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 
 
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 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 
 
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 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