Acta Horticulturae

HORTICULTURAL SCIENCE FOCUS
Control of Growth and Cropping of
Temperate Fruit Trees
Tony Webster
The majority of temperate fruit tree scion cultivars,
if allowed to develop naturally and grown
on their own roots, produce large trees of 10 m
or more in height and spread. There are exceptions,
such as peaches/nectarines and sour (tart)
cherries, which generally produce trees smaller
in stature, but apple, pear, plum and sweet
cherry scion cultivars all tend to produce large
trees. Fruit tree orchards established prior to
approximately 1950 were mainly comprised of
these large trees. Trees on dwarfing apple rootstocks
were occasionally planted between the
preferred larger trees to provide yields early in
the orchard’s life, but these were always removed
once the large trees began to crop and
fill their allotted space.
When the cost of land was relatively inexpensive
and the costs of labour, for pruning, thinning
and harvesting were also relatively small these
large trees posed no problems. However, as
land and labour prices have slowly increased,
whilst the prices in the markets for fruits, adjusted
for inflation, have stagnated, the need for
strategies focused on reducing costs of production
and increasing the value of the fruits produced
has increased. The introduction of strategies
of fruit tree management that improve
orchard profitability to the grower has become
of paramount importance to grower economic
survival.
Controlling the growth vigour of the fruit tree
has proved a very effective method of partially
achieving the aforementioned objective. Small
compact trees planted at close spacings in the
orchard have a number of advantages: Firstly,
despite the increased costs for planting stock,
they are easier and cheaper to manage, in terms
of pruning/training, spraying, thinning and,
most especially, harvesting. In addition, fruits
produced on smaller trees are often, though not
always, of higher quality and market value. This
is probably due to the improved partitioning of
water, minerals and assimilates to fruits rather
than shoots and also to the improved light
penetration into the tree canopy of well-managed
dwarfed trees. Finally, it can be argued
that there are clear environmental advantages,
in that small compact trees can be sprayed with
the most efficient use of spray chemicals and
minimal spray drift into the environment.
Reducing the intrinsic vigour of fruit trees is a
prime objective in almost all of the world areas
currently producing dessert quality fruits of
apples, pears, sweet cherries and plums.
Nevertheless, some fruit crops benefit from
being grown on large trees. For instance, fruit
grown for the processing market, such as for
fruit juice, cider, canning and drying are invariably
produced on large trees that are mechanically
harvested. Orchards of large trees are
cheaper to establish and lend themselves more
easily to the rigors of mechanical harvesting
than trees on dwarfing rootstocks, which are
often poorly anchored. In these situations large
vigorous trees can offer distinct economic
advantages. Moreover, environmentalists have
begun to suggest that large trees provide better
habitats for wild birds and other animals than
dwarf trees and this has led to government
schemes in some countries aimed at aiding the
retention of large old orchard trees.
Control of tree cropping is of equal importance
to the control of tree vigour in improving
orchard profitability. If modern orchards are to
prove profitable they must begin cropping
rapidly following planting in the orchard, crop
abundantly and consistently every year and the
fruit produced must be of the sizes and qualities
required by the markets. Trees of many cultivars,
if grown naturally on their own roots and
with minimal management inputs, are usually
slow to begin cropping and may develop a
biennial cycle of excessive fruit production in
one year followed by almost no crop in the subsequent
year. Excessive cropping inevitably
leads to very strong competition between fruits
for the tree’s limited available resources and to
reduced fruit size and quality and to poor market
prices. Cost effective strategies to increase
yield precocity and abundance as well as fruit
quality and size will continue to be essential if
profitability is to be sustained in the future.
TRADITIONAL METHODS
For hundreds of years some fruit growers have
tried to use one of several traditional methods
to control the excessive vigour of their trees.
Where available, as in apple, they have used
dwarfing rootstocks or interstocks. In addition,
they have employed various pruning/training
techniques or, very occasionally, root pruning.
Dwarfing Rootstocks and Interstocks
Rootstocks that reduce the vigorous growth of
fruit tree scions have a number of advantages
compared with other methods of growth control.
They provide a permanent control of
vigour, they are relatively inexpensive compared
with other strategies of control and they are
ideal for environmentally sustainable systems of
culture.
Until quite recently dwarfing rootstocks were
available only for apples, although it was
known that dwarfed pears could be grown if
grafted on several other Rosaceous genera (e.g.
Cotoneaster and Cydonia). Over the last 60
years, many new dwarfing rootstocks have
been produced for apples, greatly extending
the range of vigour control possible. In addition,
dwarfing quince rootstocks have been
developed for pears and the first dwarfing rootstocks
for sweet cherries, plums and peaches
have been released. Rootstocks, especially
dwarfing rootstocks, usually stimulate increased
production of flower buds and fruit set,
especially on young trees and may also increase
yield efficiency and fruit quality on mature
trees. Although this is partly linked to a change
in the partitioning of assimilates and minerals in
the tree towards flower buds and away from
shoot growth, this does not entirely explain the
effect of rootstocks. There are invigorating
rootstocks that also increase flower production
(e.g. the apple rootstock M.25) and dwarfing
rootstocks that reduce flowering and fruit set
(e.g. the sweet cherry rootstock Damil).
Despite this progress at the practical level of
rootstock selection, there is still no clear understanding
of the metabolic processes that underpin
how these rootstocks bring about the dwarfing
of scions grafted onto them. Initially, it was
thought to be a result of resistances at the graft
union, which limited the flow of water or mineral
nutrients from the soil to the scions. This may
be a contributory factor but more recent theories
suggest the role of hormones, particularly
auxins and cytokinins, in the dwarfing process
and abscisic acid (ABA) may also play a role.
ISHS • 20