Physical controls are long-lasting and need little maintenance.
Some physical methods are expensive to set up.
Warm water treatment
This method is used for pests such as stem and bulb nematodes in
narcissus bulbs. Immersion of bulbs for 2 hours at 44°C controls the
pest without seriously affecting bulb tissues. Chrysanthemum stools and
strawberry runners may be similarly treated, using temperature and time
combinations favourable to each crop. Viruses (such as aspermy virus
on chrysanthemum) are more difficult to control, since viruses are more
intimately associated with the plant nuclei. Virus concentrations may
be greatly reduced in meristems of stock plants grown at temperatures
of 40°C for about a month. This has enabled the production of tissuecultured
disease-free stock material of both edible and non-edible crops
(see tissue culture
are used for the control of weeds when other methods,
such as cultivation, hand-weeding or herbicidal control are not
Partial soil sterilization
Commercial greenhouse soils are commonly sterilized by high-pressure
steam released to penetrate downwards into the soil, which is covered
by heat resistant plastic sheeting (sheet steaming). The steam condenses
on contact with soil particles, and moves deeper only when that layer
of soil has reached steam temperature. Some active soil pests, such as
symphilids, may move downwards ahead of the steam ‘front’.
The temperatures required to kill most nematodes, insects, weed seeds
and fungi are 45°C, 55°C, 55°C and 60°C respectively. Beneficial
bacterial spores are not killed below 82°C, and therefore growers
attempt to reach, but not exceed, this soil temperature. Most mycorrhizal
fungi are unfortunately killed by this process.
In this way, organisms difficult to sterilize, such as fungal sclerotia,
Meloidogyne and Verticillium in root debris, may be killed. Sheet
steaming is effective only to depths of about 15 cm, and its effect
is reduced when soil aggregates are large and hard to
penetrate, or when soils are wet and hard to heat up. When soil pests
and diseases occur deep in the soil, heating pipes
may be placed below
the soil surface, as grids or spikes, to achieve a more thorough effect.
The ‘ steam-plough ’ achieves a similar result, as it is winched along
the greenhouse. If soil is to be used in growing composts it should
be sterilized (see sterilizing equipment). The clear advantage of soil
sterilization may occasionally be lost if a serious soil fungus (such as Pythium
) is accidentally introduced into a crop where it may quickly
spread in the absence of fungal competition.
A physical barrier
such as a fence sunk into the ground deters rabbits
and deer. Fine screens placed over ventilation fans help prevent the entry
of pests, such as fungus gnats, from outside a greenhouse or mushroom
house. Pots placed on small stands in water-filled trays are freed from
the visitations of red spider mite and adult vine weevils. Peach leaf curl
is a difficult disease to control. A plastic sheet placed over the peach or
almond over winter will greatly reduce both arrival of spores and the
moisture needed for infection of the buds.
Pheremone traps containing a specific synthetic chemical similar to the
attractant odour of a female moth are used in apple orchards to lure male
codling and tortrix moths onto a sticky surface, thus enabling an accurate
assessment of their numbers and therefore more effective control.
Comparable traps are available against plum moth and pea moth.
|Figure 16.2 (a) Horticultural fleece used to protect
(b) P lastic
guard used against
(c) Lettuce planted to attract slugs
tomatoes (d) Slug trap that avoids
A rodent trap is available that entices the rat or mouse into a container
with suitable bait and then the pest is killed humanely with a high
Allotment owners sometimes use containers such as plastic milk bottles
or jam jars sunk into the ground that when part-filled with beer, attract slugs. Between two rows of tomatoes, a ‘sacrificial’ row of lettuce can
be grown to attract slugs (see Figure 16.2), which can then be controlled
in a trap (see Figure 16.2).
Measures here warn off the pest in a chemical or physical way. Ultrasonic
devices create high frequency sounds, unheard by humans,
but offensive to animals such as rats and mice. The odour from onions
inter-planted with carrots may deter the carrot fly from attacking its host
crop. Marigolds planted in amongst crops deter whitefly and aphids.
A spray repellent using an extract from a Yucca
species is used to deter