Influence of Temperature and Darkness on Apple Fruit Abscission and Chemical Thinning

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1 This article was downloaded by: [University of Stellenbosch] On: 22 December 2014, At: 01:13 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Journal of Tree Fruit Production Publication details, including instructions for authors and subscription information: Influence of Temperature and Darkness on Apple Fruit Abscission and Chemical Thinning R. E. Byers a a Alson H. Smith, Jr. Agricultural Research and Extension Center, Virginia Polytechnic Institute and State University, 595 Laurel Grove Road, Winchester, VA, 22602, USA Published online: 15 Oct To cite this article: R. E. Byers (2002) Influence of Temperature and Darkness on Apple Fruit Abscission and Chemical Thinning, Journal of Tree Fruit Production, 3:1, 41-53, DOI: /J072v03n01_04 To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or

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3 Influence of Temperature and Darkness on Apple Fruit Abscission and Chemical Thinning R. E. Byers ABSTRACT. Stayman/M.27 (Malus domestica Borkh.) placed in the dark for 60 hours at 21 days after full bloom lost more fruit if the dark period was at 21.1 C than if it was at 4.4 C. Spraying these trees with carbaryl or NAA prior to the dark treatment caused no additional fruit loss. Ethephon caused defruiting at all temperatures of darkness. Spraying Braeburn /M.27 trees with a combination of carbaryl + Accel caused increased pygmy fruit development when trees were placed in the dark for 63 hours, 32 days after full bloom at temperatures from C. In a field experiment, applications of carbaryl at 2-hour intervals from 6:00 AM to 8:00 PM to Empire /Mark trees caused similar thinning even though application temperatures ranged from C. In another field experiment, spraying Starkrimson Delicious /Mark trees with ethephon, Accel, or carbaryl at 6:00 AM, 2:00 PM, or 8:00 PM caused similar thinning even though temperatures were 13.9 C, 33.3 C, or 19.8 C, respectively. Regression analysis of 20 years of thinning trials on five spur Delicious strains using 50 different spray treatments indicated that higher night temperatures were more highly related to chemical thinning (accounting for about 20% of the variation) than daytime temperatures or the average of the day and night time temperatures. Degree of thinning R. E. Byers is Professor of Horticulture and Director, Alson H. Smith, Jr. Agricultural Research and Extension Center, Virginia Polytechnic Institute and State University, 595 Laurel Grove Road, Winchester, VA The author extends his appreciation to Jennifer Huffman, Graduate Assistant, Department of Statistics, Virginia Polytechnic Institute and State University, Blacksburg, VA, and to David Carbaugh, Leon Combs, Chad Garber, Maurice and Harriet Keeler, Bill Huehn, and Seth Combs for assistance in conducting these experiments. This project was partially supported by the Virginia Horticultural Society Apple Research Program. Journal of Tree Fruit Production, Vol. 3(1) by The Haworth Press, Inc. All rights reserved. 41

4 42 JOURNAL OF TREE FRUIT PRODUCTION was not significantly related to days after full bloom or fruit diameter; however, a ranking of spray treatments by chemical combination chosen for degree of expected thinning accounted for 70% of the variation. In addition, a ranking of the strains of Delicious by ease of thinning was highly significant. [Article copies available for a fee from The Haworth Document Delivery Service: HAWORTH. address: <getinfo@ haworthpressinc.com> Website: < by The Haworth Press, Inc. All rights reserved.] KEYWORDS. Growth regulators, Malus domestica CHEMICALS USED. Potassium salt of naphthalene acetic acid (NAA, K-200); 2-chloroethylphosphonic acid (ethephon); 1-naphthy N-methylcarbamate (carbaryl); mixture of 6-benzyladinine and gibberellin A 4+7 (Accel) INTRODUCTION Apple trees (Malus domestica Borkh.) grown in the eastern U. S. environment often are exposed to low light conditions from cloudy/rainy periods that can reduce photosynthetic active radiation (PAR) to 10 to 15% of full sun for 2 to 3 successive days. In a previous report (Byers et al., 1991), 2 or 3 consecutive days of artificial shade (92% polypropylene shade material) reduced fruit set of Delicious apple trees when shaded in the day period after bloom. Three to four days of low light from cloudy periods were calculated to be equivalent to 2 to 3 days of 92% artificial shade. In one test, less fruit abscission occurred if 1 or 2 days of sunlight separated each day of the 2 or 3 days of artificial shade. In addition, natural June fruit drop appeared to be related to 2 to 3 days of cloudy weather. When carbaryl was applied on the first day of artificially shaded trees, more thinning occurred than if trees were not shaded. In addition, if artificially shaded trees were exposed to one day of full sun before the thinner was applied, the thinner caused less thinning than if applied during the shade event on the next day. In these studies, 2 days of artificial shade typically induced more fruit drop than common chemical thinners (carbaryl, ethephon, or NAA) (Byers et al., 1991). Chemical thinning reviews suggest that more thinning typically occurs at higher temperatures (Westwood, 1978; Williams, 1979). Jones

5 R. E. Byers 43 and Koen (1985) showed a marked increase in thinning when trees were sprayed with ethephon at increasing temperatures from 4 C to 24 C (trees were kept in growth rooms 4 hours prior to treatment and kept for 24 hours after spraying in rooms). Greene and Bukovac (1971) also showed substantial increases in penetration of NAD by increasing temperatures, as did Edgerton and Haeseler (1959) for NAA. Increased thinning at higher field temperatures has been suggested to be caused by increased chemical absorption. However, Greene (personal communication) has found little evidence of absorption differences of 14 C-NAA as temperatures increase in the field, presumably because drying time decreased as humidity decreased at higher temperatures. Little evidence has been presented on the effects of temperature on the natural susceptibility of the tree to thinning. The objective of these experiments was to investigate (1) the influence of temperature and low light on fruit abscission with or without chemical-thinning sprays, (2) the influence of daytime temperatures on fruit thinning of carbaryl sprays, and (3) the significance of temperature and timing on chemical thinning results using data gathered from previous field experiments. MATERIALS AND METHODS To magnify or concentrate the effects of temperature during lowlight stress, fruit trees were moved from the field into total darkness for 2 days at various temperatures and then returned to the field. This provided low-light stress while eliminating the need for treatments involving various levels of light and temperature during the daylight hours. To reduce transplanting shock, bearing apple trees on M.27 rootstock were grown in the field in 25-cm diameter root bags (Expt. 1 Root Control, Inc. Oklahoma City, OK; Expt. 2 Lacebark, Inc, Stillwater OK). After initial fruit set (at about 15 mm fruit diameter), trees were dug and transplanted into 19-L buckets and moved for 2 days into various dark environments and/or sprayed with various thinning chemicals prior to placement in growth chambers at 4.4 C, 10 C, 15.6 C, or 21.1 C in the dark. Immediately after treatment, trees were returned to the field, placed in the original holes and watered. Experiment 1. In 1996, 48 field grown 5-year-old Braeburn /M.27 apple trees were transferred as described above, placed into four groups by bloom density and one tree from each group was assigned to the following treatments: (1) Control trees were not disturbed and remained

6 44 JOURNAL OF TREE FRUIT PRODUCTION in the field; (2) Control trees were not disturbed but sprayed with carbaryl (600 mg?l 21 ) + Accel (46 mg?l 21 ); (3) trees were dug, placed in buckets, and remained outside; (4) trees were dug, placed in buckets and remained outside, but sprayed with carbaryl (600 mg?l 21 ) + Accel (46 mg?l 21 ); (5-8) trees were moved into growth chambers and kept in the dark for 64 hours beginning at 4:00 PM on 17 May at a constant temperature of 4.4 C, 10 C, 15.6 C, or 21.1 C, respectively; (9-12) trees were sprayed with carbaryl (600 mg?l 21 ) + accel (46 mg?l 21 ) and moved into growth chambers and kept in the dark for 64 hours beginning at 4:00 PM on 17 May at a constant temperature of 4.4 C, 10 C, 15.6 C, or 21.1 C, respectively. Trees were removed from growth chambers at 8:00 AM on 20 May and were transplanted in the field and watered. Fruit numbers/tree were counted before placement into the growth chambers, May 17, and again after fruit abscission on 11 June. Experiment 2. In 1997, 72 field grown 6-year-old Stayman /M.27 apple trees were dug from the field and placed in root bags in the dormant season of 1996 for acclimation purposes to reduce transplant shock. Two days prior to moving trees into growth chambers, trees were dug and placed in 19-L plastic buckets. Trees were blocked into four groups by bloom density and four trees each were assigned to the following treatments: (1) Control trees were undisturbed and remained in the field, (2) trees were dug placed in buckets and remained outside; (2-6) trees were moved into growth chambers and held in the dark for 63 hours at a constant temperature of 4.4 C, 10 C, 15.6 C, or 21.1 C respectively; (7-10) trees sprayed at 1:00 PM on 27 May with carbaryl (600 mg?l 21 ) and then were moved into growth chambers at 5:00 PM on 27 May and held in the dark for 63 hours at a constant temperature of 4.4 C, 10 C, 15.6 C, or 21.1 C, respectively; (11-14) trees sprayed at 1:00 PM on 27 May with NAA (10 mg?l 21 ) + Regulaid (0.125%) and then were moved into growth chambers at 5:00 PM on 27 May and kept in the dark for 63 hours at a constant temperature of 4.4 C, 10 C, 15.6 C, or 21.1 C, respectively; (15-18) trees sprayed at 1:00 PM on 27 May with ethephon (405 mg?l 21 ) and then were moved into growth chambers at 5:00 PM on 27 May and kept in the dark for 63 hours at a constant temperature of 4.4 C, 10 C, 15.6 C, or 21.1 C, respectively. Trees were removed at 8:00 AM on 30 May. Fruit numbers/tree were counted before placement into the growth chambers, 27 May, and again after fruit abscission on 17 June. Experiment 3. In 1996, 36 field grown 6-year-old Empire /Mark apple trees were blocked into four blocks by bloom density, and one tree from each block was assigned to each of the following treatments:

7 R. E. Byers 45 (1) Control unsprayed, or (2-9) sprayed with carbaryl (600 mg?l 21 ) every 2 hours beginning at 6:00 AM until 8:00 PM, respectively. Temperatures varied from a low of 17.8 C to a high of 35.6 C. The number of fruit/tree were counted and expressed as fruit/cm 2 trunk cross-sectional area. Experiment 4. In 1998, sixty 7-year-old Starkrimson Delicious /Mark apple trees were grouped into six blocks by bloom density, and one tree from each block was assigned to each of the following treatments: (1) Control unsprayed; (2,5,8) sprayed at 6 AM with ethephon (270 mg?l 21 ), Accel (50 mg?l 21 ) + superior oil (2.5 ml?l 21 ), or carbaryl (600 mg?l 21 ) + superior oil (2.5 ml?l 21 ), respectively; (3,6,9) sprayed at 2:00 PM with ethephon (270 mg?l 21 ), Accel (50 mg?l 21 ) + superior oil, or carbaryl (600 mg?l 21 ) + superior oil (2.5 ml?l 21 ), respectively; or (4,7,10) sprayed at 8:00 PM with ethephon (270 mg?l 21 ), Accel (50 mg?l 21 ) + superior oil, or carbaryl (600 mg?l 21 ) + superior oil (2.5 ml?l 21 ), respectively. Temperatures were 13.9 C at 6:00 AM, 18.9 C at 2:00 PM, and 33.3 C at 8:00 PM. The number of fruit/tree were counted and expressed as fruit/cm 2 trunk cross-sectional area. Experiment 5. Data variables from 19 thinning experiments from 1975 to 1994 involving five strains of spur Delicious and 50 different chemical treatments were studied. The chemical treatments were evaluated by regression analysis for correlation with fruit abscission from 101 treatments (included carbaryl, ethephon, 6-BA, Accel, oxymyl, and various combinations/6 single tree replicates per treatment, applied from petal fall (4 mm) to 32 mm fruit diameter, and applied over a 16-day period after full bloom with a Julian date from 109 to 125). Most of these experiments were conducted by selecting some of the best days for thinning; thus, the high and low temperatures would be expected to be higher than the average temperatures during the thinning period. The percentage fruit abscission was based on the percentage difference between the fruit/cm 2 cross-sectional area (FCA) of the treatment and the untreated control and was compared by regression analysis to: (1) high temperature on the day of application, (2) low temperature on the day of application, (3) average of the high temperature of the application day plus the next day, (4) average of the low temperature of the application day plus the next day, (5) the average temperature for the application day plus the next day, (6) a rank order of the spray treatments from least to most effective based on chemical rate and combinations (i.e., 50 mg?l 21 Accel < 100 mg?l 21 Accel < carbaryl + 50 mg?l 21 Accel, etc.), (7) a rank order of difficulty to thin of five strains of Delicious,

8 46 JOURNAL OF TREE FRUIT PRODUCTION (8) application date as the Julian date, (9) application date as the number of days after full bloom (AFB), and (10) fruit diameter on the application date. RESULTS Experiment 1. In 1996, Braeburn /M.27 apple trees placed in the dark for 64 hours and returned to natural sunlight conditions abscised all of their fruit at 15.6 C and 21.1 C dark temperature, but some were retained if the dark temperature was 4.4 C or 10 C (Table 1). These data indicate that the lower temperatures during the dark period were TABLE 1. Effects of dark temperature and chemical thinner on Braeburn /M.27 fruit set (1996). z No. Dark temp (C) Chemical spray y Tree removed from the soil y Fruit set/cm 2 trunk cross-sectional area (17 May) (11 June) Fruit set (%) x Full size fruit (%) x Pygmy fruit (%) x 1. Control outside not dug 12.0 v Control carbaryl + Accel outside not dug Control dug (outside) Control carbaryl + Accel dug (outside) C dug C dug C dug C dug C carbaryl + Accel dug C carbaryl + Accel dug C carbaryl + Accel dug C carbaryl + Accel dug ANOVA Chem *** *** *** ns Temp ** *** * * Temp 3 Chem * ns * ns Regression w no chemical thinner (4.4 C, 10.0 C, 15.6 C, 21.1 C) L**Q ns L**Q ns Regression w carbaryl + Accel (4.4 C, 10.0 C, 15.6 C, 21.1 C) L**Q ns L**Q ns z Full bloom occurred 26 April Spray treatments were: Carbaryl 600 mg?l 21 + Accel 46 mg?l 21. y Treatments 1 and 2 were trees in rootbags but not removed from their location in the orchard and thus were under normal field light/dark conditions.treatments 3 and 4 were trees in rootbags that were dug but left outside in buckets. Treatments 5 to 12 were dug and placed in growth chambers in the dark for 64 hours (starting 21 days AFB). x Percentage data (fruit number 11 June/17 May) were transformed to arcsin square root before analysis. w L = linear, Q = quadratic. v ***, **, *, ns: Significance at P = 0.001, P = 0.01, P = 0.05, or nonsignificant, respectively.

9 R. E. Byers 47 very important for fruit retention. In addition, carbaryl + Accel caused the development of more pygmy fruit at all temperatures than if not sprayed with thinner. Pygmy fruit development also appeared to be related to the combined effects of tree digging plus treatment with carbaryl + Accel (trt#4 vs. trt#2 and trt#3) and cooler temperatures. I suspect that higher temperatures caused higher respiration rates that promoted fruit abscission of pygmy and full sized fruit (whether treated with a thinner or not). Experiment 2. In 1997, Stayman /M.27 apple trees placed in the dark for 63 hours and returned to natural sunlight conditions abscised most of their fruit at the highest temperature and partially retained fruit at the lowest temperatures (Table 2). Additional fruit thinning was not significantly promoted by carbaryl or NAA + Regulaid over the range of temperatures (4.4 C to 21.1 C). Unexpectedly, ethephon caused nearly 100% fruit abscission at all temperatures from 4.4 C to 21.1 C. Experiment 3. Carbaryl sprays applied at 2 hour intervals beginning at 6 AM until 8 PM to Empire /Mark trees in the field caused similar thinning when applied at temperatures ranging from 17.8 C to a high of 35.6 C (Table 3). Experiment 4. Ethephon, Accel + oil, or carbaryl + oil sprays applied at 6 AM, 2 PM, or 8 PM to Starkrimson /Mark trees in the field caused similar thinning even though temperatures at application time were 13.9 C at 6 AM, 33.3 C at 2:00 PM, and 18.9 C at 8:00 PM (Table 4). Experiment 5. Regressions of 20 years of thinning trials indicated that the night temperatures the day of the thinning spray plus the next day were more highly correlated to chemical thinning response (R 2 = 0.192; R 2 = 0.210, respectively) than daytime temperatures the day of the thinning spray plus the day following (R 2 = 0.122; R 2 = 0.109, respectively) or the average of the day and night time temperatures (R 2 = 0.156) (Table 5). It should be stressed that the spray treatments generally were made on sunny and warmer days during the thinning period. If the date for thinning had been selected at random over this 20-year period, it is possible that a better temperature coefficient of variation may have been found. The R 2 s for days AFB, Julian date, or the fruit diameter when sprayed were not significant factors and had very low quadratic R 2 s(r 2 = 0.031; R 2 = 0.069; R 2 = 0.042, respectively) (Table 5). A ranking of the spray treatments by degree of thinning for the 50 different treatments resulted in a highly significant R 2 = Thus, selecting chemicals for increased thinning appears to be important in obtaining the degree of thinning.

10 48 JOURNAL OF TREE FRUIT PRODUCTION TABLE 2. Effects of dark temperature and chemical thinners on Stayman M27 fruit set (1997). z No. Dark temp ( C) Chemical spray y Fruit set/cm 2 trunk cross-sectional area Fruit set (%) x Number of fruit/tree (27 May) (17 June) (17 June) (27 May) (17 June) 1. Control/not dug 2. Control/dug C none C none C none C none C carbaryl C carbaryl C carbaryl C carbaryl C NAA + Regulaid C NAA + Regulaid C NAA + Regulaid C NAA + Regulaid C ethephon C ethephon C ethephon C ethephon ANOVA Chem *** *** *** Temp *** *** *** Temp 3 Chem * ns * Temperature regressions Regression (trt 3,4,5,6) (4.4 C,10.0 C,15.6 C, 21.1 C) no thinner L*Q ns L*Q ns Regression (trt 7,8,9,10) (4.4 C,10.0 C,15.6 C, 21.1 C) carbaryl L** L***Q* Regression (trt 11,12,13) (4.4 C,10.0 C,15.6 C, 21.1 C) NAA L*Q ns L*Q ns Regression (trt 15,16,17,18) (4.4 C,10.0 C,15.6 C, 21.1 C) ethephon ns ns Contrasts 3,4,5,6 vs. 7,8,9,10 no thinner vs. carbaryl ns ns ns (all temperatures) 3,4,5,6 vs. 11,12,13 no thinner vs. NAA (all temperatures) ns ns ns 3,4,5,6 vs. 15,16,17,18 no thinner vs. ethephon (all temperatures) *** *** *** z Full bloom occurred 25 April Four whole tree reps/treatment. Chemical rates were: carbaryl 600 mg L 21 (Sevin XLR); NAA mg L -1 + Regulaid 0.125%; ethephon 405 mg L 21. y Treatments 1 and 2 were trees in rootbags but not removed from their location in the orchard and thus were under normal field light/dark conditions. Treatments 3 to 18 were dug and placed in growth chambers in the dark for 63 hours (32 days AFB). x Percentage data (fruit number 17June/27 May) were transformed to arcsin square root before analysis, and thus were under normal field light/dark conditions. ***, **, *, ns: Significance at P = 0.001, P = 0.01, P = 0.05, or nonsignificant, respectively.

11 R. E. Byers 49 TABLE 3. Effect of application time on fruit thinning of Empire /Mark by carbaryl (Sevin XLR) (1996). Fruit set/cm 2 trunk No. Application time z Relative humidity Temperature ( C) cross sectional area (%) 20 May (18 June) 1. No treatment :00 AM :00 AM :00 AM :00 AM y 6. 2:00 PM :00 PM y 8. 6:00 PM :00 PM ANOVA (P > 0.05) Regression Temperature ns Time of day ns z Applications were made on May 20, 1996 when fruit diameter was 14.9 mm. y Spray treatments were: Carbaryl 600 mg L 21. Significantly different from the control by Dunnett s test (P = 0.05). ***, **, *, ns: Significance at P = 0.001, P = 0.01, P = 0.05, or nonsignificant, respectively. TABLE 4. Effects of application temperature at spraying on fruit thinning of Starkrimson /Mark by ethephon, Accel + Oil, and carbaryl (Sevin XLR + Oil) (1998). No. Treatment Application time (%) Temperature ( C) cross sectional area y z Relative Humidity Fruit set/cm 2 trunk (15 May) (15 May) (15 May) (1 June) 1. Control no treatment ethephon 6:00 AM ethephon 2:00 PM ethephon 8:00 PM Accel+ oil 6:00 AM Accel+ oil 2:00 PM Accel+ oil 8:00 PM carbaryl + oil 6:00 AM carbaryl + oil 2:00 PM carbaryl + oil 8:00 PM x ANOVA (P > 0.05) Factorial (less the control) Chem ns Time ns Chem 3 Time ns z Applications were made on May 15, 1998 when fruit diameter was 19.2 mm. Full bloom = April 15, y Spray treatment concentrations were: carbaryl 600 mg L 21 ; Accel 50 mg L 21 ; ethephon 270 mg L 21 ; Superior oil 0.25%. x Significantly different from the control by Dunnett s test (P = 0.05). ***, **, *, ns: Significance at P = 0.001, P = 0.01, P = 0.05, or nonsignificant, respectively.

12 TABLE 5. Regression analysis of fruit abscission of 50 different chemical treatments, sprayed on five spur Delicious strains, from experiments conducted over a 20-year period (101 data points). Factor X (Pr > F) R 2 X 2 (Pr>F) R 2 X+X 2 (Pr>F) R 2 Mean Minimum Maximum Day temp of application day *** *** ** C 12.8 C 33.3 C Night temp of application day *** *** *** C 0.6 C 21.7 C Day temp of application (2 days) ** ** ** C 11.4 C 32.5 C Night temp of application (2 days) *** *** *** C 3.3 C 19.2 C Night and day temp of application (2 days) *** *** **** C 7.4 C 25.4 C Days AFB Fruit diameter (mm) Spray treatment *** *** *** Strain of Spur Delicious *** *** *** Julian date * ***, **, *: Significance at P = 0.001, P = 0.01, P = 0.05, respectively. Values not in boldface italics are nonsignificant. 50

13 R. E. Byers 51 DISCUSSION Natural fruit drop during the 30 days after bloom (June drop) has been thought to be related to competition between fruits and between fruits and vegetative shoots, roots, and all tissues throughout the tree (Byers et al., 1991). Polomski et al. (1988) showed a reduction in fruit carbohydrates by shading limbs or whole trees. Previous work (Byers et al., 1991) indicated that increased thinning occurred when chemical thinners were applied during or shortly after artificial shade. Data in this report clearly indicate that higher temperatures during short periods of darkness cause additional fruit abscission. The application of NAA or carbaryl did not significantly increase chemical fruit thinning over the temperature range when subsequently exposed to 2 days of darkness. A trend to increased thinning was evident with NAA and carbaryl but not significant (Table 2). Fruit abscission by ethephon sprays was not inhibited by low temperatures. Jones and Koen (1985) showed a strong thinning effect by ethephon when applications were made in a growth chamber at higher temperatures and maintained at those temperatures for 24 hours (which suggests the temperature at the time of application and for the day thereafter was very important). In our experiments, since ethephon defruited all trees at all temperatures, it is possible the thinner had its primary action after the 2-day period in the growth chamber. Trees may have responded equally to the higher temperatures in the days in the field environment subsequent to the 2-day dark period. These studies showed that both chemically thinned trees and those not sprayed maintained fruit equally on trees during the 2-day dark period. Carbaryl and NAA did not have an additive effect on fruit abscission beyond that of the warm-dark effect of this short 2-day period. It is possible that chemical thinners have their action over a longer period of time at low temperatures than at higher temperatures. It is possible that the thinner did not affect fruit abscission until the trees were taken outside and brought into similar environmental conditions at the conclusion of the dark period. In 1998, preliminary data indicated that fruit and leaf ethylene levels of Braeburn /M.27 trees that were sprayed with ethephon (in the field) were greatly inhibited if trees were placed in the dark at 4.4 C in comparison to trees placed in the dark at 21.1 C. In addition, trees kept at 4.4 C for 4 days did not evolve ethylene, but when they were moved to 21 C high levels of ethylene were produced from leaves for several days thereafter. In Experiment 2, ethylene evolution (and action) in ethephon-sprayed trees kept in the dark for the 2-day exposure time (at

14 52 JOURNAL OF TREE FRUIT PRODUCTION lower temperatures) was delayed until after the trees were removed to higher outdoor temperatures. Since all fruit abscised even at the lowest dark temperature of 4.4 C, a critical temperature may have been required for adequate metabolism of ethephon to obtain thinning activity. Olien and Bukovac (1978) showed a dramatic influence of temperature on ethylene evolution from detached cherry leaves. The results of the ethephon studies also raise the possibility that cool temperatures may inhibit or delay chemical action of carbaryl and/or NAA by an inadequate plant metabolism rate that may subsequently reduce the effectiveness of the thinners to induce fruit abscission. Theoretically, if a chemical thinner has been absorbed and is still present in the tissue, its action may be delayed until some future time that temperatures reach a critical level to increase metabolism and promote fruit abscission. Observation of thinning by growers suggests that the combination of low light and warmer temperatures could cause serious over thinning, but on the other hand, these same thinners applied under low light and cool conditions are much less effective. In addition, June drop appears to be triggered by the combination of low light and warm temperature periods (Byers et al., 1991), but if temperatures are cool, June drop may be delayed or reduced. An analysis of temperatures in Winchester, VA from 1984 to 1993 indicated that during the 21 days after full bloom, 3 or more days of temperature above 29.4 C occurred in 7 out of 10 years. In the 1-week period from 15 to 21 days after full bloom, 3 or more days of temperatures above 29.4 C occurred only 3 out of 10 years. To obtain the same degree of probability (7 out of 10 years) for this period, the temperature would have to be lowered by 5.6 C degrees, to 23.9 C. Since the fruit diameter, Julian date, or days AFB were not found to be important factors within the range of values in these experiments (Experiment 5), thinning should be based on temperature. It appears (Table 5) that the chemical combination chosen for thinning was the most important factor followed by temperature and strain of Delicious. Many factors contribute to the chemical thinning results, but none appear more important than the thinner chosen. REFERENCES Byers, R. E., D. H. Carbaugh, C. N. Presley, and T. K. Wolf The influence of low light levels on apple fruit abscission. J. Hort. Sci. 66:1-17. Edgerton, L. J. and C. W. Haeseler Some factors influencing the absorption of naphthaleneacetic acid and napthalene acetamide by apple leaves. Proc. Amer. Soc. Hort. Sci. 74:54-60.

15 R. E. Byers 53 Greene, D. W. and M. J. Bukovac Factors influencing the penetration of napthaleneacetamide into leaves of pear (Pyrus communis L.). J. Amer. Soc. Hort. Sci. 96: Jones, K. M. and T. B. Koen Temperature effects on ethephon thinning of apples. J. Hort. Sci. 60: Olien, W. C. and M. J. Bukovac The effect of temperature on rate of ethylene evolution from ethephon and from ethephon-treated leaves of sour cherry. J. Amer. Soc. Hort. Sci. 103: Polomski, R. F., J. A. Barden, R. E. Byers, and D. D. Wolf Apple fruit nonstructural carbohydrates and abscission as influenced by shade and terbacil. J. Amer. Soc. Hort. Sci. 113: Westwood, M. N Temperate-zone pomology. W. H. Freeman and Company, San Francisco, CA. Williams, M. W Chemical thinning of apples. Hort. Rev. 1: