THE EFFECT OF G R O W T H REGULATORS ON BLOSSOM THINNING W I T H SPECIAL R E F E R E N C E TO APPLES A N D PEACHES By CLARENCE ANTON LANGER A THESIS Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of D O C T O R OF PHILOSOPHY Department of Horticulture 1952 ProQuest Number: 10008358 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10008358 Published by ProQuest LLC (2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 4 8 1 0 6 - 1346 THE EFFECT OF GROWTH REGULATORS ON BLOSSOM THINNING WITH SPECIAL REFERENCE TO APPLES AND PEACHES By Clarence Anton Langer An Abstract of A THESIS Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Horticulture 1952 ABSTRACT Fruit-thinning by hand has been practiced in the commercial orchards for many years# Recently fruit thinning with growth regulators has shown promise with apples and peaches. The experimental work with growth regulators presented in this study was begun in 19^6 to determine the usefulness of these materials to orchardists and to determine the underlying principles in­ volved# Naphthaleneacetic acid was tested at concentrations of 5 to 100 ppm, maleic hydrazide was tested at concentrations of 50 to 500 ppm, and sodium thiozole and ndark" were each tested at concentrations of 20 to 100 ppm on apples and peaches at full bloom and at calyx time# Growth regulators were tested by spraying several complete trees at different concentrations; also, by spraying several limbs of the same tree each with a different concentration and, also spraying individually marked spurs with several concen­ trations# Some trees were sprayed with naphthaleneacetic acid at 20 ppm and 100 ppm and random samples of spurs were removed and analyzed for reducing sugars, non reducing sugars, starch and nitrogen content compared to control spurs# Naphthaleneacetic acid has been shown to be effective as a blossom-thinning spray for apples in Michigan. Such factors as climate, variety, soil, nutrition, physiology, spray con­ centration, and time of application influenced the degree of thinning. A direct relationship was found between spur size and the degree of thinning, large spurs (k mm. in diameter) thinning with greater difficulty than smaller spurs (2-3 in diameter). Although there were visible responses of the apple to applications of NAA as indicated by leaf curvatures, defloration, and persistance of certain floral parts, there was no apparent affect upon chemical composition as measured by content of ni­ trogen, starch, and sugars. It is concluded that any chemical alterations are minute in quantity, and of the nature of growth regulators, enzymes, or vitamins; which are not determined by gross chemical analysis. Other regulating sprays tested for thinning action on apples were either ineffective, such as malele hydrazide, sodium thiozole, and "clark11, or they caused excessive visual damage to the tree and fruit as did 2,^-dichlorophenoxyacetic acid. More visual wilting and petiole curvature from spraying at full bloom in comparison to later spra^ys was noted on Olden­ burg variety than on Wealthy variety. jl/ i Naphthaleneacetic acid sprays were undesirable for thin­ ning peaches, and Is considered still in the experimental stage. Maleic hydrazide was found to thin peaches quite well at 500 ppm during the seasons of 1950 and 1951» although the set of fruit was still too heavy in 1951 after thinning. No visable injury was observed from the use of maleic hydrazide on peaches at concentrations up to and including 500 ppm. Thinning peaches with maleic hydrazide appears promising, but must be considered still in the experimental stage. ACKNOWLEDGEMENTS The author wishes to express his most sincere appreci­ ation to Dr. Charles L. Hamner, Professor of Horticulture, for his untiring assistance in guiding this investigational work. His suggestions and constructive criticisms were of m u c h help in the carrying out of this study. The writer further wishes to extend sincere appreci­ ation to Dr. H. B. Tukey for his suggestions and ideas per­ taining to the investigational portion of this work, as well as the ever-present words of encouragement which sustains one through m a n y trying times. The writer wishes to thank Dr. H. M. Sell, Professor, Agriculture Chemistry, for arranging to secure the analysis of blossoms and leaves of fruiting apple spurs, and to Dr. Erwin J. Benne, who m a d e the analysis. The writer wishes to thank J. C. K r e m e r of the Horti­ culture Department for his assistance. The writer is particularly grateful to the several g r o w ­ ers mentioned in the text, for trees and spraying equipment used in carrying out the tests. TABLE OF CONTENTS Page I N T R O D U C T I O N ....... REVIEW OF LITERATURE 1 ......................... Early History of H o r m o n e Development 2 ........ 2 ............... 4 Use of Growth Regulators in Preventing Abscission of Plant P a r t s ...................... 8 Growth Regulators and Fruit Set Fruit-thinning .................................. Timing of Thinning Sprays 11 ..................... 15 Concentration of Naphthaleneacetic Acid for Thinning ................................... 19 Environmental Factors Affecting T h i n n i n g ....... 19 Growth-Regulator Thinning Sprays for Peaches 21 .. M A T E R I A L S A N D M E T H O D S ........................ 23 Description of Orchards, Soil, Tree Vigor, and Pollination.................................. 23 Environmental Factors 35 ......................... Weather at blossom time .................. 35 Rainfall..................................... 36 Mechanical and chemical practices 36 iv Page Choosing and Marking of Experimental Trees and L i m b s ...................................... 37 Choosing, Measuring, and Tagging of Apple Spurs ....... 38 Thinning Sprays, Concentrations, and Application ........ 40 Per Cent Set, Fruits per One Hundred Blossom Spurs or Buds ........................ 43 Collection and Treatment of Spurs in Preparation for Chemical A n a l y s i s .............. 44 RESULTS ..................... 49 Spur Size in Relation to Blossom-thinning....... 49 Effectiveness of Maleic Hydrazide, Sodium Thiozole, and T,Clark" ......................... 52 Relation of Vitamin K to Blossom-thinning ...... 54 Thinning with Naphthaleneacetic Acid on the Tree Basis ..................................... 55 Chemical Analysis of Apple Spurs Following Application of N A A as Thinning S p r a y s .......... 58 Peach-thinning Trials in 1950 60 Peach-thinning with Maleic Hydrazide, 1951 .... 63 D I S C U S S I O N ......................................... 102 Spray Thinning of Apples ....................... Climatic factors............................ 102 102 V Page Factors of soil and nutrition............... 104 Physiological factors 105 ......... Varietal f a c t o r s ............................ 107 Economic f a c t o r s ........................... 109 Spray Thinning of Peaches ..................... 110 S U M M A R Y .............. 113 L I T E R A T U R E CITED 115 ............................... LIST O F TA B L E S Table I. II. III. IV. V. VI. Page Thinning effect of naphthaleneacetic acid sprays in relation to size of spurs of McIntosh, Wealthy, Wagener, Jonathan, and Northern Spy apples during late bloom in several areas ofM i c h i g a n ......... 65 Thinning effect of maleic hydrazide, thiozole, and " d a r k ' 1 on Northern Spy, Jonathan, Wealthy, and Wagener a p p l e s ................. 74 Thinning effect of naphthaleneacetic acid in combination with Vitamin K on two varieties of apples during blossom­ time ....................................... 76 Thinning effect of naphthaleneacetic acid sprays on Wealthy, Jonathan, McIntosh, and Yellow Transparent apples at several locations in Michigan ........... 78 Effect of naphthaleneacetic acid sprays on the nitrogen and carbohydrate content of Wagener apple-blossom spur leaves, petioles, and cluster bases ................................... 80 Effect of naphthaleneacetic acid sprays on the nitrogen and carbohydrate content in Wagener apple-blossom spur flowers, young fruits, and peduncles ............................... 82 vii Table VII. VIII. IX. Page Effect of naphthaleneacetic acid sprays on the nitrogen and carbohydrate content of entire Wagener appleblossom s p u r s ........................... 84 Thinning effect of naphthaleneacetic acid, "dark," thiozole, and maleic hydrazide, on Hale hav en, Redhaven, and Kalhaven peaches when sprayed at different stages of bloom and fruit development . . . 86 Thinning effect of maleic hydrazide on Redhaven and Halehaven peaches sprayed during blossoming at two locations in 1951 91 LIST O F FIGURES Figure 1. 2. 3. 4. 5. 6. 7. Page Plot layout of segment treatments for testing of biossom-thinning sprays in relation to size of blossom spurs ...................................... 45 Plot layout of "entire tree" treatment used in testing naphthaleneacetic acid, sodium thiozole, "dark," and maleic hydrazide as bios som-thinning sprays .................................... 46 Plot layout of limb treatments for testing of growth-regulator materials and their concentrations .................... 47 Branch of Wagener apple tree showing tags used to identify the spur sizes 2, 3, and 4 m m ........................... 48 Thinning effect of several concentrations of naphthaleneacetic acid on several sizes of fruit spurs of the McIntosh apple ................................... 68 Thinning effect of several concentrations of naphthaleneacetic acid on several sizes of fruit spurs of the Wealthy apple ................................... 69 Thinning effect of several concentrations of naphthaleneacetic acid on several sizes of fruit spurs of the Wealthy apple ............................... 70 ix Figure 8. 9. 10. 11. 12. 13. 14. Page Thinning effect of several concentrations of naphthaleneacetic acid on several sizes of fruit spurs of the Wagener apple ...................................... 71 Thinning effect of several concentrations of naphthaleneacetic acid on several sizes of fruit spurs of the Jonathan .............................. apple 72 Thinning effect of several concentrations of naphthaleneacetic acid on several sizes of fruit spurs of the Northern Spy apple ............................... 73 Effect of several concentrations of maleic hydrazide in thinning Halehaven and Redhaven peaches when sprayed at different stages of bloom and fruit development (1950) 90 Effect of several concentrations of maleic hydrazide in thinning Redhaven, Hale­ haven, and Kalhaven peaches sprayed after 90 per cent of the blossoms were fertilized ( 1 9 5 1 ) .................... 93 Tagged limb showing m o r e vigorous foliage on limb of Halehaven peach thinned heavily in 1950 with maleic hydrazide at 500 p p m .................... 94 Fruits removed from spurs 2 m m . and 3 m m . in size of fruitful Wealthy limb by a naphthaleneacetic acid spray of 20 p p m applied at calyx­ time ............... 95 X Figure 15. Page Wealthy tree spray thinned at calyxtime with 10 p p m of naphthaleneacetic acid at full bloom in 1946 and again in 1947, Orchard I X ............ 96 Baldwin tree spray thinned at calyx with 20 p p m of naphthaleneacetic acid using a "Lo wb oy Mast" .................. 97 17. Baldwin tree not thinned by spraying but hand thinned about June drop t i m e ....... 98 16. 18. B o x 19. 20. of apples from Baldwin check tree and from spray-thinned tree ............ 99 Varietal response to naphthaleneacetic acid sprays at 20 p p m at calyxtime (48 hours after application) .......... 100 Effect of 2,4-Dichlorophenoxyacetic acid on Stayman variety of apple ............. 101 INTRODUCTION Fruit-thinning has been practiced in one form or another ever since fruit has been grown. Heavy crops necessitate hand thinning, a slow and expensive operation even when labor is plentiful. Within the past decade chemical sprays have been dis­ covered that will reduce fruit set by destroying some of the flowers, thus permitting a smaller, m o r e evenly spaced crop to develop. Recently, plant-growth regulators in low concentra­ tions have proved effective in bios som-thinning of apples and pears. The experimental work with growth regulators reported in this thesis was begun in 1946 to determine the usefulness of these materials to orchardists and to determine the underlying principles involved. REVIEW OF LITERATURE Early History of H o r m o n e Development The father of plant hormones, Darwin (7), showed as early as 1850 that some influence m o v e s from the upper to the lower part of the coleoptile of grass seedlings when they are exposed to illumination from one side. Fitting (8, 9) was one of the very first to determine the nature of growth regulators. He used orchid pollen and found it caused swelling of the ovary when the pollen was brought in contact with the stigma. work in the early 1900's. Fitting discontinued his If he had continued his work, the development of growth regulators might have been m u c h m o r e rapid. However, there were reasons for discontinuance of the work. Necessary techniques had not yet been developed, and the isolation of the active growth-regulating substance could not supersede necessary physiological procedures. In 1919 Paal (28) showed that the active substance was produced in the tip and m o v e d downward in the stem. Went (39) collected the active substance from tips removed from the 3 coleoptile and placed on gelatin blocks. Then by placing the blocks on the side of a coleoptile, from which the tip had been removed, the gelatin block containing growth substance caused the coleoptile to bend, proving that the auxin had m o v e d out of the coleoptile tips into the gelatin blocks. Improvements in methods of testing for auxin-like substances were continually being developed. The use of the Avena test speeded up the procedure for growth-substance isolation from plants. Indole- acetic acid was isolated from urine extraction by Kbgl and Haagen-Smit (19) of Holland. After Kbgl's and Haagen-Smit1s work, Thimann (3 7) in the United States isolated indoleacetic acid from Rhizopus suinus. Following the discovery of indoleacetic acid in urine and fungi, it was thought for some time that auxin was only present in lower forms of plants. This was substantiated by Kbgl and Kostermans (20) with yeast, and Thimann (3 7) with Rhizopus suinus. However, it was later found in wheat (3, 13) and in immature corn (14). It is now known that indoleacetic acid occurs in m a n y different parts of the plant, but mostly at the growing points. It is also found in flowers, buds, pollen, leaves, and even secreted by aphids and perhaps m a n y other insects. 4 It is interesting to note that according to Larson (Zl) and Van Overbeek (Z7) auxin m o v e m e n t in the plant is corre­ lated with temperature. A s the temperature increases from 0° C. to 30° C., each 10° C. rise increases the production of indoleacetic acid three times. Auxins are thought to be deacti­ vated by ultraviolet rays, certain enzymes, and various environ­ mental conditions. Indoleacetic acid stimulates cell division and m a y be the controlling m e c h a n i s m in meristematic activity. Growth Regulators and Fruit Set ( Early in the 1900's there were several scientists-namely, Fitting (8, 9), Hartley (15), Winge (40), Yasuda (45) and Massart (Z3)--who were interested in producing seedless fruits. B y using various pollens and extracts of pollen they were able to produce seedless fruits in some species of plants. The production of parthenocarpic tomato fruits attracted the attention of a large nu mb er of workers using several growth regulators. Although Gustafson (12), W o n g (44), Schroeder (32), Z i m m e r m a n and Hitchcock (48, 49), Luckwill (22), and H e y and Hopf (17) were interested primarily in parthenocarpic tomatoes, 5 they also worked with such other crops as melons, squash, cu­ cumbers, pineapple, apple, and pear. Growth regulators used and the m e a n s of application varied a great deal, but the results were quite encouraging from the several materials regardless of the methods used in application. The materials were applied in lano­ lin paste on the ovaries as a spray, aerosol and dust to the buds and flowers. These methods of application were used in the greenhouse with excellent results. The following materials have shown promise for the production of parthenocarpic fruit: Indole­ acetic acid, indolebutyric acid, and beta-naphthoxyacetic acid, alpha-naphtaleneacetic acid, and 2,4-dichlorophenoxyacetic acid (2,4-D). Wittwer (41) in 1949 used a vibrating apparatus plus weekly sprays of a hormone solution containing beta-naphthoxyacetic acid at 4 p p m and para-chlorophenoxyacetic acid at 10 ppm. Wittwer applied his spray to the flower clusters with a hand sprayer. A n increase in fruit set resulted both in the spring and fall crop of greenhouse tomatoes. Inasmuch as regulators were so beneficial for increasing the set of the early blooms of tomatoes in the greenhouse, as well as overcoming the problem of self-sterility in greenhouse tomato production, they were considered by investigators to 6 have possibilities in the field. Paddock (29), working in Texas, found adverse conditions from spraying with growth-regulating sprays on the set of tomatoes in the field. fruits than, the untreated. The sprayed plots picked less It was later determined that the night temperature range is too high in Texas and does not favor the use of hormone sprays for fruit set. Zalik, Hobbs, and Leopold (46) found that they could induce parthenocarpic fruit by applica­ tion of para-chlorophenoxyacetic acid at several locations on the flower; namely, stigma, side of ovary, and the abscission layer of the pedicel. Singletary and Warren (33) found that hormones aided in the set of early tomatoes when night temperatures were low, but as the season progressed and night temperatures in­ creased, no definite value could be attributed to the hormone sprays. Wittwer and Schmidt (42) stated that night temperatures below 59°F. are not conducive to good fruit set of early tomatoes. This low night temperature can be overcome by the use of growth regulators. Wittwer and Schmidt (42), testing fourteen varieties of tomatoes of various ripening dates, showed an increase in all varieties of the early crop over the controls. H e y and Hopf (17) have been interested in the interreac­ tion of growth-regulating substances as they enter the plant, and 7 the function of these parts. They have been concerned with fruit set and frost damage prevention by combining vitamins and hor­ mones. They have found that plants containing Vitamin K respond to 2,4-D in herbicidal concentrations m u c h m o r e readily than do those not containing Vitamin K. They have concluded that vita­ mins of different kinds undoubtedly are necessary for the reaction of hormones. If there are sufficient vitamins in the plants, an addition of vitamins will not improve the effect of the hormone spray. This has been determined by the use of A r bo rm on e C which is a material combining Vitamin K with certain hormones. He y and Hopf (17) have tried other co-reactants such as paraaminobenzoic acid plus aryl thiorea derivatives and nicotinic acid plus halogenated quinoline derivatives. They have hinted that there are probably any numb er of these co-reactants in the plant kingdom which, if discovered, might lead to the an­ swer to some of the complex physiological problems still not answered in fruit set, abscission of fruits and blossoms, and frost resistance of various plants and their parts. 8 Use of Growth Regulators in Preventing Abscission of Plant Parts The original work on fruit abscission of apples byGardner, Marth, and Batjer (10), of the U. S. Horticulture Station, Beltsville, Maryland, stood for thirteen years without m u c h change. Their work showed that concentrations of 5 and 10 p p m were effective in delaying the abscission of most varieties of apples. Their findings on timing of the spray are still practiced as far as naphthaleneacetic acid (NAA) and its related substances are concerned. Generally, they found the McIntosh variety to be less affected by the spray than the other apple varieties. Most varieties were affected for two or three weeks, but McIntosh very seldom was affected for longer than eight or nine days. The effect of coverage and spray placement found by Gardner in his original work has been sub­ stantiated by m a n y workers. The greatest effect from naphthaleneacetic acid sprays is at the point of application. Accordingly, a thorough wetting of the fruit and the fruit stem is necessary if one is to secure the m a x i m u m results from the spray of N A A . It is believed that the effects of the N A A spray are not translocated in the 9 plant, as is 2,4-D. There apparently was little or no effect from N A A abscission-inhibiting sprays on the respiration of the apple trees for the foliage of the sprayed trees was dropped at the s am e time as the control trees. Burkholder and M c C o w n (4), Batjer (2), M u r n ee k (26), and m a n y others have substanti­ ated the work of Gardner, Marth, and Batjer (10) and added varietal characteristics and worthwhile suggestions as to the possible working m e c h a n i s m of the growth regulators in prevent­ ing of the abscission of the fruits at harvest time. Batjer and T h o m p s o n (1), in search for a better material than N A A , have tried 2,4-D, along with other substances, on several varieties of apples, including the Winesap, Winesap were outstanding. The effects of 2,4-D on It increased the effective period m u c h beyond that of N A A , but it did not take effect quite as rapidly as N A A . The effects were so outstanding on the Winesap variety that it was tried on all commercial varieties and at various concentrations, but apparently 2,4-D as an abscissioninhibiting spray is as selective in variety of apples as it is in killing weeds. The only varieties effectively influenced belong to the Winesap group, such as Stayman and Turley. 10 A n e w material which was introduced by the D o w C h e m i ­ cal C o m p a n y (5) in 1950, under strict research conditions--as an abscission-inhibiting apple spray--was released for c o m m e r ­ cial use during the. 1951 harvest season. It appears to be supe­ rior to any abscission-delaying spray yet discovered for all the standard commercial varieties of apples. It is marketed under the n a m e of '‘Color-Set'' and chemically is alp ha-2,4,5-trichlorophenoxypropionic acid. It apparently is a better inhibitor of the maturation of the abscission layer than NA A. Its value lies in a longer effective period of inhibition of abscission plus the fact that it can be applied before the fruit begins to drop. Whether or not it remains superior to N A A will depend on its effect on stor­ age life of the fruit, residual tree effects such as delayed blos­ soming or an upset of annual bearing. It is believed that unlike N A A , 2,4-D and M Color-Set‘‘ can transmit their effects over m u c h greater distances from point of application. The ability to affect abscission by transmission over considerable distance within the plant tissues is a desirable characteristic for any abscission inhibitor. 11 F ruit-thinning Thinning of most varieties of apples and peaches is nec­ essary to obtain increased fruit size, color and quality. Thin­ ning by hand is becoming obsolete because of the tremendous costs involved and the scarcity of skilled labor to cope with the task. Hand thinning is undoubtedly one of the most expen­ sive operations of fruit production in the West and would be in the Central West and East if proper thinning of fruit was prac­ ticed as it is in the West. Hand thinning is not usually early enough to prevent alternate bearing, especially with varieties having strong biennial bearing characteristics, as Wealthy, Gol­ den Delicious, Gr imes Golden, Duchess, and Yellow Transparent. The size of fruits is correlated with the earliness and thorough­ ness of the fruit-thinning procedure. According to Tukey and Einset (38), thinning of peaches resulted in an increase in size of fruit regardless of the time of thinning. The greatest benefit, however, was secured from hand thinning of blossoms or hand thinning of fruit during Stage I of peach development. of blossom-thinning of peaches. In fact, this is the first record Over the three-year peiod that IE this thinning was carried on, the trees thinned early responded with stronger growth and better annual bearing. M a n y other early investigations on thinning of both apples and peaches showed similar results. The discovery that the growth regulator, N A A , was ef­ fective in the thinning of apples shifted attention immediately in the direction of thinning sprays. Murne ek (24) has set forth the advantages of spray thinning over hand or pole thinning by sayingr (l) "It is fast." In other words, thinning can be ac­ complished per tree as fast as a thorough scab spray could be applied to the same tree. (2) "It is less expensive." To hand thin a 20-bushel tree with labor available today would cost sev­ eral dollars at the best and then the tree would not likely be thinned enough. O n the other hand, spray thinning of the s a me tree would cost about 30 cents. heavier load of good fruit." (3) "Permits trees to carry a This aspect is easily conceivable in that the excess load of fruit is removed before it takes from the tree its supply of available nutrients. It has been found in the western states, as well as in the Midwest, that apples thinned at calyxtime can carry and mature from 10 to 25 per cent m o r e apples than can hand-thinned trees in June. (4) "It preserves the tree's growth." B y removal of the fruits at calyxtime, the strength and food that would normally be consumed by the excess fruits can be used in the development of strong terminal and spur growth. (5) "It will help to break alternate bearing." Through research and careful observation on this phase of apple tree behavior, it has been noted that removal of fruits early after they have been set tends to bring about m o r e con­ sistent annual bearing. Schneider and Enzie (30, 31) showed in chemical thin­ ning that concentrations between 100 and 300 p p m applied dur­ ing the full bloom period practically destroyed all fruit set on Delicious and Gano apple trees. There was visual evidence of leaf- and petiole-curling for several weeks after the spray ap­ plication. The blossoming the following spring was apparently about the same on the control as on the sprayed trees. Greene (11), using N A A dissolved in a 0.1 per cent polyvinyl alcohol-water solution, sprayed the Starking variety of apple at concentrations of 10 to 100 p p m during the bloom period. The apples were thinned at all concentrations, with the greatest thinning being at the higher concentrations. He also used indolebutyric acid dissolved in the same alcohol-water 14 solvent at concentrations varying from 10 to 50 p p m on the same variety of apple with no significant results. Davidson, H a m m e r , Reimer, and Dutton (6), in their earliest work in apple-thinning, incorporated N A A at 10 ppm, along with the regular insecticide and fungicide sprays on several varieties of ap­ ples in 1945. Similar plots of the same varieties of apples were only given their regular spray treatment which did not include N A A . No methods were established for obtaining records on thinning rates, as the plot was not for that purpose. However, as harvest time grew nearer, it was quite evident that there were less fruits per tree on those receiving the N A A along with the regulator spray treatment over those not receiving N A A . Tree counts were m a d e at harvest, and there were significantly less fruits on the trees receiving N A A spray. Since this early work, Davidson et al. (6) have worked with this apple-thinning growth regulator each year. A good deal of emphasis was placed on the sodium salt of N A A for thinning during 1942-4. Favorable results in thinning were ob­ tained on the standard Michigan varieties at concentrations vary­ ing for the most part from 10 to 20 ppm. Sprays were applied separately and in conjunction with insecticides and fungicides with good results in all cases. Sprays were applied in full bloom, 15 when 75 per cent of petals were off, and two weeks after all petals were off, with satisfactory thinning. It was found that a corresponding higher concentration was necessary to give the s a m e degree of thinning as the stage of blossom period progressed. N o thinning occurred when spraying was done at four weeks after calyxtime with concentrations mentioned above. Varieties did not all respond the same in these trials, and the s a m e variety did not always respond the s am e each year, especially when it was in a different environment. vary in their reaction to the N A A Varieties sprays as regards wilting, petiole curvature, and curling of leaves. The time of the spray application seems to be a factor governing this affect on the foliage. Timing of Thinning Sprays The investigations that have been carried out in the past several years on the thinning of apples have been for the most part carried out when petals were 75 per cent off, or very near to this stage of fruit development. S o m e investi­ gators have found that apples will thin satisfactorily later than this. A few of these workers are Hoffman, Southwick, and Edgerton (18), Southwick and W e e k s (36), and Davidson _et al. (6). 16 According to the findings of Batjer (2), the earlier one can safely thin the fruit, the larger and better quality one can expect at harvest. H e also feels that early thinning will m o r e successfully create a favorable condition for a m o r e desirable annual bearing habit. Batjer (2) considers that it is risky to thin in blossom, or even after 75 per cent of petals have fallen. H e feels frost m a y create a problem in m a n y of the appleproducing sections of the United States. A s long as N A A will thin apples as late as two weeks after calyxtime, this is the better commercial practice to follow. It is pointed out that in 1949 m a n y apple areas were frosted quite severely when most of the petals were off; but some of the frosted areas, especially in Michigan, needed thinning. S o m e were thinned successfully at two weeks after the calyxtime with N A A spray. This later time of spraying (2) is an aid in preventing wilting and curling of the leaves, which is quite prevalent when some varieties are sprayed in full bloom. Batjer believes that the cause for less distortion of the foliage is that the spur leaves are older and probably coated with cuticle so the entrance of N A A is not accomplished as easily. He has also observed that weather conditions and varietal difference are factors which 17 influenced this leaf-dwarfing, curling, and wilting. Cool, damp, and cloudy weather during blossom period was m o r e likely to bring about m o r e severe dwarfing, curling, and wilting of the foliage than when the weather had been bright and warm. An explanation of this was that the unfavorable weather delayed leaf maturity. The varieties that have appeared m ost suscep­ tible to the dwarfing of foliage are Duchess, Yellow Transparent, Early McIntosh, Delicious, and Wine sap. The m o r e tolerant varieties to leaf-dwarfing and wilting are Wealthy, Jonathan, Grimes, Baldwin, and Golden Delicious. It has been observed that the abscission of apples will take place m o r e easily before June drop than after. There is not too m u c h information available on the effect of materials on apple abscission after June drop, but what there is provides in­ formation that it would be uneconomical and perhaps injurious to the trees and fruit to thin apples with N A A after the June drop, as it would be necessary to go into quitehigh concentra­ tions in order to be effective. Batjer (2) continues with his observations, and states that still there is no one who has been able to prove h o w N A A reduc­ es the set of fruit. There has been work done to indicate that 18 tree vigor plays a part. Southwick and Weeks (36) observed that the smaller, weaker wood in the interior of a thick tree thinned m o r e readily than the rest of the tree. showed dormant Wealthy buds, 4.7 m m m o r e easily than buds 4.8 m m They also and smaller, thinned and larger. Heincke (16), working just with apple wood of several varieties of apples and of various degrees of vigor, concluded that a certain degree of vigor was necessary for the storage of sufficient nutrients to bring about a good set of fruit. S m o c k and Gross (34), in their investigations of abscissioninhibiting sprays of N A A , found an increase in respiration for several days after spraying. If this increased respiration should occur when applying N A A as a thinning spray, it m a y partly aid in the answer to how it thins. For instance, if respiration was increased then the food normally used by the young apple would be depleted and the apples on the weaker spurs would be re­ m o v e d first. Regardless of the cause, it appears that the less desirable fruits are removed first by the N A A thinning sprays. 19 Concentration of Naphthalene ace tic Acid for Thinning Batjer (2) feels that for the m ost part, 5 to 20 p p m of N A A is the concentration that will meet most conditions of apple-thinning year after year, for mo st commercial varieties. He is quick to point out that the concentration will depend on variety, stage of blossom, or fruit development, and of course, on the m a n y environmental factors. The degree of thinning is undoubtedly affected by concentration, ability of the variety to absorb N A A , and the susceptibility of the fruit or blossom to abscission. Environmental Factors Affecting Thinning Batjer (2) s um s 1. case of adverse climatic In the up these factors asfollows:. conditions which are conducive to poor fruit set generally, but in a well managed orchard, there m a y be a good set, but the apples will only contain two or three seeds. It has been determined that under conditions of low seed count per apple, the N A A sprays will m o r e easily thin than when the seed count is high, and preceeded by good cross-pollination weather. zo Z. It has also been observed, under uniform conditions of pollination and seed fertilization, orchards on well-drained sites are m u c h m o r e difficult to thin than those on wet, cold, poorly drained locations. Quite often in commercial plantings the apples are overthinned by as little as 10 p p m of N A A on the poorly drained site. 3. The factor of food reserves and its correlation with the degree of thinning with N A A has been pointed out by various workers. It has been observed that trees lacking in nitrogen seldom need to be thinned. If trees are deficient in nitrogen to the point where growth is not as vigorous as it should be, they are very likely to be overthinned by N A A treatment. It is wise to consider all factors that might reduce the leaf area of a tree during the previous growing season, as it very likely could be a factor in the ease of thinning the following season. 4. It is continually questionable whether thinning sprays should be used in combination with the regular fungicide and insecticide sprays around calyxtime, or other cover sprays. It is thought better to apply the thinning spray separately, in that the regular spray compounds are likely to contain wetting agents and cause excessive runoff, or m a y react with the N A A 21 compound and cause it to be less effective, or affect its m a k e ­ up in such a w a y as to cut down the absorption by the tree and its parts. In other words, other limiting factors are add­ ed when the thinning spray is mixed with a regular fungicidal and insecticidal spray. Growth-Regulator Thinning Sprays for Peaches The literature on the thinning of peaches with growth regulators is limited. The investigations carried out on the thinning of peaches with N A A during the bloom period have yielded no positiveresults. Southwick, Edgerton, and Hoffman (35), using N A A on Valiant and Elberta peaches, found nega­ tive thinning effect at full bloom, calyxtime, or a week after petals were off. M ur ne ek and Hibbard (25), using only the Elberta variety, obtained the s a m e results. In 1950 Murneek found that peaches sprayed approximately thirty days after full bloom were thinned at June drop. He obtained fair thin­ ning results with 15 p p m N A A on Golden Jubilee, 20 p p m on Raritan Rose, and 30 p p m on Halehaven. It apparently was not consistent, as no further work has been published on peachthinning with N A A which improves or substantiates this work. 22 It is certainly plausible to expect that the results m a y vary from year to year for a given variety because of the complex environmental conditions above and below the ground. N o investigational work on peach-thinning with maleic hydrazide has appeared in the literature to the best of the writer's knowledge. However, according to Wittwer and Sharma (43), the sprouting of onions was inhibited by foliage sprays of maleic hydrazide. It is generally considered a growth regulator of the inhibiting type. MATERIALS AND METHODS Description of Orchards, Soil, Tree Vigor, and Pollination The fourteen orchards concerned in this thesis will be described in numerical order so as to furnish an accessible m e a n s of reference for the investigation. Orchard I_ is located on the Horticulture Farm, Michigan State College, East Lansing, Michigan. The orchard has been growing under sod culture method since 193 7, when it was seeded to Kentucy bluegrass. This soil is a heavy Hillsdale loam, which is an ideal fruit soil. Hillsdale soil is mainly characterized by the reddish brown clay layer about 3 l/2 to 4 feet below the surface. This layer m a y vary from 2 inches to several feet in depth. The orchard consists of some fifteen to twenty varieties of apples. The general vigor of the M c I n ­ tosh trees used in 1949 and Wagener trees used in 1950 was excellent. They were all moderately pruned annually, and ex­ hibited terminal growth of nine to twelve inches on the lower half of the tree, and ten to eighteen inches on the upper half of the tree. 24 Orchard II is located in the north central portion of the "Old Mission Peninsula," Grand Traverse County. The Old Mission Peninsula extends north (fifteen miles) into Lake M i c h ­ igan, forming the east and west Grand Traverse bays. bay is several miles across. a Coloma type of sand. Each All the soil on this peninsula is These Wealthy trees are located on top of a flat hill which is higher than any of the surrounding orchards. The orchard has been growing under a system of sod culture since 1944. Each tree has been fertilized each spring with four pounds of a m m o n i u m nitrate applied in a ring at the outer periphery of the limbs. cover was light. again in 1949. The Kentucky bluegrass The trees were severely pruned in 1948 and The orchard in which the Wealthy trees were located consisted of four other compatible pollination varieties. Orchard III includes both apples and peaches. It is located in Newaygo County, five miles west and one mile south of Fremont. This area is not considered one of the best fruit counties, but there are several comparatively small localities which rate a m o n g the best orchard sites in Michigan, of which Orchard III is one. This site is one of the two highest eleva­ tions in the county. In addition, it has a rolling topography. The apple orchard was fifteen years old and the peach orchard was ten in 1951. The soil is a very fertile Isabella sandy loam. The apples were ma naged under a trash cultivation system their first twelve years, and converted to a sod and straw mulch sys­ tem in 1948. cultivation. The peaches have been continually under trash Complete fertilizers have been used almost exclu­ sively for the life of the orchard. Three hundred pounds of 0-20-20 have been applied in the fall each year since 1948, and in the spring from three to five pounds of a m m o n i u m ni­ trate per tree depending on size and need. The apple orchard cover is dense and consists mostly of bluegrass. The trees are large for their age and have never been pruned heavily. Lateral branches are numerous, and pruning has been the "Thin W o o d Method." The single row of Wealthy trees in this orchard is bordered on both sides with compatible pollination varieties. The peaches are annually pruned, and are in a vigorous state of growth. The space between the peach rows is well cultivated until July, when weeds and grass are again allowed to grow. Orchard IV is located in Oakland County. The county is fast becoming a suburban residential community. It was at one time one of the leading apple-growing counties of the State. 26 There are s o m e excellent sites in this county. Orchard IV is located on one of these, situated about four miles north and one mile west of Rochester. A rolling topography with a superior elevation gives it good air and soil drainage. The soil is a heavy type of Hillsdale sandy loam, which has been under a sod culture since 1939. The sod is fertilized with four hundred pounds of 4-16-4 per acre every third year, and four to six pounds of a m m o n i u m nitrate per tree every year. The 4-16-4 fertilizer is applied broadcast late in the fall for the benefit of the sod cover. The last application was in 1948. is applied in a ring at the periphery of the trees. The nitrogen Pruning in one of the two blocks of Jonathan variety was very heavy. The other block of Jonathan was in need of pruning, being very brushy. These will be referred to as Jonathan (A) and Jonathan (B) of Orchard IV. The Northern Spy trees were pruned very lightly. The pollination conditions were ideal for the Northern Spy apples, with compatible varieties on all four sides. The Jonathan (A), on the other hand, were bordered on one side by a woods, the other side, by Northern Spy trees. One end was bordered by McIntosh, and the other end, by young Northern Spy trees. Jona­ than (B) were interplanted with three compatible pollination varieties. 27 Orchard V is located in Oakland County near the city limits of Birmingham. sandy loam. The orchard soil is a heavy Hillsdale The site is quite rolling, but the elevation is not quite as high as s o m e neighboring sites. However, this site is higher than s o m e of the neighboring sites, which affords good air and soil drainage. This peach orchard is ten years old, and is in an excellent physical condition. The trees are growing vigorously, ten to fifteen inches of terminal growth a year. It has set heavy crops in 1948 through 1951. The trees have been grown under a system of clean cultivation and cover crop the first six years, then switched to a trash cultivation. The fertilizer progr am of horse man ure with plenty of straw was used in the 1951 season. This was applied practically as a two-inch straw mulch over the entire orchard and then chopped up with the disk. Prior to 1951, the program was one of c o m ­ plete fertilizer 10-10-10 applied in the spring and cultivated into the soil. The orchard has been pruned lightly every year, and needs to be headed back quite severely in order to allow for better air circulation during harvest time. Varieties used in this orchard were Redhaven, Halehaven, and Kalhaven during the years 1950 and 1951. 28 Orchard VI is located in Berrien County, three miles south and two miles east of Benton Harbor. This orchard is situated on a plateau and is generally higher than the neighbor­ ing land expanse, which gradually tapers off to the Saint Joseph River Valley, one and one-half miles away. The soil is an average Col om a sand which has been clean cultivated and cover croped with rye for the past 10 years since the trees were planted. yard. Before that, it was part of the h o m e grounds and barn­ The trees have been fertilized annually with a 10-6-4 fertilizer in the spring. Each tree has received from five to ten pounds of 10-6-4 annually during the years 1949, 1950, and 1951, depending on size and vigor of the tree. The trees have always been pruned on the heavy side since they were five years old. The trees are vigorous and free of any cankers. They are a solid block of Redhaven variety, surrounded by m a n y other varieties, and near the bee yard. Orchard VII is located in Genesee County, four miles south of Linden. Genesee County can not be considered one of the good peach-growing counties of Michigan, but, on the other hand, there are two locations that have lost but one peach crop by winter freeze or spring frost since 1920. This is one of the 29 two really favorable peach locations of Genesee County. The orchard is situated on a high ridge that is bordered on both sides by broad, deep valleys of several miles in extent. are numerous inland lakes in the valleys. There The soil is a rich Hillsdale sandy loam that has been fertilized and cover cropped for the past eight years since the peach trees have been planted. Complete fertilizer has been used in the form of three hundred pounds of 0-20-20 applied with cover crop in the fall, and two to four pounds of a m m o n i u m nitrate per tree each year in the spring. Trees in this orchard are not grown quite as vigor­ ously as in the other peach orchards. Terminal growth of five to eight inches is the goal of this grower. The trees are m o d ­ erately pruned each year, with most attention being placed on removing those limbs that have been dwarfed by a lateral out­ growing it. This orchard consists of Early Halehaven, Redhaven, and Elberta varieties. Orchard VIII is located on the Old Mission Peninsula in Grand Traverse County. The general description of this loca­ tion and soil has been given under Orchard II. This apple site is at the very north end of the peninsula and has been clean cultivated with cover crops from 1939 through 1951. This 30 twelve-year period represents the age of the McIntosh and Jon­ athan trees. The orchard is interplanted with peaches which are seven years old. The McIntosh variety has been pruned m o d ­ erately during the years 1949, 1950, and 1951. The Jonathan trees have not been pruned since 1949, except for the removal of broken limbs. The apples have been fertilized with a c o m ­ plete fertilizer (10-10-10) in the spring, since they were five years old. The spring of 1951 they received three pounds of 10-10-10 per tree. Both varieties are bordered on both sides by a compatible and suitable pollinating variety of apple. Orchard IX is located eight miles east of Bake Michigan on the west side of U. S. Highway 31, halfway between Hart and Shelby. The soil is a light type of Isabella sandy loam. The orchard is on very rolling ground, but is for the most part, a side hill and valley. The orchard has been growing under a sod culture for seven years prior to the year 1946, at which time this investigational work started. except the area under the trees. The sod cover is sparse, All fertilizer treatments were applied broadcast under the spread of the trees. Ammonium sulphate was used in the spring at the rate of two to three pounds per tree. During the two years that records were taken at this 31 orchard, no other form of fertilizer was used. is a ten-acre block of solid Wealthy apples. The orchard Many swarms of bees are employed for pollintion on this farm, but none are placed in this orchard. The general vigor of the trees is slightly below average for Michigan, and about average for Oceana County. The Wealthy trees do not grow very large from Oceana County north, except for an occasional location. Terminal growth will average about six to eight inches on bearing trees. Heavy pruning is needed to stimulate vigorous bearing wood, and it is not u n c o m m o n to see the open head or clover-leaf type of pruning in the Wealthy orchards in Oceana County. Orchard X is located seven miles south of Saint Joseph and six miles east of Lake Michigan. clay loam. The soil is a M i a m i The orchard site is flat, and the surrounding area is rather level, with an occasional knob or hill. The orchard was maintained, for the ten years prior to this research in 1948, under the sod culture method. The soil is fertile and the trees vigorous and large for twenty-year-old Wealthy trees. The trees were forty feet apart and alternately planted in rows of Jonathan, McIntosh, and Wealthy varieties. The orchard received barnyard man ur e until it was fifteen years of age, but since 1942, only nitrogen has been applied in the spring. This nitrogen was either in the form of a m m o n i u m sulphate or a m m o n i u m nitrate. The trees received five pounds of a m m o n i u m nitrate the spring this work was done. The sod is a heavy Kentucky bluegrass, and was cut twice during 1948. Orchard XI is located at the very northern tip of the lower peninsula, eight miles east of Cheboygan, on the banks of the Black River. The soil is a very light sand which can be termed a Coloma type of sandy loam. This orchard soil has been m ana ge d under the clean cultivation method until 1945, when it was seeded to Chewings fescue. spotted and short. The grass was The grass cover was fertilized with three hundred pounds of 0-20-20 when it was seeded. The trees were fertilized every year with a m m o n i u m sulphate. The spring of 1946, the trees received three pounds of a m m o n i u m sulphate per tree. Trees in this section of the state are not large, and are not forced as they are farther south. Trees are moderately pruned, and, as a result, are moderately vig­ orous. Most good apples are grown on three to five-year-old wood. This orchard is a ten-acre solid block of Wealthy apple 33 but the block is surrounded by other compatible pollination varieties. T w o s w a r m s of bees were used per acre during bloom. Orchard XII is located in Cass County, six miles northwest of Dowagiac on a three-mile flat plateau above Indian Lake. This soil is a rich Fox loam. The apple trees that were twenty years old in 1946 were in need of cutting back from the sides and top. feet on the square. The trees were planted 36 The McIntosh trees were vigorous, making twelve to eighteen inches of terminal growth during the year 1945. Of course, all of southwestern Michigan was frozen out in 1945. This orchard has excellent cross pollination, with Delicious and Jonathan bordering the four rows of McIntosh. This orchard has been growing under sod cover for twelve years prior to 1945. Complete fertilizer has not been necessary for this orchard, but nitrogen has been applied some years. The year 1946, the McIntosh trees received no fertilizer. O r chard XIII is located in Oceana County, four miles west of Shelby on a rolling, sandy site about four miles east of Lake Michigan. Soil is a Coloma sandy loam which has been under sod culture since 193 7. The cover is mostly quack grass with 34 so me Kentucky bluegrass mixed in. The cover was thin and short, and was not clipped in 1947. Trees were moderately vigorous and heavily pruned. Terminal growth of the 1946 season was eight to ten inches, average. These Wealthy trees were not adjacent to any good pollination block, but there was dispersed in the orchard one McIntosh tree for every ten Wealthy trees. Orchard XIV is located in Jackson County on a gently rolling topography. The elevation of the site is satisfactory, especially concerning air and soil drainage. The soil is a heavy Hillsdale sandy loam which has been under sod culture for nine years prior to 1947. The cover is a mixture of Kentucky bluegrass and quack grass which is very heavy, and was clipped twice in 1947. The orchard has been fertilized in the past with barnyard manure, but during the five years prior to 1947, it received only nitrogen in the spring. The trees received five pounds of a m m o n i u m nitrate in the spring of 1947, spread in a ring at the periphery of the tree. The trees are vigorous, showing as m u c h as 24 inches of terminal growth in the tops of the trees. These Yellow Transparent apples were well pollinated by four compatible varieties which 35 were interplanted. Trees were in need of pruning. None had been done for three years. Environmental Factors Weather at blossom time. Weather conditions during blossom time in 1946 in Orchard XII were rainy and cool. There were no killing frosts in any of the three orchards, IX, XI, or XII. Orchard XI had plenty of sunshine, but the temper­ ature barely reached 70° F. at noon on two of the eight days of the bloom period. Every night the temperature would fall to 40° F., and on three nights the temperatures were 33° to 34° F. Orchard IX had similar weather to Orchard XI. In 1947, Orchards IX, XIII, and XIV had several days of sunshiny weather during bloom, and temperatures above 70° F. before, during, and after spraying. During the blossom period in 1948, the weather was cool and d a m p before spraying, with very few hours of temperatures of 70° F. during the entire ten-day blossom period for Orchard X. Orchard I in 1949 had temperatures above 70° F. for 48 hours prior to spraying in full bloom. blossom were falling. F e w petals of the center One night, two days after treatment, 36 temperatures dropped to freezing in low areas of the orchard. The McIntosh on top of the ridge were unharmed. In Orchard II, in 1949, temperatures of 70° F. and above occurred for 72 daylight hours before being treated, when 95 per cent of the petals were off. N o lower temperature than 40° F. was re­ corded during or after blossoming. During 1950 and 1951 seasons, no frost was experienced for any of the orchard loca­ tions, and temperatures were 70° to 80° F. for most of the blossom period for both peaches and apples. Rainfall. The rainfall during the period from 1946 through 1951 over an area from Berrien County east to Detroit and north to Cheboygan, was above normal, with the following exception. In 1946 and 1947, during peach harvest in Berrien County, lack of rain b e c am e serious on the light sand just prior to Redhaven peach harvest. This was remedied, however, by several good rains before Halehaven harvest. In other words, moisture has not been a limiting factor during this six-year period. Mechanical and chemical practices. The spraying prac­ tice for the control of insects and diseases in the orchards, and 37 especially the trees which were used for investigational work, was exceptionally good. Scab control was held to less than 10 per cent on apples, and all diseases including brown rot on peaches were held to less than 0.1 per cent. Control of red banded leaf roller, curculio, codling moth, oriental peach moth, peach tree borer, mites, and other minor insects was better than 95 per cent. Choosing and Marking of Experimental Trees and Limbs Orchards IX, XI, and XII in 1946; IX, XIII, and XIV in 1947; and X in 1948, were all arranged so that complete trees were sprayed with each concentration of fruit-thinning spray, and similar trees in every way were used for control trees. Several limbs were used to obtain records which will be de­ scribed further in detail in the following: Orchards V, VII, and VIII in 1951, and III, IV, V, and VI in 1950 were all similarly arranged, using limbs at various locations on the tree for the several concentrations of one material. L i m b s were chosen only after careful consideration of location on the tree; diameter and age of the limb; whether the limb was growing downward, horizontally, or at a vertical 38 angle; and whether each limb selected possessed a similar n u m ­ ber of fruit buds. Enough limbs were chosen to correspond to the nu mb e r of concentrations or combinations that were to be tested. Enough trees were then picked so that each concentra­ tion would be located at each of the several positions on the tree as north, south, east, and west. This was necessary to alternate every spray material combination and limb position on the tree at least once (Fig. 3, page 47). In the case of the apples, the blossom spurs were counted, and in the case of the peaches, the fruit buds were counted. Counting was always started at the terminal end of the branch and proceeded to two-, three-, four-, five-, and six-year-old wood. No older than six-year-old wood has been used in gather­ ing these data. Limb s were m a r ke d with colored wax pencils, and a marked, weatherproof tag was fastened at that spot with strong cord. Choosing, Measuring, and Tagging of Apple Spurs Orchards I, II, III, and IV, and varieties McIntosh, Wealthy, Wagener, Jonathan, and Northern Spy were used in this phase of fruit-thinning research. O n each of three trees 39 of each variety chosen for this work, it was necessary to di­ vide the tree into three segments; one for check, and one each for the two different concentrations tested. The segments were alternated so that the check would be in each of the three di­ visions. ure 1. This plan is shown in graphic form on page 45, Fig­ Twelve fruit spurs of each size, 2 m m . , 3 m m . , and 4 m m . in diameter, were distributed from the very low limbs to those twenty feet from the ground, for each of the three seg­ ments of each tree. Each spur was measured with calipers which were calibrated to 0.1 millimeter. The spurs were m e a s ­ ured just below the flower and leaf cluster base. The reason that this particular part of the spur was used rather than the blossom bud was that by measuring buds and last year's spur growth at intervals throughout the spring dormant season, it was found that the bud size was a less reliable .index measure of tree vigor. The bud continually differentiates, and is m o r e af­ fected by dehydration and moisture absorption than the spur growth. Late spring measurements are necessary to be cer­ tain that buds being measured are fruit buds. The correlation of bud size to tree vigor in late spring is less reliable than it would be in winter dormancy. Measurements were m a d e on 40 last year's fruit spur growth, at intervals from D e c e m b e r through full bloom, with no change taking place in the diameter of last year's spur growth. Each spur, when it was measured, was tagged with a small wood tag with fine wire. Tags for each size were of a different color to m a k e it easier to find them later in the season. Thinning Sprays, Concentrations, and Application Orchards IX, XI, and XII in 1946; IX, XIII, and XIV in 1947; and Orchard X in 1948 were all treated with napthaleneacetic acid at concentrations of from 5 to 20 ppm. In Orchard X, on the varieties Wealthy and Winesap, 2,4-dichlorophenoxyacetic acid were sprayed at concentrations of 2.5 to 10 ppm. These plots were all sprayed with power sprayers of the high pressure type. To all varieties in all orchards mentioned above, the num be r of gallons per tree applied for the blossom-thinning spray is the regular amount applied for scab and insect con­ trol at this very same time of apple-blossom development. In Orchard X in 1948, the quantity of spray per tree was doubled, using a concentration of 10 ppm. 41 In Orchards I, II, III, and IV, naphthaleneacetic acid was used at concentrations of 10 to 40 ppm. This work in 1949 and 1950 required spraying of individual marked spurs. sprayer was used with a very fine nozzle. was thoroughly sprayed. A knapsack Each tagged spur Every spur on each tree of each vari­ ety tagged to be sprayed with a certain concentration was sprayed before changing to a new concentration. In Orchard I in 1950, the Wagener variety used for c h e m ­ ical analysis of the spurs was sprayed with 2 0 and 100 p p m of naphthaleneacetic acid, using a power sprayer equipped with a 3 5-gallon-a-minute pump, and spray distributed with a multiplegun spray mast. These 25-year-old trees received spray from all sides as they were circled with the sprayer continually forcing spray through the tree. Trees were dripping spray from all limbs, leaves, and blossoms. In 1950 in two orchards, namely III and IV, four apple varieties, Northern Spy, Wealthy, Jonathan, and Wagener, were treated with three hormones, sodium thiozole at 20 and 100 ppm, " d a r k " * at 20 and 100 ppm, and maleic hydrazide at 50 and 500 * A commercial sulfhydryl product obtained from the B. F. Goodrich Chemical Company. 42 ppm. The s am e year, in three peach orchards, namely III, V, and VI, Redhaven, Halehaven, and Kalhaven varieties were treated with naphthaleneacetic acid at 20, 60, and 100 ppm; with Clark at 20, 60, and 100 ppm; and with maleic hydrazide at 50, 100, and 500 ppm. In 1951, in two orchards, V and VII, Redhaven and Halehaven varieties were treated with only maleic hydrazide at 100, 200, 300, 400, and 500 ppm. In 1951, in apple Orchard VIII, Jonathan and McIntosh varieties were treated with 5, 15, and 30 p p m of naphthaleneacetic acid, and each naph­ thaleneacetic acid concentration was also combined with 5, 10, 100, and 1000 p p m of Vitamin K. All thinning research since and including 1949 was done on a limb basis, as earlier explained. The spraying of these limbs was done by using a knapsack sprayer. The limb was well wetted, and every precaution was taken to prevent drift to other tagged limbs. Each limb on all trees in the orchard that received the s am e concentration was sprayed from the one m i x ­ ture. Spray treatments were applied in full bloom or at calyx­ time for the most part. A full bloom spray will be considered in this thesis as the time when the center blossom petals would 43 fall when jarred. The calyx spray will be thought of when 75 per cent or m o r e petals have fallen. There was one series of thinning sprays applied to Redhaven peaches in 1950, Orchard VI, when the fruits were nearing the end of the first phase of fruit development. There were several intermediate spraying positions between full bloom and calyxtime since 1949. How­ ever, these are clearly described in other portions of this thesis when and where they appear of importance. Per Cent Set, Fruits per One Hundred Blossom Spurs or Buds In all orchards except I, II, III, and IV which dealt with spur size, the tagged limbs were carefully examined, and the apples and peaches that continued to mature after the June drop were recorded. The limbs were all re-examined for fruit counts again when fruit was half grown, and a final count was m a d e just prior to harvest. The total fruits were used in calculating fruits per one hundred blossom spurs. In those apple orchards dealing with spur size, the number of spurs to set fruit were counted and recorded according to size of spur. N u m b e r of spurs to set fruit divided by number of spurs measured and sprayed gave the per cent set of each spur size. Collection and Treatment of Spurs in Preparation for Chemical Analysis In Orchard I, in 1950, using the apple variety Wagener, ran do m spurs were gathered in one hundred-spur samples. Four samples were taken immediately prior to spraying. Each of four samples was taken by a random picking of spurs from all trees in the experiment. Forty-eight hours after spraying, four samples were taken in the same random manner, but this time from each of the three divisions: and sprayed 100 ppm. check, sprayed 20 ppm, After seven days, the same manner of spur-sampling was carried out. Immediately after gathering the spur samples, they were separated into two parts; the leaves and cluster bases m a d e up one part, and peduncles and flowers or fruits m a d e up the other part. Care was taken to not dis­ card any portion of the 1950 season spur growth. A s soon as the samples were divided and weighed, they were frozen at -20° F. They remained in this frozen condition until all samples were taken; then they were dried and each sample weighed, ground, and thoroughly mixed. T en-gram samples were used for the chemical analysis of the spurs for nitrogen, reducing sugars, nonreducing sugars, and starch. Row Number 20 ppm Check 10 ppm 10 ppm Check 20 ppm 20 ppm Check 10 ppm Check 10 ppm 10 ppm S Check 10 ppm 20 ppm 20 ppm 10 ppm Check 20 ppm Check Check 10 ppm 20 ppm Check 20 ppm Flgu re 1. Plot layout of* segment treatments for testing of blossom thinning sprays In relation to size of blossom spurs. Naphthaleneacetic acid applied at calyx time at concentrations Indicated. Thirty-six blossom spurs selected at random from (a) high limbs, (b) low limbs, made up of three sizes - 2 mm.; nun and U mm. 46 ,0 0 Rov/ Number [ ppm ] 15 ppm 20 ppm 2 5 ppm 30 ppm Check 25 PP« 30 ppm Tre*1 Numbe 3 Check 20 ppm U 5 0 © 0 0 © Check Figure 2* Plot layout of "entire tree" treatment used In testing naphthaleneacetic acid and thlozole as blossom-thinning sprays at concentrations noted; "Clark" at 20, 60, 100 ppm.; and maleic hydrazide at $ 0 , 100, 200, 3 0 0 f h o o t and 5 0 0 ppm. 47 Row Number Thi exol e **Cfark" <3 Figure 3* Plot layout of limb treatments for testing of growth regulator materials and their concentrations, ae indicated* Figure h* Branch of Wagener applo tree showing tagc used to identify the spur sized of 2, 3* and k mm, Fruit has been removed by thinning sprays. RESULTS The results gathered, on fruit-thinning during the sixyear period, starting in 1946, are presented in the form of tables, graphs, and pictures. Spur Size in Relation to Blossom-thinning Data gathered during the years 1949 and 1950, with respect to the correlation of fruit spur size of several apple varieties with the concentration of naphthaleneacetic acid (NAA) necessary to decrease the set of fruit, are presented in Table I. Spurs Z millimeters in diameter were consistently thinned the most, and those 4 millimeters in diameter were thinned the least, regardless of variety, year, stage of floral development, and weather conditions. O n the other hand, there were no ap­ parent differences in thinning of spurs located on the lower branches of the tree as compared with those on upper branches. Nor were there any apparent differences with respect to the side of the tree upon which the spurs were located. The effect of invigorating pruning on fruit-thinning is reflected in the Wealthy variety in Orchards II and III, as shown 50 in Table I and Figures 6 and 7. In the case of Orchard II in 1949, the Wealthy trees were exceptionally vigorous, due partly to severe pruning in the winter of 1947 and to the ideal g ro w­ ing season of 1948. The fruit spurs were not only larger, but undoubtedly stronger than average Wealthy fruit spurs of other years. O n the other hand, in Orchard III, the pruning in years past has been very light. The trees were quite brushy, and the fruit spur size was generally smaller. A s shown in Table I and Figure 6, the improved vigor in Orchard III was reflected in a higher percentage set. The question whether age of a tree is a factor in thinning depends upon the criteria that are used for age; that is, chrono­ logical age or physiological age. Generally speaking, older trees twenty years or m o r e of age thin m o r e readily than younger trees. This is borne out by the results here reported. Thus, in Table I, the ages of the trees vary from 14 to 35 years of age; yet, the trends in thinning appear similar. Since thinning results as shown in Table I were based upon the size of spur, which is a measure of vigor, the data are really comparisons of physiological age, and not chronological age. Even a young 51 tree m a y be low in vigor and older physiologically than an older tree of high vigor. Pollination and fertilization also bear s o me relation to the effectiveness of thinning sprays. Thus, in Jonathan Orchard IV and Figure 9, there was a lack of good pollinating varieties, and the set was accordingly low. Jonathan was thinned m o r e severely than Northern Spy when blossoms were sprayed with N A A at 10 ppm. Ordinarily, the Northern Spy variety thins m o r e easily than does the Jonathan. There m a y be a correla­ tion between number of ovules fertilized and the ease of thinning. There is also a difference between varieties. As seen in Table I, blossom applications of N A A at 10 p p m eliminated nearly all fruits from the 2-millimeter spurs of McIntosh, Jona­ than, and Northern Spy varieties, and reduced the set between 25 and 95 per cent on 3- and 4-millimeter spurs of these varieties. Applications of N A A at 20 p p m thinned these same varieties ex­ cessively. O n the other hand, applications of N A A at 20 p p m to blossoms of the Wealthy and Wagener varieties resulted in thin­ ning of most of the 2-millimeter spurs, and reduced the set of 3- and 4-millimeter spurs as m u c h as 50 per cent. During the 1949 and 1950 seasons, 20 p p m of N A A was a suitable concentration 52 for thinning of these varieties in Orchards II and III. It is of some interest to point out that when the concentration of 40 p p m of N A A was used on Wealthy blossoms in the vigorous orchard (Orchard II), it did not overthin as it did in Orchard III the following year. In Orchard IV with the Jonathan variety, ap­ plication of 20 p p m of N A A completely eliminated fruit on all spurs measured and tagged, and similar results were obtained with Northern Spy variety in the same orchard. with Jonathan m a y be traced to poor pollination. The results However, in the case of Northern Spy, the facility for thinning undoubtedly is due to a varietal characteristic, as this tendency has been observed m a n y times under commercial thinning procedures. Effectiveness of Maleic Hydrazide, Sodium Thiozole, and M ClarkT' Three relatively n e w materials were tried in 1950 for their effectiveness as blossom-thinning agents; namely, maleic hydrazide, sodium thiozole, and "dark." The last-named m a ­ terial is a commercial product obtained from the B. F. Goodrich Chemical Company. It is a sulfhydryl compound. Results with four varieties in two orchards are shown in Table II. The m a ­ terials were not effective as blossom-thinning agents. O n the 53 contrary, maleic hydrazide seemed to increase the set on Northern Spy, Jonathan, and Wagener trees, but did reduce fruit set to some degree with Wealthy. Applications of maleic hydrazide and " d a r k " produced no evident injury to foliage, fruit, or tree, either immediately after application or at any time during the growing season. The performance of ''dark11 as a fruit-thinning spray was inconsistent; it showed m o r e indication of thinning at 20 p p m on Jonathan and Wealthy than it did at 100 ppm. The material failed to thin Wageners at either concentration, and thinned Northern Spy mostly at the higher concentrations. Sodium thio­ zole appeared quite effective on Jonathan and Northern Spy, but on Wealthy and Wagener, it was less effective. Sodium thiozole, unlike " d a r k 11 and maleic hydrazide, did cause visual damage to the foliage at 100 ppm. and curled slightly inward. Edges of the leaves were burned Abscission of flowers was inhibited, and the green color was retained until the time of the June drop, when these flowers dropped along with other partially developed fruits. 54 Relation of Vita.rn.in K to Blossom-thinning It has been postulated, by some of the English workers that the response of a variety to a blossom-thinning material such as N A A m a y be related to certain materials present in variable amounts in the variety (17), fC. A m o n g these is Vitamin Accordingly, Vitamin K was applied in combination with several concentrations of N A A , with the results shown in Table III. The Jonathan trees were sprayed when just past full bloom with 5, 15, and 30 p p m of N A A in combination with 5, 10, 100, and 1000 p p m of Vitamin K. The sprays of N A A without the addition of Vitamin K increased thinning as concentrations rose. Because the orchard was young and in good vigor, the effectiveness of N A A was not as great as has been shown in orchards of lower vigor. The addition of Vitamin K to the sprays of N A A appeared to decrease thinning at the 100-ppm level of Vitamin K for the Jonathan variety. However, at 1000 p p m of Vitamin K. there appeared to be an increase in the thinning, as compared to N A A alone. This can be partially explained by the fact that on nearly every limb that was sprayed with 1000 p p m of Vitamin K, slight foliage-burning was observed, as well as some injury to pistils. The McIntosh block of Orchard VIII was treated exactly as the 55 Jonathan, except that it was sprayed, when about 70 per cent of the petals were off. The results were quite similar to those obtained in the Jonathan block, except that Vitamin K at 1000 p p m seemed to counteract the effectiveness of N A A at 3 0 ppm. This can possibly be explained by the fact that m o r e of the blossoms had been already fertilized before they were sprayed, so that the slight burning damage did not affect the set. Never­ theless, there appeared to be some countering action of Vitamin K on N A A thinning sprays during the 1951 season on two varie­ ties, as shown in Table III. Thinning with Naphthaleneacetic Acid on the Tree Basis Data on thinning apples with naphthaleneacetic acid over a three-year period are presented in Table IV. The Wealthy variety was used each of the three years; and in addition, Jona­ than, McIntosh, and Yellow Transparent were used one year each. In 1946, in Orchard IX, blossoms from Wealthy trees sprayed with N A A at 10 p p m set 17 per cent, compared with 26 per cent for the controls. In 1947, blossoms from the same trees set 36 per cent on the checks, as compared to 15 per cent 56 set when sprayed with N A A at 15 ppm, and 6 per cent when sprayed with N A A at 3 0 ppm. In 1946, there were no frosts, but the temperatures were cool, and weather rainy, during Wealthy bloom, and the orchard is a solid block of one variety. In 1947, the weather was m u c h m o r e favorable for pollination and fertilization. Wealthy Orchard XIII is located on a neighboring farm, and is quite similar to Orchard IX. Blossoms from check trees in this orchard set m o r e fruit than in Orchard IX. However, when the blossoms were sprayed with N A A at a concentration of 15 ppm, set was reduced to 16 per cent, as compared to 15 per cent in Orchard IX with the same treatment. This appears to be an excellent comparison of the materials, procedure, weather, personal observations, and choice of trees. Wealthy blossoms in Orchard XI set fruit fairly well in spite of cool weather during bloom, and when sprayed with NAA, were thinned sufficiently for the type of trees grown in the Cheboygan area, which are small and heavily pruned. Wealthy blossoms in Orchard X showed an increasing gradation in thinning with increasing concentrations of N A A . so happened that in Orchard X, no thinning was necessary It 57 because of the poor set due to cold, rainy weather throughout the bloom period. Blos so m set in Jonathan Orchard XI was low, due to cool weather and low vigor. Most of the set was on the large, 4-millimeter spurs which ordinarily do not thin as readily as do blossoms on small- and medium-sized spurs. The 17 per cent set caused from the sprays of N A A was about the correct percentage for a good crop. The blossoms in the McIntosh Orchard XII were thinned sufficiently when sprayed with N A A at 5 p p m in 1946. The ease of thinning in this orchard in 1946 was undoubtedly due to weather conditions during blossoming. A 27 per cent set of blossoms was a little high for the very heavy bloom which prevailed, but on the other hand, 12 per cent was a little low. Too m a n y three-inch McIntosh fruits developed on the overthinned trees, and reduced the production a little m o r e than is considered economical for mo st years. Yellow Transparent blossoms in Orchard XIV were thinned satisfactorily when sprayed with N A A at 15 ppm. Applications of N A A at 10 p p m did not thin blossoms sufficiently, whereas at 20 ppm, far too m u c h thinning occurred for an economical orchard 58 practice. It appears that during the seasons of 1946, 1947, and 1948, the cool weather which prevailed at blossom time in all sections where investigations were carried out was conducive to the ease of thinning. Chemical Analysis of Apple Spurs Following Application of N A A as Thinning Sprays Observation of numerous blocks of various varieties of apples which were thinned with N A A sprays during full bloom or at calyxtime exhibited wilting 24 to 48 hours after the treat­ ment. Retardation of spur growth occurred for several weeks on several varieties when spray thinned at full bloom, and occasionally, this happened when the spraying was done at calyx­ time. S o m e slight twisting of the leaf petioles occasionally ac­ companied the wilting. With the above observations in mind, the N A A spraythinned Wagener apple spurs were analyzed for nitrogen, total sugars, reducing and nonreducing sugars, and starch. The re­ sults of the analysis are on a dry unit weight basis, and given in Tables V, VI, and VII. Examination of Table V shows no definite trend of the effect of N A A calyx sprays on the nitrogen and carbohydrate content in Wagener apple blossoms, spurs, and 59 leaves. There was a slight proportional increase in the re­ ducing sugars in the check spurs collected seven days after treatment over the treated spurs. The amount of difference does not appear to be significant, as the variation in the rep­ licate samples could account for part of the difference. Con­ sidering the uniformity of the analyses in Table V, it can be concluded that any seeming injury to leaves, petioles, or cluster bases is not sufficient to affect the composition of the materials for which the analyses were made. Table VI shows the results of analysis of the flower parts and young apples for the same constituents as in Table V. Again the table is quite uniform in reference to the analysis of nitrogen, sugars, and starch for the 48-hour period. There was an in­ crease in dry weight of the check spurs seven days after treat­ ment, due to the growth of the apples which were not removed by the N A A spray. The treated spurs had very few fruits re­ maining, as they were thinned severely at 20 ppm. The addir tional fruits could also be the reason for the slight increase in \ nitrogen and reducing sugar as shown in Table VI. Table VII shows results of the analysis of the entire fruiting spur. No definite trend seems established. Minor 60 fluctuations m a y be partly due to errors in sampling. As a whole, the results again indicate no serious effect on the c o m ­ position of the parts analyzed. Further, the sprayed trees bore a full crop of apples the following year, and required thinning. Peach-thinning Trials in 1950 The thinning investigational work on peaches with "clark,” sodium thiozole, maleic hydrazide, and naphthaleneacetic acid at p H 2.5 was carried out with Halehaven, Redhaven, and Kelhaven varieties, at three distinctly different geographical areas. The results are given in Table VIII. N A A did not reduce the set of fruit in Orchards III or V on either the Halehaven and Kalhaven varieties. However, in Orchard VI, an excellent range of thinning was accomplished on Redhaven peaches. This thinning occurred after fruit was approaching the end of Stage I of fruit development. This thinning was accompanied by excessive d a m ­ age at all concentrations used at this stage of fruit development. However, the s a m e sprays used at an early bloom stage caused no damage, although they also resulted in no thinning. The in­ jury in Orchard VI was a very severe leaf-burning, and at the high concentrations, removed practically all the leaves. 61 Applications at 20 p p m resulted in severe damage, but the fruit matured to a fair quality. However, at the higher concentra­ tions, fruits failed to mature. With 11d a r k 11 there was no definite indication of thinning in Orchards III, V, or VI. sodium thiozole. Similar results were obtained with "Clark” did not affect the foliage, blossoms, or fruits at any stage of growth. Sodium thiozole, on the other hand, did burn foliage at high concentrations in Orchard VI, but gave very little injury, if any, where the spray was applied in bloom. Also, those limbs sprayed with sodium thiozole could be located very easily by the presence of adhering "shucks," which remained characteristically green and continued to grow until the time of the June drop. Sodium thiozole and " d a r k " were discarded from further trial. Maleic hydrazide, as shown in Table VIII, caused very distinct thinning in all stages of bloom, but had no effect after peaches were 3/4 inch in diameter. Thinning was effected at concentrations of 100 and 500 ppm. The reduction in set in Orchard V on Halehaven variety when sprayed at 90 per cent of full bloom was 64 per cent. In Orchard V, on the Kalhaven variety, which was sprayed when in 70 per cent of full bloom, 62 the set was reduced by 83 per cent. Twenty-seven fruits re­ mained for each one hundred fruit buds in Orchard V on the Halehaven variety, which was too m a n y for these vigorous tenyear-old trees to carry. Seventeen fruits left for each one hundred buds in Orchard III did not reduce the total yield per tree, since the increase in size of fruit compensated for the slight overthinning, the supply of fruit buds being light in this orchard. O n the other hand, nine fruits per one hundred fruit buds was slightly too m u c h thinning on the Kalhaven trees of Orchard V. The most desirable number of fruits per hundred fruit buds will vary some with age, size, vigor, pruning, and variety of peach, but the accepted range is from twelve to twenty. The effective period during which maleic hydrazide ap-^ parently thinned the fruits is a promising suggestion for further investigation. It appears from the results shown in Table VIII that this material will thin peaches when applied from 70 per cent of full bloom to calyxtime. Thus, in the 1950 season, as shown in Table VIII, maleic hydrazide did no damage. The foliage a week after spraying, at June drop, at harvest time, and also the following year, showed no ill effects from the 63 material whatsoever. In fact, the foliage on the limbs sprayed with 500 p p m was actually larger and m o r e luxurient than on the rest of the tree. This condition was undoubtedly brought about by the early removal of the heavy load of fruit that was taxing the tree. This is illustrated in Figure 13. Peach-thinning with Maleic Hydrazide, 1951 Maleic hydrazide was used again during the season of 1951 on the Halehaven and Redhaven varieties. Table IX shows clearly the sa me definite trend as that observed in 1950, although the reduction in set was less in 1951 than in 1950. It is evi­ dent that the concentration of maleic hydrazide desired for thin­ ning must be in the neighborhood of 500 ppm. The number of fruits to set per hundred fruit buds on the unsprayed trees was m u c h lower in 1951 (Tables VIII and IX). The thinning in O r ­ chard VII on both varieties was not sufficient even at 500 ppm. Although the per cent reduction of set was significant for the Halehaven peaches of Orchard VII, there were still too m a n y fruits set per hundred fruit buds. The situation was similar for the Redhaven variety, although the number of fruits set per 64 hundred, fruit buds was within the limits acceptable for a c o m ­ mercial crop. It should be explained that the heavy set in 1951 was due to a combination of factors. First, the trees were heavier with buds than in previous years. Second, no pruning was done in 1951, so that there was a larger producing area. The per cent reduction of set, the number of fruits to set per hundred fruit buds, and the pruning practices in Orchard V on both Halehaven and Redhaven varieties were all similar to Orchard VII. Even with a 16 and 13 per cent set respectively, it was necessary to do s om e hand thinning. There was no visual damage to any part of the tree or to the fruit. The luxurient growth of the thinned limbs in 1950 was, however, not evident in 1951. This can probably be ex­ plained by the fact that the reduction in set was not as great in any case as it was in 1950. Also, the trees were pruned in 1950 and not in 1951; pruning would help improve the general vigor of the entire tree. An additional consideration is the fact that all three of the varieties used responded similarly, indicating that maleic hydrazide m a y have a wide varietal range of usefulness. 65 Table I. No. of Spurs Thinning effect of naphthaleneacetic acid, sprays in relation to size of spurs of McIntosh, Wealthy, Wagener, Jonathan, and. Northern Spy apples during late bloom in several areas in Michigan. Size of Spurs* (mm.) Spray Concen­ tration (ppm) No. of Spurs to Set Fruit Set (per cent) Conditions of Blossoms at Time of Spraying Mclnto sh Orchard I, 1949 31 31 32 2 3 4 Check Check Check 2 13 17 6.45 41.93 53.12 32 33 33 2 3 4 10 10 10 0 6 12 6.45 18.18 36.36 36 34 36 2 3 4 20 20 20 0 3 9 0.00 9.00 25.00 Full bloom, only few center, blos­ soms dropping petals; temper­ atures above 70° F. during the daylight hours for 48 hours prior to spraying; one night frost in low areas. W ealthy Orchard II, 1949 34 35 34 2 3 4 Check Check Check 3 11 25 9.00 31.43 73.53 31 32 36 2 3 4 20 20 20 0 8 16 0.00 25.00 44.44 32 35 34 2 3 4 40 40 40 0 7 14 0.00 20.00 41.18 Late bloom, 95 per cent petals off; temperature: 70° F. or above during the day­ light hours for 3 days prior to spraying; t e m ­ peratures 40-50° F. for 48 hours following sprays. 66 Table I (Continued) No. of Spurs Size of Spurs* (mm.) Spray Concen­ tration (ppm) No. of Spurs to Set Fruit Set (per cent) Conditions of Blossoms at Time of Spraying Wealthy Orchard III, 1949 36 36 36 2 3 4 Check Check Check 14 28 35 38.88 77.77 94.44 36 36 36 2 3 4 20 20 20 7 7 14 19.44 19.44 38.88 36 36 36 2 3 4 40 40 40 0 3 3 0.00 5.55 8.34 Late bloom, 85 per cent petals off; temperatures above 70° F. dur­ ing the daylight hours for 72 hours prior to spraying; 60 per cent center blos­ soms already fertilized. W agener Orchard III, 1950 36 36 36 2 3 4 Check Check Check 5 16 32 14.00 44.44 88.88 36 36 36 2 3 4 20 20 20 1 5 10 2.80 14.00 28.00 36 36 36 2 3 4 40 40 40 0 2 7 0.00 5.55 19.44 Late bloom, 95 per cent petals off and 65 per cent of center blossoms ferti­ lized; tempera­ tures 70° F. during daylight hours for 72 hours prior to spraying. 67 Table I (Continued) No. of Spurs Size of Spurs* (mm.) Spray Concen­ tration (ppm) No. of Spurs to Set Fruit Set (per cent) Conditions of Blossoms at Time of Spraying Jonathan O rchard IV, 1950 36 36 36 2 3 4 Check Check Check 7 13 12 19.44 36.11 33.34 36 36 36 2 3 4 10 10 10 0 6 6 0.00 16.67 16.67 36 36 36 2 3 4 20 20 20 0 0 0 0.00 0.00 0.00 Late bloom, 50 per cent of center blossoms fertilized; 75 per cent of petals off; t e m ­ peratures 70° F. or above during day­ light hours of the blossom period. Northern Spy Orchard IV, 1950 36 36 36 2 3 4 Check Check Check 4 7 16 11.10 19.44 44.44 36 36 36 2 3 4 10 10 10 2 5 12 5.55 14.00 33.67 36 36 36 2 3 4 20 20 20 0 0 0 0.00 0.00 0.00 Full bloom, 25 per cent of center blossoms fertilized; t e m ­ perature above 75° F. during the daylight hours for 2 days prior to spraying. * Diameters of the previous year's growth. Orchard Orchard Orchard Orchard No. No. No. No. I II Ill IV - Ingham County - Grand Traverse County - Newaygo County - Oakland County 68 Mc INTOSH Orchard I Percent Set 2 mm Buds* 3 mm Buds 4 mm Buds • • 1949 \ 1949 \ Parts Per Million WEALTHY Orchard IT 2 mm Buds 3 mm Buds 4 mm Buds Percent Set 7 0 -*, 1949 30 0 20 30 Parts Per Million WEALTHY Orchard HI 100 £ 50 • 1950 30 Parts Per Million 10 40 71 WAGENER Orchard H I 100 2 mm Buds'* 3 mm Buds^ 4 mm Buds^ 90 » 80 ■• 70 Percent Set 60 t 50 \I9 5 0 10 20 30 Parts Per Million JONATHAN Orchard ET 70 2 mm Buds 3 mm Buds 4 mm Buds 60 Percent Set 50 40 •N 30 1950 20 10 O 10 Parts Per Million 20 73 NORTHERN SPY Orchard 1ST 70r 2m m Buds 3 mm Buds 4 mm Buds 60 Percent Set 50 ••*1 9 5 0 40 - 1950 10 Parts Per Million 20 74 Table II. Thinning effect of maleic hydrazide, thiozole, and " d a r k " on Northern Spy, Jonathan, Wealthy, and Wagener apples. T reatment F ruits per 100 Blossoming Spurs Stage of Blossoms when Sprayed Northern Spy Orchard IV, 1950 Check 20 p p m " d a r k " 100 p p m " d a r k " 45 40 32 Check 20 p p m thiozole 100 p p m thiozole 44 32 29 Check 50 p p m maleic hydrazide 500 p p m maleic hydrazide Blossom petals 25 J per cent off;* r center blossom fertilized. 41 47 47 Jonathan O rchard IV, 1950 Check 20 p p m " d a r k 11 100 p p m " d a r k " Check 20 p p m thiozole 100 p p m thiozole Check 50 p p m maleic hydrazide 500 p p m maleic hydrazide 72 57 58 68 50 37 64 56 86 Blossom petals 90 per cent off; 75 per cent of center blossoms fertilized. 75 Table XI (Continued) Fruits per 100 Blossoming Spurs Treatment Stage of Blossoms when Sprayed Wealthy Orchard III, 1950 Check 20 p p m " d a r k " 100 p p m " d a r k " Check 20 p p m thiozole 100 p p m thiozole Check 50 p p m maleic hydrazide 500 p p m maleic hydrazide 56 50 53 55 60 60 Blossom petals 85 per cent off; 65 per cent of center blossoms fertilized. 49 39 47 Wagener Orchard III, 1950 Check 20 p p m " d a r k " 100 p p m " d a r k ” Check 20 p p m thiozole 100 p p m thiozole Check 50 p p m maleic hydrazide 500 p p m maleic hydrazide 51 60 66 52 60 59 50 45 58 Orchard IV - Oakland County Orchard III - Newaygo County "Clark" = sulfhydryl compound Thiozole = sodium thiozole M. H. = maleic hydrazide Blossom petals 95 per cent off; 75 per cent of center blossoms fertilized. 76 Table III. Thinning effect of naphthaleneacetic acid in combi­ nation with Vitamin K on two varieties of apples during blossomtime. Naphthalene acetic Acid (ppm) Vitamin K (ppm) F ruits per 100 Blossom Spurs Conditions of Blossoms at Spraying Time Jonathan Orchard VIII, 1951 5 5 5 5 5 5 0 0 5 10 100 1,000 48 30 31 29 45 30 0 15 15 15 15 15 0 0 5 10 100 1,000 45 26 25 31 33 28 0 30 30 30 30 30 0 0 5 10 100 1,000 46 15 11 20 27 15 30 per cent petals off, 85° F., dry, clear. 77 Table III (Continued) Naphthalene acetic Acid (ppm) Vitamin K (ppm) Fruits per 100 Blossom Spurs Conditions of Blossoms at Spraying Time McIntosh Orchard VIII, 1951 0 5 5 5 5 5 0 0 5 10 100 1,000 55 32 31 34 36 22 0 15 15 15 15 15 0 0 5 10 100 1,000 52 23 19 19 28 19 0 30 30 30 30 30 0 0 5 10 100 1,000 57 9 12 16 23 33 Orchard VIII - Grand Traverse County 70 per cent petals off, 80° F., dry, clear. 78 Table IV. Spray Concen­ tration (ppm) Thinning effect of naphthaleneacetic acid, sprays on Wealthy, Jonathan, McIntosh, and Yellow Trans­ parent apples at several locations in Michigan. F ruits per 100 Blossom Spurs Reduction in Fruit Set (per cent) Condition of Blossoms at Time of Spraying Wealthy Orchard IX, 1946 Check 10 26 17 35 Full bloom Full bloom Wealthy Orchard XI, 1946 Check 10 15 41 27 24 34 42 1'Calyxtime'' '1Calyxtime '1 1'Calyxtime'' Wealthy Orchard XIII, 1947 Check 10 15 20 44 25 16 3 43 64 93 ''Calyxtime'' ''Calyxtime'' ''Calyxtime'' 1'Calyxtime '' Wealthy Orchard IX, 1947 Check 10 15 20 36 18 15 6 50 60 83 1'Calyxtime'' ''Calyxtime '' ''Calyxtime'' ’'Calyxtime'' Wealthy Orchard X, 1948 Check 10 10* 15 20 23 11 7 6 3 52 70 74 87 Full Full Full Full Full bloom bloom bloom bloom bloom 79 Table IV (Continued) Spray Concen­ tration (ppm) F ruits per 100 Blossom Spurs Reduction in Fruit Set (per cent) Condition of Blossoms at Time of Spraying Jonathan Orchard XI, 1946 Check 10 15 17 14 10 — 18 41 '1Calyxtime'' ''Calyxtime '' ''Calyxtime1' McIntosh Orchard XII, 1946 Check 5 10 39 27 12 _ _ _ _ _ 31 70 Full bloom Full bloom Full bloom Yellow Transparent Orchard XIV, 1947 45 32 18 6 Check 10 15 20 — 29 60 86 * Twice as m a n y gallons per tree. Orchard Orchard Orchard Orchard Orchard Orchard IX X XI XII XIII XIV ^ - Oceana County Berrien County Cheboygan County Cass County Oceana County Jackson County ''Calyxtime 11 1'Calyxtime'1 ''Calyxtime '' 1'Calyxtime'1 V. Effect of naphthaleneacetic acid sprays on the nitrogen and carbohydrate content of Wagener apple-blossom spur leaves, petioles, and cluster bases, (Applications mad e at 1'calyxtime,M Orchard I, 1950.) 80 ,P o p d + -> CO to P* to d > d d i— I J, p * S o ™ P CO p d A H Pa CO vO o o 00 03 o m vQ m o O o o O o o 'JD O f t • f t • o ft co 1— 1 in m m o o o o o o o o o t • *o • • f i— i r- t— r-H o rH o o oo o r-H 1— 1 i— i o i—H r-H f— i o o o of t o. * • o o c r *o O o r-H r-H o o o -• ft o* o• r-H 0o o 00 r— o o r- in c0 CO oo o CO CO o o o o ro o r- CO m o o o o ft ft • • • 00 ro o o o o o of o O o t ft ft ft ft 04 CO tn 03 0~ - 00 00 vO r CO 0o o vQ P •d to GO O Pi a p O "P Pi PQ ^p o Q o p P qp CO NO o ft o • vO vO o o ■ P! CD d d Pi H o sO o o • ft o oo o o 03 r~ oo o o ft o • o • r CO 0o o 0-. ooft o ft ft o CO CO 03 ft CD CO o r—f O' vO • ft f t ft ft rH 04 *-H I-H 03 03 03 04 03 o 00• m« oo• CO 04 CO • * cO CO 1— 1 04 04 03 03 2 O 03 r— 1 04 < C r-H 00 Pi w d d u A d + j d d p CP w PP % P ^ M P2 h d . P d d a 00 CP CO Ji p P d o o d p f—• r-P o N 04 03 03 > > < to CD CO ft to 03 co vO oo to o o ft o • o ft o • o o CO c t i d to PQ d p d d 0vO o o • -+j r-H Table oo vO vO cO r~cO CO CO CO o o O O o o o O o o • * • o • 81 J3 O Pi t 4-U > co cd co O o o o o o o o o o o CO PQ O 'O . —i eg N N N N eg 00 eg o o * rcO o o • o (M O O• CO eg o o • t"rg o o • vO vO . — L eg o o o■ o eg (Nl rg rg fM o o o o o o p CO P u rt CO Pi 40 -) > r— I ) 4f-t> i p o £ § 00 p Pi cj o c00 p P X5J co Jh •H 00 •*H m 4) 00 vO o o o O o o o o O o ft • ■ m in o o• rfl r-H O' m o o o o• * — t 00 t'- t'» o o o •_o * in CO rH O' in HP o o o o o • of t o • o ft m in n o CO ID 00 ^ rP o o o o o o o o o o o CO ft) 00 .aO chcj ft) a, P 00 TP P ft) CO CO ffj ft) Pi o 00 co m H* vO o o o o o• o• • rg ro o* o 00 o o• in ro o• 00 -4) O O« 00 M 00 o o o o o• o o o C O ■ t H ft) •iH P O O rP p< o3 o £p f—j ^ r* co Pi P PM CO rH l> o H-1 0s o r-H r-H r— H o rH rH H of t o • o• o• o ♦ CO 4> rg CO in CO rH r-H r-J rg r-H rH rH rH rH o* o o o o• * * • co CO rg m r- in o i— t rH rH rH o * o O o ft o ft • rH rH rH rH ft) P ft) +-» PM r-H P P co ft) 00 o rg O' r- %o r- m o o o o o o ft o • o * • o u Pi •H O <+H H O o *i s £ 5 >.« a § a co o Pi Pi a i T3 nJ £ co CO ^0 o o r- vO ft • • • * in o .in O ' rg rH rg rg rg rg CO o rg r~ oo 00 r- r- i — o o o o o o ft o o * o ft o CO o Pi •H 00 < ft) m so ^ oo ^ h-< ^ rg• nO0 • i—«i S0 vO rg CO rg rg rg rg rg W > > < PM a nj V0 o H 1 i—i • • ft • O' o co in rrg CO rg rg N > isoa a) rH O' rg r- 00 o o o ft o ■ Wt) "g f Pi pi o to o ft p p ft) CQ p H o O' o o • _j H ft) SO o o • O' oo o o • ft) i> ro oj » « 0) Pi J ft) + ■> u CO p pH T" CO o -M nO r- ^ +J £ S'■m Pft) C fli P PM £ p. « * P P <3 r-H ft) oo a r -) H rH o U VI, Effect of naphthalene ace tic acid sprays on the nitrogen and carbohydrate content in Wagener apple-blossom spur flowers, young fruits, and peduncles. (Applications made at "calyxtime,M Orchard I, 1950.) 82 u JU -cfdj CO TJ (U a, TJ cd bo j, .a cd ti u O CJ bo J £ - > <3 r-H H > < < CO 00 u c u *3 Jd 0) u cu 2 r^ O O & 01 cd cu h H 0) CU a .—i Table co o o o cd CO m vO r - oo 00 0 ^ l4 £H d H g a cd cu cd cu a mI CU CU 01 o 2o c^u CM A £ 9U 3 O O ^ rd O cO 'f m -vO O ■ —i fM o 83 -4 u u Grams per Spur Blossoms, Fruits, and Peduncles — ----------------------------------------------- c d m 0o o o • 00 o o o ° O'' o o o • no o o a r-H 00 o o o • o o o • 04 o o o • r-H o o o a oi rH i-H CO o o o o o o o o o o o o o o o o o o o o o a • CO N h +c-u> -c+dj CO Pi cd GO a u ctdoo 3 3 5 in •rl m o o o o o o o 04 * u GO £ . o 3 cd Go TJ 2 CO O o a 00 in o o a r- o vO o H 4 t-H C O 0 4 o o o o o o o o • * • a o H 4 o o a d 4 cO oa 04 CO o o a 04 o oa o o o O' v£> CO vO r-H o o o o o o o o m CO o o • CO • • a CO d 4 o o a d 4 CO o o • r-H r-H 04 o o a d 4 o o • 03 cd PU CU p a PC. $ O g> E 17H1 C5O 03 ro o a o o a m o o o a t-H d 4 o o * a cu U cu u 5 pj cu GO o u ON d 4 O o a O' o -d4 m o o o o a a ON ■d4 o o a r-H m o o a ■ 00 CO o o a ON vO CO CO o o o o a a ON CO o o a CO cO o o • rH H 4 o o a CO ,d 4 o o a m cO o o a 03 O o u o •H o o GO < ■d4 • u O' o vO o > O O' N O' uD o O'- oo o CO • * • • > > < <1 co A ±cd ^ cd cu H Q JO CO Jd * u -M (U TJ ^ 1 fH cd ti < vO C" CO ^ ^ ^ ^ O' O H M m ixi m ro ^ cn ^ in io m m cu GO CU r^H #-H o U Table VII. Effect of naphthalene ace tic acid sprays on the nitrogen and carbohydrate content of entire Wagener apple-blossom spurs. (Applications made at " calyxtime,ff Orchard I, 1950.) 84 Ao u -4-> CO u b0 I' *H d ” d o 03 o d g> d P CO s 0 A « CO 00 CO CO 0 0 0 0 . ♦ 0 0 0 0 0 0 i—4 CM co sO Lfl lfl Lfl 0 0 0 0 0 0 0 « Lfl 0 • 0 • O O • xh MO t"lfl lfl 0 0 0 0 1— 1 MO O O bo d .^ ,U§ gS i-H rH I> i—4 CM H 1— 4 O i-H i— 4 i— 4 r— l 1—4 O O O O 0 00 • CO • « * Lfl 00 O CM• 00 . • —1 CM co (M CM 1 co CO CO CM CO O 00 CO rH r- 0 M0 0 pH • • • 6 • CO cO CM r-| co CO CO CO CO > U rH W Q 2 o •“ I • 1— t 00 l— t CM rH i — t rH p H pH 0 O O O • 0* * . • 0 CM rH lfl O pH I— t CM O pH O • pH O'- r- CM « • • • CM 0 0 CM CM CO CO co « > > < < < pH rH pQ •+J d < u u 00 01 + 0j H <11 d rb r 1 U p°d o o CM 43 in o t- co O' O a a 00 tH ^ <0 -M h d p P £ d cd * ^I aco Q^ pH i-H in 00 . a CM CO O' CO o CM CO rH CO CO CO P -t-> CU H c u P i-H I CO cu d fl p MCU P cu S ^. s ^ P cd P O Cd u CM X -2 U Tl +J if ) sO N O' o t"- ^rtl ^P p CO CM m MU m u o CO CO CO CO CO > < > c O 4Sh o rH pq c d H CO l (O 'ct* lO O lf l if i lf l Department, Michigan tap I •PH p o cd bo O P P £ S3 ^ P CO P -t M Chemistry u i> oo m oo o by Dr. E. J. Benne, Agriculture ,P o J4 ■c +d j CO State 85 if i o UJ x< UJX J O < -O36 a «H >s O d < dp P V* ft c © TO « r » iH > t p t * 0 © u P ©0 33 , , 0 a sP © ftK 2 ;o f "J £ ■ f C; p< pvo e cd ( —1 C d c U _ '*d p.« K C D © P g i s o Pi G < 5 5 © c P" J U e ga: T ; > ~3 C ■«. ;:;-j, -r- 36 «€ > O’ -S ~ r t ri« 4> t x . ; ? ■ ? “ *X *K f * >>P UH *H c i y-> C r iC % '-+u u. 4 r«' r< p., ** 101 DISCUSSION Spray Thinning of Apples It seems evident that such factors as climate, soil and nutrition, physiology, variety, and economics must be consid­ ered in any problem dealing with growth-regulating substances ! as blossom-thinning materials for the apple and the pear. Climatic factors. Temperature, rainfall, and sunlight can hardly be considered separately. They are certainly inter­ related in their effect on set of fruit under Michigan conditions. L o w temperatures (below 65° F.) are not conducive to pollina­ tion or fertilization of the flower. Anthers will not mature and discharge viable pollen until temperatures approach 65° F. Rain­ fall at blossomtime is invariably accompanied by a cool period of three to five days with temperatures below 60° F. Further, rain m a y destroy viable pollen, as pollen in water will swell and 11explode'1 in a very short time. Light is also a factor in fruit set. It is involved in various physiological processes, energy relationships, photo­ period, and other complex and perhaps little understood phenomena 103 involved in plant behavior. During the six years that this inves­ tigational work has been carried on, various degrees of climatic factors have been observed to be very important. For example, temperatures below 60° F., excessive rainfall, and very little sunlight are conducive to easier thinning, often resulting in over­ thinning. L o w e r concentrations of the thinning agent, as naphtha- leneacetic acid, must be used under such conditions. Batjer (2) and others have observed that well cared for orchards of cer­ tain varieties will set well, regardless of adverse weather con­ ditions during bloom, but that poorly cared for trees m a y thin very easily. Inspections of fruit from easily thinned trees has shown an average of one to three seeds per fruit, compared to four to seven seeds in fruit set under favorable weather condi­ tions. Frost is another factor in fruit-thinning; in fact, it be ­ haves in a m an n e r suggestive of N A A . Thus, in the spring of 1949, observations were m a d e of frost effects in Wealthy O r ­ chard I. Trees located in the hollow or low spot in this or­ chard were frosted at different degrees of intensity at differ­ ent heights on the tree. The topmost blossoms of the tree were very slightly frosted, and the bottom blossoms were very heavily 104 damaged by frost. S o m e of these blossoms had been tagged according to spur diameter preparatory to spraying with N A A . Because of the frost damage, the trees were not sprayed. When the fruit was half grown, it was observed that there was an oc­ casional apple on the upper half of the trees, and these fruits were c o m m o n l y found on large spurs (4 m m . in diameter) as compared with smaller-sized spurs (2 to 3 mm.). Frost is thus also a selective thinning agent, removing the blossoms from weak spurs first. This observation suggests that fruit growers should be less concerned about loss of a crop from frost injury follow­ ing the use of thinning sprays, inasmuch as it is the weakest blossoms which are removed in either event. servation was m a d e in an orchard in 1949. A confirming ob­ Part of the orchard had been sprayed with a thinning spray when frost forecasts were received. The remainder of the orchard was left unsprayed. Three nights of frost followed, and slight to heavy damage oc­ curred; yet the fall harvest showed no difference in crop b e ­ tween the thinned and unthinned frosted trees. Factors of soil and nutrition. Soil and nutrition are also factors in determining the effectiveness of biossom-thinning sprays. 105 Both of these factors, in turn, affect the vigor of the tree, and it has been shown that trees in good vigor are less easily thinned than trees in poor vigor (Table I). Well cared for or­ chards which receive annually sufficient fertilizer to satisfy tree requirements, and which are provided with sod covers or sod and mulch systems to preserve moisture and prevent ero­ sion and loss of valuable top soil, develop strong vigorous fruit spurs. In the orchards included in the present study, no drain­ age problems were found, but according to other investigators (3), trees growing on poorly drained soil have thinned at m u c h lower concentrations of growth-regulating sprays than trees on well-aerated soils. It is probably that soil aeration is affected under these conditions, with accompanying reduction in oxygen supply, respiration, and nutrient uptake. Reduced vigor and reduced fruit set would follow. Physiological factors. The effect of pruning is associated with vigor, age of spurs, and productive wood of the tree. No one has explained satisfactorily the physiological effects of pruning on the set of fruit. However, a correlation has been shown in the present studies between spur size and fruit set, repeated on several varieties at several geographical locations 106 and for different degrees of pruning. Thinning in every case was m o r e easily attained as the size or vigor of fruit spurs decreased. The observation that weaker wood or smaller buds thin m o r e easily than vigorous wood is substantiated by Batjer (3) and M urneek (48). Whether young trees would thin with greater difficulty than older trees was not an important consideration in the pres­ ent studies. However, it has been found that an overly vigorous, succulent young tree may, under certain conditions and with cer­ tain varieties, thin as easily as a weak tree. There is evidently m u c h m o r e yet to be explained as to the relation between the physiology of the tree and the responses observed from thinning with growth regulators. It is interesting to note in the tables of chemical analysis (Tables V, VI, and VII), concerning Wagener apple spurs sprayed with 20 and 100 p p m of naphthaleneacetic acid that no signifi­ cant differences appear in nitrogen, starch, or sugar content of the spurs, either 48 hours or seven days after treatment. Nev­ ertheless, wilting and petiole curvature was evident at 48 hours, and curvatures persisted for seven days (Figure 19). It seems reasonable to postulate that naphthaleneacetic acid might alter 107 the carbohydrate-nitrogen ratio in treated plants. Yet such does not s e e m to have been the case (Tables V, VI, VII). It appears that the thinning action by growth regulators is m o r e likely as­ sociated with some sensitive mechanism in the plant such as complex hormone, enzyme, or vitamin relationships which ac­ co mp an y fertilization and fruit set. The idea of such relationships prompted the use of Vita­ m i n K in conjunction with naphthaleneacetic acid thinning sprays on two varieties in 1951, the results of which are given in Table V. While there is some indication of an antagonistic effect of Vitamin K at the higher concentrations, m o r e research on this phase of the problem will need to be done before any conclusions can be drawn. The Wagener apple variety contains three times as m u c h Vitamin C as the McIntosh variety of apple. The Stur- m e r variety contains m o r e Vitamin C than do citrus fruits. This fact is mentioned to indicate the wide variation in content of particular constituents in the apple. It is reasonable to sug­ gest that as wide a variation exists also with other vitamins, hormones, and enzymes in different apple varieties. Varietal factors. It has been definitely shown that s ome varieties thin m o r e easily at lower concentrations of growth- 108 regulating sprays than do others. It is shown in Table I and in the six following graphs illustrating Table I, and also in Table VI, that Jonathan, McIntosh, and Northern Spy blossoms were thinned m o r e heavily at lower concentrations than were Wealthy, Wagener, and Yellow Transparent. It has been noted in other parts of this thesis that there are m a n y variations in the same variety under different conditions. All varieties tested are m o r e susceptible to thinning and wilting and petiole-curling when sprayed at full bloom than when sprayed at calyx. However, some vari­ eties are m o r e resistant to wilting than others. For example, Figure 19 shows the difference between untreated Wealthy and Oldenburg branches and those sprayed with N A A . The Olden­ burg variety shows m u c h m o r e wilting and petiole curvature than the Wealthy variety. This condition has been observed to be accentuated when spraying is done during full bloom as c o m ­ pared to spraying at calyxtime. observations. Davidson (6) has m a d e similar It has also been observed that wilting and epi- nastic curvature are m o r e evident in years of cool, d a m p blos­ s o m periods than in sunny, w a r m blossom periods. This m a y be associated with greater synthesis of other hormones, vita­ mins, or enzymes by the plant under conditions of sunshine and 109 w a r m e r weather. Riboflavin, for example, is known to inac­ tivate indoleacetic acid in the presence of light. The selective action of 2,4-dichlorophenoxyacetic acid is of s om e interest. Used at 2.5, 5, and 10 ppm, this material did not affect the McIntosh and Jonathan varieties, but on Staym a n the effects were severe. The Stayman trees were not only thinned, but tree growth was seriously checked for six weeks. Figure 20 is f rom a photograph taken six weeks after spraying. Economic factors. One of the features of thinning at blossomtime or soon after is the strong tendency that this treat­ ment has to develop annual bearing. Thus, Wealthy Orchard IX has been spray thinned with growth regulators from 1946 through 1951. This orchard, not being too vigorous, has thinned excel­ lently every year with 10 to 15 ppm, and has not missed a good crop of fruit during this six-year period (Figure 15). The Wealthy variety has a strong biennial bearing tendency, but this early thinning, before the fruit has set, has resulted in r e m a r k ­ able uniformity in cropping. It has been observed, however, that as this hormone thinning is continued for several years, the trees b e co me m o r e difficult to thin. This is because the vigor of the trees has been improved and larger diameter spurs 110 have been found which, as is shown in Table I, are m o r e diffi­ cult to thin than small-diameter spurs (2 to 3 mm.). The ob­ servations have also been m a d e that spray-thinned trees carry a greater load of larger apples than do similar hand-thinned trees. Co mmercial growers and investigators in several sec­ tions of Michigan, and in other apple-growing areas of the United States, have reported similar experiences. Annual bearing, larger size fruit, and greater yield are significant contributions to c o m ­ mercial fruit-growing. The cost of thinning has been reduced tenfold by spray thinning as compared to hand thinning. This saving in itself could easily be the difference between profit and loss in a large commercial apple orchard. In 1951 nearly one-quarter of the apple orchards of Michigan were spray thinned. Spray Thinning of Peaches M u c h less investigational w ork has been done in the thin­ ning of peaches with growth regulators than with apples. Such trials as have been m a d e have not given satisfactory results. Likewise, three of the materials reported in this thesis (NAA, sodium thiozole, and "dark") failed to give the desired results. Ill O n the other hand, maleic hydrazide showed promise when used at 500 ppm. But there is no indication that the thinning is se­ lective as regards vigor; Otherwise there would have been m o r e severe thinning in 1951 of trees which had been weakened by winter injury in the latter part of November, 1950. It should be noted that the greatest thinning in 1950 was when the spray was applied before 75 per cent of the buds were open. In 1951, no spraying was done until practically all the blossoms were fertilized. It will be necessary to m a k e m o r e definite m e a s u r e ­ ments of vigor, stage of development, concentrations of spray, climatic factors, soil factors, and perhaps complex physiological factors before it will be reasonably safe to r e c o m m e n d maleic hydrazide for use as a general thinning agent for peaches. It is known, and has been demonstrated, that maleic hydrazide will remove all of the blossoms f ro m s o me deciduous shade trees at 1,000 p p m without apparent damage to the tree. It m a y be possible to use naphthalene ace tic acid for thin­ ning peaches if applied at the time of the June drop. Table III shows in Orchard VI that thinning was accomplished with this material although with excessive damage to foliage. However, it is felt the low p H used m a y have accounted for some of the 112 burning. Further, other investigators have obtained occasional success with naphthaleneacetic acid without excessive burning. Correlation studies should be m a d e of the length and thickness of terminal growth and stages of bud and flower development in relation to concentration of maleic hydrazide. It is felt that the limited success with maleic hydrazide reported here merits detailed study. SUMMARY Naphthaleneacetic acid has been shown to be effective as a blossom-thinning spray for apples in Michigan. Such factors as climate, variety, soil, nutrition, physiology, spray concentra­ tion, and time of application influenced the degree of thinning. A direct relationship was found between spur size and the degree of thinning, large spurs (4 m m . in diameter) thin­ ning with greater difficulty than smaller spurs (2 to 3 m m . in diameter). Although there were visible responses of the apple to applications of N A A as indicated by leaf curvatures, defloration, and persistence of certain floral parts, there was no apparent effect upon chemical composition as measured by content of nitrogen, starch, and sugars. It is concluded that any chemical alterations are minute in quantity, and of the nature of growth regulators, enzymes, or vitamins; which are not determined by gross chemical analysis. Other regulating sprays tested for thinning action on ap­ ples were either ineffective, such as maleic hydrazide, sodium 114 thiozole, and "dark," or they caused excessive visual damage to the tree and fruit as did 2,4-dichlorophenoxyacetic acid. M o r e visual wilting and petiole curvature from spraying at full bloom in comparison to later sprays was noted on Olden­ burg variety than on Wealthy variety. Naphthaleneacetic acid sprays were undesirable for thin­ ning peaches, and is considered still in the experimental stage. Maleic hydrazide was found to thin peaches quite well at 500 p p m during the seasons of 1950 and 1951, although the set of fruit was still too heavy in 1951 after thinning. N o visible injury was observed from the use of maleic hydrazide on peaches at concentrations up to and including 500 ppm. Thinning peaches with maleic hydrazide appears promis­ ing, but must be considered still in the experimental stage. L I T E R A T U R E CITED Batjer, L. P., and Thompson, A. H. Further studies with 2,4-dichlorophenoxyacetic acid sprays in retarding fruit drop of Wine sap apples. Ame r. Soc. Hort. Sci. Proc. 49l . 45-48. Batjer, L. P. Fruit thinning with chemical sprays. Circular Bui. 867. 1951. USDA Berger, J., and Avery, G. S. Isolation of an auxin precursor and an auxin (indoleacetic acid) from maize. Amer . J. Bot. 31: 199-203. 1944. Burkholder, C. L., and M c C o w n , M. Effect of scoring and of alpha-naphthaleneacetic acid and amide sprays upon fruit set and preharvest drop. Am er. Soc. Hort. Sci. Proc. 38: 117-120. 1941. Davidson, J. H. Progress report on a pre-harvest spray. Mich. State Hort. Soc. Proc. 81: 59-62. 1951. Davidson, J. H., H a m m e r , O. H., Reimer, C. A., and Dut­ ton, W. C. Thinning apples with the sodium salt of naphthaleneacetic acid. Mich. Agr. Expt. Sta. Quart. Bui. 27: 352-356. 1945. Darwin, C. and F. The power of m o v e m e n t in plants. London, 1880. Fitting, H. Die Beeinflussung der Orchideenbluten durch die Bestaubung und durch andere Umstande. Z. Bot., 1: 1-86. 1909. Fitting, H. Weitere Entwicklungsphysiologische Untersuchungen an Orchideenbluten. Z. Bot., 2: 225-66. 1910. 116 10. Gardner, F. E., Marth, Paul C., and Batjer, L. P. Spray­ ing with plant growth substances for control of preharvest drop of apples. A m er . Soc. Hort. Sci. Proc. 39: 415-428. 1939. 11. Greene, L. Growth regulators and fruit set with starking apples. A m e r . Soc. Hort. Sci. Proc. 42: 149-150. 1943. 12. Gustafson, F. G. Inducement of fruit development by growthpromoting chemicals. Nat. Acad. Sci. Proc., Wash. 22: 628-636. 1936. 13. Haagen-Smit, A. J., Leech, W. D., and Bergren, W. R. Estimation, isolation and identification of auxins in plant materials; Amer . J. Bot. 29: 500-506. 1942. 14. Haagen-Smit, A. J., Dandliker, W. B., Wittwer, S. H., and Murneek, A. E. Isolation of 3-indoleacetic acid from immature corn kernels. A m e r. J. Bot. 33: 118-120. 1946. 15. Hartley, C. P. Injurious effects of premature pollination with general notes on artificial pollination and the setting of fruit without pollination. U S D A Bui. 22. 1902. 16. Heinicke, A. J. Factors influencing the abscission of flow­ ers and partially developed fruits of the apple (Pyrus Malus). N. Y. Cornell Agr. Expt. Sta. Bui. 393, 114 pp. 1917. 17. Hey, G. L., and Hopf, P. P. A n e w theory of the action of plant hormones. The Grower, 35: 2, 3 and 4. 1951. 18. Hoffman, M. B., Southwick, F. W., and Edgerton, L. J. A comparison of two types of material for the c h e m ­ ical thinning of apples. Amer. Soc. Hort. Sci. Proc. 49: 37-41. 1947. 117 19. Kogl, F., and Haagen-Smit, A. J. Uber die Ghemie des Wuchsstoffs. V e r h . A k a d . W e t . Amst. 34: 1,4111,416. 1931. 20. Kogl, F., and Kostermans, D. G. Uber die Konstitutionsspe zifitat des Heteroauxins. Z. Physiol. C h e m . 235: 201-216. 1935. 21. Larsen, P. The thermolabile substance that oxidized growth substance in Phaseolus seedling. Planta, 30: 673681. 1940. 22. Luckwill, S. C. Fruit-setting sprays for tomatoes. culture, 53: 262-265. 1946. 23. Massart, J. Sur la pollination sans fecondation. Bot. Bruxelles 3: 89-95. 1902. 24. Murneek, A. E. Chemical thinning of apples. Expt. Sta. Bui. 553. 1951. 25. Murneek, A. E., and Hibbard, A.D. Investigations on thinning of peaches by mea ns of caustic and hormone sprays. Amer,. Soc. Hort. Sci. Proc. 50: 206-208. 1947. 26. Murneek, A. E. Reduction and delay of fruit abscission by spraying with growth substances. A me r. Soc. Hort. Sci. Proc. 37: 432-434. 1939. 27. van Overbeek, J. The growth hormone and the dwarf type of growth in corn. Nat. Acad. Sci. Proc., Wash. 21: 292-299. 1935. 28. Paal, A. Uber phototropische Reizleitungen. 58: 406-58. 1914. Agri­ Bui. Jard. Mo. Agr. Jb. W i s s . Bot. 29. Paddock, Elton F. Effects on tomatoes of field application of two hormone-insecticide-fungicide mixtures. A mer. Soc. Hort. Sci. Proc. 52: 365-367. 1948. 118 30. Schneider, G. W., and Enzie, J. V. The effect of certain chemicals on the fruit set of the apple. A m er . Soc. Hort. Sci. Proc. 42: 167-176. 1943. 31. Schneider, G. W., and Enzie, J. V. Further studies on the effect of certain chemicals on the fruit set of the apple. A m e r . Soc. Hort. Sci. Proc. 45: 63-68. 1944, 32. Schroeder, R. A. Application of plant hormones to tomato ovaries. A m e r . Soc. Hort. Sci. Proc. 35: 537-538. 1938. 33. Singletary, C. C., and Warren, G. F. Influence of time and methods of application of hormones on tomato fruit set. A m e r . Soc. Hort. Sci. Proc. 57: 225-230. 1951, 34. Smock, R. M., and Gross, C. R. The effect of some hor­ m o n e materials on the respiration and softening rates of apples. A m e r . Soc. Hort. Sci. Proc. 49: 67-77. 1947. 35. Southwick, F. W., Edgerton, L. J., and Hoffman, M. B. Studies in thinning peaches with blossom removal sprays. Am er, Soc. Hort. Sci. Proc. 49: 26-32. 1947. 36. Southwick, F. W., and Weeks, W. D. Chemical thinning of apples at blossom time and up to four weeks from petal fall. A m e r . Soc. Hort. Sci. Proc. 53: 143147. 1949. 37. Thimann, K. V. O n the plant growth hormone produced by Rhizopus suinus. J. Biol. Chem. 109: 279-291. 1935. 38. Tukey, H. B., and Einset, O. Effect of fruit thinning on size, color and yield of peaches and on growth and blossoming of trees. Amer. Soc. Hort. Sci. Proc. 36: 314-319. 1938. 39. Went, F. W. Growth auxin and tropisms in decapitated Avena coleoptiles. Plant Physiol. 17: 236-49. 1942. 119 40. Winge, C. The chromosomes: their ntimber and general importance. C. R. Trav. Lab. Carlsberg 13: 131266. 1917. 41. Wittwer, S. H. Effect of fruit setting treatment, variety and solar radiation or yield and fruit size of green­ house tomatoes. A m e r . Soc. Hort. Sci. Proc. 53: 349-354. 1949. 42. Wittwer, S. H., and Schmidt, W. A. Further investigations of the effects of hormone sprays on the fruiting re­ sponse of outdoor tomatoes. A m e r . Soc. Hort. Sci. Proc. 55: 335-342. 1950. 43. Wittwer, S. H., and Sharma, R. C. The control of storage sprouting in onions by pre-harvest foliage sprays of maleic hydrazide. Science 112: 597-598. 1951. 44. Wong, Cheong-yin. Induced parthenocarpy of watermelon, cucumber and pepper by use of growth substances. A m e r . Soc. Hort. Sci. Proc. 36: 632-636. 1938. 45. Yasuda, S. Further report on parthenocarpy caused by the stimulus of pollination in some plants of solonaceae. Agr. and Hort. 9: 647-56. 1934. 46. Zalik, Saul, Hobbs, G. A., and Leopold, A. C. Partheno­ carpy in tomatoes induced by para-chlorophenoxyacetic acid applied to several loci. Amer . Soc. Hort. Sci. Proc. 58: 201-207. 1951. 47. Z i m m e r m a n , P. W., and Hitchcock, A. E. Substances ef­ fective for increasing fruit set and inducing seed­ less tomatoes. Amer,. Soc. Hort. Sci. Proc. 45: 353-361. 1944. 48. Z i m m e r m a n , P. W., and Hitchcock, A. E. The aerosol method of treating plants with growth substances. Contr. Boyce T h om ps on Inst. 13: 313-322. 1944.