Discuss crop diseases control precautionary measures that should be exercised when growing crops

Resistant plants
An important method of avoiding plant disease is to use resistant plants. A plant
immune to a particular disease is highly valued by the grower if its quality is as good as
those which are susceptible to the disease. Growing immune varieties is the simplest
way of avoiding disease and it is a pity that such varieties are so limited in number.

Soil sterilization
One of the most important precautions taken to avoid disease in horticultural crops is
the practice of soil sterilization. In this the soil in a greenhouse intended for tomatoes,
cucumbers, lettuces etc, is treated by passing hot steam through it (a commercial
practice) or by watering with chemicals (eg formaldehyde or cresylic acid) before the
crop is planted so that any dormant spores of disease are killed. However, at the
temperature used, the spores of beneficial bacteria such as the nitrogen fixers are not
killed, so that after the process is completed these can begin to enrich the soil without
any immediate competition from other organisms. Small glasshouses can be washed
down inside with formalin or other disinfectants and the same sort of treatment can be
given to garden frames, pots, boxes, seed trays and tools. Cresylic acid is one
substance used for this purpose and in some instances also for sterilizing the surface of
the oil, but there are several other good disinfectants available for the gardener.

Preventing the spread of disease
When, despite the precautions referred to, a disease makes its appearance it is
necessary to act quickly and to take direct action measures. The chief of these is to
cover the plant with a protective film of a chemical which will kill the fungus or at least
prevent the germination of its spores. Diseased parts can be removed before treatment,
but it must be remembered that in most diseases an affected plant is often doomed
.The objective of the treatment is to protect still healthy tissues, so that it is wise to
spray or dust early. Various chemicals, known as Fungicides, which will deal with all
types of disease, are widely obtainable on the market. These are applied as a fine misty
spray or as dust (most people hold that spraying is more efficient than dusting). Many
types of spraying and dusting machines are in use. There are also smokes which are lit
to fumigate glasshouses, which have previously been cleared of all plant life. Seeds,
bulbs and corms are also treated by dusting or by immersion in a liquid fungicide to
give them protection from soil-borne diseases, etc., after planting.

Fungal diseases
These are diseases which are caused through the attack of various parasitic fungi.
Some fungi obtain food by living on the decaying organic matter which we call humus in
the soil, but some obtain their food by attacking living plants and injure or kill them in
the process—these are called parasites. In general, the harmless saprophytes are large
and easily seen but the parasites are very small and need a microscope for their proper
identification, although their presence may be detected because of some whitish or
greyish mould or furry growth (eg rose mildew). These parasitic fungi grow
microscopically inside plants but some have also a smothering effect (mildews) and
grow on the outside of leaves and stems etc. They reproduce and spread themselves by
means of innumerable spores.

Fungicides
Fungicides are chemical substances which are used in the control of those diseases of
plants which are caused by fungus parasites (see above). The ideal fungicide is a
substance which will kill a fungus or prevent its spores from germinating, without doing
any harm to the host plant. These substances are used in various forms and in various
ways and many chemicals have been tried in the search for the most effective safe
fungicide. Sulphur and copper are two of the oldest elements used for this purpose and
in various forms are still used against some diseases. For instance copper sulphate is
used to make the well-known Bordeaux mixture which
has been in use for a century, while sulphur probably dates from Biblical times.

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NatalieR answered the question on May 24, 2022 at 06:23