Japanese Plum Cultivars Research Articles

Chemical Characterization of Pruning Wood Extracts from Six Japanese Plum (Prunus salicina Lindl.) Cultivars and Their Antitumor Activity.

The Japanese plum tree (Prunus salicina Lindl.) is mainly cultivated in temperate areas of China and some European countries. Certain amounts of wood (from pruning works) are generated every year from this crop of worldwide commercial significance. The main objective of this work was to value this agricultural woody residue, for which the chemical composition of pruning wood extracts from six Japanese plum cultivars was investigated, and the antiproliferative activity of extracts and pure phenolics present in those extracts was measured. For the chemical characterization, total phenolic content and DPPH radical-scavenging assays and HPLC‒DAD/ESI‒MS analyses were performed, with the procyanidin (-)-ent-epicatechin-(2α→O→7,4α→8)-epicatechin (5) and the propelargonidin (+)-epiafzelechin-(2β→O→7,4β→8)-epicatechin (7) being the major components of the wood extracts. Some quantitative differences were found among plum cultivars, and the content of proanthocyanidins ranged from 1.50 (cv. 'Fortune') to 4.44 (cv. 'Showtime') mg/g of dry wood. Regarding the antitumoral activity, eight wood extracts and four phenolic compounds were evaluated in MCF-7 cells after 48 h of induction, showing the wood extract from cv. 'Songold' and (‒)-annphenone (3), the best antiproliferative activity (IC50: 424 μg/mL and 405 μg/mL, respectively).

Machine Learning-based Differentiation of Japanese Plum Cultivars Based on Metabolite Profiling

This study investigates the metabolite diversity of twelve Japanese plum cultivars grown in South Africa to understand their differential organoleptic characteristics and nutritional properties. The goal is to differentiate or associate these plum cultivars based on their metabolic profiles. Metabolite profiling was conducted using gas chromatography-mass spectrometry (GC-MS) at different postharvest ripening stages. Different unsupervised machine learning algorithms were applied: hierarchical clustering, K-means clustering, Density-Based Spatial Applications with Noise, and principal component analysis (PCA). Results revealed that each cultivar contains a unique combination of 13 amino acids, 4 sugars (contributing to organoleptic characteristics), and numerous phenolic compounds and antioxidant activities (contributing to nutritional value). The levels of these compounds are cultivar-dependent and vary with postharvest stages. The number of clusters of plum cultivars varied with both the clustering algorithm and postharvest stages. However, certain cultivars were consistently grouped regardless of the clustering method, indicating similar characteristics and responses to storage and shelf-life conditions. Similar consistent groupings were observed after cold storage and shelf life. Our findings also showed that K-means clustering is the most effective model for plum cultivar differentiation based on the Silhouette Score and Davies-Bouldin Index. This study enhances our understanding of how metabolites evolve over different postharvest stages and provides a robust framework for differentiating plum cultivars, which can aid in sorting and grading operations. The research offers actionable insights to improve postharvest handling and storage practices, which are critical for maintaining the nutritional quality of plums, an important fruit for human health.

Insights into tissue-specific anthocyanin accumulation in Japanese plum (Prunus salicina L.) fruits: A comparative study of three cultivars

In the present study, three matured Japanese plum cultivars with different colored peel and flesh were selected to mine the key transcription factors regulating anthocyanin formation in tissues. Results showed that PsMYB10 was correlated with structural genes C4H, F3H, and ANS. PsMYB6 could positively regulate C4H (r = 0.732) and accumulated anthocyanins in Sanhua plum’s flesh. Sanhua plum has the highest phenolic and anthocyanin contents (10.24 ± 0.37 gallic acid equivalent mg g−1 dry weight (DW) and 68.95 ± 1.03 μg g−1 DW), resulting itself superior biological activity as 367.1 ± 42.9 Trolox equivalent mg g−1 DW in oxygen radical absorbance capacity value and 72.79 ± 4.34 quercetin equivalent mg g−1 DW in cellular antioxidant activity value. The present work provides new insights into the regulatory mechanism of tissue-specific anthocyanin biosynthesis, confirming the pivotal role of anthocyanins in the biological activity of plums, providing essential support for the development of horticultural products enriched with anthocyanins.

S-Locus Genotyping in Japanese Plum by High Throughput Sequencing Using a Synthetic S-Loci Reference Sequence.

Self-incompatibility in Prunus species is governed by a single locus consisting of two highly multi-allelic and tightly linked genes, one coding for an F-box protein-i.e., SFB in Prunus- controlling the pollen specificity and one coding for an S-RNase gene controlling the pistil specificity. Genotyping the allelic combination in a fruit tree species is an essential procedure both for cross-based breeding and for establishing pollination requirements. Gel-based PCR techniques using primer pairs designed from conserved regions and spanning polymorphic intronic regions are traditionally used for this task. However, with the great advance of massive sequencing techniques and the lowering of sequencing costs, new genotyping-by-sequencing procedures are emerging. The alignment of resequenced individuals to reference genomes, commonly used for polymorphism detection, yields little or no coverage in the S-locus region due to high polymorphism between different alleles within the same species, and cannot be used for this purpose. Using the available sequences of Japanese plum S-loci concatenated in a rosary-like structure as synthetic reference sequence, we describe a procedure to accurately genotype resequenced individuals that allowed the analysis of the S-genotype in 88 Japanese plum cultivars, 74 of them are reported for the first time. In addition to unraveling two new S-alleles from published reference genomes, we identified at least two S-alleles in 74 cultivars. According to their S-allele composition, they were assigned to 22 incompatibility groups, including nine new incompatibility groups reported here for the first time (XXVII-XXXV).

Potential of Myrobalan Plum as a New Pollinizer for Japanese Plum Cultivars

Potential of Myrobalan Plum as a New Pollinizer for Japanese Plum Cultivars

Sugar composition, total phenolic content, and radical scavenging activity of 12 Japanese plum cultivars grown in South Africa: from harvest to shelf life

ISHS V International Symposium on Pomegranate and Minor Mediterranean Fruits Sugar composition, total phenolic content, and radical scavenging activity of 12 Japanese plum cultivars grown in South Africa: from harvest to shelf life

Whole Transcriptome Analyses of Apricots and Japanese Plum Fruits after 1-MCP (Ethylene-Inhibitor) and Ethrel (Ethylene-Precursor) Treatments Reveal New Insights into the Physiology of the Ripening Process.

The physiology of Prunus fruit ripening is a complex and not completely understood process. To improve this knowledge, postharvest behavior during the shelf-life period at the transcriptomic level has been studied using high-throughput sequencing analysis (RNA-Seq). Monitoring of fruits has been analyzed after different ethylene regulator treatments, including 1-MCP (ethylene-inhibitor) and Ethrel (ethylene-precursor) in two contrasting selected apricot (Prunus armeniaca L.) and Japanese plum (P. salicina L.) cultivars, ‘Goldrich’ and ‘Santa Rosa’. KEEG and protein–protein interaction network analysis unveiled that the most significant metabolic pathways involved in the ripening process were photosynthesis and plant hormone signal transduction. In addition, previously discovered genes linked to fruit ripening, such as pectinesterase or auxin-responsive protein, have been confirmed as the main genes involved in this process. Genes encoding pectinesterase in the pentose and glucuronate interconversions pathway were the most overexpressed in both species, being upregulated by Ethrel. On the other hand, auxin-responsive protein IAA and aquaporin PIP were both upregulated by 1-MCP in ‘Goldrich’ and ‘Santa Rosa’, respectively. Results also showed the upregulation of chitinase and glutaredoxin 3 after Ethrel treatment in ‘Goldrich’ and ‘Santa Rosa’, respectively, while photosystem I subunit V psaG (photosynthesis) was upregulated after 1-MCP in both species. Furthermore, the overexpression of genes encoding GDP-L-galactose and ferredoxin in the ascorbate and aldarate metabolism and photosynthesis pathways caused by 1-MCP favored antioxidant activity and therefore slowed down the fruit senescence process.

First Report of Fruit Rot of Sweet Cultivars of Japanese Plum caused by Alternaria alternata, A. arborescens, and A. tenuissima in Chile.

During the last two seasons, an unusual fruit rot was observed in four orchards of sweet Japanese plum (Prunus salicina) cultivars located in the Chilean Central Valley (30°00'S, 70°42'W). The incidence was 5% in Black Majesty, 4% in Red Lyon, and 6% in Sweet Mary cultivars in 2020. Fruits in the field showed a firm, dehydrated, and slightly sunken rot on the blossom end, along with rough and irregular epidermis in the affected area. Internally, the fruit flesh appeared light to dark-brown or olive-green. Symptomatic fruits (n=119) were superficially disinfected (75% ethanol) and, pieces of the pericarp (3 x 3 mm) were removed and placed on potato dextrose agar (PDA). Isolates of Alternaria spp. were obtained and 9 of these were selected for identification. Colonies were dark olive to gray-brown with white margins, small, catenulate and muriform conidia, produced in single or branched conidiophores. Isolates produced brown to golden-brown, ovoid, ellipsoidal to obclavate conidia with dimensions of 19.7 to 26.7 × 10.0 to 11.9 μm with two to four transverse and zero to three longitudinal septa on 0.05× PDA (Pryor and Michailides 2002) after 7 d at 20°C under 10/14 h light/dark cycles. A molecular analysis was performed by sequencing the nuclear genes RNA polymerase II subunit (RPB2), plasma membrane ATPase (ATP), and the calmodulin (Cal) gene using primers RPB2-5F2/fRPB2-7cR, ATPDF1/ATPDR1, and CALDF1/CALDR1, respectively (Lawrence et al. 2013; Woudenberg et al. 2013). A BLAST search revealed the presence of Alternaria spp. with a 99% to 100% identity with the reference sequences of A. alternata (JQ905182, JQ671874, JQ646208), A. arborescens (JQ646487, JQ671880, JQ646214), and A. tenuissima (JQ811961, JQ811989, JQ646209). Maximum parsimony phylogenetic analysis confirmed the identifications. Sequences were deposited in GenBank as numbers MW514249 to MW514257, MT872324 to MT872332, and MT872314 to MT872322 for RPB2, ATP, and Cal sequences, respectively. All these Alternaria isolates were deposited in the Colección Chilena de Recursos Genéticos Microbianos - INIA, Chillán Chile (RGM3069 to RGM3077). Pathogenicity of A. alternata (n=4), A. arborescens (n=3) and, A. tenuissima (n=2) was tested in Red Lyon plum fruits. Plums were disinfected in 1% sodium hypochlorite for 2 min, rinsed in sterile distilled water for 1 min and dried on absorbent towels in a laminar flow hood. Then, the plums were wounded on the blossom end with a sterile needle (1 x 0.5 mm), inoculated with 10 µl of a conidial suspension (106 conidia/ml), wrapped with Parafilm and maintained in a humid chamber (>95% relative humidity). An equal number of fruits wounded and inoculated with sterile water were used as a control. After 7 days at 20°C, all inoculated fruits developed a dark-brown firm rot with lesion lengths of 24.4 (±3.0) mm, 19.6 (±0.7) mm, and 16.8 (±2.4) mm for A. alternata, A. arborescens and A. tenuissima, respectively. A. alternata was the most aggressive species (P < 0.001). Control fruits remained asymptomatic. Koch's postulates were fulfilled after the re-isolating the causal agent from the border of the lesions. Leaf spots and fruit rots caused by Alternaria isolates have been reported in stone fruits, including plums (Kim et al. 2005; Long et al. 2021; Moosa et al., 2019; Yang et al. 2014). To our knowledge, this is the first report of A. alternata, A. arborescens, and A. tenuissima associated with fruit rot in sweet Japanese plum cultivars in the field, in Chile.

‘lucía myrtea’ and ‘victoria myrtea’: Two New Japanese Plum Cultivars with Low Chilling Requirements, Early Ripening, and High-quality Fruit for the Fresh Market

"‘lucía myrtea’ and ‘victoria myrtea’: Two New Japanese Plum Cultivars with Low Chilling Requirements, Early Ripening, and High-quality Fruit for the Fresh Market" published on Nov 2021 by American Society for Horticultural Science.

Heat requirements for flowering of European and Japanese plum cultivars in the Belgrade region

Heat requirements for flowering of European and Japanese plum cultivars in the Belgrade region

SSR marker-based paternity tests to determine suitable pollenisers for the Japanese plum cultivar ‘Sweet Pekeetah’

SSR marker-based paternity tests to determine suitable pollenisers for the Japanese plum cultivar ‘Sweet Pekeetah’

New promising Japanese plum cultivars for warm areas from CEBAS-CSIC/IMIDA breeding programme

New promising Japanese plum cultivars for warm areas from CEBAS-CSIC/IMIDA breeding programme

Characterization of Japanese Plum (Prunus salicina) PsMYB10 Alleles Reveals Structural Variation and Polymorphisms Correlating With Fruit Skin Color.

The red to blue hue of plant organs is caused due to anthocyanins, which are water-soluble flavonoid pigments. The accumulation of these pigments is regulated by a complex of R2R3-MYB transcription factors (TFs), basic-helix-loop-helix (bHLH), and WD-repeat (WDR) proteins (MBW complex). In Rosaceae species, R2R3-MYBs, particularly MYB10 genes, are responsible for part of the natural variation in anthocyanin colors. Japanese plum cultivars, which are hybrids of Prunus salicina, have high variability in the color hue and pattern, going from yellow-green to red and purple-blue, probably as a result of the interspecific hybridization origin of the crop. Because of such variability, Japanese plum can be considered as an excellent model to study the color determination in Rosaceae fruit tree species. Here, we cloned and characterized the alleles of the PsMYB10 genes in the linkage group LG3 region where quantitative trait loci (QTLs) for the organ color have been mapped to other Prunus species. Allele segregation in biparental populations as well as in a panel of varieties, combined with the whole-genome sequence of two varieties with contrasting fruit color, allowed the organization of the MYB10 alleles into haplotypes. With the help of this strategy, alleles were assigned to genes and at least three copies of PsMYB10.1 were identified in some varieties. In total, we observed six haplotypes, which were able to characterize 91.36% of the cultivars. In addition, two alleles of PsMYB10.1 were found to be highly associated with anthocyanin and anthocyanin-less skin. Their expression during the fruit development confirms their role in the fruit skin coloration. Here, we provide a highly efficient molecular marker for the early selection of colored or non-colored fruits in Japanese plum breeding programs.

Pomological Characteristics and Ploidy Levels of Japanese Plum (Prunus salicina Lindl.) Cultivars Preserved in Poland.

Research on the resistance to frost, susceptibility to sharka, flowering biology, fruit setting, yield, and ploidy levels of 36 Japanese plum cultivars (mostly hybrids of Prunus salicina with Prunus cerasifera) were carried out in 2015–2020 at the Experimental Orchard located in Dąbrowice near Skierniewice. Relatively mild winters with sporadic temperature drops to nearly −21 °C in January of 2017 and 2018 caused slight damage to several cultivars of Japanese plum insufficiently resistant to frost. The trees of most cultivars remained healthy, with no signs of damage. ‘Barkhatnaya’ and ‘Tatyana’ cultivars turned out to be very susceptible to sharka. ‘Herkules’ trees were the most vigorous. ‘Barkhatnaja’, ‘Blue Gigant’, ‘Shater’, and ‘Tatyana’ trees were characterized by weak growth. The trees of Japanese plum started flowering early, usually in the first or second decade of April. Most of the cultivars belonged to early season cultivars, the fruits of which ripened in July. Based on the assessment of tree productivity, ‘Barkhatnaya’, ‘Inese’, ‘Shater’, ‘Tatyana’, and ‘Vanier’ are the best for growing in the climate of Central Europe. ‘Tsernushka’, ‘Chuk’, ‘Dofi Sandra’, ‘Early Golden’, ‘Ewierch Rannyj’, ‘Yevraziya’, ‘Gek’, ‘General’, ‘Kometa’, ‘Kometa Late’, ‘Maschenka’, and ‘Naidyona’ trees also yielded well. ‘Blue Gigant’, ‘Black Amber’, and ‘Herkules’ had the largest fruits, and ‘Chuk’ and ‘Inese’ cultivars produced the smallest fruits. Among the assessed Japanese plum cultivars, those with round fruit, dark skin with various shades of purple, yellow flesh, and A cytometric analysis showed that almost all cultivars are diploid, except for ‘Herkules’ (possibly pentaploid) and ‘Yevraziya’ (possibly hexaploid or aneuploid).

Physiological and transcriptomic changes derived of the application of ethylene biosynthesis regulators 1-MCP® and Ethrel® in different apricot (Prunus armeniaca L.) and Japanese plum (Prunus salicina L.) cultivars

Physiological and transcriptomic changes derived of the application of ethylene biosynthesis regulators 1-MCP^® and Ethrel^® in different apricot (<i>Prunus armeniaca</i> L.) and Japanese plum (<i>Prunus salicina</i> L.) cultivars

First report of Cherry virus F infecting Japanese plum in Korea.

Cherry virus F (CVF) is a tentative member of the genus Fabavirus in the family Secoviridae, consisting of two RNA segments (Koloniuk et al. 2018). To date, CVF has been documented in only sweet cherry (Prunus avium) in the Czech Republic (Koloniuk et al. 2018), Canada, and Greece. In May 2014, we collected leaf samples from four symptomatic (leaf spots and dapple fruits) and two asymptomatic Japanese plum cultivars (Sun and Gadam) grown in an orchard in Hoengseong, South Korea, to identify viruses and viroids infecting plum trees. Total RNA from individual plum trees was extracted using two commercial kits: Fruit-mate for RNA Purification Kit (Takara, Shiga, Japan) and RNeasy Plant Mini Kit (Qiagen, Hilden, Germany). We generated six mRNA libraries from the six different plum cultivars for RNA-sequencing using the TruSeq RNA Library Preparation Kit v2 (Illumina, CA, U.S.A.) as described previously (Jo et al. 2017). The mRNA libraries were paired-end (2 X 100 bp) sequenced with a HiSeq 2000 system (Macrogen, Seoul, Korea). The raw sequence reads were de novo assembled by Trinity program v. 2.8.6, with default parameters (Haas et al. 2013). The assembled contigs were subjected to BLASTX search against the non-redundant protein database in NCBI. Of the two asymptomatic cultivars, the transcriptome of asymptomatic plum cv. Gadam contained five contigs specific to CVF. Two and three contigs were specific to CVF RNA1 (2,571 reads, coverage 42.15%) and RNA2 (2,025 reads, coverage 53.04%), respectively. The size of these five contigs ranged from 241 to 5,986 bp. Contigs of 5,986 and 3,867 bp in length, referred to as CVF isolate Gadam RNA1 (GenBank MN896996) and RNA2 (GenBank MN896995), respectively, were subjected to BLASTP search against NCBI's non-redundant protein database. The results showed that the polyprotein sequences of RNA1 and RNA2 shared 95.3% and 93.11% amino acid identities with isolates SwC-H_1a from the Czech Republic (GenBank acc. no. AWB36326) and Stac-3B_c8 from Canada (AZZ10055), respectively. To confirm the infection of CVF in cv. Gadam, RT-PCR was conducted using CVF RNA1-specific primers designed based on the CVF reference genome sequences (MH998210 and MH998216), including 5'-CCACCAAATAGGCAAGAGGTCAC-3' (position 3190-3212) and 5'-CACAATCACCATCAATGGTCTCTGC-3' (position 3742-3766), and CVF RNA2-specific primers, including 5'-CTGCTTTATGATGCTAGACATCAAGATG-3' (position 1015-1042) and 5'-ACAATAGGCATGCTCATCTCAACCTC-3' (position 1594-1619). We amplified 577-bp RNA1-specific and 605-bp RNA2-specific amplicons that were cloned and then performed Sanger sequencing. Sequencing of the cloned amplicons for isolate Gadam RNA1 (GenBank MN896993) and RNA2 (GenBank MN896994) revealed values of 99.48% and 99.17% nucleotide identity to that of RNA1 and RNA2 determined by high-throughput sequencing, respectively. Additionally, we tested five plants for each of the six plum cultivars grown in the same orchard. The detection of CVF was carried out through PCR using the primers and protocol described above. Of the 30 trees, CVF was detected in three trees of cv. Gadam by both primer pairs. To our knowledge, this is the first report of CVF infecting Japanese plum and the first report of the virus in Korea. However, its prevalence in other Prunus species, including apricot, European plum, and peach, should be further elucidated.

Evaluation of Soil Water Content Measurements with Capacitance Probes to Support Irrigation Scheduling in a “Red Beaut” Japanese Plum Orchard

Advances in electromagnetic sensor technologies in recent years have made automated irrigation scheduling a reality through the use of state-of-the-art soil moisture sensing devices. However, correct sensor positioning and interpretation of the measurements are key to the successful implementation of these management systems. The aim of this study is to establish guidelines for soil moisture sensor placement to support irrigation scheduling, taking into account the physiological response of the plant. The experimental work was carried out in Vegas Bajas del Guadiana (Extremadura, Spain) on a drip-irrigated experimental orchard of the early-maturing Japanese plum cultivar “Red Beaut”. Two irrigation treatments were established: control and drying. The control treatment was scheduled to cover crop water needs. In the drying treatment, the fruit trees were irrigated as in control, except in certain periods (preharvest and postharvest) in which irrigation was suspended (drying cycles). Over 3 years (2015–2017), a series of plant parameters were analyzed in relation to the measurements provided by a battery of frequency domain reflectometry probes installed in different positions with respect to tree and dripper: midday stem water potential (Ψstem), sap flow, leaf stomatal conductance, net leaf photosynthesis and daily fraction of intercepted photosynthetically active radiation. After making a comparison of these measurements as indicators of plant water status, Ψstem was found to be the physiological parameter that detected water stress earliest. The drying cycles were very useful to select the probe positions that provided the best information for irrigation management and to establish a threshold in the different phases of the crop below which detrimental effects could be caused to the crop. With respect to the probes located closest to the drippers, a drop in the relative soil water content (RSWC) below 0.2 would not be advisable for “non-stress” scheduling in the preharvest period. When no deficit irrigation strategies are applied in the postharvest period, the criteria are similar to those of preharvest. However, the probes located between the dripper at 0.15 and 0.30 m depth provide information on moderate water stress if the RSWC values falls below 0.2. The severe tree water stress was detected below 0.1 RSWC in probes located at 60 cm depth from this same position.

Establishing Pollination Requirements in Japanese Plum by Phenological Monitoring, Hand Pollinations, Fluorescence Microscopy and Molecular Genotyping.

The Japanese plum cultivars commonly grown are interspecific hybrids derived from crosses between the original Prunus salicina with other Prunus species. Most hybrids exhibit gametophytic self-incompatibility, which is controlled by a single and highly polymorphic S-locus that contains multiple alleles. Most cultivated hybrids are self-incompatible and need pollen from a compatible donor to fertilize their flowers. Establishing pollination requirements in Japanese plum is becoming increasingly important due to the high number of new cultivars with unknown pollination requirements. In this work, a methodology for the determination of pollination requirements in Japanese plum-type hybrids is described. Self-(in)compatibility is determined by hand-pollinations in both the field and in the laboratory, followed by monitoring pollen tube elongation with fluorescence microscopy, and also monitoring fruit maturation in the field. Selection of pollinizer cultivars is assessed by combining the identification of S-genotypes by PCR analysis with the monitoring of flowering time in the field. Knowing the pollination requirements of cultivars facilitates the selection of cultivars for the design of new orchards and allows the early detection of productivity problems related with pollination deficiency in established orchards.

First Report of Fire Blight of Apricot (Prunus armeniaca) Caused by Erwinia amylovora in Korea.

During a survey in May 2020, symptoms of blight were observed on apricot (Prunus armeniaca cv. undetermined) in an orchard (37°06´01.5″N 127°57´44.9″E) in Chungju, South Korea, where fire blight of apple occurred. Three apricot trees in the apple orchard were heavily diseased and showed severe shoot blight and stem canker symptoms. Bacterial isolates were recovered on King's medium B from leaves and twigs that were surface-sterilized with 70% alcohol. Colonies with uniform mucoid, smooth surfaces were collected. DNA from nine isolates did not yield an amplicon in a PCR assay for detection of Erwinia pyrifoliae using primer set CPS1/CPS2c (Kim et al. 2001). Each isolate was positive in PCR assays for E. amylovora using primer sets A/B (Bereswill et al. 1992) and AJ75/76 (Llop et al. 2000) that target pEA29. Sequencing of the PCR products resulted in 99.9% (929 bp out of 930 bp) and 100% (747 bp out of 747 bp) identity with sequences of E. amylovora FB20 (GeneBank: CP050240), respectively. Amplifications of the partial 16S rRNA (GeneBank: LC557153) and hrpN (GeneBank: LC575997) genes were performed, and the products were sequenced. The primers used to amplify 16S rRNA were 518F: 5'-CCAGCAGCCGCGGTAATACG-3' and 800R: 5'-TACCAGGGTATCTAATCC-3', and those for the hrpN genes were HRPN1: 5'-ATGAGTCTGAATACAAG-3' and HRPN3c: 5'-GCTTGCCAAGTGCCATA-3'. BLAST analyses showed 99.8% (1439 bp out of 1442 bp) and 100% (1136 bp out of 1136 bp) identities, respectively, to the sequences of E. amylovora FB20. The ability of the isolates to induce a hypersensitive reaction on tobacco (Nicotiana tabacum cv. Xanthi) leaves was also evaluated. Bacterial suspensions (1.5 ⅹ 108 CFU) of 2 isolates were injected into tobacco leaves, and after 48 h, both isolates caused a hypersensitive response. To confirm pathogenicity of isolates, 3-mm-deep holes in five immature apricot (cv. Goldcot) and five immature apple (cv. Fuji) fruits were inoculated with 10 µl bacterial suspension (1.5 ⅹ 108 CFU/ml). The inoculated fruits were placed in a humid plastic box. After 7 days at 27℃, severe necrosis and bacterial ooze were present at the inoculated sites in three repeated tests. No symptoms were observed on fruits inoculated with sterile water. To complete Koch's postulates, bacteria were reisolated from the inoculated apricot and apple fruits. PCR using the specific primer sets stated above confirmed the identity as E. amylovora. Thus, based on disease symptoms, sequences, and pathogenicity, the bacterium causing blight of apricot was identified as E. amylovora. Natural infections of E. amylovora on apricot trees have been reported in the Czech Republic and Hungary (Korba and Sillerova 2011; Vegh and Palkovics 2013). Fire blight was observed in the Czech Republic on apricot trees near pear seedlings, which are highly susceptible to E. amylovora (Korba and Sillerova 2011). Natural infections of E. amylovora on Japanese plum planted adjacent to an apple orchard with severe fire blight has been reported in the United States (Mohan and Thomson 1996). Moreover, susceptibility to fire blight has been reported for apricot and Japanese plum cultivars (Mohan and Bijman 1999). To our knowledge, this the first report of fire blight of apricot caused by E. amylovora in Korea. This report is important because it provides evidence that apricot may be an overlooked reservoir for E. amylovora, in addition to apple, pear, and other rosaceous plants, in Korea. An intensive survey for additional host plants for the fire blight pathogen will be continued in Korea. This work was supported by a grant from the Agenda program (PJ01530202) of Rural Development Administration, Republic of Korea.

Influence of Prunus rootstocks and spacing on performance of Japanese plum grown under sub–tropical conditions

In India as well as in other significant regions of the world, plum production is focussed on a very small pool of varieties that inhibits its expansion and limits the availability of fruits all year round. This situation emphasizes on the urgent requirement of evolving alternative cultivars of rootstock and scion, with better performance under local situation and contrasting spacing. Japanese plum cultivar ‘Satluj Purple’ (Fla 1–2) is known for its hardy nature, precocity, high productivity and regularity in bearing appropriate for both the juice manufacturing industry and fresh fruit marketplace. In spite of having good fruit quality under sub–tropical scenario and early market availability when other fruits are not accessible, only a few rootstock studies for ‘Satluj Purple’ have been executed in India. Consequently, a field study was conducted at Punjab Agricultural University, Ludhiana state, to assess the horticultural output of ‘Satluj Purple’ Japanese plum grafted on four rootstocks: ‘Kala Amritsari’ (Prunus salicina L.), ‘Kabul Green Gage’ (Prunus salicina L.), Sharbati (Prunus persica Batsch) and Flordaguard (Prunus persica Batsch) over two consecutive seasons. The experimental grove was planted in 2015 using a 6 × 6 m and 6 × 1.5 m spacing, in a randomized block design, with three replications and two trees per replication under climate conditions typical of a semi–arid and sub–tropical area. Fruiting, yield and fruit quality were assessed for each rootstock at different spacing. Reproductive growth (fruit set, fruit retention and fruit yield per tree) was found to be higher on ‘Kala Amritsari’ rootstock. Fruits harvested from trees grafted on ‘Kala Amritsari’ were superior in fruit quality (fruit weight, fruit colour, total soluble solids (TSS), titratable acidity (TA) and ripening index (TSS/TA). Irrespective of the rootstocks, the performance of plum trees in terms of fruiting, yield and fruit quality was better in 6 × 6 m planted trees as compared to trees planted at 6 × 1.5 m distances. Yield was found to be positively correlated with fruit weight, chroma, TSS, ripening index, fruit set and retention while it was negatively correlated with the TA values. Overall, the results from this study indicated that the rootstocks ‘Kala Amritsari’ and ‘Kabul Green Gage’ are the most suitable rootstocks for ‘Satluj Purple’ under the conditions investigated herein. Finally, the ‘Satluj Purple’ trees at spacing of 6 × 6 m performed better as compared to 6 × 1.5 m planted trees regardless of the rootstocks.