This situation is similar for many other important tropical crops, which are now mainly
grown in regions far from where they originated. Soya is now mainly grown in the
Americas, though it originated in northeast Asia. Groundnuts are now found mainly in
India and China even though they originated in South America. Pressure from disease
and insects is the main reason for this global redistribution of crops.
Causes of Plant Diseases
live as parasites on plants. Fungal diseases are very often the biggest problem for plants.
When and Why do Diseases Develop?
- Weak plants are always more liable to be damaged by diseases (and insects), so
unfavourable growing conditions, such as poorly drained or infertile land, drought,
overcrowded plants, heavy weed growth and so on will lead to a higher incidence
of disease. - If susceptible crops are grown. Some crops, and some varieties of these crops, are
resistant to diseases (they are not affected) or are tolerant (they are not seriously
affected). Others are susceptible (they are likely to be affected). Unfortunately this
resistance or tolerance is often quickly overcome by the disease organisms, and a
new variety which is released on to the market by plant breeders as “resistant” or
“tolerant” when it is launched may become “susceptible” in just a few years. - If the climate is favourable for the spread of disease; in general diseases spread
more quickly in hot and humid weather than in cold and dry weather. Some fungi
however, such as Powdery Mildew and other wind-borne diseases spread more in
dry weather. - If there is an imbalance of plant nutrients in the soil, for example if there is too
much or too little nitrogen or potassium. - If the same crop, or crops that are related, are grown continuously on the same
land, ie without rotation, or if the rotation is badly planned.
Disease Control
Farmers and gardeners have several ways of controlling and preventing diseases; in
most cases, preventing diseases is easier and cheaper than curing them, by using:
- Resistant or tolerant crops or varieties.
- Crop rotations—soil-borne diseases and diseases carried over in plant residues can
often be controlled if their life cycle is broken by an appropriate crop rotation. - Timing—crops can often be planted at certain times of the year so as to avoid
harmful temperatures and/or rainfall at critical stages of plant growth. - Crop husbandry—strong and healthy plants can escape diseases altogether, while
weak plants in the same growing conditions may be badly infected. All infected
plants and residues should be removed and destroyed. Heavy infestation of weeds
also can increase the spread and damage done by diseases. - Clean seed—disease-free plants should always be chosen by farmers, either from
their own crops or when seed is bought in from outside, or “off the farm”.
82 TONY WINCH
Fungi, viruses and bacteria can cause diseases in plants - three harmful organisms that