Acta Horticulturae

One of the most difficult problems facing fruit
growers currently is how to reduce the excessive
numbers of fruitlets that frequently set on
trees and that lead to the production of small,
unmarketable fruits. Traditionally, the only
method available was to thin fruits by hand and
this is still the most reliable method giving an
even distribution of fruits throughout the tree.
Unfortunately, hand thinning is very expensive;
it is, together with pruning and harvesting one
of the three most costly operations in the
orchard.
Until a few years ago it was possible to use a
range of chemical sprays for thinning fruitlets
on apple, pear, peach and plum trees. Most of
these sprays were either types of synthetic
auxins (e.g. NAA, NAD, 3-CPA) or chemicals
that released ethylene when applied to the tree
(e.g. ethephon). On apples another chemical,
the carbamate pesticide carbaryl (Sevin), also
proved extremely effective as a fruitlet thinner.
Unfortunately, most of these chemicals are in
the process of being withdrawn from use for
thinning on environmental grounds. Indeed,
carbaryl is already banned for use as a thinner
in most European countries. In a few countries,
such as the UK and Germany all of the above
chemicals are officially withdrawn from use.
Although growers in many countries continue
to use these products it is only a matter of time
before the ban becomes complete.
In the future growers will be forced to rely on
certain flower thinning chemicals that are considered
more environmentally acceptable.
Chemicals, such as lime sulphur and ammonium
thiosulphate, are still approved in some
countries even for use in organic growing systems.
Other newer flower thinners (such as
endothallic acid, pelargonic acid and Wilthin)
have a less secure future unless they are accepted
by the environmental lobby. Current
research in the USA, France and the UK is focusing
on the use of more naturally occurring products
such as fish oils and oil seed rape oil
(Colza) for flower thinning of apples.
CURRENT STRATEGIES
New Scion Cultivars or Clones
Natural mutations of apple cultivars such as
‘Red Delicious’ and ‘McIntosh’, which exhibit
reduced growth, have been available for
approximately 50 years. These have been classified
into either compact types with reduced
internode length or spur types that tend to produce
more fruiting spurs and less shoot growth.
Although the ‘Red Delicious’ spur types have
proved popular (e.g. ‘Red Chief’), attempts to
artificially produce similar spur or compact
clones of the other principal apple scion cultivars
and of other fruit species (e.g. sweet cherries)
have proved less successful. Use of breeding
techniques involving treatment of scion
wood with irradiation or treatment of cultures
with chemical mutagens has produced compact
mutants but, like their natural counterparts,
Root restriction: A. Restricted roots of 4-year-old ‘Bramley’s Seedling’ apple (membrane removed);
B. Three-year-old sweet cherry showing partially restricted roots.
these have almost always proved to be unstable
chimeras. This means that the trees frequently
mutate back to the original vigorous form when
planted in the orchard; this problem is worsened
if severe pruning is used either at the
nursery or orchard stage of production. If compact
or spur type scions are to prove successful
in the future it will be essential to develop techniques
that create solid mutants and not periclinal
chimeras, as is the case currently.
One mutant of ‘McIntosh’, found in Canada,
has been shown to be solid and the effects to
be transferable in conventional breeding techniques.
This is the ‘Wijcik’ columnar clone,
which produces trees with a single stem or
trunk and little or no side branching. Pollen
from this original mutant has been used to produce
a range of new columnar scion cultivars in
the UK (e.g. ‘Bolero’ and ‘Waltz’), Canada and,
more recently Germany. Currently, these columnar
scions are only of value as trees for the
home gardener, since their fruit quality is not
sufficient for them to be taken up by the large
multiple retailers. Also, the ‘McIntosh’ parentage
has resulted in sensitivity amongst the offspring
to canker (Nectria galligena) and a tendency
to biennial cropping. Columnar trees
could have clear advantages in the future if the
above defects with the cultivars can be overcome.
They require little or no pruning and
techniques of mechanical harvesting would be
easy to develop leading to potentially significant
savings in the costs of production.
Most new scion cultivars are bred with the aim
of improving yields, fruit quality or, more
recently, disease resistance. However, one novel
strategy explored in France is to breed new cultivars
that are self thinning to single fruits per
spur cluster, so avoiding the problems of thinning.
French researchers have already made
some progress in producing apples with this
novel characteristic and it is hoped that this
strategy will be continued.
Root Manipulation/Restriction
If the root growth of trees is constrained by
limiting, in some way, the soil volume available,
significant reductions in shoot growth can be
achieved. This is a component of the ancient
technique of bonzai culture. Limiting root
growth can be achieved by growing trees
within semi-permeable membranes buried in
the soil or by growing the trees on mounded
soil placed over polythene buried in a trench
within the soil, so isolating the roots from the
bulk of the available soil volume. Part of the
shoot reduction achieved is attributable to
reductions in water availability. However, even
where water supply is abundant growth reduction
is still apparent, possibly due to reductions
in the amounts of cytokinins produced and
transported to the scion by the roots.
Root restriction allows growers very precise
control of tree growth but does require very
careful management of water and nutrient supply
and very firm anchorage supports for the
trees. Also, if root restriction is too severe, fruit
sizes are reduced. Control of tree growth by
root restriction is only likely to be popular in the
future for fruit species where suitable dwarfing
rootstocks are not available and where
management skills in irrigation nutrition and
thinning are of a high level.
Regulated Deficit Irrigation
Experiments in Australia and elsewhere have
shown that if water supply to trees is withheld
early in the season, during the period of maximum
shoot growth, but then applied in abundance
during the period of fruit swelling, shoot
growth can be reduced with no ill effects on
fruit yield or quality. Whilst this technique has
advantages when used with peaches, pears and
some apple cultivars, problems with fruit splitting
have also been noted. The technique is
only relevant in production areas that expe-
CHRONICA HORTICULTURAE •VOL 46 • NUMBER 3 • 2006 • 25