Thinning of ’Golden Delicious’ apples with the combination of
ethephon and CPPU
Red~enje plodi~ev jablane sorte Zlati deli{es s kombinacijo 
etefona ter CPPU
Matej STOPAR
Agricultural Institute of Slovenia, Hacquetova 17, 1000 Ljubljana, Slovenia
Abstract. A synthetic cytokinin forchlorfenuron (CPPU) 5 ppm was sprayed at
the time of petal fall (PF) on eight years old ’Golden Delicious’/M.9 apple trees
with the aim of enlarging the fruit growth. Since CPPU is not recognized as a thin-
ning agent which increases the return bloom, a low dose of ethephon was sprayed
on CPPU treated trees to enhance flower bud formation and increase flowering in
the next season. Ethephon was applied in successive treatments (alone or in the
combination with CPPU) 3 x 30 ppm or 5 x 50 ppm, starting at PF, and continued
in week intervals.
The single application of CPPU 5 ppm or the combination of ethephon 3 x 30
ppm with CPPU 5 ppm decreased the fruit set and enhanced the fruit growth to
reach the commercial demand. The combination spraying of ethephon 5 x 50 ppm
+ CPPU 2 x 5 ppm resulted in a too strong thinning response while the yield of big-
ger size fruit was not enhanced. The alone application of ethephon 3 x 30 ppm had
no influence on the fruit thinning or fruit weight while the application of ethephon
5 x 50 ppm reduced the fruit set but, consequently, did not enhance the fruit growth.
None of the treatments influenced the return bloom.
Key words: apple thinning, CPPU, ethephon, alternate bearing
Abbreviations: N-(2-chloro-4-pyridyl)-N’-phenylurea (CPPU),
2-chloroethylphosphonic acid (ethephon)
Izvle~ek. Z namenom pove~anja velikosti plodov, smo {kropili osem let stare
jablane ’Zlati deli{es/M.9’ s sinteti~nim citokininom forklorfenuronom (CPPU), v
koncentraciji 5 ppm ob koncu cvetenja (KC) dreves. Ker CPPU ne izbolj{uje
povratno cvetenje jablan, so CPPU tretiranim drevesom dodali {e etefon, kateri naj
bi spodbudil nastanek cvetnega brstja ter tako izbolj{al cvetenje v naslednjem letu.
Etefon je bil nane{en v ve~ zaporednih nanosih (sam ali v kombinaciji s CPPU), v
tedenskih intervalih z za~etkom ob KC jablan in sicer 3x30 ppm ali 5x50 ppm.
Samostojni nanos CPPU 5 ppm ali pa kombinacija etefona 3x30 ppm s CPPU
5 ppm je zmanj{ala rodni nastavek in pove~ala rast plodov kot zahteva komercial-
ACTA BIOLOGICA SLOVENICA
LJUBLJANA 2005 Vol. 48, [t. 1: 45-48
Sprejeto (accepted): 2005-09-27
stevilka 1_05.qxp  13.12.2005  12:32  Page 45
ni nivo pridelave. Kombinacija {kropljenj etefona 5x50 ppm + CPPU 2x5 ppm je povzro~ila
premo~an osip plodi~ev, tako da se pridelek plodov ve~jega velikostnega razreda ni izbolj{al.
Samostojni nanos etefona 3x30 ppm ni imel vpliva niti na red~enje plodi~ev niti na te`o plodov,
medtem ko je etefon 5x50 ppm sicer zmanj{al rodni nastavek, kar pa posledi~no ni povzro~ilo
pove~anja te`e plodov.
Klju~ne besede: red~enje plodi~ev jablane, CPPU, etefon, izmeni~na rodnost
Okraj{ave: N-(2-chloro-4-pyridyl)-N’-phenylurea (CPPU),
2-chloroethylphosphonic acid (etefon)
Introduction
Forchlorfenuron (CPPU), a synthetic cytokinin, has a strong influence on apple fruit growth if
applied at petal fall (PF) or a few days later at the concentration below 10 ppm (GREENE 1989,
GREENE 2001). Undesirable side effect, a reduced return bloom, was observed on ’Delicious’ and
’McIntosh’ apple trees after CPPU 5 ppm applications (CURRY & GREENE 1993, GREENE 1989). By
contrast, an old thinner, ethephon, is known to enhance the return bloom of apple trees because its
post bloom application mostly results in good flower bud induction (KNIGHt & BROWNING 1986). The
application of ethephon mostly thins apple fruitlets while the growth of the remaining fruit, frequent-
ly, does not respond with an enhanced growth rate (EBERT & BENDER 1986, LINK 2000, STOPAR
2000). In the experiment we tried to combine the positive effect of both growth regulators: applica-
tion of ethephon a few times in a low concentration to enhance the return bloom and not to thin the
fruitlets, and the use of CPPU for the enhancement of fruit growth on the high loaded trees.
Material and methods
Eight years old ’Golden Delicious’/M.9 apple trees were selected according to high bloom den-
sity and homogeneous growth vigor. In the field trial a standard randomized block design with six
replications and a single tree per plot were used.
Successively, ethephon and CPPU were applied at weekly intervals and the first spraying of both
agents started at petal fall (PF) time. The treatments were as follows:
1) Control – no thinning
2) Hand thin (done just after the June drop time)
3) CPPU 1x5 ppm = 5 ml Sitofex (SKW, Trostberk, Germany) / L water;
4) Ethephon 3x30 ppm = 0.06 ml Ethrel (Chromos, Zagreb, Croatia) / L water
5) Ethephon 5x50 ppm = 0.10 ml Ethrel / L water
6) Ethephon 3x30 ppm + CPPU 1x5 ppm;
7) Ethephon 5x50 ppm + CPPU 2x5 ppm;
The spraying was done with a hand sprayer to the point of drip. No surfactant was used. When
ethephon was combined with CPPU spraying, first, ethephon was applied and an hour later, after the
leaves were dry, CPPU was applied. At maturity time the fruit was harvested, counted, weighed and
graded into two size classes, < 68 mm and > 68 mm fruit diameter. Phytotoxic effect on trees was
estimated a month after spraying with a visual scale (1 = no effect observed, 5 = suppressed shoot
growth, bigger and wrinkled leaves with yellow green spots, more transparent crown). The return
Acta Biologica Slovenica, 48 (1), 2005 46
stevilka 1_05.qxp  13.12.2005  12:32  Page 46
bloom was estimated visually next spring at bloom time using the scale 1-10 (1 = no flower clusters
present on the trees; 10 = abundant flowering). During the experiment the trees received standard
pest and disease management program. Data were subjected to statistical analysis using the statisti-
cal program Statgraphics 5.0 (STSC, Rockwille, USA).
Results and discussion
The single PF application of CPPU 5 ppm significantly reduced the final fruit set (Table 1) and
increased the fruit growth to satisfy the commercial demand for ’Golden Delicious’ apples (Table 2).
When GREENE (2001) sprayed CPPU 8 ppm on ’McIntosh’ at PF he got no thinning effect while a week
later spraying thinned the trees appropriately. STOPAR (1999) did not observe an enhanced fruit drop of
small fruited apple cultivars ’Jonathan’, ’Elstar’ and ’Gala’ when CPPU 5 ppm was sprayed a week after
PF. It looks that CPPU is not a thinner with a consistent fruitlet abscission response if sprayed either at PF
or a week later. In this trial the return bloom after CPPU 5 ppm single spraying was not influenced too.
Matej Stopar: Thinning of ’Golden delicious’ apples... 47
Treatment * 
Flower
clusters 
(No./tree) 
Fruit  
(No./tree) 
Fruit 
(No./100
clusters) 
Return   
bloom 
(1-10)** 
  1) Control 191 a 142 cd 74 b 3,17 ab 
  2) Hand thin 219 a 101 bcd 47 a 1,83 a 
  3) CPPU 1x5 ppm 223 a   74 ab 33 a 4,0 ab
  4) Ethephon 3x30 ppm 190 a 148 d 83 b 1,93 a 
  5) Ethephon 5x50 ppm 215 a   96 bc 44 a 4,38 ab 
  6) Ethephon 3x30 ppm + CPPU 1x5 ppm 221 a   76 ab 34 a 4,67 b 
  7) Ethephon 5x50 ppm + CPPU 2x5 ppm 196 a   42 a 22 a 4,00 ab 
Table 1: The number of flower clusters at the start of the experiment, the final fruit number and the return
bloom of ’Golden Delicious/M.9’ apple trees after the application of thinning agents.
Treatment * 
Yield 
(Kg/tree) 
Fruit    
> 68mm 
(Kg/tree) 
Fruit  
weight   
(g) 
Phytotoxicity
(1-5)**
  1) Control 16.0 b   5.5 ab 115 a 1,0 a 
  2) Hand thin 12.7 ab   8.4 ab 125 a 1,0 a 
  3) CPPU 1x5 ppm 10.9 ab   8.7 ab 161 b 2,2 b 
  4) Ethephon 3x30 ppm 15.5 b   4.5 a 106 a 1,0 a 
  5) Ethephon 5x50 ppm 10.8 ab   4.2 a 110 a 1,2 a 
  6) Ethephon 3x30 ppm + CPPU 1x5 ppm 12.7 ab 11.2 b 174 b 2,0 b 
  7) Ethephon 5x50 ppm + CPPU 2x5 ppm   7.2 a   6.8 ab 169 b 4,0 b
Table 2: Total yield, yield of commercial (> 68 mm) fruit, mean fruit weight and the effect on phy-
totoxicity after the application of thinning agents on ’Golden Delicious/M.9’ apple trees.
stevilka 1_05.qxp  13.12.2005  12:32  Page 47
Application of ethephon 3x30 ppm did not affect the fruit set, yield or fruit growth of ’Golden
Delicious’, while spraying of ethephon 5x50 ppm reduced the fruit set but did not enhance the fruit
growth (Table 1,2). None of the ethephon alone treatments affected the return bloom in this trial sig-
nificantly. The combination spraying of ethephon 3x30 ppm + CPPU 5 ppm reduced the fruit set and
enhanced the fruit growth while the return bloom was not increased, i. e. all similar effects as those
which happened after a single CPPU 5 ppm spraying. A stronger combination of both regulators
(ethephon 5x50 ppm + CPPU 2x5 ppm) induced the overthinning of ’Golden Delicious’ trees. The
fruit growth was enlarged but the yield of bigger size fruit was not increased because the fruit was
not numerous. A very similar overthinning occurred when CPPU 5 ppm was combined with ethep-
hon 3x70 ppm (PF + weekly intervals) on the cultivar ’Elstar’ (STOPAR 1998). In spite of overthin-
ning the return bloom was not increased. This can be speculated as a diminishing effect of CPPU on
the return bloom observed by GREENE (1989) and CURRY & GREENE (1993).
A month after spraying the phytotoxic effect of treatments was estimated on trees. A week shoot
growth, fewer but bigger leaves with green yellow spots and wrinkled surface were observed on
CPPU treated crowns (Table 2). This phytotoxic effect was more pronounced on crowns on which
CPPU was applied twice and the effect remaining on trees until harvest.
Spraying of ’Golden Delicious’ with CPPU 5 ppm at PF thinned trees in our experiment, how-
ever, bearing in mind CPPU trials reported by others no consistent thinning response was observed.
If a weak phytotoxic CPPU 5 ppm effect is taken into consideration, no recommendation should be
given to apple growers for the use of CPPU. For a more complete understanding of CPPU action the
concentration and time response studies should be done on different apple cultivars.
Literature
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