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A set of nine Simple Sequence Repeat (SSR) loci, approved by the ECPGR Prunus working group, are proposed as a standard set for genotyping European plum accessions. These loci show sufficient reliability in spite of problems caused by hexaploidy. Polymorphism in the loci is high and enables differentiation between unique plum accessions as well as analyses of genetic grouping and overall genetic structure. A set of seven reference accessions are described. A compiled dataset with allelic information for 165 accessions is presented. Genetic structure reveals three different K-values (2, 4 and 9) demonstrating a major dichotomy between Prunus insititia-related accessions and cultivars belonging to Prunus domestica sensu stricto, as well as differentiation among minor subgroups defined by pomological traits and geographical origin.

The genotyping of European fruit tree collections has helped to identify synonyms, determine parentage, reveal key specimens in the collections and provide information on the development of modern cultivars from one or several progenitors. However, studies on European plum Prunus domestica L. accessions have been lagging behind, mainly because of the hexaploid chromosome number. In this co-operative study, 104 accessions conserved by 14 partners across Europe were phenotyped for 20 descriptors, and genotyped for 8 SSR loci together with 8 reference cultivars. Based on the descriptors and additional information supplied by the partners, as well as the scientific and horticultural literature, each accession was assigned to one of six pomological groups; (1) egg plums sensu lato (E), (2) prunes of the French d’Agen type (P/A), (3) prunes of the Central-Southeast European Zwetschen type (P/Z), (4) greengages (G), (5) mirabelles (M) and (6) bullaces, damsons and var. pomariorum (D/B). A MANOVA conducted on descriptor data revealed significant differentiation among the pomological groups as well as a geographic impact on the differentiation of local plum accessions in Europe. SSR data showed that two trios and seven pairs of genotypes had very similar allele profiles and possibly are genetically identical in spite of different accession names. An AMOVA indicated sparse genetic differentiation when accessions were grouped according to geographic origin whereas significant differences were obtained among pomological groups. A Bayesian analysis of genetic structure, as well as a discriminant analysis of principal components (DAPC), further revealed levels of similarity among and within the different pomological groups, suggesting that egg plums sensu lato (E) and greengages (G) can be referred to subsp. domestica while damsons and bullaces (D/B) but also Central-Southeast European prunes (P/Z) show more affinity to subsp. insititia. The small and possibly heterogeneous groups with mirabelles (M) and prunes of the d’Agen type (P/A) take an intermediate position suggesting a hybridogenic origin.

‘Ingeborg’ is currently the main commercial pear cultivar grown in Norway. However, fruit set and subsequent yields of this cultivar have proven to be variable and overall low averaging 10–20 t·ha−1. Pear seeds found in ‘Ingeborg’ fruits are often underdeveloped, suggesting that incomplete fertilization might be a major cause of poor fruit set. In some years, sporadically unfavorable environmental conditions during and immediately after pollination in Hardanger district, western Norway, have resulted in poor fruit set of ‘Ingeborg’. In this study, the pollinizer efficacy of several pollinizers, namely ‘Clara Frijs’, ‘Herzogin Elsa’, ‘Anna’, ‘Colorée de Juillet’, and ‘Belle lucrative’, from several orchards located in the Hardanger district was investigated using 12 microsatellite markers for two growing seasons (2014 and 2016). Pollinizer efficacy was estimated by genotyping ‘Ingeborg’, each individual pollinizer, as well as normally developed seeds from ‘Ingeborg’ fruit, and conducting gene assignment analyses to identify the pollen contribution from each of the pollinizer cultivars. In addition, S-allele genotyping was conducted, and only one pollinizer, ‘Anna’, was identified as being semicompatible with ‘Ingeborg’, whereas all other pollinizers were fully compatible. ‘Clara Frijs’ and ‘Belle lucrative’ were identified as the most efficient pollinizers probably because these cultivars were abundant compared with all other pollinizers within all, but one of the examined orchards. Higher yields could not be attributed to a particular pollinizer, and genetic effects associated with the triploid nature of ‘Ingeborg’ are most likely implicated as a cause behind the low and variable yield of this cultivar.

a b s t r a c t Different approaches for germplasm conservation can have considerable impact on the amount of genetic diversity captured. Nine SSR loci were used to compare genetic diversity and genetic structure of tradi- tional Nordic plum accessions maintained using two different approaches; ex situ (germplasm collection, Balsgard in Sweden), and on-farm (Southern Norway). In addition, eight international reference cultivars were included. A total of 204 distinct SSR alleles were amplified, revealing high overall diversity. Only one duplicate was found in the ex situ collection while numerous duplicates were found in the on-farm material. There was little differentiation between the three groups of samples, i.e. the Swedish ex situ col- lection, the Norwegian on-farm collection and the international reference cultivars. However, a Bayesian analysis of genetic structure revealed a clear difference in genetic structure between the Swedish ex situ collection and the Norwegian on-farm collection. After classifying the ex situ plum accessions into four different morphological groups/species, based on their pedigree data and phenotype, a more pronounced clustering was noted. The results of the Bayesian analyses thus indicated that 'Krikon' (subsp. insititia) and some possibly related genotypes from Norway form the most divergent cluster. Other morphological groups/species that could be discerned although with some overlap, are greengages (e.g. 'Reine Claude'), common European plum (exemplified with 'Victoria' and 'Stanley') and two Norwegian accessions with unknown affinity. © 2015 Elsevier B.V. All rights reserved.