Ilze Grāvīte, Edīte Kaufmane, Laila Ikase and Edgars Cirša
All growers prefer to obtain a first harvest as soon as possible. The aim of this study was to identify an effective tree training system for new Latvian plum cultivars, which provides the highest yield and fruit quality. The trial was established in 2012 at the Institute of Horticulture (formerly Latvia State Institute of Fruit-Growing) and included cultivars ‘Ance’, ‘Adelyn’, and ‘Sonora’; and the controls ‘Victoria’ and ‘Jubileum’. Planting distances were 4 × 2.5 m, and the rootstock was Prunus cerasifera. Tree training was done using four systems: two systems with branch bending (Heka espalier and spindle); and two systems without bending (standard (round) and flat crown). Evaluation of average fruit weight and yield per trunk cross section area (TCSA) was done from 2015 to 2017. The Heka espalier and spindle systems had first yield in 2015, but systems without bending had first yield only in 2016. Average yield per TCSA was significantly higher for the Heka espalier system (0.49 kg·cm−2) than for the spindle system (0.24 kg·cm−2), standard crown system (0.17 kg·cm−2), and flat crown system (0.30 kg·cm−2). Cultivar ‘Victoria’ had the highest yield among all systems. Average fruit size showed the same results: the biggest fruits were obtained with the Heka espalier system, and the smallest for systems without bending in the standard crown.
Dzintra Dēķena, Jānis Lepsis, Ina Alsiņa, Līga Lepse and Kersti Kahu
The issue of the influence of rootstock on winter-hardiness of plum (Prunus × rossica Erem.) tree flower buds in the Baltic region is becoming important. The choice of rootstock is the main precondition for obtaining a high yielding and sustainable plum orchard. Freezing of flower buds is one of the most significant damages in winter for stone fruits. The aim of the investigation was to determine the relationship between concentration of dry matter and reducing sugars in annual shoots during winter and wintering ability of trees. The dynamics of reducing sugar concentration in one-year-old shoots during winter was investigated during two successive seasons in two locations. Orchards were planted in 2001 in Latvia and in Estonia. The well-known plum cultivar ‘Kubanskaya Kometa’ (Prunus rossica Erem.) was grafted on eight clonal rootstocks (‘St. Julien A’, ‘Brompton’, ‘Ackermann’, ‘Pixy’, GF8/1, G5/22, GF655/2, and ‘Hamyra’) and eight generative propagated rootstocks (‘St. Julien INRA 2’, ‘St. Julien d’Orleans’, ‘St. Julien Noir’, ‘Brompton’, ‘Wangenheims Zwetsche’, ‘St. Julien Wädenswill’, ‘Myrobalan’ and Prunus cerasifera var. divaricate). Shoot samples were harvested two times during winter — at the end of January and at the end of March. Dry matter concentration (mg·g−1) and the concentration of reducing sugars (mg·g−1 DM) by Fehling’s solution method was determined. Tree flowering intensity was scored using a scale from 1 to 5, where 1 = no flowers and 5 = abundant flowering. Dry matter concentration in plum shoots varied among rootstocks, years and growing location. In Pūre, Latvia, the largest differences in dry matter concentration were found for trees grafted on ‘St. Julien INRA2’ (in 2011–2012) and ‘Brompton’ cuttings (in 2012–2013) but in Polli, Estonia for trees grafted on G5/22 (in 2011–2012) and ‘Myrobalan’ (in 2012–2013). One of the most stable rootstock/graft combinations in the trial when GF655/2 was used as rootstock, where dry matter concentration was between 491 and 525 mg·g−1, and reducing sugars between 37.5–49.2 mg·g−1, and flowering intensity between 2.5 and 4.
Ingrida Mazeikiene, Darius Kviklys, Jurate Brone Siksnianiene, Dainius Zinkus and Vidmantas Stanys
Prunus necrotic ring spot ilarvirus (PNRSV) and Apple chlorotic leaf spot trichovirus (ACLSV) are common in plum orchards. The aim of the study was to obtain virus-free planting material of Prunus domestica L. by chemotherapy in vitro. Ribavirin at concentrations of 10 to 50 mg·l−1 was added to Murashige–Skoog (MS) nutrition medium for virus eradication from microshoots. After a two-week period of chemotherapy, meristems were subcultured monthly on MS medium and proliferation index of shoots was estimated. Microshoots were retested by reverse transcription polymerase chain reaction for presence of virus. At lowest concentrations of 10 mg·l−1 ribavirin was entirely ineffective for ACLSV and 10 to 30 mg·l−1 was ineffective for PNRSV elimination. Ribavirin concentrations of 40 and 50 mg·l−1 destroyed both pathogens. However, at higher concentrations of 40 and 50 mg·l−1 ribavirin exhibited some signs of phytotoxicity on microshoots in the first sub-cultivation period. In order to test the genetic stability of the microplants after chemotherapy the amplified fragment length polymorphism (AFLP) method was applied. Plant genome stability in ‘Magna Glauca’ at concentrations of 40 mg·l−1 was damaged, as the presence of polymorphic AFLP markers were observed.
Dzintra Dēķena, Ina Alsiņa, Valda Laugale and Kersti Kahu
The selection of appropriate rootstock is the main precondition for obtaining a high yielding and sustainable plum orchard. In the Northern climate, plum overwintering is especially important, where winter hardiness of flower buds is one of indicators. This investigation was carried out during three wintering periods (2010–2013) at the Institute of Horticulture in Pūre (Latvia) and the Polli Horticultural Research Centre (Estonia), in orchards planted in 2001. The aim of the investigation was to evaluate the influence of different rootstocks on the viability of flower buds during winter for two plum cultivars in two growing regions. European plum ‘Victoria’ and hybrid plum ‘Kubanskaya Kometa’ grafted on eight clonal and eight seedling rootstocks were used in the investigation. Bud samples were taken two times during winter: end of January and end of March. The viability of flower buds and flowering intensity were determined in the laboratory. The viability was determined as dehydrogenase activity using triphenyl tetrazole chloride (0.5%), where in living cells the colourless substance due to enzymatic activity turns into a brightly coloured product — formasan. The optical density of colour was determined with a spectrophotometer at 485 nm. Both cultivars ‘Kubanskaya Kometa’ and ‘Victoria’ had higher flower bud viability in Polli compare to Pūre. In both growing regions, the highest activity of dehydrogenases for cultivar ‘Kubanskaya Kometa’ was on rootstocks ‘Myrobalan’, ‘St. Julien INRA 2’, ‘Wangenheims Zwetsche’ and for cultivar ‘Victoria’ — on rootstocks ‘Ackermann’, ‘Brompton’ seedlings, and ‘St. Julien d’ Orleans’. The dehydrogenase activity of in flower buds had a tendency to decrease during winter.
Juozas Lanauskas, Darius Kviklys, Nobertas Uselis and Loreta Buskienė
Twenty plum cultivars on Myrobalan (Prunus cerasifera Ehrh.) seedling rootstock were tested in 2012–2017 at the Institute of Horticulture of the Lithuanian Research Centre for Agriculture and Forestry. Plum trees were spaced at 4.5 × 2.5 m and trained as spindles. Orchard floor management included frequent mowing of grass in the alleyways with herbicide strips along tree rows. ‘Duke of Edinburgh’, ‘Kijevas Vēlā’, ‘Dąbrowicka Prune’, ‘Čačanska Najbolje’, ‘and ‘Štaro Vengrinė’ cultivars were the most vigorous plum trees at the end of 6th year. Their trunk diameter achieved 92–96 mm. ‘Valor’, ‘Queen Victoria’, ‘Herman’, ‘Čačanska Najbolje’, ‘Favorita del Sultano’, ‘Ave’, and ‘Jubileum’ cultivars had the lowest trunk diameter — 72–78 mm. The highest cumulative yield of four fruiting years was recorded for ‘Kometa’ and ‘Violeta’ cultivars, respectively, 43.0 and 46.5 kg/tree. ‘Favorita del Sultano’, ‘Rausvė’, and ‘Kijevas Vēlā’ produced the largest fruits — 52–60 g. The smallest fruit occurred on ‘Dąbrowicka Prune’ and ‘Herman’ cultivars, respectively, 22 and 25 g. ‘Dąbrowicka Prune’ had the highest soluble solids content (SSC) — 19.5%. The least SSC was recorded in ‘Kometa’, ‘Violeta’ and ‘Herman’ fruits — 10.6–11.8%. ‘Renklod Rannij Doneckij’, ‘Čačanska Najbolje’, and ‘Valor’ cultivars had the highest fruit flesh firmness.
This article reviews the history of plum cultivation and cultivars in Sweden with the aim to describe important heirloom cultivars and to explain how they are conserved in the Swedish National Gene Bank. Commercial plum production in Sweden started around 1890 and was initially in part based on small-fruited local cultivars grown on their own roots. Along with the development of a modern Swedish nursery industry and experimental research the use of grafted trees grew in importance. In the mid-1950s, the yearly plum production in Sweden was estimated to be approximately 2000 tonnes. Since the mid-1980s, production has declined and it is now only about 250 tonnes per year. The work to safeguard heirloom cultivars began with a national inventory in 2005 and since 2012, so-called mandate cultivars have been planted in the Swedish National Gene Bank at Alnarp. Today 45 plum cultivars are preserved with two trees in the gene bank at Alnarp and two trees in local clonal archives.
Madalina Butac, Mihai Botu, Madalina Militaru, Craisor Mazilu, Ion Dutu and Silvia Nicolae
In Romania, work on identification, conservation and evaluation of fruit genetic resources activities was initiated in 1970 in order to limit the loss of biodiversity. There are rich sources of germplasm located in two research centres: RIFG Pitesti with 642 accessions and UCv-SCDP Vâlcea with 361 accessions, representing wild species, local populations, named cultivars, breeder’s selections and rootstocks. Observations were made according to the IBPGR Prunus descriptors updated by the ECP/GR Prunus Working Group. The following genetic resources from the Piteşti and Vâlcea collections were used in a breeding programme in the development of several plum cultivars: ‘Grase de Becs’, ‘Carpatin’, ‘Ialomiža’, ‘Kirke’, ‘Wilhelmina Späth’ (for resistance / tolerance to Plum pox virus); ‘Vinete romāneşti’, ‘Tuleu timpuriu’, ‘Anna Späth’ (for late blooming), ‘Tuleu gras’, ‘Vâlcean’ (for fruit quality), ‘Stanley’, ‘Pescăruş’, ‘Centenar’ (for productivity), and ‘Diana’ (for self-fertility). The plum rootstock breeding programme used the following genotypes as sources of genes: ‘Rosior văratec’, ‘Brompton’, ‘Renclod Verde’, ‘Pixy’, ‘Saint Julien A’, ‘Albe mici’, ‘Scolduş’, ‘Porumbar’, etc. Breeding using the germplasm in these collections resulted in the release of 40 cultivars and 11 generative and vegetative rootstocks.
Plums have been commercially grown in Latvia since the 19th century. Plantations expanded especially in the 1920s–1930s. At that time, many cultivars were introduced, mostly from Western Europe. After the severe winters of 1939/40, 1941/42, 1955/56, and 1978/79, the plum orchard area significantly decreased because of a lack of winter-hardy cultivars. For this reason, cultivars from Russia, Belarus, and Estonia were introduced. Among the old landraces, highly winter-hardy ‘Latvijas Dzeltenā Olplūme’ is still important. Previously it used to be productive and had good fruit quality. Unfortunately, nurseries often propagated the hardiest clones, which lacked productivity. As Latvian consumers love yellow plums, at present breeding and propagation of productive clones from old orchards is ongoing. In the mid-20th century, active plum breeding started in Latvia, crossing hardy genotypes with high quality donors. From this period ‘Lāse’ and ‘Minjona’ are still grown. Since the 1980s, plum breeding has been undedrtaken at the Institute of Horticulture. The crossing work in plums concentrated on the hexaploid group, aiming to combine large fruit size with good flavour, different time of ripening (in particular, early to medium late) and and good winter-hardiness. New cultivars include ‘Sonora’, ‘Ance’, and ‘Adelyn’, which have good quality, and productivity 20–30 kg per tree, and in some years up to 70 kg per tree.
The most widely used rootstocks are seedlings of Prunus cerasifera Ehrh. ssp. divaricata C.K. Schneid., which is more hardy that the common myrobalan plum. These rootstocks have good compatibility with most cultivars except gages, and are adapted to different soil types and are disease tolerant. Their drawback is a long growth season which reduces winter-hardiness of grafted cultivars. Also, they tend to form suckers around the stem. A hardy seedling PU-20651 (P. salicina ssp. ussuriensis × P. cerasifera) was bred at the Institute, which was shown to slightly reduce tree vigour. Testing of several Western European and Russian rootstocks did not result in their introduction into production. Trials are presently being carried out with size-reducing root-stocks originated from cultivar Wangenheim VVA-1, Weiwa, S766, and M633.
Commercial production of plums in Latvia is relatively small, as the area of orchards is the smallest among fruit trees. Plums are grown mostly for fresh consumption, with a small part for jams, yoghurt, and ice-cream additives. Fruits are sold mostly in small shops, markets and at farms. The main reason is the climate, which allows growing of a limited range of cultivars that tolerate the –30 °C winter temperatures in some years (about every five years). About 20 cultivars are grown commercially; the share of the six most popular plum cultivars is 69% of the total plum production. In recent years, plantations of new cultivars developed at our Institute have expanded, especially regarding the early ripening cultivar ‘Ance’.
The main objective of this work was to verify the possibility of shortening the time needed to produce nursery trees of plums, in view of the economic profitability in nursery production. To achieve this goal, rootstocks Citation®, Ishtara®, Penta®, Torinel®, Pumiselekt and St. Julien (from Wädenswil) were chip budded in the springtime with ‘Shiro’, ‘Angeleno’®, ‘Black Amber’, SLE2014/1, ‘Fertility’ (Asian type plums) and ‘Stanley’ (European type plum) as a control. At the end of the vegetation period, height (from the grafting position), trunk diameter above the place of budding and the number of shoots was recorded for each tree. The Penta® rootstock was evaluated as the most efficient, while ‘Shiro’ variety was evaluated as the most suitable variety. The best variety/rootstock combination was the combination of ‘Shiro’ on Ishtara® rootstock, where 100% of the budded trees reached an average height of 33.0 ± 3.6 cm. The highest trees of an average of 68.3 ± 4.6 cm were recorded for the combination of SLE2014/1 on Torinel® root-stock.
Sharka (PPV) is one of the most dangerous viruses in fruit growing. More and more fruit growers are unsatisfied with the resistances or tolerance of plum varieties cultivated today. With the utilisation of the hypersensitive reaction to PPV in breeding we developed ‘Jojo’ as the first absolutely resistant cultivar in the field. Most interesting now is the development of new sharka-resistant cultivars, especially in three points: extension of ripening range, better fruit size, and high fruit quality. In a new breeding programme, we obtained many hypersensitive clones. After testing in different regions, some interesting resistant clones and cultivars were selected and are presented here.