Rootstock Breeding Programs Research Articles

High resolution melting analysis for early identification of citrus hybrids: A reliable tool to overcome the limitations of morphological markers and assist rootstock breeding

Citrus rootstocks are highly polyembryonic. Seeds usually produce many nucellar seedlings that are identical to the mother plant and sometimes one zygotic plant. Nucellar seedlings hamper the identification of hybrids if the morphological markers that distinguish sexual and nucellar plants are missing, and competition among embryos might impair zygotic embryo viability. Accurate morphological and/or molecular screening methods are needed to identify and discard the nucellar plants, which is labor-intensive and increases the costs of rootstock breeding programs. This study describes an affordable and reliable method for an early identification of hybrids in progenies derived from highly polyembryonic female parents based on DNA isolation from pooled leaf samples (2 to 4 plants per sample) coupled with a high resolution melting (HRM) analysis at two SNP loci. The method was validated on more than one thousand progeny derived from the crosses alemow (Citrus macrophylla Wester)×Swingle citrumelo [Citrus paradisi Macf.×Poncirus trifoliata (L.) Raf.] (A×S), and Cleopatra mandarin (Citrus reshni Hort. ex Tan)×Swingle citrumelo (C×S). Bulking reduced the number of DNA isolations for population genotyping, while HRM unequivocally identified, in a single step, the bulks containing the unifoliate hybrids. A consistent difference was found between A×S and C×S populations in the frequency of uni- and multifoliate hybrids (69% unifoliate in A×S vs. 10% in C×S), suggesting that the combination of parents, rather than just the male parent carrying the trifoliate character, influenced the early multifoliate leaf expression. Moreover, several multifoliate nucellar seedlings were found among the C×S progenies, making the multifoliate leaf character inappropriate for the early identification of all hybrids.

CLONING AND CHARACTERIZATION OF THE DAGGER NEMATODE RESISTANCE GENE XIR1

The dagger nematode, Xiphinema index, feeds aggressively on grape roots and in the process vectors grapevine fanleaf virus (GFLV) leading to the severe viral disease known as fanleaf degeneration. Disease symptoms consist of disrupted fruit set caused by GFLV and depressed plant growth caused by root damage from X. index. The use of fumigants to control X. index in vineyards is no longer recommended because of their high cost, detrimental environmental effects and lack of effective soil penetration to control nematodes on a deep perennial root system. Therefore, resistance to X. index has been an important objective in grape rootstock breeding programs. We previously demonstrated that resistance to X. index derived from a Vitis arizonica/girdiana hybrid b42-26 was largely controlled by a major quantitative trait locus, XiR1 (X. index Resistance 1) located on chromosome 19. Genetic studies leading to the isolation and characterization of the genes conferring resistance to X. index would further our understanding of resistance and assist molecular and classical breeding efforts to control X. index. In this report, we present the development of high resolution genetic and physical maps in the XiR1 region as well as the isolation of the XiR1 locus by a positional cloning approach. This study has identified the first locus responsible for ectoparasitic nematode resistance. The markers developed from this study are being used to expedite the breeding of resistant grape rootstocks.

Dw2, a New Dwarfing Locus in Apple Rootstocks and Its Relationship to Induction of Early Bearing in Apple Scions

The ability of certain apple rootstocks to dwarf their scions has been known for centuries and their use revolutionized apple (Malus ×domestica) production systems. In this investigation, several apple rootstock breeding populations, planted in multiple replicated field and pot experiments, were used to ascertain the degree of dwarfing when grafted with multiple scions. A previous genetic map of a breeding population derived from parents ‘Ottawa 3’ (O.3) and ‘Robusta 5’ (R5) was used for quantitative trait locus (QTL) analysis of traits related to scion vigor suppression, induction of early bearing, and other tree size measurements on own-rooted and grafted trees. The analysis confirmed a previously reported QTL that imparts vigor control [Dw1, log of odds (LOD) = 7.2] on linkage group (LG) 5 and a new QTL named Dw2 (LOD = 6.4) on LG11 that has a similar effect on vigor. The data from this population were used to study the interaction of these two loci. To validate these findings, a new genetic map comprised of 1841 single-nucleotide polymorphisms was constructed from a cross of the dwarfing, precocious rootstocks ‘Geneva 935’ (G.935) and ‘Budagovsky 9’ (B.9), resulting in the confirmation and modeling of the effect of Dw1 and Dw2 on vigor control of apple scions. Flower density and fruit yield data allowed the identification of genetic factors Eb1 (LOD = 7.1) and Eb2 (LOD = 7.6) that cause early bearing of scions, roughly colocated with the dwarfing factors. The major QTL for mean number of fruit produced per tree colocated with Dw2 (LOD = 7.0) and a minor QTL was located on LG16 (LOD = 3.5). These findings will aid the development of a marker-assisted breeding strategy, and the discovery of additional sources for dwarfing and predictive modeling of new apple rootstocks in the Geneva® apple rootstock breeding program.

Detection of SNP and validation of a SFP InDel (deletion) in inverted repeat region of the Prunus species chloroplast genome

In order to control tree size, disease, precocity and stress most Prunus varieties are cultivated as composite plants grafted onto desirable rootstocks that impart all the afore-mentioned traits. Several Prunus rootstock breeding programs have been focused on the production of interspecific hybrids. The pedigree of most of these rootstocks remains unknown due to the lack of parental information necessitating the application of DNA-based knowledge in breeding programs. The amplification and sequencing of the chloroplast inverted repeat B (IRB) region spanning 25,960bp from P. cerasifera (myrobalan plum) Ehrh., P. amygdalus (almond) and P. persica (peach) using the ASAP method revealed a single nucleotide polymorphisms (SNP) in the rps19-rpl2 IRB region in myrobalan when compared to almond and peach. In addition, a prominent and an easily identifiable single feature polymorphism (SFP-InDel (deletion)) of 18 nucleotides was discovered in reference to the peach chloroplast genome in the ycf1 gene in the IRB region. In this work, it has been developed a highly useful polymorphic molecular marker to characterize the maternal parent in interspecific hybrids of Prunus rootstocks as a first step toward developing pedigree information. The ycf1 SFP-InDel (deletion) has been successfully used in several 3-way hybrids generated in the stone fruit rootstock breeding program for the characterization of new interspecific plant material. This SFP is expected to be highly utile in characterizing the maternal lineage of Prunus hybrids in other breeding programs.

Horticultural evaluation of exotic watermelon germplasm as potential rootstocks

Watermelon grafting is on the rise worldwide as an agrotechnology aimed at preventing soilborne-pest damage. Grafting watermelon on Cucurbita rootstocks may negatively affect fruit size, shape and quality. However, grafting watermelon on watermelon rootstocks can prevent these negative effects. Twenty-one exotic watermelon accessions were evaluated as potential sources for watermelon rootstock breeding programs. Most of the watermelon accessions tested in the field as rootstocks for the mini-watermelon ‘Extazy’ gave yields similar to the nongrafted and self-grafted ‘Extazy’. Four accessions: WAN, PI 457916, PI 307750 and PI 307609, produced significantly lower yields. The accessions BDA, CON, MAL, PI 296341 and PI 307609 were selected for detailed evaluation due to their previously found tolerance to soilborne pathogens. No difference was found in the total fruit quality index between nongrafted ‘Extazy’ fruit and ‘Extazy’ fruit from plants grafted on the different watermelon accessions. Fruit weight from plants grafted on Cucurbita rootstocks was higher than that from plants grafted on ‘Extazy’ or on the other watermelon accessions. No bitter flavor and no cucurbitacin were present in ‘Extazy’ fruits of plants grafted on bitter fruit watermelon accessions. Thus the examined exotic watermelon accessions did not adversely affect fruit quality and can be used as a basic germplasm for watermelon rootstock breeding. The most promising accession is PI 296341.

Cultural Conditions and Propagule Type Influence Relative Chloride Exclusion in Grapevine Rootstocks

Breeding for salt resistance in grapevines and other crops has made slow progress despite decades of research. One factor contributing to this problem in grapevines is the weak or nonexistent correlation of field and greenhouse performance observed in some studies when salt resistance is assessed by chloride accumulation in leaf tissue. To develop a rapid chloride exclusion assay for use in rootstock breeding, multiple systems were tested. Results were obtained wherein the well-established field performance of specific genotypes was augmented, equalized, or reversed when in containerized culture. One assay using fritted clay media and herbaceous cuttings yielded a rank order and relative chloride uptake among the tested genotypes that was similar to published values from long-term studies in experimental vineyards. This assay used only 14 days of high salt exposure, was inexpensive, required relatively little space and maintenance, and has continued to provide reliable data in subsequent experiments. The results demonstrate the potential for considerable plasticity in chloride exclusion exhibited by ungrafted grapevines when assayed in containers. This underscores the importance of system design wherein genotypes of known capacity for chloride exclusion are accurately calibrated to their established field performance. This study describes an empirically derived assay that replicates these conditions closely enough to be used in a rootstock breeding program for improved chloride exclusion.

Morphological characterization of cherry rootstock candidates selected from Samsun Province in Turkey

Sweet cherry and sour cherry production in Turkey occupies an important place in worldwide production and is still of considerable importance in terms of providing further fruit species and gene resources. In this study, 88 sweet cherry, 16 sour cherry, and 9 mahaleb types displaying potential for cultivar rootstocks were selected from wild cherry populations in Samsun Province of the Central Black Sea Region in Turkey. The morphologic characteristics of the studied genotypes were compared with the standard clone rootstocks PHL-A, MaxMa 14, Montmorency, Weiroot 158, Gisela 5, Gisela 6, and SL 64. A total of 22 morphological and phenotypic characteristics were evaluated in the selected genotypes and clonal rootstocks. The obtained data were analyzed by using principal component analysis, and 7 principal component (PC) axes accounted for 70.37% of the total cumulative variation. The first PC axis, which accounted for 22.4% of the total variation, was represented by leaf blade length, leaf blade width, petiole length, ratio of 1-year-old shoot internodes length, and 1-year-old shoot branching, while the second PC axis, which accounted for 13.55% of the variation, was represented by leaf blade ratio of length to width, 1-year-old shoot length, 1-year-old shoot thickness, and leaf blade shape. Average linkage cluster analysis was also performed, and 7 main clusters were identified. According to the diversity analysis of coefficients, the C 0032 and C 0094 genotypes were identified as being very similar, while the C 0002 and S 0012 genotypes were determined as the most distant genotypes in terms of morphology. In conclusion, the genotypes evaluated in this study may be useful for rootstock breeding programs.

Eleven Year Evaluation of American (Geneva®) and Polish Rootstocks with ‘Golden Delicious Reinders’ Apple in Poland

Abstract A field study of dwarf apple rootstock performance using ‘Golden Delicious Reinders’ as scion cultivar was conducted during the year 2001-2011. The study included 6 rootstocks from Cornell Geneva rootstock breeding program (G.11, G.16, G.41, G.202, CG.3007 and CG.4013) 4 rootstocks from the Polish rootstock breeding program (P 14, P 16, P 59 and P 60) and 3 Malling rootstocks as control (M.9 T337, M.9 Pajam 2 and M.26). During the eleven years of growth (2001-2011), there was no tree loss due to root system or cultivar damage caused by frost or diseases including fire blight (Ervinia amylovora Burr. Winsl.) infestation on the rootstock. Trees growing on P 59 and P 16 had the weakest growth while the strongest growing trees were on P 14, M.26, P 60, G.202 and CG.4013. The size of the trees on M.9 Pajam 2, G.16, G.41, CG3007 and CG.4013 were similar to that of the trees growing on the standard rootstock M.9 T337, while the trees on P 59, P 16 and G.11 were significantly smaller from standard trees on M.9 T337. After eleven years of the growth, the cumulative yield varied from 56.7 kg (on P 59) to 237.8 kg (on CG.4013). High yields were also obtained from vigorously growing trees on: G.202, P 14, G.41 and M.26. The cumulative yield expressed as yield per cm2 of trunk cross-sectional area was highest for trees on G.11, P 16, M.9 Pajam 2 and G.41 and lowest for vigorously growing trees on P 60, P 14 and M.26. In 2009, a year of good cropping, the mean mass of fruits varied from 140 g on G.16 to 213 g on P 60. The fruit size from trees on P 14, M.26 and P 16 was statistically similar to fruits from trees on M.9 T337. The results of the eleven-year study lead to the conclusion that the rootstocks G.11 and G.41, which are highly resistant to fire blight, can be recommended for growing ‘Golden Delicious Reinders’ apple in the Polish climate, similarly to trees on M.9 T337, P 16 and M.9 Pajam 2. For orchards located on light soils the following rootstocks are also promising: G.202, CG.3007, CG.4013 and P 14 and P 60 rootstocks of polish selection.

Testing of 'Gisela 5' and 'Santa Lucia 64' cherry rootstocks in Bosnia and Herzegovina

Cherry cultivation has a long tradition in Bosnia and Herzegovina mainly due to favorable climatic conditions for cherry growing in this region. However, current cherry production is insufficient because of prevailing old cultivars and rootstocks. Modern intensive production of sweet cherry (Prunus avium L.) requires planting of high quality cultivars on dwarfing rootstocks planted in high density orchards. Cherry rootstock breeding programs worldwide require data on tolerance and performance of their rootstocks in different climatic conditions. Therefore, the influence of two cherry rootstocks ('Gisela 5 and 'Santa Lucia 64') on phenological events (blooming), growth and pomological properties of two cherry cultivars ('Stella' and 'Burlat') planted in modern orchard (managed according to standard commercial practice for integrated fruit production), near Sarajevo was evaluated. The trees grafted on 'Gisela 5' rootstocks were planted in 2004 and on 'Santa Lucia 64' in 2005. All tress were trained in a spindle system and the analyzed parameters were monitored in 2010. Rootstocks greatly influenced blooming time, growth and fruit characteristics of both investigated cultivars. The weaker rootstock was 'Gisela 5', which stimulated earlier blooming and caused statistically significant better fruit characteristics (fruit size, % flesh of fruit as well as total soluble solids content in fruit). The results of the analysis showed that both cherry cultivars reached better fruit quality on 'Gisela 5'. 'Stella' had better fruit quality than 'Burlat'. 'Santa Lucia 64' proved a better rootstock for 'Burlat' than for 'Stella'.

Characterization of Genetic Diversity of Stone Fruit Rootstocks Used in Chile by Means of Microsatellite Markers

Stone fruit (Prunus L.) production in Chile covers ≈43,000 ha and includes a wide variety of soils and climates requiring a large diversity of rootstocks. The most commercially important rootstock cultivars are 26 genotypes from three different taxonomic groups belonging to the subgenera Amygdalus (L.) Benth. Hook. (peach group), Prunus Focke [= Prunophora (Neck.)] Focke (plum group), and Cerasus (Adans.) Focke (cherry group) with eight, seven, and 10 individuals, respectively. To determine their genetic diversity, characterization by microsatellite markers [simple sequence repeat (SSR)] was conducted. Of a total of 20 SSR markers evaluated, 12 generated amplified products that were consistent in the three taxonomic groups. The number of alleles per marker ranged from 18 for PSM-3 to four in CPPCT-002. Clustering analysis, by both traditional hierarchical and model-based approaches, indicate that all genotypes are clustered in their respective taxonomic groups, including the interspecific hybrids. Genetic diversity, measured as the average distances (expected heterozygosity) between individuals in the same cluster, was higher in Cerasus (0.78) followed by Prunus (0.72) and Amygdalus (0.64). Total number of alleles observed was 133, of which 14, 33, and 35 from six, 10, and 10 loci were unique for the peach, plum, and cherry rootstock groups, respectively. Alleles shared among peach/plum, plum/cherry, and peach/cherry rootstock genotypes were 13, 14, and 18 from nine, seven, and seven loci, respectively. Only six alleles from five loci were common to the three taxonomic groups. In addition, to develop a rootstock identification system based on SSR markers, a minimum set of three markers (PMS-3, BPPCT-037, and BPPCT-036) able to differentiate the 26 genotypes was identified. This study is the first step toward establishing a stone fruit rootstock breeding program in Chile.

Graft union formation and cell-to-cell communication via plasmodesmata in compatible and incompatible stem unions of Prunus spp.

Graft compatibility with at least most commercial cultivars is an essential trait in rootstock breeding programs for orchard performance and longevity. However, its range of application is restricted by physiological and biochemical factors that produce incompatible grafts. As a biological process, the incompatibility reaction is difficult to study because of the wide range of different scion–rootstock interactions produced when grafting. The aim of this study was to characterize the early cellular signs of graft incompatibility based mainly on histochemical tests and the quantification of cell-to-cell communication via plasmodesmata using a novel method termed in vitro ‘stem unions’.Results from different Prunus graft combinations demonstrated that adhesion and callus proliferation occurred in both compatible and incompatible combinations 1 week after grafting. Nevertheless, the new cambium formation derived from the callus tissue appeared to be delayed in incompatible heterografts. A different cell wall composition as well as an enhanced metabolism was detected in the incompatible unions. Using fluorescence redistribution after photobleaching (FRAP) of a symplastic tracer, we determined the plasmodesmal coupling within the tissues primarily involved in the grafting processes. We found that the plasmodesmal coupling was higher between callus cells than between cortex cells. In addition, the plasmodesmal coupling was significantly higher within interfaces from compatible graft combinations compared with that of incompatible unions when analysed 2 weeks after grafting. These new findings strengthen the idea that callus cells are playing a central role in scion/rootstock interaction and that late rejection is predetermined already at the initial steps of union formation.

Testing of ‘Gisela 5’ and ‘Santa Lucia 64’ cherry rootstocks in Bosnia and Herzegovina

Cherry cultivation has a long tradition in Bosnia and Herzegovina mainly due to favorable climatic conditions for cherry growing in this region. However, current cherry production is insufficient because of prevailing old cultivars and rootstocks. Modern intensive production of sweet cherry (Prunus avium L.) requires planting of high quality cultivars on dwarfing rootstocks planted in high density orchards. Cherry rootstock breeding programs worldwide require data on tolerance and performance of their rootstocks in different climatic conditions. Therefore, the influence of two cherry rootstocks ('Gisela 5 and 'Santa Lucia 64') on phenological events (blooming), growth and pomological properties of two cherry cultivars ('Stella' and 'Burlat') planted in modern orchard (managed according to standard commercial practice for integrated fruit production), near Sarajevo was evaluated. The trees grafted on 'Gisela 5' rootstocks were planted in 2004 and on 'Santa Lucia 64' in 2005. All tress were trained in a spindle system and the analyzed parameters were monitored in 2010. Rootstocks greatly influenced blooming time, growth and fruit characteristics of both investigated cultivars. The weaker rootstock was 'Gisela 5', which stimulated earlier blooming and caused statistically significant better fruit characteristics (fruit size, % flesh of fruit as well as total soluble solids content in fruit). The results of the analysis showed that both cherry cultivars reached better fruit quality on 'Gisela 5'. 'Stella' had better fruit quality than 'Burlat'. 'Santa Lucia 64' proved a better rootstock for 'Burlat' than for 'Stella'.

Assessment of the genetic diversity of the Tunisian citrus rootstock germplasm

BackgroundCitrus represents a substantial income for farmers in the Mediterranean Basin. However, the Mediterranean citrus industry faces increasing biotic and abiotic constraints. Therefore the breeding and selection of new rootstocks are now of the utmost importance. In Tunisia, in addition to sour orange, the most widespread traditional rootstock of the Mediterranean area, other citrus rootstocks and well adapted to local environmental conditions, are traditionally used and should be important genetic resources for breeding. To characterize the diversity of Tunisian citrus rootstocks, two hundred and one local accessions belonging to four facultative apomictic species (C. aurantium, sour orange; C. sinensis, orange; C. limon, lemon; and C. aurantifolia, lime) were collected and genotyped using 20 nuclear SSR markers and four indel mitochondrial markers. Multi-locus genotypes (MLGs) were compared to references from French and Spanish collections.ResultsThe differentiation of the four varietal groups was well-marked. The groups displayed a relatively high allelic diversity, primarily due to very high heterozygosity. Sixteen distinct MLGs were identified. Ten of these were noted in sour oranges. However, the majority of the analysed sour orange accessions corresponded with only two MLGs, differentiated by a single allele, likely due to a mutation. The most frequent MLG is shared with the reference sour oranges. No polymorphism was found within the sweet orange group. Two MLGs, differentiated by a single locus, were noted in lemon. The predominant MLG was shared with the reference lemons. Limes were represented by three genotypes. Two corresponded to the 'Mexican lime' and 'limonette de Marrakech' references. The MLG of 'Chiiri' lime was unique.ConclusionsThe Tunisian citrus rootstock genetic diversity is predominantly due to high heterozygosity and differentiation between the four varietal groups. The phenotypic diversity within the varietal groups has resulted from multiple introductions, somatic mutations and rare sexual recombination events. Finally, this diversity study enabled the identification of a core sample of accessions for further physiological and agronomical evaluations. These core accessions will be integrated into citrus rootstock breeding programs for the Mediterranean Basin.

Advances in apple culture worldwide

Over the last 60 years, planting densities for apple have increased as improved management systems have been developed. Dwarfing rootstocks have been the key to the dramatic changes in tree size, spacing and early production. The Malling series of dwarfing rootstocks (M.9 and M.26) have been the most important dwarfing rootstocks in the world but are poorly adapted in some areas of the world and they are susceptible to the bacterial disease fire blight and the soil disease complex, apple replant disease which limits their uses in some areas. Rootstock breeding programs in several parts of the world are developing improved rootstocks with resistance to fire blight, and replant disease, and improved cold hardiness and yield efficiency. A second important trend has been the increasing importance of new cultivars. New cultivars have provided opportunities for higher prices until they are over-produced. A new trend is the "variety club" in which variety owners manage the production and marketing of a new unique cultivar to bring higher prices to the growers and variety owners. This has led to many fruit growers being unable to plant or grow some new cultivars. Important rootstock and cultivar genes have been mapped and can be used in marker assisted selection of future rootstock and cultivar selections. Other important improvements in apple culture include the development of pre-formed trees, the development of minimal pruning strategies and limb angle bending which have also contributed to the dramatic changes in early production in the 2nd-5th years after planting. Studies on light interception and distribution have led to improved tree forms with better fruit quality. Simple pruning strategies and labor positioning platform machines have resulted in partial mechanization of pruning which has reduced management costs. Improved plant growth regulators for thinning and the development of a thinning prediction model based on tree carbohydrate balance have improved the ability to produce the optimum fruit size and crop load. Other new plant growth regulators have also allowed control of shoot growth, control of preharvest fruit drop and control of fruit softening in storage after harvest. As we look to the future, there will be continued incremental improvement in our understanding of plant physiology that will lead to continued incremental improvements in orchard management but there is likely to be dramatic changes in orchard production systems through genomics research and genetic engineering. A greater understanding of the genetic control of dwarfing, precocity, rooting, vegetative growth, flowering, fruit growth and disease resistance which will lead to new varieties and rootstocks which are less expensive to grow and manage.

THE RESULTS OF THE ESTONIAN APPLE ROOTSTOCK BREEDING PROGRAM

THE RESULTS OF THE ESTONIAN APPLE ROOTSTOCK BREEDING PROGRAM

THE HISTORY OF APPLE BREEDING IN PEOPLE'S REPUBLIC OF CHINA

China has more than 50 years of history in apple breeding. There have been 260 apple breeders from 40 universities or institutes all over the country, including the provinces of Liaoning, Hebei, Shaanxi, Shandong and Shanxi. In the past 50 years China released 270 apple varieties obtained by crossing, using ‘Fuji’, ‘Ralls’, ‘Delicious’, ‘Golden Delicious’, ‘Jonathan’, and ‘Toko’ as parents. Among them, ‘Qinguan’ and ‘Yanfeng’ bred in Shaanxi, ‘Huaguan’ and ‘Huashuai’ in Henan, ‘Liaofu’, ‘Yueshuai’, ‘Hanfu’, ‘Qiujin’, and ‘Huahong’ in Liaoning, ‘Guohong’ and ‘Yanshanhong’ in Hebei, ‘Danxia’ in Shanxi, and ‘Dailv’ in Shandong have a significant planted area. China has also initiated a program for breeding red flesh and juice apples with high acid content. Traditional crossing, sport selection, and open-pollinated seedling selection combined with marker-assisted selection, anther culture, transgenic engineering, are the approaches used for breeding new apple varieties. Research on inheritance of important traits, pre-selection and early evaluation of seedlings, methods to shorten the juvenility, resistance to apple canker and apple ring rot, and genetic engineering was also conducted in the apple breeding programs. China began apple rootstock breeding program thirty years ago. Traits of dwarfing, high disease resistance, drought tolerance, and cold hardiness, are the primary consideration in the apple rootstock breeding programs. China has released several apple rootstocks as S19, S20, SH15, 63-2-19, 77-34, Liaozhen No. 2, GM2 56, CX3, 75-9-5, and 75-7-1. A National Apple Breeding Association in China has been formed to coordinate the breeding groups across the country.

EVALUATION OF DWARFING ROOTSTOCKS IN WASHINGTON APPLE REPLANT SITES

Apple replant disorder is an increasingly significant factor challenging apple (Malus × domestica) orchard establishment and consistent production. Standard fumigation practices using Telone or Vapam offer effective control, but must be applied correctly and safely, are expensive, subject to restrictive regulation, and not acceptable in organic systems. Dwarfing rootstocks with resistance or tolerance to apple replant disorder offer a clear alternative to producers in tree fruit production areas of the Pacific Northwest of the USA, as long as their genotypes are suitably adapted to the diverse production conditions of the region. In collaboration with the USDA-ARS/Cornell University National Apple Rootstock Breeding Program in Geneva, NY, the Washington Tree Fruit Research Commission (WTFRC) has conducted eight rootstock trials in commercial Washington orchards since 2003. These trials collectively included 65 rootstocks and three scion cultivars (‘Gala’, ‘Fuji’ and ‘Honeycrisp’) planted in modern, high density systems in known replant sites. They featured 46 Geneva (G.) series selections, as well as 19 other genotypes. Trials were designed as split block (fumigated vs. non-fumigated) randomized complete blocks with a minimum of four replications and typically five or more trees per plot. Most Geneva rootstocks outperformed the industry standards of Budagovski (B.) 9, Malling (M.) 9 Pajam 2, and M.26 for yield, trunk cross sectional area, and fruit size. Supporter 1, Supporter 2, and Supporter 3 have performed similarly to M.9 clones and have shown no advantage over standard commercial rootstocks. Several Geneva selections G.41 (tested as CG.3041), G.935 (tested as CG.5935), CG.4214, CG.4814 have performed well in non-fumigated treatments across sites. Trees with G.41 and G.11 rootstocks have shown less vigor than trees on G.935, CG.4214, G.202, and CG.4814. Many of the Geneva series also show significant improvement over currently available rootstocks for other traits including wooly apple aphid resistance, fire blight resistance, ease of propagation. Our collaborative trials under commercial conditions clearly indicate the potential of improved genotypes for apple production in the Pacific Northwest of the USA.

Multivariate analysis of Prunus subgen. Cerasus germplasm in Iran using morphological variables

Knowledge of relationships among the cultivated and wild species of Cerasus, a subgenus of Prunus L., is important for recognizing gene pools, identifying pitfalls in germplasm collections and developing effective conservation and management strategies. In this study, morphological characterization of wild Cerasus subgenus species naturally growing in Iran, including P. avium L. (mazzard), P. mahaleb L., P. brachypetala Boiss., P. incana Pall., P. yazdiana Mozaff., P. microcarpa Boiss. subsp. microcarpa, P. microcarpa Boiss. subsp. diffusa and P. pseudoprostrata Pojark. and three commercial species, sweet cherry (P. avium L.), sour cherry (P. cerasus L.) and duke cherry (P. × gondouinii Rehd.) was investigated. Twenty-nine variables were recorded in 95 accessions of these species to detect similarities among them. All studied characteristics were showing a high degree of variability, but it was highly pronounced for plant height, leaf characters especially for dimensions and pubescence as well as for petiole and peduncle length. The majority of significant correlation coefficients were found in the characteristics representing plant size and growth habit with leaf size. Principal component analysis was performed for phenotypical diversity determination and grouping of species. Characters with high discriminating values were those related to growth habit, height, compactness, trunk diameter, leaf dimensions, petiole length, leaf tip and serration, flower color, peduncle length, fruit shape and stone shape. Based on the cluster analysis, species included in two main clusters, first those with large structure (sweet, sour and duke cherries, mazzard and mahaleb), second the other species with small structure which might be potentially suitable for rootstock breeding programs. Scatter plot using two first factors also confirmed the grouping by cluster analysis. The conservation of the highly diverse native populations of Iranian wild Cerasus species is recommended.

Morphological and molecular variability in some Iranian almond genotypes and related Prunus species and their potentials for rootstock breeding

In this study, in order to know the variability for a rootstock breeding program genetic diversity and relationships among 55 Iranian almond genotypes and seven related Prunus species were investigated. Morphological and molecular analyses were used. Principal component analysis showed that three components explained 67.6% of the total morphological variation for the first year and 68.06% for the second year of the study. Leaf traits were predominant in the first component and contributed most of the total variation. Leaf length and width, as well as, leaf area were highly correlated with each other and correlated to vigor. Also a negative correlation was found between leaf length/width ratio and vigor. Ward's method was used to construct cluster from morphological data which allocated individuals into their respective species. Out of 100 pre-screened RAPD primers, 16 with reproducible bands and maximum polymorphism were selected. Two-hundred and sixty bands were scored of which 250 of them were polymorphic. Average value of polymorphism per primer was 95.81% and maximum value for polymorphism (100%) was obtained from TIBMBA-14, TIBMBA-17, TIBMBB-05, TIBMBB-08, TIBMBD-09, and TIBMBD-10. On the other hand, the minimum value was obtained from TIBMBB-16 (86%). Primer TIBMBB-5 gave the maximum number of bands (25 fragments) and the minimum obtained from TIBMBE-18 (11 fragments). Genetic similarity based on Jaccard's coefficient ranged from 0.28 to 0.79 with an average of 0.53. Molecular analysis revealed a high degree of separation among samples regarding their geographical origin. Correlation between two approaches was low ( R = −0.38). High molecular and morphological variability indicated that this collection includes rich and valuable plant materials for almond rootstock breeding.

PLUM ROOTSTOCKS FOR INTENSIVE PLUM CULTURE

At SCDP Valcea a rootstock breeding program started 30 years ago and its main targets were to obtain plum rootstocks suited to the soil conditions from the hill area of Romania (clay soils, thin, wet and with low natural fertility). The breeding program released 7 plum rootstocks for intensive culture. Two of the rootstocks originated in clonal selections carried out on local populations of Prunus domestica ('Otesani 8') and Prunus insititia ('Otesani 11') and 5 were obtained through different hybrid crosses ('Miroval', 'Rival', 'Oltval', 'Corval' and 'Pinval'). 'Otesani 8' is a low vigor rootstock suited for plum orchards with 1000-1200 trees/ha. 'Otesani 11', 'Rival', 'Oltval' have higher vigor then 'Otesani 8' and are suited for densities of 500-700 trees/ha in plum orchards. 'Corval' and 'Pinval' have high vigor and can be used as rootstocks for plum orchards with 400-500 trees/ha. This large new rootstock assortment assures the possibility of establishing plum orchards in all the favorable ecological areas of Romania.