Agriculture Reference
In-Depth Information
plants are as healthy and disease-free as possible. Crop
failure in a greenhouse as a result of plant pests (mainly
insects) or disease can carry a high cost and should, of
course, be avoided if at all possible.
Disease and pest control can be achieved by adopt-
ing good management practices, including sensible
breaks in production along with appropriate steriliza-
tion strategies. However, the application of chemical
insecticides and fungicides is also a frequently needed
practice. Application can be by spraying plants or pests
or by fumigating a whole area within the greenhouse.
The main advantage of chemical control of disease and
pests are that they can be applied in anticipation that
a problem will exist. Therefore they offer
preventative
disease and pest control. The disadvantage is that many
of these chemicals are indeed harmful both to humans
and other plant and insect life and it is therefore always
desirable to minimize their use.
There are now many types of biological controls that
can be used to control insect pests within a green-
house. A well-known example is the release of ladybugs
(ladybirds) which are natural predators of aphids into
greenhouses. There are many other predator insects
available that can offer effective control of other insect
pests. A sample of specific predator types available
and the pest they attack include:
Amblysieus cucumeris
against thrips;
Aphidoletes aphidimyza
against aphids
and
Encarsia formosa
whitefly, while ladybugs and green
lacewings are used as general insect predators.
The major difficulty of biological predatory control
relates to the fact that the pest must in fact be there,
even if at a low level, before the predators are released
(otherwise how will they survive!). It is therefore diffi-
cult to avoid some insect damage and almost impossible
to achieve complete preventative control.
The risk of soil-borne diseases can be avoided (or at
least substantially reduced) by using only sterile soil,
or soil mixes, in the greenhouse. However, unless an
inert, synthetic soil substitute is used (e.g. 'Perlite'
or sand/gravel/Perlite) the possibility that disease will
occur as a result of infected soil cannot be entirely
avoided. Often the sterilization procedure fails to
remove all disease or fails to kill weed seeds. In addition
if peat moss is used in soil mixes it is almost impossible
to ensure the mix is free from insect pests that have a
reproductive cycle in the peat moss.
Achieving good disease and pest control in green-
houses can be achieved by other means. For example,
good insect-proofing throughout the house will reduce
the risk that insects will enter the greenhouse. However,
it should be borne in mind that people are very effec-
tive spreaders of plant disease in greenhouses. Personnel
from the breeding programme are likely to be in contact
with plants outside the greenhouse (i.e. will visit field
plots) and so there is a great risk that these staff will
transmit disease or carry in insect pests prevalent to the
crop with them while visiting the glasshouse. Simple
rules, such as any greenhouse operations are carried out
first thing each day and other field tasks are done later,
can help in reducing disease incidence and spread.
Plant viruses can cause particularly serious problems
in plant breeding schemes as many virus diseases are
transmitted through the planting material (e.g. seed
viruses in cereals and tuber borne viruses in clonal
crops). Many viruses can be eliminated by avoiding
the virus vectors, which are often insects (particularly
aphids). Workers in the breeding programme can also be
responsible for carrying insect vectors into greenhouses
on their hands or clothes. Again the risks of infection
can be reduced by applying simple rules (e.g. protective
clothing, sterile gloves etc.).
Economics
Despite the attraction of greenhouses as an integral part
of any plant breeding programme, there is no doubt
that this facility can be responsible for a high propor-
tion of the overall cost of operating a breeding system.
In addition, due to the high cost of building and main-
taining greenhouse facilities the actual space available
will be limited. In the practical world (the one in which
we unfortunately all live) economic use of greenhouse
space will become a major factor.
Plants in greenhouses are grown either in pots (or
some other individual unit) or in beds (where many
plants are propagated together). The size of pot used
(or the plant density in seedling beds) will have a large
influence on the number of plants that can be grown in
a unit area. It is therefore necessary to choose a density
pot size that will allow good plant health and growth. If
small pots are used then more plants can be propagated
at lower cost. If they are, however, too small, then plant
health or reproductive efficiency can be affected.
It is necessary to allow access to plants grown in green-
houses. Increased efficiency of greenhouse space can be
