Agriculture Reference
In-Depth Information
Biological control is practised by the encouragement or introduction of the
predatory mite
Typhlodromus pyri
(Scheuten), strains of which are resistant to
some common pesticides.
Two-spotted spider mites also reduce net photosynthesis of apple leaves
(Ferree
et al.
,
). 'Golden Delicious' and 'Jonathan' show appreciable effects
under conditions where the photosynthesis of 'Paula Red' and 'McIntosh' is
not affected and only slightly affected, respectively.
Replant problems
One of the most serious problems in apple production is the poor growth of
trees planted in land previously occupied by apple trees. The reduction in
growth in old apple soil compared with fresh soil maybeupto
% (Hoestra,
). In pot experiments in which 'apple soil' and fresh soil were mixed in
varying proportions, apple seedling growth was related to the proportion of
apple soil in a non-linear way:
% of 'apple soil' gave
% growth reduction,
%a
% reduction and
%a
% reduction. Trees tend to show recovery
starting
) found that fumigation of
land previously cropped with apples using a broad spectrum biocide (chloro-
picrin) led to a
or
years after planting, but Oehl (
years from
planting. The delayed cropping on 'replant' soils has a particularly adverse
effect on the economics of high-density planting systems. Geldart (
% increase in accumulated crop over the first
) and
Peterson and Hinman (
) demonstrated that in British Columbia, Canada,
and Washington State, USA it was not economically viable to plant apple trees
on land previously cropped with apples unless remedial measures were taken.
Treatments with a broad spectrum of action against all categories of soil
organisms (nematodes, fungi, bacteria and actinomycetes) restore the growth
potential of replant soils to that of 'fresh' soils, the response being much greater
on the former than the latter (Hoestra,
). Such broad-spectrum treatments
include chloropicrin, steaming and heating to
◦
C. The effect of chloropicrin
fumigation of soil samples in pot tests of seedling growth is taken as an index
of the potential problems in replanting and the need for pre-planting orchard
treatments.
Studies on poor growth of newly planted apple trees in the absence of
soil sterilization have indicated two distinct, though sometimes interacting,
problems. The first is caused by nematodes. The second is found even when
nematode numbers are low and cannot be overcome by the use of nemati-
cides (Hoestra,
). This was initially referred to as specific apple replant
disease (SARD) but is now generally referred to as apple replant disease, ARD
(Utkhede,
) following evidence, discussed below, that it can be induced by
crops other than apple.
