Plums - Prunus domestica, Prunus salicina

TAXONOMY

    Plums are placed within the Prunoideae subfamily of the Rosaceae, which contains all of the stone fruits such as peach, cherry, and apricot. The subgenus Prunophora contains plums and apricots. Hybrids between plums and apricots have been produced recently which are said to be finer fruits than either parent. A "Plumcot" is 50% plum, 50% apricot; an "Aprium" is 75% apricot, 25% plum; and the most popular hybrid, the "Pluot" is 75% plum, 25% apricot.

Clockwise from top: Japanese plum, apricot, and their hybrid the pluot.

Plum species:

    1. European plums - Prunus domestica L. Worldwide, this is one of the main species grown. Fruit are generally oval, smaller, and more variable in color than Japanese plums. In the USA, P. domestica is used for prunes or fruit cocktail or other products, and rarely eaten fresh.

    2. Japanese plums - P. salicina Lindl. and hybrids. These are the most common fresh eating plums in the USA. They are larger, rounder (or heart shaped), and firmer than European plums and are primarily grown for fresh market.

    3. Damsons, Bullace plums, St. Juliens, and Mirabelles - P. insititia L. These are the small, wild plums native to Europe, cultivated their prior to the introduction  of P. domestica. The 'St. Julien' types are used as dwarf rootstocks for plums. Fruit are small and oval (1 inch), purple and clingstone for Damsons and yellow and freestone for Mirabelles, with heavy bloom. They are used primarily for jams/jellies/preserves.

jap flesh colors mirabelle
28.2 right Plums are particularly diverse in terms of size, color, and flavor. At top left, Japanese plum hybrids. Top right, Mirabelle plums (P. insititia). Lower left,  ‘Stanley', a major prune cultivar in the USA.

European Plum Cultivars
    These are placed into 4 groups, based mostly on fruit color and/or size, and use (processed or fresh).

    1. Reine Claude or Greengage - Round, green or golden plums used for canning and fresh market. 'Reine Claude', 'Imperial Gage', 'Hand'.
    2. Yellow egg - Large, yellow, oval plums primarily used for canning. 'Yellow egg', 'Golden drop'.
    3. Lombard - Large, oval, red or pink plums, used for fresh market in western Europe. 'Victoria', 'Lombard', 'Pond'.
    4. Prunes - Oval, dark blue or purple, freestone cultivars, dried postharvest. 'French' (syn. Agen), 'Stanley', 'Italian' (syn. 'Fellenberg'), ‘Blufre', and ‘President'.

Japanese Plum Cultivars
  'Santa Rosa', ‘Burbank', ‘Shiro', ‘Beauty', ‘Gold', ‘Methley', ‘Red Beaut', and ‘Ozark Premier' are grown in several countries. In addition, ‘Friar' and ‘Simka' are popular in the USA. Like European plums, many flesh and skin colors occur in Japanese plum cultivars.

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ORIGIN, HISTORY OF CULTIVATION

P. domestica
    This species is native to western Asia, in the Caucasus mountains adjacent to the Caspian Sea. It was brought to North America by Spanish Missionaries (west coast) and English Colonists (east coast). Today, most production is in western states, with the vast majority in California's central valley, where climate disfavors disease and rain cracking.

P. salicina
    Contrary to the name, this species originated in China, where it was cultivated for thousands of years. It was brought to Japan 200-400 years ago, where it then spread around the world, being falsely called "Japanese plum". In the USA, it is grown primarily in California, and is the major fresh market plum seen in grocery stores.

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PRODUCTION

World (2004 FAO) - 9,836,859 MT or 21.6 billion pounds. Plums are produced commercially in 81 countries on 6.4 million acres. Yields average 3640 lbs/acre worldwide.
        
Top 10 countries
(% of world production)
1. China (45) 6. France (2)
2. USA (7) 7. Chile (2)
3. Serbia (6) 8. Turkey (2)
4. Romania (6) 9. Spain (2)
5. Germany (5) 10. Italy (2)

United States (2004 USDA) - 327,900 MT or 656 million pounds. Unusually low; production is generally double this amount. The industry value was $153 million in 2002, on the low side of the range of $200-320 million over the last decade. Most commercial plums are produced in only 5 states (CA, OR, WA, MI, ID) with California producing 95-98% of all plums in the USA.

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BOTANICAL DESCRIPTION   

Plant
    Small to medium sized trees, similar to but more erect growing than peach. European plums are larger and more erect than Japanese plums. Leaves ovate or elliptic with acute or obtuse tips, short petioles, crenulate margins. Japanese plum trees have rougher bark, more persistent spurs, and more numerous flowers than European plums. They are also more precocious, disease resistant, and vigorous than European plums.
Flowers of European (left) and Japanese (right) plums are basically the same in structure, but more abundant in Japanese types.

Flowers
    Flowers are similar in morphology to peach, but white, smaller, and have longer pedicels. Flowers are borne mostly in umbel-like clusters of 2-3 individuals on short spurs, and solitary or 2-3 in axils of 1-yr-old wood. European plums bloom much later than Japanese types, and are therefore less frost prone.

Pollination
    Honey bees are the major pollinator. For Japanese plums, pollinizers are necessary for commercial production for most cultivars. 'Bruce', 'AU Producer', ‘Beauty', 'Santa Rosa' (and its sports), ‘Simka', ‘Casselman' and 'Methley' do not require cross pollination. In P. domestica, about ½ of the major cultivars require pollinizers, but most of the major prunes produced in the USA do not.  ‘Stanley', 'French', ‘Italian', and 'Sugar' prune types are self-fruitful.

Fruit
    A drupe. Oval shaped in European types, round to conical in Japanese types. Bloom (epicuticular wax) is usually present on glabrous surface (thus, the fruit surface is termed "glaucous"). Plums require 2.5 to 6 months for fruit development, with most Japanese ripening in relatively short periods (3 months), and some prune & canning cultivars ripening in autumn. Thinning is necessary for proper size development for Japanese plums, but not always necessary for European plums, particularly prunes, since they are not as floriferous, and fruit set is generally lighter.

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GENERAL CULTURE

Soils and Climate
    Deep, well-drained soils with pH 5.5 - 6.5 give best results. However, plums are the most tolerant of all stone fruits with respect to heavy soils and waterlogging.
    Plums are adapted to a wide range of climatic conditions; at least some cultivars can be grown in almost every state in the US. European plums have a more northern adaptation, and Japanese do better in southern areas of the temperate zone or in Mediterranean climates, Commercially, Japanese plums and prunes are grown where rainfall during the growing season is minimal, and humidity low to prevent diseases; this is why most production is in California. Cold hardiness is excellent for European plums, similar to apple and pear, but Japanese plums are less cold hardy (similar to peach).  Plums have chilling requirements ranging from 550-800 hrs for Japanese, and >1000 for European. Rainfall during the growing season can reduce production by accentuating diseases and causing fruit cracking.

Propagation
    Plums are T- or Chip-budded onto rootstocks as are other stone fruits.

Rootstocks
    Since plum scions are genetically diverse, many different species/selections are used as rootstocks. In the USA, Myrobalan 29C (Prunus cerasifera) and Marianna 2624, a hybrid between ‘Myrobalan' and a Native American plum, are used most frequently since they are widely compatible with most cultivars. Myrobalan 29C produces large trees with slightly delayed ripening, and is not particularly resistant to diseases or other root related problems. Marianna 2624 produces a somewhat smaller tree with slightly earlier ripening, and is resistant to a number of problems confronting other stocks.

Planting Design, Training, Pruning
    Japanese plums, like peaches, are typically small, spreading trees. They are planted at relatively close in-row spacings (10-20 ft), leaving about 18-20 ft between rows depending on equipment size. Pollinizers are planted in alternate row arrangements or distributed about every 3rd tree in every 3rd row. The larger  European plum requires wider spacings than Japanese in many cases, and those grown for prunes do not require pollinizers. Plums are trained to open center, but usually more upright than peach due to the natural growth habit. Pruning during formative years is light; interior branches and waterspouts are thinned, and growing scaffolds are headed to induce branching. At maturity, vigorous upright shoots are removed, since fruiting occurs increasingly on spurs on older wood as trees age.



A Japanese plum hybrid in full bloom, trained to an open center system (left). A prune orchard in California also trained to an open center, with higher crotches to allow shaker attachment (right).

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HARVEST, POSTHARVEST HANDLING

Maturity
    A variety of indices are used for plum maturity, depending on use, species/cultivar and location. Japanese plums and European plums for fresh market are harvested based on skin color and firmness, although sugar content and sugar to acid ratio has been used. Flesh color, firmness, and sugar content are the most reliable indicators for prunes; flesh turns from green-yellow to amber, solids reach 25-35%, and firmness of 1-2 lbs.

Harvest Method
    Plums for fresh consumption must be hand harvested, and require 2-4 pickings for optimal maturity over a 7-10 day period, as for peach. Prunes for canning or drying are harvested by shake and catch methods like sour cherries.

Postharvest Handling
    Fresh plums are handled postharvest just like peaches. Prunes used to be dried in the sun like raisins, but now are dried in forced air tunnels for a more uniform product.

Storage
    Plums have similar storage characteristics and problems as peaches, cherries, and apricots. They can be stored about 2-3 weeks at 32  F and 90% RH. Neither species is susceptible to chilling injury in normal storage conditions. Brown rot and Rhizopus rot, and blue and grey molds are the most common storage problems. Once prunes are dried, they are relatively resistant to postharvest diseases, and can last for months.

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CONTRIBUTION TO DIET

    Most Japanese plums are marketed as fresh fruit. European plums have a much wider variety of uses. In California, almost all European plums are dried for prunes. In other plum producing states, utilization is reported as: 30-50% fresh , 18-25% dried (prunes) , 20-25% canned , 1-3% frozen. Plums are used for jelly/jam/preserves, brandy and cognac, pies, cakes, tarts, and in confectionery. Per capita consumption of plums 1.3 lbs.

Dietary value, per 100 gram edible portion

Fresh plum Prune
Water (%) 87
 28
Calories 48
 255
Protein (%) 0.5
 2.1
Fat (%) 0.2
 0.6
Carbohydrates (%) 12
 67
Crude Fiber (%) < 1 1-3

% of US RDA*
Vitamin A 5
 32
Thiamin, B1 2.1
 6.4
Riboflavin, B2 1.9
 10.6
Niacin 2.8
 8.9
Vitamin C 13.0
 6.7
Calcium 1.5
 6.4
Phosphorus 2.2
 9.9
Iron 5.0
 39
Sodium --- ---
Potassium 3.6
 15
* Percent of recommended daily allowance set by FDA, assuming a 154 lb male adult, 2700 calories per day.

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