Horticulturists, in their everyday activities, may remove or reduce
damaging organisms in many different ways and thus protect the crop.
Below are described some of the more important methods used.
Fit in with daily routines. Have a long-lasting effect.
May be time-consuming.
of soils enables a physical improvement in
soil structure as a preparation for the growing of crops. The improved
drainage and tilth may reduce damping-off diseases, disturb annual and
perennial weeds, such as chickweed and couch-grass, and expose soil
pests, such as leatherjackets and cutworms, to the eager beaks of birds.
may be necessary to deplete the underground
rhizomes or tap roots of perennial weeds, such as couch-grass. Hoeing
annual weeds is an effective method, provided the roots are fully
exposed, and the soil dry enough to prevent root re-establishment.
While the correct content and balance of major and minor nutrients
(see Plant nutrition
) in the soil are recognized as vitally important for
optimum crop yield and quality, it should be remembered that plant
resistance to pests and diseases is also affected by nutrient levels in the
plant. Excessive nitrogen levels, causing soft tissue growth, encourage
the increase of insects such as peach-potato aphid, fungi such as grey
mould, and bacteria such as fireblight. Adequate levels of potassium, on
the other hand, help control fungal diseases, such as Fusarium wilt on
plants such as peas, tomatoes and carnation, and tomato mosaic virus.
Fertility provided by composted material usually provides nutrients to
the plant at the correct concentrations.
Club root disease of brassicas is less damaging in soil with a pH greater
than 6, and lime may be incorporated before planting these crops
to achieve this aim. Amenity horticulturists apply mulches, such as
composted bark, grass cuttings or straw, to bare soil in order to control
annual weeds by excluding the source of light. Black polythene sheeting
is used in soft fruit production to achieve a similar objective.
Some important soil-borne pests and diseases attack specific crops, such
as potato cyst nematode on potato and club root on cruciferous plants.
As they are soil-borne, they are slow in their dispersal, but are difficult to
control. By the simple method of planting a given crop in a different plot
each season, such pests or diseases are excluded from their preferred
host crop for a number of years, during which their numbers will slowly
decline. A gardener often creates five or six plots (sometimes bounded
by wooden planking) to achieve successful rotation. Plants belonging to
the same plant families fit into the rotation system. They have the same
sensitivity to particular pests and diseases. Potatoes, tomatoes, peppers
and aubergines are all members of the Solanaceae. Melons, marrows,
courgettes and cucumbers all belong to the Cucurbitaceae. When
considering a rotation plan, it is advisable to confine members of the
same family to the same plot in the same growing season. Rotation does
not work well against unspecific problems, such as grey mould, which
may attack a wide range of plant families. Rotation is also not likely to
be effective against rapidly spreading organisms such as aphids.
|Figure 16.3 Rotation plots used to reduce pest and disease
stage of white rot disease
) on onion and related
crops, such as leeks, garlic, chives and
shallots, is able to survive in the soil for 15
years or more. It can be seen that a very long
rotation period would be necessary to remove
this serious disease. A six year rotation is not
Planting and harvesting times
Some pests emerge from their overwintering
stage at about the same time each year, such
as cabbage root fly in late April. By planting
early to establish tolerant brassica plants
before the pest emerges a useful supplement
to chemical control is achieved. The
deliberate planting of early potato cultivars enables harvesting before
the maturation of potato cyst nematode cysts, so that damage to the crop
and the release of the nematode eggs is avoided. Annual weeds may be
induced to germinate in a prepared seedbed by irrigation. After they
have been controlled with a contact herbicide, such as paraquat, a crop
may then be sown into the undisturbed bed or stale seedbed, with less
chance of further weed germination.
Clean seed and planting material
Seed producers take stringent precautions to exclude weed seed
contaminants and pests and diseases from their seed stocks. While weed
seeds are, in the main, removed by mechanical separators, and insects
can be killed by seed dressings, systemic fungal seed infections, such as
celery leaf spot disease in celery seed, are best controlled by immersion
of dry seed in a 0.2 per cent thiram solution at 30°C for 24 hours
(thiram soak treatment
). The seed is then re-dried. A similar treatment
is often given for carrot and parsley seed.
Equal care is taken to monitor seed crops likely to carry virus disease
(such as tomato mosaic). Curative control by dry heat at 70°C for
four days is usually effective, although it may reduce subsequent seed
Vegetative propagation material
Vegetative propagation material is used in all areas of horticulture,
such as bulbs (tulips and onions), tubers (dahlias and potatoes), runners
(strawberries), cuttings (chrysanthemums and many trees and shrubs)
and graft scions in trees. The increase of 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, virus on nursery
stock, are all symptoms that would suggest either careful sorting, or
rejection of the stocks.
Accurate and rapid methods of virus testing (using test plants, electron
microscopy and staining by ELISA
techniques) now enable growers
to learn quickly the quality of their planting stocks. Fungal levels in
cuttings (such as Fusarium
wilt of carnations) can be routinely checked
by placing plant segments in sterile nutrient culture.
The quality of vegetative material is monitored in the UK
by the Plant Health Propagation Scheme
. In particular, it supervises the provision
of six quality levels of plant material (Foundation, Super Elite, Elite,
A, Approved and Healthy grades) for the soft fruit, top fruit and bulb
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
), but not so excessive that root diseases (such as damping
off in pot plants, club root of cabbage and brown root rot of conifers) are
Water sources can be analyzed for Pythium
if damping off diseases are a constant problem. Covering and regular
cleaning of water tanks to prevent the breeding of these fungi in rotting
organic matter may be important in their control. Seed trays and pots
should be washed to remove all traces of compost that might harbour
damping off disease.
Conifer nursery stock grown on raised gravel beds is less likely to suffer
the water-borne spread of conifer brown rot. Many protected crops are
grown in isolated beds or peat modules to reduce spread of wilt-inducing
organisms (such as Fusarium
spp.). Gooseberry sawfly caterpillars can
be removed in spring from leaves found on the lower centre stems of the
gooseberry. This action helps prevent subsequent invasions by the pest
in the summer months.
High humidity encourages many diseases. In greenhouses, the careful
timing of daily overhead irrigation and of ventilation (to reduce
overnight condensation on leaves or flowers) may greatly reduce levels
of diseases, such as grey mould on pot plants or downy mildew on
lettuce. The slow drawing-back of motorized thermal screens
above commercial glasshouse crops (so as to prevent condensation
problems) has contributed greatly to the reduction of disease.
Reducing spread of pests and diseases
|Figure 16.4 A sowthistle acting as alternate host to
The spread of pests and diseases from plant to plant or field to field
can be slowed down. Tomato mosaic virus spread may be reduced by leaving suspect plants till the end of de-leafing or
harvesting. Washing knives and hands regularly in
warm, soapy water will reduce subsequent viral
spread. Soil-borne problems, such as club root,
eelworms and damping off diseases, are easily
carried by boots and tractor wheels. Foot and wheel
dips, containing a general chemical sterilant, such
as formaldehyde, have been successfully used,
especially in preventing damping off problems in
Alternate hosts harbouring pests and diseases should
be removed where possible. A few examples of
many alternate hosts are given here. Soil-borne
problems, such as club root of cabbage and freeliving
eelworms on strawberries are harboured by
shepherd’s purse (Capsella bursa-pastoris
chickweed (Stellaria media
) respectively. Groundsel (Senecio vulgaris
is an alternate host of rust on cinerarias, while docks (Rumex spp.
) act as
a reservoir of dock sawfly, which damages young apple trees.
Removal of infected plant material
With rapid-increase problems, such as peach-potato aphid and white
rust of chrysanthemum fungus in greenhouses, removal of affected
leaves is practicable in the early stages of the problem, but becomes
progressively unmanageable after the pest or disease has increased
and dispersed throughout the plants. Slow-increase problems, such as
Fusarium wilt disease on tomatoes or carnations and vine weevil larvae
found damaging roots of plants such as primulas and begonias, may
be removed throughout the crop cycle, but the infected roots and soil
must be carefully placed in a bag to prevent dispersal of the problem.
In commercial outdoor production, labour costs usually prevent such
removal during the growing season. However, removal is achieved
chemically in some situations. The destruction of blight-infected potato
foliage with herbicide such as diquat prior to harvest reduces infection
of the tubers. Burning of post-harvest leaf material and lifting of root
debris after harvest (against grey mould on strawberries and club root on
brassicas respectively) may help prevent problems in the next crop.
In fruit tree species such as apple, routine pruning operations may
remove serious pests such as fruit tree red spider mite, 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 reduction in humidity
provides a microenvironment less favourable to disease increase.
Tree stumps harbouring serious underground diseases such as honey
fungus should be removed manually or using a mechanical stump
grinder. Making a feature of an infected stump by placing a bird table on
it is one of the least recommended activities in gardening.
In physical/cultural control, some hazards
- Unsafe use of cultivation equipment, such as ploughs, rotavators,
flame throwers and steam sterilization equipment, used to control
weeds, pests and diseases.
- Unsafe removal of infected trees.
- Unsafe burning of infected plant material.
When using cultural control, risks can be minimized
- following guidelines for the safe use of ploughs and rotavators to
avoid damage to humans and adjoining crops or plantings;
- following guidelines for the safe use of flame throwers and steam
- taking safety precautions when removing infected shrubs and trees;
- carefully transferring infected plants to dumps or compost sites, thus
avoiding the spread of infections.