7–10 days. Root‐injury inoculation at 2‐week seedling stage appeared the best for early and clearer distinction between resistant and susceptible lines. The observations suggest a role played by the root system in governing genotypic resistance to the pathogen. Direct shoot inoculation is to be adopted only for selecting highly resistant lines or to thin down segregating populations during resistance breeding.     

Evaluation of Gossypium species for resistance to cotton leaf curl Burewala virus

Cotton leaf curl disease (CLCuD), caused by cotton leaf curl Burewala virus (CLCuBV), has emerged as a major threat to cotton production in Pakistan. Resistance to CLCuBV was evaluated in cultivated and wild cotton genotypes representing six Gossypium species by visual symptom scoring and virus assessment using PCR tests. Considerable variation in responses was observed when using whitefly and graft transmission to inoculate Gossypium genotypes with CLCuBV in field and greenhouse experiments. Under field evaluation, all cultivated genotypes of Gossypium hirsutum and three genotypes of G. barbadense were susceptible. Eleven genotypes that represented six wild and cultivated Gossypium species were considered to be highly resistant as they were free from infection. Similar results were obtained when these genotypes were tested using whitefly transmission. To verify these findings, 132 cultivated and wild genotypes were tested by graft inoculation. All G. hirsutum genotypes (116 cultivated, 1 wild, 1 transgenic Coker-312 and 1 non-transgenic Coker-312), three G. barbadense genotypes and one G. thurberi genotype were highly susceptible and exhibited symptoms 9–12 days after grafting. Four genotypes of G. arboreum and one genotype of G. anomalum did not express symptoms but had a detectable level of virus. One genotype of G. herbaceum and three wild genotypes of G. hirsutum showed mild symptoms (severity indexes of 1–2) and exhibited delayed disease development. These genotypes were classified as moderately resistant to resistant. Resistant genotypes that were identified in this study will be useful sources for exploitation of breeding programmes aimed at developing CLCuBV-resistant varieties and increasing genetic diversity.     

Efficient Inoculation of Rice black‐streaked dwarf virus to Maize Using Laodelphax striatellus Fallen

Maize rough dwarf disease caused by Rice black‐streaked dwarf virus (RBSDV) is the most important disease of maize in China. Although deploying disease resistant hybrids would be the most effective way to control the disease, development of resistant hybrids has been limited by virus transmission rates that are too low for effective screening. An efficient inoculation technique for RBSDV was developed using Laodelphax striatellus Fallen, in which a virus‐free planthopper colony was developed and viruliferous planthoppers were obtained by allowing a 3‐ to 4‐day acquisition access period on RBSDV‐infected wheat plants. Planthoppers were then allowed a 25‐ to 28‐day latent period on wheat seedlings followed by a 3‐day inoculation access period on two‐to‐three‐leaf stage maize seedlings. By 35 days postinoculation, susceptible hybrid ‘Zhengdan 958’, inbred lines of ‘Ye 107’ and ‘Ye 478’ plants showed 100% RBSDV infection with symptoms of stunting plants, darkening leaves and white waxy swellings on underside of leaves. At tasseling stage, average disease indices were from 96.4 to 100.0%. Enzyme‐linked immunosorbent assays were correlated with the presence of symptoms. The high efficiency of RBSDV transmission obtained using this technique provides a reliable procedure to screen for RBSDV resistance in maize.     

Somatic Hybrids Between Potato and S. berthaultii Show Partial Resistance to Soil‐Borne Fungi and Potato Virus Y

Three tetraploid somatic hybrid lines produced by protoplast fusion between a dihaploid potato, Solanum tuberosum, cultivar BF15 and the wild potato species Solanum berthaultii were evaluated here for their response to different soil‐borne pathogens, that is Fusarium solani, Pythium aphanidermatum and Rhizoctonia solani as well as to infection by potato virus Y (PVY). Both hybrid and BF15 plants grown in vitro were inoculated with the tested pathogen strains, that is R. solani, P. aphanidermatum, or F. solani. The growth level and disease severity index of these plants were compared to the susceptible commercial cultivar Spunta. A better growth of inoculated hybrid plants and restricted disease symptoms were observed in comparison with the commercial plants. Under glasshouse conditions and after inoculation with R. solani and P. aphanidermatum, improved resistance of the hybrid plants to these pathogens was confirmed. Indeed, these plants showed no significant damage following inoculation and a better development in R. solani‐infected plants. The susceptibility of the hybrid tubers to R. solani, P. aphanidermatum, and to F. solani infection was also determined. A significant reduction of tissue colonisation was observed in all the hybrid lines compared to the cultivated cultivars. The STBc and STBd hybrids also showed improved resistance to the PVY ordinary strain (PVY^o) under glasshouse conditions.     

Comparison of Different Inoculation Techniques to Screen Resistance of Pea Lines to Downy Mildew

Pea lines with partial resistance to downy mildew were tested with different inoculation techniques. Lines with superior resistance to leaf and pod infection in the field also show superior resistance to artificial inoculation of germinating seeds or seedlings at different ages in greenhouse experiments. Increased resistance of older seedlings was found for all genotypes including the susceptible control. Juvenile leaves at the top of the seedlings plants at time of inoculation, developed the strongest symptoms. Selection for partial resistance can probably be carried out in the greenhouse to improve resistance of adult plants in field.     

Plant Growth‐promoting Fungus Penicillium oxalicum Enhances Plant Growth and Induces Resistance in Pearl Millet Against Downy Mildew Disease

Plant Growth‐promoting Fungus (PGPF) Penicillium oxalicum was isolated from rhizosphere soil of pearl millet and was tested for its ability to promote growth and induce systemic resistance in pearl millet against downy mildew disease. The fungal isolate P. oxalicum UOM PGPF 16 was identified as P. oxalicum using ITS sequencing and morphological analysis and sequence was deposited at NCBI with accession number KF150220. Pearl millet susceptible seeds were treated with three different inducers (CS, CF and LCF) of PGPF P. oxalicum and all the inducers significantly reduced the downy mildew disease and enhanced plant growth. Among the inducers tested, CS treatment recorded highest seed germination of 91% and 1427 seedling vigour followed by LCF and CF treatments. The vegetative growth parameter and NPK uptake studies under greenhouse conditions revealed that the CS treatment of P. oxalicum remarkably enhanced the parameters tested when compared to control plants. A significant disease protection of 62% and 58% against downy mildew disease was observed in plants pretreated with CS of P. oxalicum under greenhouse and field conditions, respectively. The spatio‐temporal studies revealed that inducers P. oxalicum required a minimum of 3 days for developing maximum disease resistance which was maintained thereafter. The maximum Peroxidase (POX) activity (62.7 U) was observed at 24 h in seedlings treated with CS of PGPF P. oxalicum and the activity gradually reduced at later time points after pathogen inoculation. Chitinase (CHT) activity was significantly higher in inducer treated seedlings when compared to control seedlings inoculated with pathogen after 48 h and remained constant at all time points.     

Interactions Between Rhizoctonia Root Rot and the Foliar Common Bean Diseases Anthracnose and Rust

The effects of co‐inoculation of Rhizoctonia solani and Colletotrichum lindemuthianum or Uromyces appendiculatus at different inoculum levels were studied on the disease dynamics and on the growth of bean plants under greenhouse conditions. Bean seeds were sown in R. solani‐infested soil. Additional experiments in which seedlings were transplanted to infested soil were also carried out. Conidial suspensions of C. lindemuthianum or uredospores of U. appendiculatus were inoculated onto leaves at plant developmental stages V2 and V3, respectively. Interactions between root rot and the aerial diseases were observed depending on the inoculum levels and on the timing of R. solani inoculation. Anthracnose severity tended to be higher on R. solani‐infected plants. Conversely, R. solani infection significantly reduced diameter of pustules and rust severity. When seedlings were transplanted to soil infested with low levels of R. solani, root rot severity and density of R. solani in the soil were magnified at high levels of C. lindemuthianum or U. appendiculatus. In these experiments, a synergistic interaction between root rot and anthracnose was observed to affect the plant dry weight. Antagonistic effects on the plant dry weight were found for the combination root rot/rust only when seeds were sown in infested soil.     

Molecular biology of <Emphasis Type="Italic">Ganoderma</Emphasis> pathogenicity and diagnosis in coconut seedlings

The pathogenicity of Ganoderma boninense was tested on coconut seedlings under greenhouse conditions and infection confirmed by using immunological and molecular diagnostic tools. Desiccation of older leaves and the emergence of sporophores were observed from pathogen-inoculated seedlings, whereas a control seedling does not show any pathogenic symptoms. Mature sporophores were formed within 10–13 weeks after inoculation. Polyclonal antibodies raised against mycelial proteins of Ganoderma were used for detection of Ganoderma in infected field palm and seedlings through indirect enzyme-linked immunosorbent assay technique. We adopted dot-immunobinding assay for the detection of Ganoderma from greenhouse and field samples. Under nucleic-acid-based diagnosis, G. boninense (167 bp) was detected from artificially inoculated seedlings and infected field palms by polymerase chain reaction. Apart from these, histopathological studies also support the Ganoderma pathogenicity in coconut seedlings. The pathogenicity test and combination of all the three diagnostic methods for Ganoderma could be highly reliable, rapid, sensitive and effective screening of resistance in planting material in the future.     

Population Structure and Management of Podosphaera pannosa Associated with Peach Powdery Mildew in Oman

In 2004, severe powdery mildew infection on peach occurred in Al‐Jabal Al‐Akdhar, Oman, and resulted in substantial yield losses to growers. This study was conducted to investigate occurrence, causal agents, genetic diversity and efficacy of azoxystrobin in management of this disease. Powdery mildew was observed on all farms and peach trees in Al‐Jabal Al‐Akdhar. Disease symptoms were first observed on shoots in April, followed by appearance on fruits. Disease severity reached its peak between May and June. Morphological and molecular identification of 22 powdery mildew isolates indicated that all belong to Podosphaera pannosa. Podosphaera pannosa reproduced the same symptoms upon inoculation on peach leaves. Amplified fragment length polymorphisms analysis of 35 isolates of P. pannosa from five different villages using four primer pair combinations produced 688 polymorphic loci and 35 different genotypes. Populations of P. pannosa were found to have low levels of gene diversity (H = 0.1858), which suggests that P. pannosa has been recently introduced into Al‐Jabal Al‐Akdhar. Analysis of molecular variance showed low levels of genetic differentiation among populations from the different villages, implying the introduction of P. pannosa into the different villages via common sources as well as frequent movement of pathogen inoculum among the different villages. Evaluating the efficacy of azoxystrobin showed that azoxystrobin is as efficacious as thiophanate‐methyl in managing the disease, with sulphur being the least efficacious. The study is the first to report the presence of P. pannosa in Oman. Also reported are its genetic diversity and its management under commercial conditions.     

Feeding behavior of Diaphorina citri and its transmission of ‘Candidatus Liberibacter asiaticus’ to citrus

Huanglongbing (HLB), also known as citrus greening, is currently the most destructive disease of citrus, responsible for huge economic losses in the world's major citrus production areas. The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), transmits ‘Candidatus Liberibacter asiaticus’ (Clas), the pathogen responsible to cause HLB. Understanding of vector, pathogen, and host plant interactions is important for the management of this vector‐disease complex. We used the direct‐current electrical penetration graph (DC‐EPG) system to evaluate feeding behavior of Clas‐infected D. citri adults, and their potential to transmit the pathogen to healthy citrus, Citrus reticulata Blanco cv. Sunki (Rutaceae), following a 24‐h inoculation access period. Plants were tested for the presence of Clas by qPCR 6 months after inoculation. Findings suggest that inoculation was associated with salivation into the phloem sieve elements (waveform E1). The minimum feeding time for successful transmission by a single adult was 88.8 min, with a minimum E1 duration of 5.1 min. Regression analysis indicated a significant relationship between E1 duration and transmission efficiency. The adults successful in transmitting Clas to healthy citrus were able to penetrate and feed in the phloem much earlier than those which did not transmit. The minimum duration of E1 for a female was shorter than that of a male, but transmission was higher. However, durations of other EPG parameters were not significantly different between male and female. Feeding by single Clas‐infected D. citri adults on 6‐month‐old plants (Sunki) resulted in 23% HLB‐positive plants 6 months after inoculation. Multiple nymphs or adults could transmit the pathogen more efficiently than individual adults in the field, and further enhance the severity of the disease. Effective tactics are warranted to control D. citri and disrupt transmission of Clas.     

Tolerance and overcompensation to infection by Phytophthora infestans in the wild perennial climber Solanum dulcamara

Studies of infection by Phytophthora infestans—the causal agent of potato late blight—in wild species can provide novel insights into plant defense responses, and indicate how wild plants might be influenced by recurrent epidemics in agricultural fields. In the present study, our aim was to investigate if different clones of Solanum dulcamara (a relative of potato) collected in the wild differ in resistance and tolerance to infection by a common European isolate of P. infestans. We performed infection experiments with six S. dulcamara genotypes (clones) both in the laboratory and in the field and measured the degree of infection and plant performance traits. In the laboratory, the six evaluated genotypes varied from resistant to susceptible, as measured by degree of infection 20 days post infection. Two of the four genotypes susceptible to infection showed a quadratic (concave downward) relationship between the degree of infection and shoot length, with maximum shoot length at intermediate values of infection. This result suggests overcompensation, that is, an increase in growth in infected individuals. The number of leaves decreased with increasing degree of infection, but at different rates in the four susceptible genotypes, indicating genetic variation for tolerance. In the field, the inoculated genotypes did not show any disease symptoms, but plant biomass at the end of the growing season was higher for inoculated plants than for controls, in‐line with the overcompensation detected in the laboratory. We conclude that in S. dulcamara there are indications of genetic variation for both resistance and tolerance to P. infestans infection. Moreover, some genotypes displayed overcompensation. Learning about plant tolerance and overcompensation to infection by pathogens can help broaden our understanding of plant defense in natural populations and help develop more sustainable plant protection strategies for economically important crop diseases.     

Opportunities of marker-assisted selection for Plum pox virus resistance in apricot breeding programs

Evaluation of Plum pox virus (PPV) resistance is a laborious and expensive task, and the development of new accurate methods, including the use of molecular markers, would be very useful for breeding programs for resistance. In this work, the Plum pox virus resistance of 80 apricot genotypes of different genetic origins was evaluated in controlled greenhouse and natural field conditions. The genotypes for five simple sequence repeat (SSR) markers described as linked to PPV resistance were also determined. Depending on their behavior, cultivars were classified as resistant, susceptible, and uncertain, and the genotype was identified for each SSR linked to different phenotypes. Twenty genotypes were resistant and 37 susceptible in the greenhouse and in the field. However, 23 genotypes did not show clear behavior, probably due to the complex plant-virus interaction, so they were classified as uncertain. In general, results showed a narrow relationship between the SSRs PGS1.21 and PGS1.24, and resistance to PPV, although some genotypes did not show this relationship. Most of the susceptible genotypes did not show the alleles of resistance. Therefore, in most cases, marker-assisted selection (MAS) could be used as a means of screening new seedlings for early selection, making it possible to remove those that are susceptible. However, in certain cases, MAS using these markers has not proven to be completely effective. The origin of such discrepancies could be the presence of a second locus involved in PPV resistance. In addition, other factors affecting efficiency of MAS discussed in the work are the presence of null alleles and recombinant events. Resistant seedlings would have to be evaluated in greenhouse and natural conditions to confirm their actual behavior against PPV. From the breeding point of view, the use of homozygous resistant parents for the SSR resistance alleles, with good agronomic characteristics, would increase the efficiency of breeding programs, since all seedlings would be resistant regardless of the other parent. Finally, new molecular markers should be developed to accurately select resistant seedlings regardless of the resistant progenitors involved.     

Genetic variation for resistance to ergot (<Emphasis Type="Italic">Claviceps purpurea</Emphasis> [Fr.] Tul.) among full-sib families of five populations of winter rye (<Emphasis Type="Italic">Secale cereale</Emphasis> L.)

Ergot (Claviceps purpurea [Fr.] Tul.) is a serious disease of rye (Secale cereale L.) and it adversely affects the quality of grain. The present investigation was undertaken to study genotypic variability among full-sib families (FSF) of five open-pollinated (OP) winter rye populations of highly diverse origin, namely Dankovskie Selekcyine (Poland), Charkovskaja (Ukraine), NEM4 (Russia), Halo and Carokurz, both from Germany. About 50 FSF were developed at random in each population, and the FSF of each population were evaluated in separate but adjacent experiments conducted in four environments under artificial inoculation. A mixture of conidia of C. purpurea isolates was sprayed thrice during the flowering period. The materials were manually harvested at yellow-ripe stage. Resistance trait recorded was disease severity, i.e. percent ergot sclerotia in grain by weight. Mean ergot severity ranged from 2.29 to 4.08% for the five populations across environments. Significant genotypic variation (P < 0.01) due to FSF and FSF × environment interaction was observed within each population. Genotypic variation within all populations was higher than that among five populations. All populations showed high estimates of heritability (0.72–0.89). The study indicated that the evaluated OP populations are rich reservoirs of genetic variation that should also be used in hybrid breeding. Recurrent selection to further improve ergot resistance should be successful.     

Hybridization between two cryptic filamentous brown seaweeds along the shore: analysing pre‐ and postzygotic barriers in populations of individuals with varying ploidy levels

We aimed to study the importance of hybridization between two cryptic species of the genus Ectocarpus, a group of filamentous algae with haploid–diploid life cycles that include the principal genetic model organism for the brown algae. In haploid–diploid species, the genetic structure of the two phases of the life cycle can be analysed separately in natural populations. Such life cycles provide a unique opportunity to estimate the frequency of hybrid genotypes in diploid sporophytes and meiotic recombinant genotypes in haploid gametophytes allowing the effects of reproductive barriers preventing fertilization or preventing meiosis to be untangle. The level of hybridization between E. siliculosus and E. crouaniorum was quantified along the European coast. Clonal cultures (568 diploid, 336 haploid) isolated from field samples were genotyped using cytoplasmic and nuclear markers to estimate the frequency of hybrid genotypes in diploids and recombinant haploids. We identified admixed individuals using microsatellite loci, classical assignment methods and a newly developed Bayesian method (XPloidAssignment), which allows the analysis of populations that exhibit variations in ploidy level. Over all populations, the level of hybridization was estimated at 8.7%. Hybrids were exclusively observed in sympatric populations. More than 98% of hybrids were diploids (40% of which showed signs of aneuploidy) with a high frequency of rare alleles. The near absence of haploid recombinant hybrids demonstrates that the reproductive barriers are mostly postzygotic and suggests that abnormal chromosome segregation during meiosis following hybridization of species with different genome sizes could be a major cause of interspecific incompatibility in this system.     

Segregation of male‐sterility alleles across a species boundary

Hybrid zones may serve as bridges permitting gene flow between species, including alleles influencing the evolution of breeding systems. Using greenhouse crosses, we assessed the likelihood that a hybrid zone could serve as a conduit for transfer of nuclear male‐sterility alleles between a gynodioecious species and a hermaphroditic species with very rare females in some populations. Segregation patterns in progeny of crosses between rare females of hermaphroditic Schiedea menziesii and hermaphroditic plants of gynodioecious Schiedea salicaria heterozygous at the male‐sterility locus, and between female S. salicaria and hermaphroditic plants from the hybrid zone, were used to determine whether male‐sterility was controlled at the same locus in the parental species and the hybrid zone. Segregations of females and hermaphrodites in approximately equal ratios from many of the crosses indicate that the same nuclear male‐sterility allele occurs in the parent species and the hybrid zone. These rare male‐sterility alleles in S. menziesii may result from gene flow from S. salicaria through the hybrid zone, presumably facilitated by wind pollination in S. salicaria. Alternatively, rare male‐sterility alleles might result from a reversal from gynodioecy to hermaphroditism in S. menziesii, or possibly de novo evolution of male sterility. Phylogenetic analysis indicates that some species of Schiedea have probably evolved separate sexes independently, but not in the lineage containing S. salicaria and S. menziesii. High levels of selfing and expression of strong inbreeding depression in S. menziesii, which together should favour females in populations, argue against a reversal from gynodioecy to hermaphroditism in S. menziesii.     

Inheritance of resistance to leaf and glume blotch caused by Septoria nodorum Berk. in winter wheat

Sixteen crosses between eight winter wheat cultivars were screened for resistance to Septoria nodorum leaf and glume blotch in the F₁ and F₄ generations using artificial inoculation in the field. The F₁ of most crosses showed dominance for susceptibility on both ear and leaf. The effects of general combining ability were of similar magnitude as the effects for specific combining ability. On the basis of the phenotypic difference of the parents, no prediction was possible about the amount and the direction of genetic variance in the segregating populations. The variation observed in this study both within and among the segregating populations suggests a quantitative inheritance pattern influencing the expression of the two traits. The components of variance between F₂ families within a population were as high as (for S. nodorum blotch on the ear) or higher (for S. nodorum blotch on the leaf) than those between populations. Therefore, strong selection within a few populations may be as effective to obtain new resistant genotypes as selection in a large number of populations. In almost all crosses, progenies were found that were more resistant than the better parent. Thus transgression breeding may be a tool to breed for higher levels of resistance to S. nodorum blotch. Highly resistant genotypes were found even in combination with two susceptible parents. The genetic source for Septoria resistance is probably broader than is generally assumed and could be used to improve S. nodorum resistance by combination breeding followed by strong selection in large populations. Received: 18 January / Accepted: 30 April 1999     

Feeding by Scolytus bark beetles to test for differently susceptible elm varieties

Dutch elm disease (DED), caused by the fungi Ophiostoma ulmi and O. novo‐ulmi, has reduced elm populations severely in Europe and North America. Breeding programmes are in action to find less susceptible elm varieties suitable for re‐establishing elm stands. Bark beetles, mainly Scolytus spp., are the only known natural vectors of DED. During twig feeding, beetles transfer Ophiostoma spores to healthy elms. Thus, less palatable elms should run a lower risk of DED infections. In feeding preference bioassays, we offered twigs from elms exhibiting different degree of susceptibility to O. novo‐ulmi, together with non‐host trees to Scolytus beetles. Scolytus multistriatus preferred wych elm, Ulmus glabra, to 100% in two‐choice tests, whereas S. laevis did not discriminate between a tolerant and a susceptible variety of field elm, U. minor. We suggest that the feeding assay is useful as a low‐tech method in breeding programmes for evaluating the suitability of promising elm genotypes to vector insects.     

Characterization of the multiple resistance traits of somatic hybrids between Solanum cardiophyllum Lindl. and two commercial potato cultivars

Interspecific somatic hybrids between commercial cultivars of potato Solanum tuberosum L. Agave and Delikat and the wild diploid species Solanum cardiophyllum Lindl. (cph) were produced by protoplast electrofusion. The hybrid nature of the regenerated plants was confirmed by flow cytometry, simple sequence repeat (SSR), amplified fragment length polymorphism (AFLP), microsatellite-anchored fragment length polymorphism (MFLP) markers and morphological analysis. Somatic hybrids were assessed for their resistance to Colorado potato beetle (CPB) using a laboratory bioassay, to Potato virus Y (PVY) by mechanical inoculation and field trials, and foliage blight in a greenhouse and by field trials. Twenty-four and 26 somatic hybrids of cph + cv. Agave or cph + cv. Delikat, respectively, showed no symptoms of infection with PVY, of which 3 and 12, respectively, were also resistant to foliage blight. One hybrid of cph + Agave performed best in CPB and PVY resistance tests. Of the somatic hybrids that were evaluated for their morphology and tuber yield in the field for 3 years, four did not differ significantly in tuber yield from the parental and standard cultivars. Progeny of hybrids was obtained by pollinating them with pollen from a cultivar, selfing or cross-pollination. The results confirm that protoplast electrofusion can be used to transfer the CPB, PVY and late blight resistance of cph into somatic hybrids. These resistant somatic hybrids can be used in pre-breeding studies, molecular characterization and for increasing the genetic diversity available for potato breeding by marker-assisted combinatorial introgression into the potato gene pool.