Evaluation of resistance to sharka (Plum pox virus) of several Prunus rootstocks

Sharka (Plum pox virus, PPV) severely affects the production of most Prunus species in the areas affected by the disease. In this study, the resistance of 15 Prunus rootstocks to a Dideron type isolate of PPV was evaluated under controlled conditions in an insect‐proof greenhouse. After four cycles of study,‘GF677’ almond x peach hybrid,‘Myrobolan 29C plum and ‘L2’ cherry did not show any symptoms and were ELISA‐DASI and RT‐PCR negative. These were considered resistant to PPV. The rest of the rootstocks assayed showed symptoms of sharka (confirmed by ELISA‐DASI or RT‐PCR), although the level of susceptibility was different for each rootstock.‘GF305’ peach, ‘Puebla de Soto’ plum and ‘Real Fino’ apricot, were highly susceptible to PPV, showing strong sharka symptoms and being ELISA and RT‐PCR positive. ‘Marianna 2624’ plum,‘AC 9921‐07’ hybrid and ‘CP‐2’ plum showed susceptibility to PPV confirmed by ELISA‐DASI and RT‐PCR positives.‘Nemaguard’ and ‘Nemared’ hybrids, ‘Torinel’ plum and ‘STN2’ hybrid showed an intermediate susceptibility to PPV with slight sharka symptoms and were ELISA‐DASI and RT‐PCR positive. Whereas, ‘Montclar’ peach and ‘Evrica’ hybrid showed moderate resistance to PPV with slight sharka symptoms and were ELISA positive but RT‐PCR negative. The results open new possibilities in the search for different sources of resistance to PPV within Prunus.     

An extended interspecific gene pool available to peach and almond breeding as characterized using simple sequence repeat (SSR) markers

Genetic diversity, as revealed by eighteenSimple Sequence Repeat (SSR) markers inthirty almond [P. dulcis (Mill.) D.A.Webb], twenty fresh-market peach [Prunus persica (L.), Basch], fifteenprocessing clingstone peach cultivars, andten rootstocks, established the geneticrelatedness among cultivars andcharacterized the variation within andbetween species. One accession each of thewild Prunus species, P.davidiana [(Carriere) Franch] and P.webbii [(Spach.) Vieh.], was included inthe analysis. The number of presumedalleles revealed by the SSR analysis rangedfrom one to six in peach whereas almondcultivars showed a range of three to nine.Peach cultivars clustered into ten groups,which are in general agreement withdocumented origin. Most processingclingstone peach cultivars clusteredseparately from fresh-market freestonecultivars supporting a distinct origin. Twomajor clusters were observed in almond withone containing California cultivars and theother containing European cultivars and theimportant California cultivar Mission.Results establish the value of SSR markersfor distinguishing different geneticlineages and characterize an extensive andlargely unexploited inter-species gene poolavailable to peach and almond breedingprograms.     

The European Prunus mapping project Progress in the almond linkage map

Summary Six European research groups are collaborating to develop genetic markers and linkage maps for use inPrunus breeding programmes. A basic map with 200 RFLPs and 50 more markers including isozymes and RAPDs will be constructed using two highly segregating populations: an interspecific peach × almond F₂ and a cherry F₂. Then, the parents of eleven almond, cherry, peach or plum breeding progenies segregating for target characters will be screened for polymorphisms at the marker loci, and a set of reduced maps, one per progeny, will be constructed with markers spaced 20–30 cM and covering the whole genome. Cosegregation analysis of markers and characters of interest will allow us to find linkages between markers and major genes or quantitative trait loci responsible for the expression of these traits. A map with 72 markers, 7 isozymes and 65 RFLPs, has been developed at the IRTA-Cabrils laboratory using an intraspecific almond progeny, ‘Ferragnes’ × ‘Mono’. Probes for the analysis of RFLPs were obtained from almond genomic and cDNA libraries. The level of polymorphism for RFLPs and the distribution of markers in the chromosomes of almond are discussed.     

Inheritance of resistance to the root-knot nematode (Meloidogyne javanica Chitwood) in bitter almond progenies

Summary Through recurrent selection and progeny testing, five bitter almond genotypes have indicated dominance of resistance for the root-knot nematode Meloidogyne javanica.All progenies from reciprocal crosses of resistant bitter almonds x susceptible sweet almonds were found completely resistant in four out of five bitter almond progenies. One selection showed near complete dominance of resistance. Progenies of resistant peach x susceptible almond cultivars segregate as to resistance, thus pointing to a possibly different mode of inheritance of nematode resistance in resistant peach as compared with almond.     

Primers amplifying a range of Prunus S-alleles

Although various consensus polymerase chain reaction (PCR) primers have been reported for identifying Prunus S‐alleles, they have been developed from and optimized on a limited set of alleles, which may limit their applicability to a broader allele range. To develop a primer set for use across the genus, degenerate consensus primers were designed from conserved regions of 27 S‐RNase sequences available from five Prunus species. The primers were tested in 15 previously genotyped cultivars of cherry, almond and apricot, representing alleles S₁ to S₆ in each crop and also S_c in apricot. Comparisons were made with previously published primers tested in the same 15 cultivars under reported reaction conditions. The new primers generated an amplification product for each of the 19 S‐alleles whereas those previously available amplified no more than 14. The primers will be useful for genotyping and genetic studies in cultivars and wild populations.     

Inheritance of resistance to plum pox potyvirus (PPV) in 'Stella' apricot seedlings

Resistance to sharka (plum pox potyvirus) was studied in 74 seedlings resulting from open pollination of the resistant apricot cultivar ‘Stella’. Each seedling was inoculated at the base by a chip-bud from a diseased GF305 peach tree. To follow tlie spread of virus into the scedlings. a Chip-bud of a healthy GF305 and another from a healthy sensitive Manicot apricot were grafted above the inoculation point in the order described. Six observations of symptoms were made in the leaves of the GF305 used For inoculation, the GF305-control, the Manicot-control and the ‘Stella’ seedlings during four growth cycles. An enzyme-linked immunosorbent assay (ELISA) was applied to the leaves of the ‘Stella’ seedlings. No ‘Stella’ seedling showed symptoms of sharka. Only three seedlings reacted positively to the ELISA. Substantial differences were observed in the speed of virus propagation through the ‘Stella’ seedlings, some of them showing a strong resistance to virus translocation. The possible use of the ‘Stella’ cultivar in apricot breeding programmes to obtain resistant cultivars is discussed.     

Susceptibility to sharka (Plum pox virus) in Prunus mandshurica×P. armeniaca seedlings

Prunus mandshurica [(Maxim.) Koehne] from Central Asia is a species related to apricot and grown in China and Mongolia. This species has been used in apricot breeding as a source of frost resistance. In addition, P. mandshurica has been suggested as the possible origin of some North American apricot cultivars resistant to sharka (Plum pox virus, PPV). The aim of this work was to transmit the resistance to PPV from P. mandshurica to the Spanish apricot cultivar ‘Currot’ by traditional crossing. The resistance to a Dideron PPV isolate of the descendants of P. mandshuricaבCurrot’ and their progenitors was evaluated under controlled conditions in a greenhouse. The results showed the susceptibility of both, the progenitors and the offspring to PPV, as being much higher than in other apricot seedlings. The results showed that the P. mandshurica accession studied is not a good progenitor in breeding apricot for PPV resistance, but just the opposite. The possible role of P. mandshurica as a source of resistance in apricot resistant cultivars is questioned.     

Identification and characterization of new S‐alleles associated with self‐incompatibility in almond

Almond is a highly heterozygous species with a high number of S‐alleles controlling its gametophytic self‐incompatibility system (GSI). In this work, we have analysed 14 Spanish local almond cultivars for S‐RNase allele diversity. Five new S‐RNase alleles were identified by cloning and sequencing, S₃₁ (804 bp) in ‘Pou de Felanitx’ and ‘Totsol’, S₃₂ (855 bp) in ‘Taiatona’, S₃₃ (1165 bp) in ‘Pou d’Establiments’ and ‘Muel’, S₃₄ (1663 bp) in ‘Pané‐Barquets’ and S₃₅ (1658 bp) in ‘Planeta de les Garrigues’. Additionally, seven already known almond alleles could be recognized in the local cultivars studied. The high number of new alleles identified reveals the wide diversity of almond germplasm still existing and requiring characterization, and points to the possibility of new findings by a wider study focusing on other provenances. The almond S‐RNases have been compared to those of other Prunus species, showing a high identity and confirming that the S‐RNase gene in this genus presents a probable common ancestor.     

Inheritance of resistance to the peach root‐knot nematode (Meloidogyne floridensis) in interspecific crosses between peach (Prunus persica) and its wild relative (Prunus kansuensis)

The peach root‐knot nematode, Meloidogyne floridensis (MF), infects majority of available nematode‐resistant peach rootstocks which are mostly derived from peach (Prunus persica) and Chinese wild peach (P. davidiana). Interspecific hybridization of peach with its wild relative, Kansu peach (P. kansuensis), offers potential for broadening the resistance spectrum in standard peach rootstocks. We investigated the inheritance of resistance to MF in segregating populations of peach (‘Okinawa’ or ‘Flordaguard’) × P. kansuensis. A total of 379 individuals from 13 F₂ and BC₁F₁ families were challenged with a pathogenic MF isolate “MFGnv14” and were classified as resistant (R) or susceptible (S) based on root galling intensity. Segregation analyses in F₂ progeny revealed the involvement of a major locus with a dominant or recessive allele determining resistance in progeny segregating 3R:1S and 1R:3S, respectively. Testcrosses with a homozygous‐susceptible peach genotype (‘Flordaguard’ or ‘UFSharp’) confirmed P. kansuensis as a source of new resistance and the heterozygous allelic status of P. kansuensis at the locus conferring resistance to MF. We propose a single‐locus dominant/recessive model for the inheritance of resistance.     

Quantitative evaluation of the degree of sprout leaf bending of rice cultivars using P-type Fourier descriptors and principal component analysis

For decades, seedling peaches have been used as the standard rootstock in California almond orchards. Vigorous, deep rooted trees are needed in almond orchards for maximum yields and to withstand the annual tree-shaking at harvest. Currently, researchers are actively evaluating rootstocks for almonds in field trials and in various screening protocols. In this study, seedling rootstocks, obtained from male-sterile advanced generation peach-almond (PEAL) hybrid mother trees, were compared with ‘Nemaguard’ peach seedlings for emergence in the nursery row, trunk caliper at propagation time, and end of season dormant above ground tree weight. Seedling emergence was affected significantly (P ≤ 0.05) by seed source, as was trunk caliper and end of season dormant above ground tree weight. Trunk caliper and dormant above ground tree weight were also affected significantly (P ≤ 0.05) by planting year. Results obtained in this study demonstrate the enhanced first year growth from seedlings of advanced generation PEAL hybrids, as compared to ‘Nemaguard’ seedlings. Seedling emergence in the rootstock bed was not affected significantly by planting year. Advanced generation PEAL hybrid seedlings were ready for June-budding at an earlier date compared to ‘Nemaguard’ seedlings, providing the potential for larger-sized finished nursery stock by the end of the growing season. Due to the male-sterile status of the advanced generation PEAL mother trees, bloom periods of several root-knot nematode resistant rootstock cultivars were examined for their degree of synchronicity with the mother trees. The examined rootstocks and mother trees varied in both chill hour and post-chill heat requirements necessary to effect bloom. ‘Flordaguard’ peach rootstock began bloom in advance of the male-sterile mother trees, whereas the bloom period of ‘Tsukuba No. 4’ occurred well after, suggesting they would not be effective synchronous pollenizers for consistent hybrid seed production. Based on more limited flowering period data, better bloom synchronicity was achieved with a Tsukuba No. 4 X Flordaguard hybrid.     

Pollen retention following natural self pollination in peach,almond, and peach × almond hybrids

Summary Retention of pollen grains following natural self-pollination was evaluated in 15 cultivars (cvs.) of almond, 4 peach and 2 nectarine cvs., and 37 interspecific peach × almond hybrids compared to 7 almond seedlings. The level of pollen retention was presumed to reflect and integrate the degree of homogamy, the amount of pollen produced by the flower, the extent of anther-stigma contact during anthesis, and the level of pollen germination. Pollen retention averaged 5 times greater in the peach and nectarine cvs. than in the almond cvs. The greater pollen retention, characteristic of the peach, was dominant in expression in the interspecific F₁ hybrids over the lower levels of pollen retention, characteristic of the almond. Thus, gametophytic self-incompatibility is not the only trait supporting outcrossing in the almond. Our data are consistent with the concept of co-evolution of floral traits relating to different breeding strategies. The level of pollen retention could often be anticipated at anthesis on the basis of blossom phenotype. That is, stigma-anther contact was observed frequently in the blossoms of peach, nectarine, and the peach × almond F₁ hybrids, but only infrequently in almond.     

Differential cold sensitivity of pollen grain germination in two Prunus species

Summary Pollen grains from selected cutivars of almond [Prunus dulcis (Mill.) D. A. Webb] and peach (Prunus persica Batsch L.) were exposed to a range of temperatures between 1°C and 34°C on a thermogradient plate. Pollen germination at temperatures below 9°C was conspicuously greater in almond than peach. Miximal germination percentages were attained at about 16°C (almond) and 23°C (peach). The two species did not differ in their capacity for pollen tube elongation over a broad range of temperatures. Maximal pollen tube elongation occurred at temperatures 5°C to 8°C higher than maximal pollen germination. Species affiliation appeared to be of much greater consequence than chilling requirement or bloom date of the sporophyte in predicting gametophytic response to temperature.     

Clone identification in Japanese flowering cherry (Prunus subgenus Cerasus) cultivars using nuclear SSR markers

Numerous cultivars of Japanese flowering cherry (Prunus subgenus Cerasus) are recognized, but in many cases they are difficult to distinguish morphologically. Therefore, we evaluated the clonal status of 215 designated cultivars using 17 SSR markers. More than half the cultivars were morphologically distinct and had unique genotypes. However, 22 cultivars were found to consist of multiple clones, which probably originate from the chance seedlings, suggesting that their unique characteristics have not been maintained through propagation by grafting alone. We also identified 23 groups consisting of two or more cultivars with identical genotypes. Most members of these groups were putatively synonymously related and morphologically identical. However, some of them were probably derived from bud sport mutants and had distinct morphologies. SSR marker analysis provided useful insights into the clonal status of the examined Japanese flowering cherry cultivars and proved to be a useful tool for cultivar characterization.     

Evidence of a new gene for high resistance to Meloidogyne spp. in Myrobalan plum, Prunus cerasifera

The Myrobalan plum Prunus cerasifera clones ‘P,2175’ and ‘P,1079’ carry single major genes (Ma1 and Ma2. respectively) for resistance to the predominant root–knot nematode (RKN) species Meloidogyne arenaria (MA), Meloidogyne incognita (MI) and Meloidogyne jaranica(MJ). The Myrobalan plum clone ‘P.2980’ is another complete–spec–trum source bearing favourable agronomic features. The genetics of its resistance to MA, MI, MJ and to the population Meloidogyne sp. Florida has been investigated from G1 crosses with the host Myrobalan plum clone ‘P.16.5’ (recessive for both Ma genes), the resistant clone ‘P.2175’ (heterozygous for Mal) and the ‘Nemared’ peach (homozygous for resistance to MA, MI and MJ but a host for M. sp. Florida). The segregation of the Gl hybrids from the intraspecific crosses into two significantly separated resistant and host classes, independent of the RKN species, indicates that resistance to all four species in ‘P.2980’ is also controlled by a single major dominant (heterozygous) gene. This gene is designated Ma3 because it shares the same spectrum and efficiency as Ma1 and Ma2, and its relationship with these last two genes is discussed. The first hybrids between ‘P.2980’ and ‘Nemared’ tested only segregated for resistance to M. sp. Florida. These results illustrate the possibility of cumulating (pyramiding) Ma3 and the ‘Nemared’ peach resistance gene(s) into new interspecific rootstocks.     

On genetically remodeling the almond tree

Summary Dwarf peach trees require only 1/4th the space of standard trees and thus reduce the cost of experiments in which the tree is the experimental unit, by 75%. Or, put another way, they facilitate estimates of tree performance, in experiments of the same size, that are 50% more precise than those obtainable from standard trees. The objective of this study was to determine if the almond tree could be genetically compressed (dwarfed) to facilitate discrimination of genetic and cultural manipulation on its productivity, to facilitate genetic manipulation, and to increase its productive efficiency.The results obtained indicate the almond tree can be dwarfed (compressed), by the dw gene of peach. interspecific crosses of dwarf (dw/dw) peach, P. persica, with almond, P. amygdalus, followed by backcrosses to almond revealed: 1. Plant stature and node density vary widely among the dwarf (dw/dw) inter species hybrids. The heritabilities of these traits are high. 2. Spur density also varies widely. Its mean is high and its heritability is very high. 3. The dwarf inter species hybrids produce flowers copiously. However, at 22 months of age the flowers of most seedlings are sterile, apparently due to abnormal pistils. Consequently, average productivity is very low. However, some of these dwarf P. persica × P. amygdalus hybrids do express moderate to high productivity. Further, the heritability of productivity, among the seedlings expressing some degree of fertility, is very high. Consequently, further backcrosses to almond are expected to rapidly restore fertility and productivity. The yield potential of dw/dw dwarf almonds will remain unknown until fertility is restored.     

The delay of flowering time in almond: a review of the combined effect of adaptation,mutation and breeding

Regulation of flowering time in almond, as in other Prunus species, is a complex process involving both chill and heat requirements. Following exposure to appropriate consecutive periods of cold and warm temperatures, the buds break dormancy and sprout or flower depending on bud type. To maximize flowering and subsequent vegetative growth and fruit set, chilling and ensuing warm temperature requirements have to be fully satisfied. Because of its potential for very early flowering, flowering time in almond is a major determinant of its adaptation to new environments. In colder regions, Late-flowering is often necessary to avoid frost damage during and just after flowering. Consequently, the selection of delayed flowering times remains an important objective in almond improvement programs. Flowering time is considered a quantitative though highly heritable trait. In addition, a dominant gene (Late flowering, Lb), originally identified in a spontaneous mutation of the Californian almond cultivar ‘Nonpareil’, was also described. The objective of this review is a comparative analysis of the effects of regional adaptation, breeding and mutation on the delay of flowering time in new almond cultivars. Findings indicate that the adaptation of almonds from the Mediterranean basin to colder regions in Northern Europe and America has been mainly achieved through delayed flowering. These adapted late-flowering cultivars have usually been developed by selecting desired quantitative genes within each regional germplasm. Additional progress thus appears achievable with a more comprehensive understanding of the quantitative and qualitative genetics controlling this trait. The use of molecular markers for the early selection of genes conferring late flowering, including both spontaneous mutations as well as unique regional germplasm, should allow development of even later cultivars including ultra-late cultivars flowering as into April.     

Correlation of stylar ribonuclease zymograms with incompatibility alleles in sweet cherry

Summary Protein stylar extracts of 16 cultivars of sweet cherry (Prunus avium), from the 10 different incompatibility groups to which incompatibility alleles have been assigned, were separated on acrylamide gels using isoelectric focusing (IEF) and were stained for ribonuclease activity. When two cultivars from the same incompatibility group were analyzed they gave identical zymograms and the cultivars of the 10 different incompatibility groups gave in all eight distinct zymograms. The ribonuclease polymorphism could be correlated with the reported S allele constitutions of the cultivars. Three ribonuclease bands were identified that each consistently corresponded to one of the six known incompatibility alleles (S ₁, S₂ and S ₆), a fourth band apparently corresponded to S ₃ and to the combination of S ₄ and S ₅, and a fifth band to S ₄ and S ₅ in other combinations. Thus, it seems that S alleles of cherry have ribonuclease activity and that IEF is useful for distinguishing S allele constitutions. The ribonuclease pattern of Summit, a cultivar of unknown incompatibility group, indicated its incompatibility genotype to be S ₁S₂, and this was confirmed by controlled pollination. The same band corresponded to S ₄ and S ₄', the mutant allele in self-compatible cultivars. IEF and ribonuclease staining promise to be useful tools for exploring the incompatibility relationships of cherry cultivars and perhaps of other self-incompatible Prunus crops.     

Chloroplast DNA variation in cultivated and wild Prunus avium L: a comparative study

Chloroplast DNA variation in 96 Prunus avium L. cultivars was assessed and compared with the results of a previous study of cpDNA diversity in 23 wild populations of the species. The polymerase chain reaction‐restriction fragment length polymorphism method was used in these studies. Approximately 9% of the chloroplast genome was analyzed, using five universal primer pairs and three restriction enzymes. Ten polymorphic fragments were common to both the wild and sweet cherry; eight polymorphic fragments were found only in the wild cherry. In the cultivars, all mutations were small (5‐30 bp) indels. In the wild populations, a point mutation was also detected in addition to indels. The mutational combinations revealed three haplotypes in the cultivars, which are the main haplotypes in the wild cherry populations. Chloroplast DNA diversity in wild cherry is higher (16 haplotypes) than in sweet cherry cultivars (three haplotypes). The probable wild origin of the sweet cherry cultivars in the maternal line, on the basis of haplotypic similarity, was discussed.     

Morphological variation within a sour cherry collection

Summary Morphological traits of 28 full-sib sour cherry (Prunus cerasus L.) families developed with pollen from European sour cherry selections were evaluated with principal component (PC) analysis. The traits which loaded on the first PC were size characters such as lateral length, leaf area, fruit weight, and trunk diameter increase. These character loading on the first PC could be interpreted as representing gradations between morphologies characteristic of the 2 presumed progenitor species, sweet cherry (P. avium L.) and ground cherry (P. fruticosa Pall.). Mean family differences in trunk diameter increase, lateral length, leaf area, and fruit weight varied approximately 12, 3.7, 2.5, and 2 fold, respectively. These results suggest that it may be possible to select sour cherry hybrids approaching the tree and fruit size of either progenitor species. The results are discussed in reference to germplasm collection and the potential of certain cultivars and hybrids as parents.     

Independence of self-compatibility and potentiality for self-pollination in peach x almond hybrids

Summary The almond of commerce (Prunus dulcis (Mill.) D.A. Webb) is self-incompatible (SI) and requires honey-bees to effect the transfer of pollen among cultivars that flower simultaneously. Four year old trees from the F₂ generation of several peach x almond hybrids were studied to determine whether self-compatibility (SC) and the potentiality for natural, i.e., abiotic, self pollination (NSP) are genetically related or are inherited independently. Both SC and the high potentiality for NSP are characteristic of peach (Prunus persica (L.) Batsch) but not almond. Forty percent of SC genotypes exhibited adequate NSP (SC, +NSP) for good fertility i.e., without insect-mediated pollination. The remaining 60% of SC genotypes (SC,-NSP) exhibited an average 61% reduction in fruit set on limbs bagged to exclude honeybees during anthesis relative to fruit set on open pollinated limbs. Our data are consistent with the concept that fertility is dependent upon the load of compatible pollen deposited on the stigma. Fruit set reduction on bagged limbs, compared with bagged and self-pollinated limbs, was presumably due to a) lack of/insufficient pollination for fertilization and/or b) post-zygotic abortion of genetically inferior recombinants. Selection following manual self-pollination may result in SC genotypes with or without the capacity for NSP. In contrast, significant fruit set on limbs enclosed during pistil receptivity necessitates that the genotype selected express both SC and the potentiality for NSP.