Acta Horticulturae

Table 3. Rootstocks for sweet cherries.
Genus/Type Rootstock Vigour Remarks
Clones of species
Prunus avium F.12/1 Vigorous Now only used to a small extent in
Australia.
P. mahaleb SL.64 and other Vigorous Well adapted to dry environments
clones
and to cold winters.
P. cerasus Tabel/Edabriz Semi dwarfing Good induction of cropping; difficult
to establish; sensitive to stress;
unsuited to dry conditions.
Weiroot 53, 72 and Semi dwarfing Incompatibility can be a problem;
158 sensitive to viruses.
Weiroot 10 and 13 Intermediate Incompatibility can be a problem;
sensitive to viruses.
CAB 11E, 8F and 6P Intermediate Quite popular in Italy and performed
well in Spanish trials.
P. canescens Camil Intermediate Intolerant of soil pathogens.
P. cerasifera Adara Vigorous Tolerates wet, heavy soils and
alkalinity but incompatible with
some scion cultivars.
Clones of hybrids
P. incisa x P. serrula Inmil Dwarfing Induces erect habit, poor branching
and poor yield efficiency.
P. x dawyckensis Damil Semi dwarfing Poor yield efficiency and sensitive to
certain root pathogens.
P. cerasus x P. canescens Gisela 3 Dwarfing to semi As yet untested to any extent outside
dwarfing Germany.
P. cerasus x P. canescens Gisela 5 Semi dwarfing Currently the most popular dwarfing
rootstock; induces high levels of
fruit set and cropping; sensitive to
Phytophthora on heavy clay, poorly
drained soils.
P. cerasus x P. canescens Gisela 6 Intermediate Induces high levels of yield efficiency;
sensitive to Phytophthora on heavy
clay, poorly drained soils.
P. avium x P. cerasus PH-L Series e.g. 4 Intermediate Czech rootstocks still under trial in
and 6
many parts of Europe; good crop
induction compared with F.12/1.
P. avium x Colt Semi vigorous Induces better growth control with
P. pseudocerasus traditional UK cultivars than with
Stella and its derivatives; induces
good fruit size.
P. avium x P. mahaleb MxM 14, Brokforest or Semi vigorous Well suited to hot dry soil conditions;
Maxma Delbard 14
Performs poorly on some deep clay
and loam soils.
colour. However, reports that the product left
high levels of residues in processed apples used
in baby foods resulted in its withdrawal from
use. It has never been re-introduced, even
though the reports were later found to be
unproven and largely false.
A similar fate has more recently befallen cyclocel
(CCC), which had proved especially efficient
in controlling the shoot growth on pear trees.
Growers in the Netherlands and Belgium were
using many sprays of CCC and, although this in
itself was not illegal, the chemical residues that
were found in marketed fruits was shown to be
well above those permitted.
The fate of both Alar and CCC demonstrates
the current power of the multiple retailers
(supermarkets) in determining how fruits supplied
to them are grown. With access to sophisticated
analytical techniques they can rapidly
determine if residues are present in fruits sent
to them and can equally rapidly reject whole
consignments from packers or exporters, if
undesirable residues are found. They perceive,
probably correctly, that the modern consumer
does not want fruits containing these residues,
irrespective of whether it is legal or illegal to use
the chemicals in fruit production.
Paclobutrazol (Cultar/PP333) is another similar
chemical that is particularly effective in controlling
the growth of stone as well as pome fruit
trees. It is best applied as sprays to pome fruits
and soil drenches to stone fruits. The chemical
has a long half life in the soil and has been
shown to affect the growth of other crops planted
after the orchard is grubbed. This strong
persistence of paclobutrazol in the soil has been
a strong contributory factor in the failure to get
approval for its release to commerce in many
countries. Although available in the UK,
Australia and New Zealand it is not available in
the USA and many European countries. It will
probably have only a limited use in the future.
A new growth regulating chemical prohexadione-Ca
(Regalis) has been released recently in
several European countries. It has an advantage,
in comparison with paclobutrazol, in that
it has very low persistence in the tree and soil
and leaves almost no residues in fruits.
However, it is not as effective as Cultar in stimulating
flowering and it is necessary to apply it
several times during the season if growth control
is to be adequate. Interestingly, it also
appears to have beneficial side effects in reducing
the incidence of fireblight (Erwinia amylovora)
and other diseases on treated trees.
Although sprays of growth regulating chemicals,
such as paclobutrazol, stimulate the production
of greatly increased numbers of flower
buds, this can lead to problems of excessive
fruit set, which is followed by very severe fruitlet
abscission. It is probable that the canopy leaf
area cannot support the excessive numbers of
fruitlets that set and this stimulates a very
severe fruitlet drop. Interestingly, in contrast to
the effects noted with daminozide, cyclocel and
paclobutrazol, sprays of prohexadione-CA, the
newest growth regulating chemical, sometimes
reduce, rather than increase flowering.
If one takes account of current consumer attitudes
concerning pesticide residues in fruits it is
difficult to see a secure future for the use of any
of these growth regulating sprays. This is mirrored
by the lack of new research within chemical
companies on discovering and developing
these products. In the next few decades growers
and researchers may be better to focus on
use of non-chemical methods of tree size control,
if they are to be sure of long term market
acceptance for their fruits.
Increase or Decrease Fruit Set and
Retention
In countries experiencing spring frost damage
to pear flowers it has become common to apply
sprays of gibberellins to aid the set and retention
of damaged flowers. However, this is only
fully effective with cultivars that have the ability
to set parthenocarpically (e.g. ‘Conference’
pear). More recently, sprays of a mixture of gibberellins
have been used to stimulate the precocious
cropping of young pear trees in Belgium.
Gibberellin sprays, often together with an auxin
component, may also be used to aid the fruit
set of apples, cherries and plums, although
these treatments have never gained commercial
acceptance due to their significant cost.
Another strategy aimed at increasing yields is to
apply a spray in the autumn that delays flowering
in the spring; this lowers the risk of frost
damage to the vulnerable flowers. Sprays of
ethephon have mainly been effective in this respect.
ISHS • 24