PLUM AND PRUNE
Prunus spp., family RosaceaePrunes are basically plums that because of their high sugar content can be dried successfully without removal of the stone. More than 2,000 varieties of plums and prunes, comprising 15 species, have been grown in the United States. Some are native to America; however, all commercially grown cultivars in California, the major producer of plums and prunes, belong to the European plum (P. domestica L.), the Japanese plum (P. salicina Lindl.), or the hybrids of the latter (Allen 1929). The best known and most important are the European plums and prunes of which the Italian prune is the most widely grown in the world. Of the numerous species of native plums (P. americana Marsh.) (fig. 156), only a few are commercially less important. These include the 'Damson ' (P. insititia L.), myrobalan or cherry plum (P. cerasifera Ehrh.) and the Simon type (P. simonii Carr.) (Allen 1929).
In 1971, California produced an estimated 101,000 tons of plums and 131,000 tons of prunes, while Idaho, Michigan, Oregon, and Washington, produced a combined total of only 63,500 tons. The total value of the crop in all of these States was $62 million. The 1969 acreage in California was 21,770 acres of plums (producing 3.08 tons per acre) and 97,560 acres of prunes (producing 1.33 tons of fruit per acre) (Henderson and Swedberg 1970).
[gfx] FIGURE 156. - Flowers of the native plum.
Plant:
The deciduous trees of plums and prunes (fig. 157) are spaced in orchards 16 to 24 (average 20) feet apart, depending upon species, soil type, and other factors (Kinman 1943). The Japanese types are in general smaller than the European types, but, depending upon vigor and type, the height may vary from 10 to 20 feet. In California, the numerous white flowers appear ahead of the leaves from late February to mid-March, and the fruit is harvested from May to July.
[gfx] FIGURE 157. - Closeup of prune flowers.
Inflorescence:
The numerous white to cream-colored, 1 inch or smaller flowers occur in clusters of one to three along the new growth of the branches of the plum. The Japanese types bloom about the time almonds bloom. The European types bloom about the time peaches bloom. Buchanan (1903) stated that the anthers are about level with the two-lobed stigma, but Brown (1951) noted that the stigma of 'President' cv. was twice the length of the stamens (figs. 158 and 159).He also referred to the "long-styled low- nectared 'Jefferson' cv." Knuth (1908, p. 344) stated that the stigma of P. domestica projects beyond the inner stamens but is at the same level of the outer ones, but in P. insititia it exceeds the longest stamen in length. The style leads to one ovary with two ovules, one of which rarely develops. Considerable nectar is secreted by the fleshy lining of the receptacle at the base of the styler column (Buchanan 1903), and, although quite dilute in the early morning, it becomes more concentrated as the day advances. Vansell (1934) reported the sugar concentration of only 6.2 percent at 7 to 8 a.m. when the relative humidity (R.H.) was 100 percent and the weather was foggy; 8.1 percent at 9:40 a.m., when the R.H. was down to 85 percent; and 25.8 at 2 p.m., when the R.H. was down to 53 percent. Later, Vansell (1942*) reported that the sugar concentration in the nectar of the 'Gos' plum blossom increased from 20 percent at 8:30 a.m. to 37 percent at 4 p.m.
Brown (1951) found considerable differences in the amount of nectar produced per flower, with one cv. ('Kea') producing 1.7 ml per 100 flowersÑmore than 10 times as much as the lowest nectar-yielding cultivar. He reported a close correlation between nectar volume per flower and the number of bees present. Vansell (1942*) also observed bees that in one case shifted their activity from plums at about 10 a.m. to more attractive manzanita (Arctostaphylos sp.) but shifted back to plums in the midafternoon. Roberts and Congdon (1955) considered that plum pollen was not sufficiently attractive to pollen-gathering insects to insure effective pollination.
The flower is open for 5 days according to Knuth (1908, p. 344) with the stigma being receptive almost 2 days before the anthers dehisce. How long it is receptive is not clear. Backhouse (1911) said that if the flowers are not pollinated, they shed in 3 or 4 days.
As a source of pollen and nectar for honey bees, plums are considered of stimulative value but because of the short flowering period and low sugar content of the nectar little surplus honey is obtained.
[gfx] FIGURE 158. - Longitudinal section of French prune flower, x 7.
FIGURE 159. - Longitudinal section of 'Mariposa ' plum flower, x 8.
Pollination Requirements:
Rather thorough studies have been made to determine the pollination requirements of the different species of plums (Backhouse 1911, 1912; Hendrickson 1916, 1918, 1919a, 1919b, 1922, 1923, 1930; Luce and Morris 1928; Marshall 1920; MacDaniels 1942; Philp and Vansell 1932, 1944; Waugh 1898). These studies established that plum cultivars vary from completely self-incompatible, in which they set no fruit with their own pollen, to complete self-compatibility, where a full crop is set from the plants' own pollen. Some are also cross-incompatible - not receptive to pollen of certain other cultivars. The majority are self-incompatible (Backhouse 1911; Griggs 1970*; Griggs and Hesse 1963). Pollinating insects are necessary on all cultivars to transfer the pollen from the anthers to the stigmas (Alderman and Angelo 1933). Thompson and Liu (1972) concluded from their tests that the Italian prune is fully self- fruitful and bees are not necessary for pollen distribution. Dickson and Smith (1953) stated that except for the Italian prune and Stanley, all European cultivars in Canada are self-unfruitful and require mixed plantings, and those two benefit from cross-pollination in many orchards. They also stated that the 'Burbank' and the 'Shiro', the main Japanese cultivars are also self-unfruitful and concluded that insect pollination is necessary for all cultivars, both European and Japanese. Luce and Morris (1928) also noted that most cultivars are self-sterile. Dorsey (1919) concluded that pollen abortion was not the cause of sterility, but rather it was associated with genetic factors in embryo development.
To provide pollen within the orchard, Griggs and Hesse (1963) recommended that in every fourth tree location in every fourth row there should be planted a compatible cultivar that flowers consistently at the same time as the primary cultivar flowers. Free (1962) showed that fruit set on plum trees decreased sharply with increased distance from the pollenizer tree. Trees adjacent to pollenizer trees had a greater set on the sides facing the pollenizers than on their far sides, indicating that the pollen was not thoroughly distributed over the tree.
Pollinators:
The honey bee has been recognized as the primary pollinating agent of plums and prunes by numerous workers since Waugh (1898, 1900) stressed its importance (Buchanan 1903; Free 1962; Hendrickson 1916, 1930; Hooper 1936; Kinman 1938,1943; MacDaniels 1942), although bumble bees and other wild bees and blowflies and other flies are given some credit by Backhouse (1912) and Brown (1951). Wind is not a factor (Backhouse 1912, Waugh 1900). Hooper (1936) pointed out that the honey bee was best because of its strong tendency to continue foraging from one source. As with many other deciduous fruit trees, plums and prunes bloom early in the spring when few pollinating agents are active. Also, large plantings have more blooms than local pollinators can service. Kinman (1924, 1938, 1943) warned that crop failures can be expected if no bees are present. Honey bees are easy to transport and establish in the orchard at flowering time, and are essential in the commercial production of both plums and prunes. The blooms are usually attractive to bees all day but more so in the morning. The plums and prunes, like other stone fruits, require that only one viable pollen tube reach the ovary to produce a fruit, but this pollen grain must, in most cases, arrive from another compatible blossom and at the right time. To assure that such pollen reaches the maximum number of flowers to produce the plum or prune crop desired, a heavy population of pollinators is required.
Hendrickson (1916, 1918) indicated that although the number of blooms on a tree varies greatly from year to year, a set of 15 to 20 percent results in massive crops. This only occurs when proper pollenizers are interplanted and bees are present in large numbers.
Pollination Recommendations and Practices:
Hendrickson (1916) concluded that best pollination would result " . . . if the bees were brought in from some outside district and scattered about the orchard, about one hive to the acre, during the blossoming period, and then removed." Philp and Vansell (1932) stated that bees were rented for plum pollination during World War I at $5 to $7 per colony.
Allen (1929) recommended one colony per acre, but believed that a centrally located apiary might serve one or even more small orchards. Roberts and Congdon (1955) said that the groups of colonies should be no further than 150 yards apart. Philp and Vansell (1944) suggested one colony per acre, the colonies in groups of 10 to 20. The Great Britain Ministry of Agriculture, Fisheries, and Food (1958) also recommended strong colonies be placed in the orchard. Roberts (1956) stated that the number of colonies per acre necessary to insure good pollination will vary (in New Zealand), but in most circumstances one vigorous colony per acre will meet all requirements. Stephen (1961) also recommended one colony per acre, with the bees to be moved in at one-third bloom stage.
Griggs and Hesse (1963) recommended for each acre at least one strong colony of honey bees with four or five frames of brood and enough bees to cover eight frames, the colonies to be placed in the orchard in groups of 5 to 10.
Most growers take some steps to see that bee colonies are in or near their orchards.
LITERATURE CITED:
ALDERMAN, W. H., and ANGELO, E.
1933. SELF AND CROSS STERILITY IN PLUM HYBRIDS. Amer. Soc. Hort. Sci. Proc. 29: 118-121.ALLEN, F. W.
1929. PLUM GROWING IN CALIFORNIA. Calif. Agr. Ext. Sen. Cir. 34, 65 pp.BACKHOUSE, W. [O.]
1911. SELF-STERILITY IN PLUMS. Gard. Chron. 1296: 299.______ 1912. THE POLLINATION OF FRUIT TREES. Gard. Chron. 1352: 381.
BROWN, A. G.
1951. FACTORS AFFECTING FRUIT PRODUCTION IN PLUMS. Fruit Yearbook 1950 (4): 12-18.BUCHANAN, R. E.
1903. CONTRIBUTION TO OUR KNOWLEDGE OF THE DEVELOPMENT OF PRUNUS AMERICANA. Iowa Acad. Sci. Proc.: 77-93.DICKSON, G. H., and SMITH, M. V.
1953. FRUIT POLLINATION. Ontario Agr. Col. Cir. 172, 6 pp.DORSEY M. J.
1919. A STUDY OF STERILITY IN THE PLUM. Genetics 4: 417-488.FREE, J. B.
1962. THE EFFECT OF DISTANCE FROM POLLINIZER VARIETIES ON THE FRUIT SET ON TREES IN PLUM AND APPLE ORCHARDS. Jour. Hort. Sci. 37(4): 262-271.GREAT BRITAIN MINISTRY OF AGRICULTURE, FISHERIES AND FOOD.
1958. THE POLLINATION OF PLUMS AND CHERRIES. Gr. Brit. Min. Agr. Fish. and Food Adv. Leaflet 378, rev., 6 pp. London.GRIGGS, W. H., and HESSE, C. O.
1963. POLLINATION REQUIREMENTS OF JAPANESE PLUMS. Calif. Agr. Expt. Stat. Ext. Serv. Leaflet 163, n.p.HENDERSON, W. W., and SWEDBERG, J. H.
1970. CALIFORNIA FRUIT AND NUT STATISTICS. 1968-1969. Calif. Crop and Livestock Rptg. Serv., 11 pp.HENDRICKSON, A. H.
1916. THE COMMON HONEYBEE AS AN AGENT IN PRUNE POLLINATION. Calif. Agr. Expt. Sta. Bul. 274: 127-132.______ 1918. THE COMMON HONEYBEE AS AN AGENT IN PRUNE POLLINATION. Calif. Agr. Expt. Sta. Bul. 291: 215-236.
______ 1919a. PLUM POLLINATION. Calif. Agr. Expt. Sta. Bul. 310, 28 pp.
______ 1919b. FIVE YEARS RESULTS IN PLUM POLLINATION. Amer. Soc. Hort. Sci. Proc. 15: 65-66.
______ 1922. FURTHER EXPERIMENTS IN PLUM POLLINATION. Calif. Agr. Expt. Sta. Bul. 352: 247-266.
______ 1923. PRUNE GROWING IN CALIFORNIA. Calif. Agr. Expt. Sta. Bul. 328, 38 pp.
______ 1930. THE ESSENTIALS OF PLUM POLLINATION. Blue Anchor [Sacramento] 7(2): 8-9, 31-32.
HOOPER, C. H.
1936. PLUMS; NOTES ON THEIR POLLINATION, ORDER OF FLOWERING OF VARIETIES AND INSECT VISITORS TO THE BLOSSOMS. Jour. Sol-East. Agr. Col. [Wye, Kent] 38: 131-140.KINMAN, C. F.
1924. PLUM AND PRUNE GROWING IN THE PACIFIC STATES. U.S. Dept. Agr. Farmers' Bul. 1372, 59 pp. ______ 1938. PLUM AND PRUNE GROWING IN THE PACIFIC STATES. U.S. Dept. Agr. Farmers' Bul. 1372, rev., 55 pp. ______ 1943. PLUM AND PRUNE GROWING IN THE PACIFIC STATES. U.S. Dept. Agr. Farmers' Bul. 1372, rev., 55 pp.
LUCE, W. A., and MORRIS, O. M.
1928. POLLINATION OF DECIDUOUS FRUITS. Wash. Agr. Expt. Sta. Bul. 223, 22 pp.ACDANIELS, L. H.
1942. NOTES ON THE POLLINATION OF THE ITALIAN PRUNE. Amer. Soc. Hort. Sci. Proc. 40: 84-86.MARSHALL, R. E.
1920. REPORT OF THREE YEARS, RESULTS IN PLUM POLLINATION IN OREGON. Amer. Soc. Hort. Sci. Proc. 16: 42 - 49.PHILP, G. L., and VANSELL G. H.
1932. POLLINATION OF DECIDUOUS FRUITS BY BEES. Calif. Agr. Ext. Serv. Cir. 62, 26 pp.______and VANSELL, G. H.
1944. POLLINATION OF DECIDUOUS FRUITS BY BEES. Calif. Agr. Ext. Serv. Cir. 62, rev., 26 pp.ROBERTS, D.
1956. SUGAR SPRAYS AID FERTILISATION OF PLUMS BY BEES. New Zeal. Jour. Agr. 93(3): 206-207, 209, 211.______and CONGDON, N. B.
1955. THE RELATIONSHIP OF NECTAR SECRETION (VOLUME) AND SUGAR CONCENTRATION TO INSECT POLLINATION OF PLUMS (PRUNUS SPP.). New Zeal. Jour. Sci. and Tech. Sect. A, 37(3): 196206.STEPHEN, W. P.
1961. BEES AND POLLINATION OF STONE FRUITS. Oreg. State Hort. Soc. Ann. Rpt 53, pp. 78-79.THOMPSON, M. M., and LIU, L. J.
1972. POLLINATION AND ERRATIC BEARING IN 'ITALIAN PRUNES' Amer. Soc. Hort. Sci. Proc. 97: 489-491.VANSELL, G. H.
1934. RELATION BETWEEN THE NECTAR CONCENTRATION IN FRUIT BLOSSOMS AND THE VISITS OF HONEY BEES. Jour. Econ. Ent. 27: 943-945.WAUGH, F. A.
1898. POLLINATION OF PLUMS. Vt. Agr. Expt. Sta. 11th Ann. Rpt. 1897-98: 238-262.______ 1900. PROPAGATION OF PLUMS - PRELIMINARY REPORT. Vt. Agr. Expt. Sta. 13th Ann. Rpt: 333.
