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Apple Tree Management in South Africa: Practical Science
Nigel C. Cook
Department of Horticultural Science, University of Stellenbosch, Private Bag X1, Matieland
7602, South Africa.
Keywords: apple tree training, high-density planting, warm apple growing regions.
Abstract
Densities for apples appear to have stabilised at around 1500 trees/ha. The
predominant system currently used is a tree with a single central leader and fruiting
branches directly situated on the leader. The tree is similar to the “Solaxe”, but is
unique in the sense that its evolution is the result of an improved understanding of
the reaction of the apple tree to a lack of winter chilling, hot growing conditions and
poor soils. Whips (the predominant nursery tree available) are not headed and are
supported by a simple trellis. The next major obstacle is managing bud break with
sub-optimal chilling (and resultant delayed foliation problems) to obtain the correct
number of well-positioned shoots to create the complexity required for early
production. Nursery trees can be chilled in the cold room (4°C) before planting. The
requirement of scoring and notching appears to be due to increased inhibition by the
shoot tissues due to insufficient chilling. An additional problem of insufficient
chilling, which is exaggerated with progressively less chilling is the increase in basal
dominance or shoot autonomy, even on non headed whips. Shoot autonomy opposes
the development of a well defined hierarchy required for the formation of a pyramidal
shaped tree with optimal light distribution. In this respect the uneven vigour of side
shoots is aggressively managed by bud flicking and branch thinning and bending.
The shift to high density has occurred using the same rootstock, i.e. the vigorous
Merton 793. Only recently with improved soil preparation, better irrigation and more
knowledge of minimising transplant shock are more dwarfing stocks becoming a
viable option. Managing complexity, via less pruning and more manipulation
(notching, scoring, bending and bud flicking), vigour is controlled to a large extent by
early fruiting. Girdling is used to induce early flowering and fruiting. Light in the
system is primarily controlled by the complete removal of excessively vigorous
fruiting branches and removal of all laterals on the basal 20 to 50 cm of the fruiting
branch.
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INTRODUCTION
Most South African growers have adopted a pyramidal shaped tree that is optimally suited to
a good distribution of light. Densities for apples appear to have stabilised at around 1500
trees/ha (4 x 1.5 m spacing). The predominant system currently used is a tree with a single
central leader with fruiting branches directly attached to the main axis. There are no
permanent structural side branches. In appearance the South African high-density system
resembles the “Solaxe”, but is unique in the sense that its evolution is the result of an
improved understanding of the reaction of the apple tree to a lack of winter chilling, hot
growing conditions and poor soils.
During the past 15 years many aspects of fruit growing have changed to allow the
successful planting of apples at high densities:
−
Improved methods of soil preparation;
−
More knowledge of minimising “transplant shock”;
−
Improved irrigation scheduling and system design;
−
Better understanding of the negative effects of pruning;
−
More understanding on the management of branching;
−
Better understanding of tree architecture;
−
Recently, more knowledge of the use of girdling to reduce vigour and promote fruiting.
The shift to high density has occurred using the industry standard, vigorous Merton 793
rootstock. While locally M.793 is not as vigorous as in other growing regions, it is too
vigorous for plantings at spacings closer than 4 x 1.5 m. By managing complexity, via less
pruning and more manipulation (girdling, notching, scoring, bending and bud flicking), vigour
is adequately controlled to a large extent by early fruiting. With improved soil preparation,
better irrigation and more knowledge of minimising “transplant shock” the more dwarfing M7
is an option, but is not resistant to the local strains of woolly apple aphid that are apparently
more aggressive than elsewhere (Giliomee et al., 1968). M.9 is extremely dwarfed locally,
probably due to high evapo-transpiration demands of our climate.
UNDERSTANDING BRANCHING UNDER LOW CHILL CONDITIONS
Local growers are now aware of the advantages of not heading newly planted whip nursery
trees. Heading accentuates basal dominance by the stimulation of excessively vigorous
shoots (reiterations) that are more or less equal in strength, i.e., autonomous. Developing
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branches at the desired position on the axis is a problem under conditions of delayed
foliation. While heading guarantees branching in the desired position, pruning promotes
shoot autonomy and delays fruiting. These shoots are not conducive to the training of trees
with a well-defined branching hierarchy that is desired in high density plantings. The shift
towards the non-heading route has necessitated the use of a three-wire support system,
which has to be in place the day the trees are planted. This simple trellis also provides
support against wind and allows branches to be tied down and positioned. Correct branch
positioning is a highly effective method of controlling sunburn.
Shoots of the apple rootstock M.9 grown under conditions of “complete chilling” in Belgium
(52°N) developed a well-defined acrotonic (distal) bud bursting tendency before budburst in
spring (Cook et al., 1998; Table 1; Fig. 1). This “normal” acrotonic bursting tendency was
established during late winter following extended chilling via a greatly increased growth rate
of the terminal bud relative to the lateral buds. The inhibitory effects of the shoot tissues
accentuate the dominance of the terminal bud. Spring budburst on these shoots occurred
first in the terminal bud closely followed by numerous distally situated laterals. The new
terminal shoot rapidly establishes dominance (apical control) over the proximally situated
laterals, i.e., each subsequent lateral is inhibited sequentially in a proximal direction.
Definition between the dominant leader and subordinate branches results via strong apical
control that gives rise to a clearly defined acrotonic branching. It appears that when
budburst is both numerous and synchronised on a shoot axis, as occurs under conditions of
“complete” chilling, correlative phenomena between buds are conducive to the development
of stronger apical control.
A lack of winter chilling greatly modifies the normal pattern of spring budburst, resulting in
delayed foliation (Saure, 1985). Delayed foliation involves a protracted budburst between
buds on different shoots and also on buds within individual shoots (Jacobs et al., 1981), and
budburst on insufficiently chilled shoots is slower (Crabbé, 1994; Cook and Jacobs, 1999).
With delayed foliation branching is reduced. Furthermore, an increased basitonic bursting
tendency is observed, with the proximal lateral buds bursting more readily than the distal
lateral buds due to inhibition by the shoot tissues (Cook and Jacobs, 1999). The shorter
dormant period, as experienced with mild winters common to the apple growing regions of
South Africa (Table 2), impedes the development of acrotony and subsequent apical control.
In the Koue Bokkeveld winters are supposedly cold enough for “adequate” chilling (ca. 1300
Richardson Chill Units, Richardson et al., 1974). However, budburst is still reduced and
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more erratic than with “complete” chilling in a cold room (personal observation). When
training trees from unheaded whips in areas with insufficient chilling an increased tendency
towards basal dominance and a loss of apical control is observed, even with the use of rest
breaking chemicals (personal observation).
MANAGING POOR CHILLING AT PLANTING
Currently, whips are the predominant nursery tree sold in South Africa and under local
conditions it is a challenge for the grower to induce acceptable budburst and branching, in
terms of numbers and position on the non-headed whips. Firstly, it is critical to plant a large
enough whip. Larger trees have more buds, produce more leaves, and have a greater
bearing potential and increased subsequent yield. Furthermore, on larger whips there is a
better chance that sufficient branches will develop, following manipulation. Whips of 1.8 to
2.2 m long are considered ideal.
The most efficient way to induce numerous lateral branches is to artificially chill the nursery
tree before planting (Young et al., 1987). Trees are stored for two to three months in the
cold room at 4°C before planting. Two months chilling in a cold room gives ca. 1500 chilling
hour accumulation. In the past problems were experienced with chilling injury in the cold
room, most probably due to poorly hardened trees. Nurseries now produce trees with welldeveloped buds and more trees are going through the cold room. Alternatively to cold
treatment, rest-breaking agents (Dormex, DNOC winter oil, other mineral oils, and
combinations thereof) are used to promote the bursting of adequate numbers of buds
(Honeyborne, 1996). Unfortunately budburst is no guarantee for branch formation and buds
can burst only to form a spur. This is more common following the application of a restbreaking agent. To ensure branch formation the above treatments are combined with
notching, scoring, Promalin (6-Benzyl adenine and Gibberellins A4+A7) applications and
bud flicking.
ESTABLISHING A HIERARCHY
To promote a hierarchic growth habit we need to encourage apical control of the developing
leader. As discussed, acrotonic branching develops with adequate chilling. The next
challenge to the grower is to establish a desirable hierarchy, i.e., a more or less pyramidal
form. Commonly, the lateral buds directly below the terminal bud are removed as soon as
they have swelled sufficiently to be able to be flicked out easily (bud flicking). In the event
that insufficient buds burst following the above treatments, at the first signs of bud growth on
the whip the trunk can be scored 1.1 m above the ground with a single cut of a knife blade.
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Notching selected buds is also effective (Greene and Autio, 1994). A combination of
Promalin and brown acrylic paint applied to the buds at bud swell is sometimes used in
South Africa (Abbot Laboratories (SA) (Pty) Ltd, Somerset West), but can cause damage if
applied too late. Following these manipulations it is often required to use toothpicks or
clothes pegs to widen the crotch angles.
In the warmer production areas achieving the desired hierarchy is even more difficult.
Commonly, without cold room chilling, despite the above cultural inputs, insufficient shoots
develop and those originating on the basal portion of the whip easily develop excessive
autonomy. Under these circumstances dominance of the central leader (apical control) is
reduced and sometimes even lost. Excessive pruning is required to amend this problem in
the early years which delays production. In the colder production areas, without cold room
chilling, chemical rest-breaking can induce numerous lateral branches. These laterals are,
however, seldom of equal vigour and occasional laterals grow too vigorously, and become
autonomous. To maintain a hierarchy these autonomous shoots or “bulls” need to be bent
down during the growing season often at a time of the year when labour is limited. Most of
these problems are avoided by chilling the nursery trees in the cold room.
RAPID TRANSITION FROM VEGETATIVE TO REPRODUCTIVE: FRUITING BRANCHES
The combination of vigorous rootstocks in climates with long summers and warm autumn
temperatures presents the risk of late growth that opposes the formation of flower buds. By
the judicious application of fertilisers and controlled irrigation trees can be induced to slow
down and stop growth in time for the development of flower buds. A limited number of fruit
allowed to set in the second leaf can help to contain vigour. Vigour is further controlled by
girdling. Girdling in early summer enhances flowering and overcomes the negative effects of
vigorous rootstocks. It is critical that sufficient flower buds are formed in the second leaf to
produce a sufficiently large crop in the third leaf. It is equally critical that the crop in the third
leaf is regulated to prevent over cropping that can result in a dip in production in the fourth
leaf. Fruiting is thus regulated to control growth.
Pruning is kept to a minimum in the early years and the ideal is not to prune before the end
of the second leaf when only thinning cuts are made. The grower is more dependent on
bending to maintain a hierarchic architecture, however, some thinning cuts are occasionally
required to re-establish a hierarchy. Thinning cuts can be applied from the second winter
onwards to reduce excessive flower buds and maintain light distribution. Branches are not
bent before they have spurred up at the beginning of the second year. Light is managed in
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subsequent years by thinning out excessively vigorous branches on the trunk after harvest.
The 2:1 ratio pruning rule is used to identify excessively vigorous branches (Zahn, 1986),
but ultimately 20 to 22 branches are left on the tree.
The management of fruiting branches is determined by the fruiting habit of the cultivar. In
broad terms the 2:1 ratio pruning rule is applied to the fruiting branches of the more spur
bearing cultivars, i.e., Golden Delicious and Royal Gala. The 2:1 ratio rule is applied more
rigorously to cultivars dependent on light for red colour development. The spurred-up fruiting
units situated laterally on the fruiting branches are regularly cut back into the spurs to thin
fruit buds and promote fruit size (Table 3). In the more weeping cultivars, i.e. Fuji, Pink Lady
and Granny Smith, the 2:1 ratio pruning rule is less rigorously applied and fruiting branches
are allowed to fork. However, on the fruiting branches vigorous, upright vegetative shoots
(reiterations) are removed with a clean cut. These fruiting branches are allowed to weep.
It has been observed in Royal Gala that larger spurs produce larger fruit and bourse shoots
with a greater leaf area and an increased chance to set fruit in the following year
(Lespinasse and Delort, 1993). This is known as bourse-over-bourse bearing (Lauri et al.,
1995). Certain cultivars (e.g., Granny Smith and Pink Lady) have more of an ability to bear
regular crops. One-year-old shoots bear tip fruit and bend over. On this bent over shoot
fewer but longer spurs develop on the upper surface of the bent over shoot. These longer
spurs have a greater chance of forming another tip flower bud (terminal bourse). In the
following year the terminal bourse gives fruit and sufficiently long bourse shoots which can
again form a terminal bourse (bourse-over-bourse bearing). The weak spurs on the lower
surface of the bent over shoot flower but do not set fruit and no bourse shoot forms,
resulting in spur extinction. This is a natural way of pruning out weak spurs so that more
reserves can be channelled towards the more autonomous spurs.
To reduce vegetative growth and the need for pruning no fruiting branches are cut back.
Spur autonomy and bourse-over-bourse bearing is promoted by bending the fruiting
branches below the horizontal and the removal of forks and upright shoots on the fruiting
branch according to the 2:1 rule.
MANAGING LIGHT DISTRIBUTION
For the production of red colour the apples need to be exposed to light intensities of at least
70% of full sunlight (Robinson et al., 1991). To achieve this it is necessary to maintain a
pyramidal or hierarchic architecture and reduce excess vegetative growth. Generative or
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fruitful branches (as opposed to vegetative of vigorous branches) are spindled on the trunk
such that each branch occupies its own high light environment within the canopy. Eventually
only 20 such branches are needed on a tree for adequate production.
Most of the work required for maintaining fruit quality is aimed at maintaining good light
distribution and spur quality. Excessively vigourours branches are removed completely on
an annual basis. The most of the light problems develop in the lowest part of the tree and it
is here that most branch removal occurs. As a result the basal (pendant) branches slowly
“move up the trunk”. Branches are annually simplified according to the 2:1 rule.
Simplification is more critical on Royal Gala than in Fuji, Pink Lady and Braeburn. In Royal
Gala it is also necessary to improve fruit size by cutting back into weak spurred up units that
do develop on the fruiting branches over time.
It has been well documented that the most shaded part of the tree is near the trunk in lower
part of the canopy (Robinson et al., 1991). In South Africa, growers clean up all spurs and
shoots on the basal 20 to 50 cm of the branch. This creates a window or “chimney” for light
in an otherwise over shaded part of the canopy. In mature trees this canopy is opened up
throughout the trunk and dramatically improves fruit colour and quality through improved
light distribution.
REFERENCES
Cook, N.C. Jacobs, G. 1999. Suboptimal Winter Chilling Impedes Development of Acrotony
in Apple Shoots. HortScience 34: 1213-1216.
Cook, N.C. Rabe, E. Keulemans, J. Jacobs, G. 1998. The Expression of Acrotony in
Deciduous Fruit Trees: A Study of the Apple Rootstock M.9. J. Amer. Soc. Hort. Sci.
123: 30-34.
Crabbé, J. 1994. Dormancy, p. 597-611. In: C.J. Arntzen and E.M. Ritter (eds.).
Encyclopedia of agricultural science, Volume 1. Academic Press, Inc., New York.
Giliomee, J.H. Strydom, D.K. Van Zyl, H.J. 1968. Northern Spy, Merton and Malling-Merton
Rootstocks Susceptible to Woolly Aphid, Eroisoma lanigerum, in the Western Cape.
S. Afr. J. Agric. Sci. 11: 183-186
Greene, D.W. Autio. W.R. 1994. Notching Techniques Increase Branching of Young Apple
Trees. J. Amer. Soc. Hort. Sci. 119: 678-682
Honeyborne, G.E. 1996. Chemical Restbreaking in Pink Lady, Sundowner and Fiesta: A
Preliminary Report. Deciduous Fruit Grower 46: 135-141
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Jacobs, G. Watermeyer, P.J. Strydom, D.K. 1981. Aspects of Winter Rest of Apple Trees.
Crop Production 10: 103-104
Lauri, P.E., Terouanne, E., Lespinasse, J.M., Regnard, J.L. and Kelner, J.J., 1995.
Genotypic differences in the axillary bud growth and fruiting pattern of apple fruiting
branches over several years: An approach to regulation of fruit bearing. Scientia Hort.
64: 265-281.
Lespinasse, J.M., Delort, J.F. 1993. Regulation of fruiting in apple, role of the bourse and
crowned brindles. Acta Hort. 349: 239-246.
Richardson, E.A. Seeley, S.D. Walker, D.R. 1974. A Model for Estimating the Completion of
Rest for ‘Redhaven’ and ‘Elberta’ Peach Trees. HortScience 9: 331-332
Robinson, T.L., Lakso, A.N., Zhongbo, R. 1991. Modifying apple tree canopies for improved
production efficiency. HortScience 26: 1005-1012.
Saure, M.C. 1985. Dormancy Release in Deciduous Fruit Trees. Hort. Reviews 7: 239-300
Young, E. Motomura, Y. Unrath, C.R. 1987. The Influence of Root Temperature during
Dormancy on Respiration, Carbohydrates, and Growth resumption in Apple and
Peach. J. Amer. Soc. Hort. Sci. 112: 514-519
Zahn,
F.–G.
1986.
Intensivierung
von
Steinobstanlagen
durch
stärkenbezogene
Schnittbehandlung. Ewerbsobstbau 28: 124-140
Tables
1. The growth potential (1/days to 50% burburst) of terminal and lateral buds bud prior to
spring budburst under varied climatic conditions. Endodormancy of terminal buds determined
from forcing intact shoots. Endodormancy of lateral buds determined from forcing
decapitated shoots. Paradormancy (of lateral buds only) determined by forcing shoots with
disbudded shoot piece distal to a lateral bud.
Bud position
Terminal
Lateral
Dormancy
Belgium
Cold
Warm
South Africa
South Africa
Endo
3
13
33
Para
0
0
0
Total
3
13
33
Endo
5
16
15
Para
2
6
35
Total
7
22
50
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2. Long term averages of the accumulated Utah Chill Units (Richardson et al., 1974) for the
more important South African apple growing regions (INFRUITEC, Private Bag X5013,
Stellenbosch 7599, South Africa).
Region
Accumulated Richardson Chill Units
May
June
July
August
Total
Koue Bokkeveld
202
356
389
364
1311
Elgin
48
183
274
240
745
Langkloof
8
179
226
203
616
Vyeboom
-23
149
238
218
346
Piketberg
-166
69
169
131
203
115
214
203
130
662
Western Cape
Eastern Free State
Bethlehem
3. The effect of pruning on fruit mass of Royal Gala apples grown in the Koue Bokkeveld.
The control trees received only the complete removal of excessively vigorous branches that
directly impeded light distribution. The 2:1 ratio pruning rule treatment received the same
pruning as the control treatment with the further complete removal of laterals on the fruiting
branches that did not adhere to the 2:1 rule, i.e. the diameter of the laterals removed was
greater than half of the diameter of the branch upon which the lateral was situated. The third
treatment received the same as the previous treatments except that the laterals on the fruiting
branches were also headed back to a maximum of five fruit buds (weaker units were headed
back deeper). While this was a commercial trial and was not randomised fruit mass was
adjusted using fruit number at harvest as a covariate in an analysis of variance.
Pruning treatment
Mean fruit mass (g)
Control
119
3:1 ratio pruning rule
129
3:1 ratio pruning rule with heading back of laterals
142
Significance level
0.0054
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Figures
Fig. 1. Well defined acrotonic branching habit in a 2-year-apple branch.