Agriculture Reference
In-Depth Information
Responses to summer pruning are very variable, being influenced by time
of pruning, the type of buds immediately below the pruning cut, rootstock
and scion cultivar.
Pinching out of shoot tips or removal of shoot apices or new leaves greatly
reduces growth in shoot length (Table
.
).
Cultivar differences in shoot growth
Scion cultivars may show extreme contrasts in their vigour of shoot growth.
Barlow (
) reported cumulative extension growth over
years of only
m per tree of 'Miller's Seedling'/'M.
' under conditions where 'Laxton's
Superb'/'M.
m per tree. Such differences may reflect intrinsic
differences in shoot extension growth potential but also result from cultivar
differences in cropping level, in lateral budbreak, and in the angle of lateral
branches to the horizontal.
The physiological basis for intrinsic differences between cultivars with re-
spect to extension growth has not been studied to any great extent. This partly
reflects the difficulty and complexity of such studies. It also reflects the fact
that scion cultivars are selected primarily for the characteristics of their fruits.
Control of tree vigour is achieved primarily by the use of size-controlling root-
stocks, by control of the level of fruiting, by branch training and pruning and
by the use of plant growth regulators. The exception to this generalization is
the selection and utilization of spur-types.
Spur-type cultivars have been selected and propagated vegetatively follow-
ing observation of branches with short internodes and a higher than average
number of fruiting spurs per unit length of branch. These have arisen sponta-
neously by bud mutation within many of the leading cultivars, most commonly
within 'Delicious' ('Red Delicious'). Others have been selected following in-
duction of mutation by irradiation (Campbell and Lacey,
' produced
). They are
considered advantageous because they offer a measure of vigour control with
less restriction to a specific rootstock than the standard cultivars while retaining
the fruit characteristics of these.
Spur-type trees typically have about the same number of nodes per shoot as
their 'parent' cultivar but the internodes are
% shorter. There may be no
difference between the 'normal' cultivars and the spur-types in the number of
laterals produced in the first year, but subsequently a greater proportion of the
axillary buds develop into short fruiting spurs, instead of long vegetative shoots,
in the spur-types (Arasu,
-
). These differences have cumulative effects so
that in mature trees spur-types may show only half to one third as much total
annual shoot growth as their standard counterparts (Curry and Looney,
).
Since most spur-types have arisen by mutation from different 'parents' it
is not surprising that they show variation in the factors which contribute to
