Agriculture Reference
In-Depth Information
Introduction
A general overview is given in this chapter for the
various control measure available against weeds,
pests and diseases. In the three following chapters,
important weeds, pests and diseases are described
with an emphasis on symptoms, damage and
life cycles, and with brief comments on controls
relevant to the particular organism causing the
problem.
In this chapter, the human and environmental safety
aspects of different control measures are given in
some detail. Control measures that are mentioned
briefly in the narrative of the following three chapters
are covered here in more detail.
X
Soil sterilization may lead to a worse disease
situation if infected soil/plants are introduced.
Safe practice and environmental effects. In physical
control, some hazards are:
X
Unsafe use of cultivation equipment such as
ploughs, rotavators, fl ame throwers and steam
sterilization equipment, used to control weeds,
pests and diseases.
X
Unsafe removal of infected trees.
X
Unsafe burning of infected plant material.
Natural balances:
X
Of the physical controls described below, the
only method that may affect natural balances is
partial soil sterilization
. If a newly sterilized soil is
planted with some plants infected with a disease
such as 'damping off' or
Fusarium
wilt, the spread
of the disease may be more rapid than in a non-
sterilized soil. This is because naturally occurring
bacterial and fungal competitors to the disease
have been killed off by the heat process.
General comments on plant health
maintenance
Gardeners should consider the whole range of control
options (physical, cultural, biological, chemical and
resistance) when confronted by a weed, pest or
disease problem. It should be borne in mind that
for
weeds
, cultural controls such as hoeing are an
important alternative to the use of herbicides. With
the small
pests
, physical controls such as deterrents
and traps, and biological controls are increasingly
being used, rather than insecticides.
Disease
control
should, where possible, involve species and cultivars
with plant resistance.
While the environmentally preferable non-chemical
methods mentioned above may appeal to gardeners,
there will be occasions when chemical methods may
seem the only option. Control of
blight
on potato in a
wet summer would be one example.
Examples of physical controls
Barriers
Plastic sheets
laid on the ground (and plants planted
through slits in the plastic with ornamentals or
soft fruit) are a very good way of reducing weed
populations.
Biodegradable
forms of plastic used for
this purpose are becoming popular.
Risks
: when the plastic sheets are no longer needed,
they should be disposed of in the correct location at
the local council rubbish centre. They should not be
burnt, producing toxic fumes.
Horticultural fleece
placed over growing crops (see
Figure 16.1) helps prevent the entry of pests such
as cabbage root fly and cabbage white butterfly into
rotation bed crops.
Fences
using
sturdy wire-mesh sunk into the ground
to a depth of about 30 cm deter rabbits and small deer
from digging under the fence. Fine-mesh
screens
placed over ventilation fans help to prevent the entry
of pests such as fungus gnats or aphids from outside
a greenhouse. Pots placed on small stands in
water-
filled trays
are freed from flightless species such as
red spider mite and adult vine weevil.
Physical control
Physical control
is a material, mechanical or
hand control where the weed, pest or disease is
directly blocked or
destroyed
.
Benefits of physical control:
X
Once established, they often remain for a long
time, e.g. wire or wood fencing used against
rabbits and deer.
X
They usually need little maintenance, e.g. plastic
sheets placed on decorative borders to reduce
weeds.
Limitations of physical control:
X
Some physical methods are expensive to set up,
e.g. fencing used against rabbits.
Traps
Pheromone traps
containing a specific synthetic
chemical similar to the attractant odour of the female
moth are commonly used by gardeners to lure male
codling and tortrix moths onto a sticky surface, thus
