UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA CIRCULAR 341 June, 1937 DELAYED THINNING AS AN AID IN CONTROLLING THE GUMMING OF THE PHILLIPS CLING PEACH LUTHEE D. DAVIS 1 The production of gum by stone fruits and stone-fruit trees is rather common. The gumming appears most often on the branches or trunk, in association with injury caused by an organism, an insect, or a me- chanical means. Gumming on the fruit is less common than that on the tree, and gumming restricted to certain varieties occurs even less fre- quently. The Phillips Cling peach, however, suffers from gumming of the third class; and since about 1927 the losses from this cause have been a major problem to growers of this variety. The Division of Pomology in 1929 began investigating this disease in order to discover the underlying causes and to work out control methods. Although the best remedy may not be found until the cause is fully un- derstood, a method already worked out gives 80 to 90 per cent control. This method is described in this circular, together with certain related portions of the case history of the disease. 2 TYPES OF GUMMING As was recognized early in the investigation, more than one type of gum- ming occurs on the fruit. The different types have been placed in four groups, differing from each other in the time when the gumming occurs, in its position on the fruit, in the character of the gummy mass, in the damage to the flesh, and in the apparent cause. A recognition of these types is important in understanding of the method of control. Certain terms indicating parts of the fruit are convenient in discuss- ing the different types of gumming. These areas, with the term desig- 1 Assistant Professor of Pomology and Assistant Pomologist in the Experiment Station. 3 More detailed information regarding the case history is given in : Davis, Luther D. The gumming of Phillips Cling peaches. Hilgardia 11 (1) (in press). 2 University of California — Experiment Station nating them, are illustrated in figure 1. "Distal end" indicates the area immediately surrounding the tip or blossom end of the fruit. The ventral side or ventral suture is the side with the seam or the area adjacent to and along the seam. The dorsal side is the area opposite the seam or ventral suture. The peach, like most other stone fruits, will gum when subjected to an external injury such as an insect puncture, limb or leaf rub, or shot- hole infection. A wide variety of gumming from one or more of these -—DORSAL DISTAL VENTRAL Fig. 1. — This figure illustrates, in split and nonsplit fruit, the parts desig- nated as distal, dorsal, and ventral. The fruits are mature Phillips Cling. causes may be found on the Phillips Cling. The gumming does not occur at any particular place on the periphery of the fruit and may occur at any time before the fruit begins to develop the yellow undercolor prior to harvest. The character of the gummy mass seems to be determined very largely by the nature of the injury. It may vary from the small, inconspicuous threads of gum produced by an insect puncture (fig. 2) to the rather large and sometimes spectacular mass produced by a larger injury (fig. 3) . The extent of the damage to the flesh is also determined by the cause : in some cases the injury is only a few cells deep ; in others, such as limb rubs, it may be many cells deep. This type of gumming is not restricted to the Phillips Cling but may occur on other varieties. Its control would consist in eliminating or reducing the particular cause. A second type has been designated as the early suture gumming, because of the time of its appearance and the location of the gummy mass on the fruit. Gumming of this type occurs rather early in the de- velopment of the fruit — when or a little before the pit begins to harden on the tip. The mass of gum is always large — sometimes half the size of the fruit — and is always located on the ventral suture. The mass, some- Cm. 341] Gumming of the Phillips Cling Peach Fig. 2. — Small, rather inconspicuous gumming produced by insects on mature Phillips Cling peaches. Fig. 3. — Large masses of gum produced by limb rub on young Phillips Cling peaches. what smooth and globular in outline, extends through the flesh into the pit, which is split, and usually fills the pit cavity. The point of emer- gence is usually about halfway between the base and blossom end along the seam, but may be at any point along it. Figure 4 shows typical cases of this type of gumming and the manner in which the gum extends into the pit. Fruits thus affected shrivel and fall very soon after the gum has appeared on the outside. They consequently do not constitute a problem at harvest time. A third type, designated as distal and dorsal, is distinguished by a 4 University of California — Experiment Station pocket in the flesh beneath the point where the gum emerges, by the size and character of the gummy mass, by its position on the fruit, and by the time of its appearance. Gummy fruits of this sort may have the gum emerging at one of two positions on the fruit : near the tip of the blossom end — distal gumming; or on the dorsal side, one-fourth to one-third the distance toward the stem-end — the dorsal type. Gum occurring in the Fig. 4. — Gumming of the early suture type, showing the variations in the point of emergence and the gum extending into the pit, which is always split. latter position is always associated with a mound arising abruptly from the regular periphery of the fruit. The gum always exudes from the base of the mound — that is, at the place where the smooth periphery is broken by the hump in the flesh (figs. 5 and 7) . In fruits affected with the distal or dorsal type the gumming is usually twisted and segmented (figs. 6 and 7) . In young fruits it emerges from a slitlike opening, which is several times longer than wide and appears as though made by the point of a thin knife blade (figs. 6 and 7) . Later in the season the dimensions have been changed by growth, so that in mature fruits the opening is more nearly round. This opening differs characteristically from that caused by other types of injury, can be readily seen when the gummy mass is removed, and is not easily mistaken or confused by anyone at all familiar with it. Gummy fruits of the distal and dorsal types always have a pocket in the flesh beneath the point where the gum emerges (figs. 5 and 8). This pocket is so located that, when the fruit is halved for canning, each half contains a portion of the pocket. For this reason and because gumming is associated with such a high percentage of split-pits, the canners dis- criminate heavily against gummy fruits. Cir. 341] Gumming of the Phillips Cling Peach Gumming on either the distal end or the dorsal side will begin to appear during the third week after the date when the pit has begun to harden on the tip and a short distance along the ventral suture. This is two to three weeks after the early suture gumming has appeared. A fourth type of gumming has been called late ventral. This is char- acterized by a green, water-soaked discoloration of the flesh on both Fig. 5. — Mature Phillips Cling peaches showing dorsal gumming. Although no gum is emerging from the fruit shown at the upper left, the hump indicates its presence. The same fruit appears at the lower left, showing the pocket. sides of the ventral suture (fig. 9). Many very small drops of gum exude from the flesh so that the surface of the discolored area feels rough. This granular feel of the beads of gum may serve to identify this type even when the discoloration is not pronounced. Although there are no pockets in the flesh in such gumming, the discoloration renders both halves of the fruit unfit for a first-grade canned product. It is necessary to have a clear conception of the four types of gum- 6 University of California — Experiment Station ming in order to understand the problems involved in its control. The first type, resulting from external injury, is not characteristic of the Phillips Cling but may occur on any variety and has several widely different causes. Its control consists of reducing or eliminating as far as possible the particular cause. The second type, the early suture gum- ming, occurs more frequently on Phillips Cling than on other varieties. Pig. 6. — Mature Phillips Cling peaches showing variations of gumming in the distal position. The percentage of this type is small, however, compared with the third and fourth types; and since, in addition, such fruits drop from the tree very soon after gumming occurs, the second type is not a problem for the grower. Fruits of the third and fourth types remain on the tree until harvest time. These two types therefore constitute the problem of the Phillips Cling grower and will be discussed in the remainder of this circular. The gumming in the distal position accounts for a very large propor- Cm. 341] Gumming of the Phillips Cling Peach Fig. 7. — Young Phillips Cling peaches with distal and dorsal gumming. Fig. 8. — Mature Phillips Cling peaches with distal gumming, showing pockets in the flesh beneath the point of emergence of the gum. 8 University of California — Experiment Station tion of all gummy fruits. During the period of the investigation more than 30,000 gummy fruits were collected and examined. Of these, 77 per cent had distal gumming; 11 per cent, dorsal; and 12 per cent, late ventral. CONTROL OF GUMMING Control by delayed thinning is based upon the fact that gumming takes place during a relatively short period, which occurs regularly from season to season. Gumming begins at the same stage of development of the fruit year after year. The time when it will begin and the period during which a Fig. 9. — Young Phillips Cling fruits with late ventral gumming. very large proportion of it will occur can be forecast from certain refer- ence dates. Since there is a wide variation in the relative earliness of different seasons, one must select for reference something other than a calendar date. The time when the pit begins to harden on the tip and about % inch along the ventral suture has been used as the reference date. Pit hardening in the peach begins on the tip that extends up into the flesh, and it progresses along the ventral suture before beginning to harden on the dorsal side. The hardening is accompanied by a change of color of the pit from white to yellowish. The reference date can be determined by cutting successive slices at right angles to the suture, Cm. 341] Gumming of the Phillips Cling Peach starting at the tip. Before the pit has begun to harden, a sharp knife will cut through the flesh and pit without any feeling of flintiness or hesitation. After the pit begins to harden, the knife will hesitate in com- ing into contact with it. At the reference date the pit will be yellowish at the tip and for about % inch along the ventral suture ; the dorsal side and remainder of the pit will still be white and soft; the yellow portion will be flinty and difficult to cut. One may further verify the reference date by examining the inside of the ovule or kernel. At this time three structures will be plainly vis- ible when the kernel is cut open along its edges and one half carefully lifted off (fig. 10). One structure, the embryo, located at the extreme 1% "m> • • <*f*5jfr ^ \ *- #.■ V... ^endosperm Iemsryo hSEED COAT i& Ijjj^ ^wT—S^ jjtipt ^^B^^ h-NUCELUUS '& ' <s l# . •-.«. « 'Z^ '* W ^ . ■p *mw W" Fig. 10. — Kernels (ovules) of peaches showing the various structures: em- bryo, endosperm, nucellus, and seed coat, as well as the variation in size of the embryo at the time of the reference date. tip of the pointed end of the kernel, is glistening white and opaque and varies from a size just barely visible to the eye to a length of about y 1Q inch. A second structure, the endosperm, fills about half the seed coats and is semitranslucent. The embryo is embedded in it. The third structure, the nucellus, fills the remainder of the seed coats. It is a trans- parent jellylike mass, not nearly so dense as the endosperm. In very young kernels the watery nucellus is the only structure visible to the naked eye. As the fruit becomes older the endosperm becomes visible, gradually displacing the nucellus, which it absorbs. A little later the embryo is seen, gradually displacing and absorbing the endosperm, so that when the kernel has reached maturity the embryo is the only struc- 10 University of California — Experiment Station ture easily visible. The time when the embryo is first perceptible coin- cides with the time when the pit begins to harden. Figure 10 shows the condition of the seed on the reference date. The endosperm and embryo have been dissected out and are shown above the half of the seed contain- ing the nucellus. The time and rate of appearance of gumming, with respect to the reference date, for four years are shown in figure 11. The data in this 65 m _i < 60 Q £so 3 45 2 O40 < Z25 uj o a 20 uj cw a. 15 ^_^Hi: _ t / f III 'I / // 1 /// 1 II /// ! w i 1 I I935 J It -1932 1936- w -1933 / /, 1 r- A * 21 28 35 42 49 DAYS FROM REFERENCE DATE Fig. 11.- -Time and rate of appearance of gumming plotted with respect to the reference date. figure show the accumulated number of gummy fruits, as percentage of the total gummy fruits appearing, when plotted against the number of days from the reference date. The data were obtained by carefully going over certain trees each week and removing all the gummy fruits. A very small percentage of the gumming had occurred during about the first 15 days after the reference date. Then very suddenly, within a week, gumming increased very rapidly and continued at a high rate for about 20 days, so that at the end of about 35 days after the reference date 85 to 90 per cent of all the gumming had occurred. Although the reference dates vary with the seasons, the time when gumming occurs with respect Cir. 341] Gumming of the Phillips Cling Peach 11 to these dates does not. In 1932 the reference date selected was May 18; in 1933, May 31; in 1935, June 1; and in 1936, May 20. The following- data illustrate how suddenly the number of gummy fruits increases. In 1935, from 35 trees, 32 gummy fruits were removed on June 6, five days after the reference date; 117 on June 13; and 1,698 on June 20. In 1936 ten gummy fruits were removed on June 2, thirteen days after the reference date; and 1,225 on June 9. The control of gumming by delayed thinning depends upon at least two factors : (1) a heavy set of fruit remaining on the tree while gum- ming is taking place will prevent a certain number of fruits from becom- ing gummy; (2) thinning after most of the gumming has occurred allows gummy fruits to be removed during the thinning process. The following data illustrate how delaying the thinning affects the number of gummy fruits produced and the control obtained. In 1932 experiments were carried out in five orchards. Thinning was done at three different times — first about April 30, eighteen days before the reference date; second about May 28, ten days after the reference date and about five days before gum began to appear; third about June 25, three weeks after gumming began to occur and when about 90 per cent of it had appeared. In the experiment there were 258 trees, approxi- mately equally divided among the three times of thinning. The gummy fruits were counted on all the trees at three different times — first about June 13, second about July 21, and third about August 23. At the first count there were, on the average, 43 gummy fruits per tree on the early thinned, 28 on the second thinning, and 12 on the trees to be thinned June 25. These figures show how gumming is affected by leaving the crop on the trees. The early-thinned trees had almost four times as many gummy fruits per tree as those not yet thinned; those thinned about May 28, just before gum appeared, had more than two times as many as the unthinned trees. The second count was made about July 21, ap- proximately a month after the last thinning and when most of the gum- ming had occurred. At this count the early-thinned trees had an average of 55 gummy fruits per tree; the second thinning, 36; and the last thin- ning, 9. The differences between the last-thinned and the other two groups have increased, a situation that would be expected since gum was still appearing at the time of the first count and since the last thin- ning would remove a certain amount of this. At the second count the early-thinned trees had more than six times as many gummy fruits as the late-thinned ; and the second thinning, four times as many. The last count, about August 23, was made just before harvest. The early-thinned trees had an average of 54, the second-thinned 38, and the last-thinned 9 gummy fruits per tree. This count, which serves only as a check on 12 University of California — Experiment Station that made about July 21, shows that no change had occurred in the num- ber of gummy fruits. In 1933 experiments were carried out in six orchards on a total of 2,244 trees, approximately equally divided among three times of thin- ning. The first thinning was done about May 20, the second about June 20, and the third about July 5. The reference date for this year was May 31. Only two counts of gummy fruits were made this year — one about June 30, and the other about September 1. At the first counting the early-thinned trees had an average of 42 gummy fruits per tree; the second thinning 26; the trees yet unthinned 17. The effect of a crop on the tree is again evident in the low number of gummy fruits on the trees yet unthinned. At the second count, about September 1, the early- thinned trees had an average of 50 gummy fruits; the second-thinned 31; and the last-thinned 11. The data are remarkably like those obtained in 1932. In 1934 experiments were carried out on 1,683 trees approximately equally divided among three thinning dates : April 30, May 31, and June 20. The count made just before harvest showed 28 gummy fruits per tree on the first thinning, 14 on the second, and 9 on the last. The foregoing data illustrate the control affected by delaying the thinning until most of the gumming has occurred. The effectiveness of leaving the crop on the tree while gumming is taking place seems to be closely related to the amount of fruit that the tree has set. In trees with a medium-to-heavy crop little gumming will occur during this period, whereas in trees with a very light crop delaying the thinning will have small effect in preventing the appearance of gumming. The length of time which thinning should be delayed will therefore depend upon the set of fruit. In trees with light crops the thinning should be delayed until fully five weeks have elapsed after the reference date, since in this case the effective control will consist largely in removal of the gummy fruits at thinning time. In trees with heavy crops the thinning could perhaps be begun a little earlier than this, since the presence of the full set will have a depressing effect on the number of gummy fruits occurring. The important point in applying this method for controlling the gum- ming of the Phillips Cling may be summarized as follows : Need for a Clear Understanding of the Different Types of Gumming. — The grower must not be misled by the early appearance of the first or second types. Short Space of Time in Which Gumming Occurs. — Gumming occurs at a very definite time with respect to hardening of the pit on the tip and along the ventral suture. Gum will begin to appear rapidly during Cir. 341] Gumming of the Phillips Cling Peach 13 the third week after the day selected as the reference date; that is, it will begin soon after about 15 days have elapsed after the selected date. During the third, fourth, and fifth week from this time gumming will occur very rapidly, so that at the end of 35 days approximately 90 per cent of the gumming will have occurred. Establishing the Reference Date. — It is important that this date be selected so that the grower may plan his subsequent operations with ref- erence to it. It is best established by cutting successive slices at right angles to the suture, beginning at the tip, in a number of fruits. This operation should be repeated at frequent intervals until the reference date has been chosen from the color and flintiness of the pit and from the size of the embryo. During the period of the investigation there have been variations in the number of days between this date and that selected for full bloom, but an average of the years 1929-1936 inclusive shows that about 60 days have elapsed between full bloom and the ref- erence date. Examination of the fruit should begin in advance of the expected date. Calculation of the Time When Gumming Will Occur and of the Thin- ning Date. — Addition of two weeks to the reference date will give the date after which the gumming will appear, and addition of three more weeks or a total of five weeks will give the date when most of the gum will have appeared. Plans for orchard operations should agree with this latest date but should be made at the time when the reference date is selected. The value of selecting the reference date lies partly in the fact that the approximate thinning date can be forecast more than a month ahead. Deviations from the forecasted thinning date should be made with reference to the grower's judgment regarding the set of fruit or after consultation with the farm advisor. This method of controlling gumming in the Phillips Cling has been successfully used now for several years in many orchards of the state. Better methods may be found after more of the fundamental and under- lying causes are known. At present, however, the method here described seems to offer a solution of the problem by which the losses from gum- ming may be reduced by as much as 90 per cent, where the work has been carefully done, as compared with thinning at the time formerly practiced. This program is based — in part, at least — on certain vulner- able points of the disease as shown by its case history and will therefore give the best results when properly modified in accordance with the best judgment of the grower and his farm advisor. 14 University of California — Experiment Station ACKNOWLEDGMENTS For suggestions and cooperation the author is deeply indebted to R. H. Klamt, Farm Advisor of Sutter County; A. A. Jungerman, Farm Ad- visor of Stanislaus County; E. F. Serr, Assistant Farm Advisor of San Joaquin County; and numerous growers who have willingly allowed the use of their orchards. lOm-6,'37