Study the Pests, Diseases and Weeds
Regular
monitoring of pests, diseases and weeds is the basis for effective management.
To be able to manage pests, diseases and weeds, information is needed on the
specific pests, diseases and weeds present in the region, village or crop
fields and the associated damage they cause.
Typical signs of pest attacks on crop plants
Most
crop pests belong to the insects, mites and nematodes. Pest damage is obvious
and easy to identify.
Insect
damage can be categorized by biting and chewing (e.g. caterpillars, weevils),
piercing and sucking (e.g. aphids, psyllids) and boring (e.g. borer, leaf miner)
species. Some are slow moving (e.g. caterpillars), fast moving (e.g. fruit flies),
hidden (e.g. stem borer), or easy to observe (e.g. caterpillars, weevils).
Pest
damage is often species-specific: leaves with holes or missing parts is an
indication of caterpillar or weevil damage; curled leaves is an indication of
aphids; damaged or rotten fruits are often caused by larvae of fruit flies;
withering plants can also be caused by larvae of noctuids or the stem borer;
and branches or trunks with holes may be an attack by lignivorous insects.
Mites
are very small and cannot be seen with the naked eye. However, some mite
species (spider mites) weave a typical tissue on attacked plant parts and can
therefore, easily be detected. If mites are present on plants, leaves and fruits,
become yellowish.
Nematodes
are also very small and therefore, they are not easy to observe with the naked
eye. They mostly attack plant roots; plants become yellow, wither and die.
Typical signs of disease attacks on crop plants
Most
crop diseases are caused by fungi, bacteria or viruses.
Fungi
cause the great majority, estimated at two-thirds, of infectious plant
diseases. They include all white and true rusts, smuts, needle casts, leaf
curls, mildew, sooty moulds and anthracnose.
In addition, they are responsible for most leaf, fruit, and flower spots,
cankers, blights, wilts, scabs, and root, stem, fruit, wood rots among many
others. Parts of plants or the total crop plant can wither and die.
Bacteria
cause any of the four following main problems. Some bacteria produce enzymes
that breakdown the cell walls of plants anywhere in the plant. This causes
parts of the plant to start rotting (known as ‘rot’). Some bacteria produce
toxins that are generally damaging to plant tissues, usually causing early
death of the plant. Others produce large amounts of very sticky sugars; as they
travel through the plant, they block the narrow channels preventing water
getting from the plant roots up to the shoots and leaves, again causing rapid
death of the plant. Finally, other bacteria produce proteins that mimic plant
hormones. These lead to overgrowth of plant tissue and form tumours.
Viruses
mostly cause systemic diseases. Generally, leaves show chlorosis or change in
colour of leaves and other green parts. Light green or yellow patches of
various shades, shapes and sizes appear in affected leaves. These patches may
form characteristic mosaic patterns, resulting in general reduction in growth
and vigour of the plant.
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