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A brief outline of the story of the sugar prune was given in a chapter of an earlier volume. The preceding chapter gives further details of the quest of a perfect prune. In the present chapter I wish to speak more of results than of methods, and to present somewhat in detail the characteristics and merits of the four nearly perfect prunes that have been produced as the result of my long quest. While some of the details here presented appeal rather to the orchardist than to the general reader, yet the story as a whole will be found not without popular interest. The fact that the growing of prunes is an industry of great significance, and that the fruit is everywhere an important commercial product would furnish ample excuse, were excuse needed, for entering somewhat more into detail as regards the specific qualities of my quartet of prunes than has been done in the case of most other of my plant developments.


Another prune of the same parentage with the Giant (referred to in the preceding chapter), namely, the Hungarian prune, crossed with prune d'Agen, was advertised at the same time, under the number "A.P. 318" in New Creations of 1893. This was purchased by Stark Brothers of Louisiana, Missouri, who procured the entire stock for $3,000, and named it "Splendor." This prune is very much larger than the common French prune, is oblong, has a rich violet-purple skin, and the flesh is exceedingly sweet, and black when cured-a great advantage. The American people have been educated to black prunes and generally prefer them to those of lighter colors, following the fashion set by the French smoke-dried prunes. The Splendor fully answers the desire on the part of the buyer and consumer for a "black" prune, of large size and superior quality. Splendor prunes, when cooked, require little sugar, containing about five percent more sugar than the French prune, its quality and flavor are superior, and it has a perfectly free stone smaller than is usual with prunes. It ripens here two weeks earlier than the French prune. The tree is even more productive, it is a more constant bearer, and is sturdier than its French parent. The tree is a well proportioned one, requiring but little pruning. The fruit is borne in clusters commencing low down on the body of the tree. Many thought that this excellent prune would soon completely displace the prune d'Agen. Surely if quality and productiveness were all that were demanded by the grower, this would have occurred. But Splendor has one peculiarity which places it at a serious disadvantage for general commercial purposes as a drying prune: the fruit clings to the tree when ripe, where it gradually dries into a delicious, sweet prune. As prune growers like to have the prune fall as soon as ripe, to save trouble in harvesting, the clinging of the Splendor to the tree is considered a more or less serious fault. However, it is quite commonly planted wherever the German prune thrives, and gives excellent satisfaction, except for the extra trouble of picking. It is shipped East as a fresh plum from sections of California in large quantities and is unusually well adapted to shipping, on account of its large content of sugar, making a fruit which carries well.


The Splendor was the best prune I had heretofore produced, but it clearly left much to be desired. It was with intense satisfaction that I was able to offer, in New Creations of 1899, a prune that at least approached the realization of my ideal. This was another seedling of Petite d'Agen. It was christened the Sugar prune. For fourteen years I had labored to produce a large, early, productive, handsome, easily cured, richly flavored prune with a high percentage of sugar. The prize appeared in 1893, and by 1899 I had tested it sufficiently to warrant its introduction. Numerous growers had ordered $50 to $500 worth of wood for grafting-regardless of the quantity-even before grafting wood was offered. I had worked diligently and unceasingly, watching for the slightest indication of variation in the direction desired. Finally through systematic crossing and careful selection, my cherished desires were realized-after years of persevering effort and patient waiting-in the Sugar prune. In this, at last, I found a prune possessing the best qualities of all the prunes combined in one; and several of these qualities were intensified. The Sugar prune had no rival until the advent of the still newer prune, the Standard, which I introduced in 1910. When the selection of seedlings was made from which the Sugar prune originated, about one-half were at once discarded. Only those were saved which had the customary indications of good fruiting-large leaves, prominent buds, and strong, heavy wood with short joints. Grafts from the young seedlings were placed upon Japanese plum stocks. This was done because there was no other stock at hand at that time. It proved to be a costly experiment, because more than half of these new, promising seedlings died before bearing fruit. Some of the grafts did not start at all; some made a short growth and died the first season; some grew a few seasons and died. Fortunately, however, some thrived as well as on their own roots. The grafts that bore the first fruits of the prune which was later named "Sugar," made a fair but not a good union with the Japan plum. Although the first fruits of this variety were born on Japanese plum stock it is not recommended that Sugar prunes be grafted upon such stock. Roots of the Myrobolan plum make better stocks. Almond roots are also highly commended by some orchardists. The seedling bearing the Sugar prune yielded its fruit the second year after grafting. At that time I had the French Robe de Sergeant and German and Italian prunes growing on my Sebastopol place, and it was with these that the Sugar prune was compared. It proved to be superior in all respects to any of them. Some of the fruits from the other grafts of this same lot of seedlings bore good plums but not good prunes. The fruits of the others had various faults, such as cracking, too large pit, clingstones, poor drying qualities, late ripening, scant foliage, or susceptibility to disease. Several years are always required for the merits of a new fruit to gain full recognition, but the Sugar prune has gained pretty steadily in popularity. More and more growers are working their orchards into this variety, and it is taking the place it deserves, high among the leading prunes of commerce. Besides this, it is proving to be one of the most acceptable fresh fruits in the eastern markets as well as extremely profitable when cured. The growers at Vacaville, California, the most important early fruit shipping center, are becoming more enthusiastic as they see the fruiting of these trees, the ease with which the larger prunes can be harvested, and the greater price per ton. About 2,500 new trees of this variety were planted in Vaca Valley in 1913. Growers there received $17 to $25 per ton more for Sugar prunes in 1913 than for French prunes grown on the same farm at the same time. One of the growers reports that his French prunes averaged 57 to the pound last year-when cured-while his Sugar prunes averaged 39 per pound. The larger prunes always bring the best prices. Not only did the Sugar prunes bring exceptional prices, the whole crop was dried perfectly, while the French and Imperial prunes, ripening later, were caught by the rains and many of them spoiled. The Imperial prune often dried to almost nothing but skin and stone. One pound of green Sugar prunes makes seven and one half ounces of dry fruit. It contains six percent more sugar than the French prune and is far superior to it in flavor. It is so much more productive that it may be grown for less than half the cost of producing the French prune. The Sugar prune has a great advantage over the other varieties in ripening early in August, three weeks before the French prune, and about a month earlier than the Imperial. It ripens at a time when the weather is hot and dry, so that it can be cured bright and glossy in a short time and before there is any danger from fall rains. A month or so later, when the last of the older varieties are maturing, the weather is often cloudy and foggy, or sometimes even rainy and in any case the days are much shorter, so that curing is carried on under difficulties, often (as in the cases just cited) with serious loss. In 1912, prune shippers estimated that rain damaged the crop of French prunes in this county twenty-five percent. The Sugar prunes were all cured and packed before the rains, so there was no loss of this variety.


The fruit of the Sugar prune is usually even in size and very large, averaging thirteen to fifteen to the pound fresh, which is at least three to four times as large as the French prune grown here under the same conditions. It has excellent curing qualities, standing the lye bath better than most other prunes. The tree is very far superior to the French prune tree in every respect; better grower, better bearer, better foliage, better form. It requires less careful but abundant pruning; and it will carry and mature more than double the quantity of fruit. The wood is somewhat brittle, but the chief cause of the breaking of the limbs, which sometimes occurs, is prolific bearing. It must be thinned when the fruit is about half grown, to prevent damage to the tree. I have found that a very satisfactory and simple device for doing this is to tap the limbs gently with a piece of ordinary three-quarters inch rubber hose five to six inches long, fastened on the end of a cane or bamboo pole. The hose causes no injury to the branches, and, by striking just hard enough, the fruit can be made to fall evenly and leave the amount desired. The need of thinning, however, may be largely obviated by proper winter pruning. When this variety was first offered, grafting wood was sold at $10 per foot. That the investment was a profitable one even at that price is shown by the following quotation from a letter written by one of the first purchasers: "I was one of the first to introduce this fine fruit into our locality, the first year the grafting wood was placed on the market. I bought seven feet of wood for $70. The same was grafted into Tragedy prune trees, using one bud for each cion. The following fall and winter I sold about $600 worth of buds and cions from the ten trees which I had grafted with the Sugar prune cions."


Pre-eminent as are the qualities of the Sugar prune, there is always room for improvement. I endeavored to make such improvement by the usual method of crossbreeding. About 1897 I combined the Sugar prune with the Tragedy. There were only twelve or fifteen seedlings from the cross. But these were carefully grafted upon older trees, on larger branches where they would be in less danger of injury. This, of course, made the bearing of fruit a year later than if they had been placed upon the smaller branches. But it seemed worth while to wait for fruits of such high promise. The whole tree was given over to each of the seedlings. Nor was this exceptional solicitude unavailing. For among these carefully nurtured cions was one that bore a fruit that surpassed even the hitherto matchless Sugar prune. After a period of trial, in which it met the severest tests, this superlative prune was introduced as the "Standard." It is rather curious to record that, with a single exception, all the remaining cions of this patrician sisterhood have proved wholly worthless as prunes. But that, of course, was a matter of no consequence. It sufficed that one cion came to fruitage with the paragon of prunes. The Standard prune surpasses the Sugar prune in quality. It also has a stone that is entirely free from the flesh, being the first prune ever produced that combined superior qualities of flesh with this desirable characteristic. In the opinion of a number of the best known growers, it is the best prune ever produced. The trees are enormous and never-failing bearers, and good, healthy growers, better than the French prune though not as strong as the Sugar. Well grown fruits measure nearly six inches around one way by four and a half inches the other. On older standard orchard trees the size may average larger than this, and when the crop is not too heavy the fruits are really enormous. The skin is purple with a heavy blue bloom; flesh honey-yellow, fine grained, juicy, yet firmer than most drying prunes, and very sweet. The stone, which is free, is only five-eighths of an inch in diameter. Standard is without doubt the best combination drying and shipping prune ever produced. It ripens with the French prune in September. It has been kept fully a month in good condition in a basket in an ordinary living room during our warm fall weather. It can be successfully shipped after it becomes dead ripe, to any part of the United States. And the final test as a prune is that when dipped in the ordinary lye solution the skin cracks properly, so that the result is a big, quickly-dried prune of superlative quality. The following comparison of the French and Standard prunes, made by G. E. Colby of the University of California, gives a good idea of the value of the Standard prune:


  • The "Standard" / French Prune
  • Average weight in grams 49.7 / 23.6
  • Number per pound...... 9.1 / 19.1
  • Flesh, percent .......... 96.5 / 94.2
  • Pit, percent ............ 3.5 / 5.8
  • Sugar, percent ......... . 18.9 / 18.5

In case any one wishes to change a prune orchard over to a more profitable variety, whether for drying or shipping fresh, I would strongly recommend the Standard for grafting. The Standard was offered to orchardists in my catalog of 1911-12. The trees were sold at $3 each, and thousands of trees have been distributed, but it will be a good many years before the real value of this superior prune is fully appreciated.


The most striking individual peculiarity of the Standard prune is its freestone quality, already referred to. The development of this character is of such interest and importance that it calls for more than passing mention. At first, it is very probable, all fruits were clingstones. The stone was probably firmly attached to the flesh from the time of the forming of the meat to the final decay of the fruit. The stone in fruit acts as a support to the flesh, to which it is attached and around which it grows. The clingstone feature was evidently an advantage to the fruit, as plum and prune seeds will not germinate if thoroughly dried, and the clinging meat in most of the fruits keeps the seed moist for a longer time, thus helping to conserve its vitality until the proper season for germination. Where the flesh is attached to the pit, the circulation between the pit and the surrounding flesh is less interrupted, probably an advantage to the development of both. The clingstone is thus the more normal condition of fruits. Most fruits are clingstone until brought under cultivation. All fruits, both wild and cultivated, are clingstone until towards the time the ripening process commences. That many cultivated fruits are freestone is no doubt the result of artificial selection to meet a very natural demand. Nuts furnish analogies that help us to understand the relations of seed-stone and fruit. The case of the almond, which was perhaps more nearly the parent form of stone fruits, is particularly instructive. In place of the rich surrounding meat which we see in peaches, apricots, and plums, the almond has a leathery skin, which is inedible. This generally clings to the stone persistently in the wilder forms, but with the best cultivated almonds the nut drops readily from the husk or outside covering. Similar to the persistency with which the flesh of the plum clings to the stone is the attachment of the husk in the walnuts and the chestnut, in each of which the husk separates with more difficulty in the wild than in the best cultivated varieties. From the standpoint of protection and reproduction of the almond, the clinging husk is an advantage rather than an objection. The seed of the almond will germinate after being thoroughly dried. It needs no flesh to tide it over, as do the pulpy stone fruits. But for men's use the clinging husk is a disadvantage, and the clingstone habit has been eliminated in all the best cultivated varieties of the almond. In the plum a similar change has been developed by selection. The meat does not cling to the stone, in many cultivated varieties. In the almond the quality of the meat has been greatly improved, while the husk or immediate covering has not been improved in any respect, as no use is made of it. Even a freestone fruit does not start as a freestone, but the flesh tends to leave the stone as the fruit approaches maturity, very much as a leaf ripens away from its supporting stem in the fall when it has performed its annual function, or the fruit parts from the tree when it is fully ripe. The flesh parts from the stone by a natural process. This leaves the stone either "free" or partially free. Some individual trees, among a lot of seedlings -chestnuts in particular-will hold their leaves persistently all winter (this persistence is especially common with crossbred chestnuts) even when thoroughly dead and dried, giving an untidy appearance to the tree, while the leaves of other seedlings fall at once and leave the branches clean and free. This is a similar process to the parting of the flesh from the pit in fruits, both being ripening processes. There is every gradation between the complete attachment we call "clingstone" and the "free-stone" condition. In some fruits there is a single point of attachment; in others the flesh adheres over a part of the surface while the remainder may be wholly free from the stone. There is also another form of partial separation found in some fruits where the flesh clings tenaciously to the stone until fully ripe, when it parts readily, while in others it may separate from the fruit and be shaken about within it even before thoroughly ripe. There seem to be two forms of variation, one in the time of attachment and the other in the persistency of attachment. This persistency of attachment varies greatly; in some fruit it would be possible by a little work to cut around the stone and in others the flesh is attached so closely that to remove the stone satisfactorily you must have sharp tools and use them with discretion. The old hereditary tendencies make it difficult to change plum and prune heredity so that it will produce freestones instead of clingstones. Nevertheless this has been accomplished with several varieties, including the Standard prune. Of late the canners have preferred the clingstone peaches mostly, perhaps because they have a firmer flesh that does not fall to pieces when cooked, as the freestone peaches generally do. The pit is very easily removed with a sharp instrument made for the purpose. With this exception, fruits are generally more valuable when they are freestone.


But what if the fruit had no stone at all? That would, indeed, be the ideal condition. And this ideal is met, or nearly met, in the fourth member of my quartet of best prunes-the Conquest. This, the newest of my prunes, was first offered in the catalog of 1911-12. The work of producing the stoneless prune parallels that of the production of the stoneless plum, a preliminary account of which has already been given, and fuller details as to which will appear in the succeeding chapter. Here it is necessary to mention only such aspects of the work as refer specifically to this prune. The Conquest was produced by crossing a partially stoneless plum in my orchard with the French prune. The difficulty of getting a stoneless prune was about equal to the difficulty of getting a satisfactory stoneless plum. If I had crossed with a plum it would have been a hundred times more difficult to get the prune characters than it was to get stonelessness. In the Conquest the size and quality of the French prune is retained, together with the stonelessness of the other parent. This cross brought out both prunes and plums-some of the largest plums ever seen. At first they were all blue like the stoneless parent; later they took on all colors of ordinary plums. The advantages of the stoneless prune are too obvious to require elucidation. To be sure, the new prune is not yet absolutely stoneless. A small speck still persists in prunes of best quality. It has been no great trouble to eliminate the stone in a poor fruit; to combine stonelessness with good quality of fruit has been extremely difficult. But continued selection will finally produce a prune of this kind which has the quality of the best French prunes, together with entire stonelessness.


Such, then, are the four Burbank prunes that are the pick of all those that have been developed on my experiment farms. The methods used in their production are similar to those used in the development of the four best Burbank plums as told in an earlier chapter. The distinctive qualities of the four prunes themselves may be summarized thus: The Splendor prune is large, productive, has high sugar content, has a small freestone and ripens early, yet has the fault of clinging to the tree. The Sugar prune is large, productive, very early, superior in tree form, an especially good curer, and is both a sure bearer and a sure seller. The Standard prune has most superior quality of flesh, is entirely freestone, and in general is the best combination drying and shipping prune thus far produced. The Conquest prune is similar to the French prune in quality of flesh, and has the stone brought down in size to a mere speck. Because of the many characters it is necessary to combine in producing a successful prune, it is probable that the work represented by these four varieties is fully equal to the production of ten times that number of standard plums-with, probably, proportionate benefits. But from the almost numberless varieties in my orchard, the result of years of selective breeding, there will probably arise individuals year by year that will present new and superior combinations of qualities; and among these may appear at any time a prune that will surpass my best prunes of the present as markedly as these surpass their predecessors of a generation ago. This, indeed, is fully to be expected. Each of my prune trees, with its colony of selected hybrids, may be regarded as a factory admirably equipped for the turning out of new varieties of prunes. Even though it were left to be operated solely by the bees, its mechanism has been so perfected, its equipment is so complete, that it can scarcely fail of its purpose.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 5 Chapter 4