Agriculture Reference
In-Depth Information
harvesting before the maturation of
potato cyst
nematode
cysts (see p. 247), so that damage to
the crop and the release of the nematode eggs are
avoided. Annual weeds may be induced to germinate
in a prepared seedbed by irrigation. After weeds have
been controlled with a herbicide such as
glyphosate
,
a crop may then be sown into the undisturbed bed
or
stale seedbed
, with less chance of further weed
germination.
organic matter may be important in their control.
Seed trays and pots should be thoroughly washed
to remove all traces of compost that might harbour
damping-off disease.
Companion planting
An increasingly common practice in the garden is
the deliberate establishment of two or more plant
species close together with the intention of deriving
some horticultural benefit from their association.
Such a situation may seem at first sight to encourage
competition rather than mutual benefit. Supporters
of companion planting consider that plant and animal
species in the natural world show much evidence
of mutual cooperation. Some experimental results
have given support to the practice, but much of the
evidence remains unproven. It should be stated,
however, that while most commercial horticulturist
producers in Western Europe grow blocks of a single
species, millions of subsistence growers worldwide
are using two or three different crop species inter-
planted as a regular practice.
Two of many recommended companion-planting
groupings are:
X
potato, broad bean, sweet corn and onion
X
carrot, leek, broad bean and broccoli.
Vegetative propagation material
Such material (see also p. 94) is used in many areas
of horticulture, as bulbs (e.g. tulips and onions),
tubers (e.g. dahlias and potatoes), runners (e.g.
strawberries), cuttings (e.g. chrysanthemums and
many trees and shrubs) and graft scions in trees. The
increase in nematodes, viruses, fungi and bacteria
by vegetative propagation is a particular problem,
since the organisms are inside the plant tissues, and
since the plant tissues are sensitive to any drastic
control measures.
Inspection
of introduced material
may greatly reduce the risk of this problem. Soft,
puffy narcissus bulbs, chrysanthemum cuttings with
an internal rot, whitefly or red spider mite on stock
plants, and virus on nursery stock, are all symptoms
that would suggest either careful sorting or rejection
of the stocks.
Alternate hosts
Clean stock schemes
Alternate hosts harbouring pests and diseases should
be removed where possible. A few examples of weed
alternate hosts are given here:
X
Club root infects shepherd's purse (
Capsella bursa-
pastoris
), and white blister 'rust' may be seen on
charlock (
Sinapis arvensis
).
X
Black bean aphid infests fat hen (
Chenopodium
album
).
X
In the greenhouse, glasshouse red spider mite may
be found on chickweed.
X
Speedwells may harbour stem eelworm.
Quality of vegetative material is monitored in
Britain and Ireland by the government
Plant Health
Propagation Scheme (PHPS)
and
Seed Potato
Classification Scheme (SPCS)
. In this way, high-
quality potatoes, bulbs and fruit plants (with low levels
of virus diseases and nematodes) are available to the
gardener from soft fruit, top fruit, potato and bulb
industries.
Hygienic growing
Below are a few examples of the hundreds of
'
common sense
' activities that reduce pest and
disease attack in greenhouses and gardens. During
the crop, the grower should aim to provide optimum
conditions for growth. Water content of soil should be
adequate for growth (see
field capacity
, p. 151), but
not be so excessive that root diseases (e.g. damping
off in pot plants, club root of cabbage and brown root
rot of conifers) are actively encouraged.
Removal of infected plant material
In fruit tree species such as apple, routine
pruning
operations may remove serious pests such as fruit
tree red spider mite eggs, and diseases such as
canker and powdery mildew. Pruning should also
aim to reduce the density of shoots in the centre
of the tree. The resulting reduction in humidity
provides a microenvironment that is less favourable
to these diseases.
Tree stumps
harbouring serious
underground diseases such as honey fungus should
be removed manually or by means of a mechanical
stump grinder. Making a feature of an infected
Water sources
Covering and regular cleaning of water tanks to
prevent the breeding of
damping-off fungi
in rotting
