Agriculture Reference
In-Depth Information
Virus and viroids
Viruses are very simple microscopic agents that possess either RNA or
DNA that is encapsulated in a protein coat. They are capable of replicating
themselves inside the plant using the plant cell's own metabolism. Viroids
dif er from viruses in being composed of only a single strand of ribonucleic
acid but are not protected by a protein coat. Virus diseases are normally
transmitted by contaminated propagation material, such as seeds, cuttings,
layers, budwood or entire plants, hands or tools that are contaminated or by
insect transmission. Insect transmission is probably the most important way
viral diseases are spread, and aphids are one of the most important vectors, as
well as some other insects such as thrips.
Once a plant is infected with a virus, it is not possible to cure that plant.
Sanitation is by removal of the diseased plants and using plants that serve as
host vectors for the virus and insect. Careful disinfection of hands and tools
such as grafting knives, pruning shears or saws is a must. Seed disinfection
can also be ef ective with some plants of the
Solanaceae
family. If an economic
analysis can justify its use, an anti-aphid screen or fabric can be used, as is
done in some papaya fi elds.
Viruses can kill a plant or reduce production and quality of fruit. Some
cause dif erent types of leaf spots such as mosaics, while others can cause
leaf yellowing, stunting and curled leaves. Some species will show no obvious
visible symptoms but are still infected. Clean material can be obtained by
meristem culture combined with thermotherapy, micro-grafting, ovule and
nucellar embryo culture. Some viral diseases are not seed-transmitted and
the seeds act as fi lters. Care has to be taken with the plant material taken to
the farm and to keep it as clean as possible. At country level, strict quarantine
is important to avoid the importation of new viruses carried unknowingly
by people.
Nematodes
Nematodes are non-vertebrate animals that can be plant parasites. Most of
them attack the roots, causing galls, wounds, tumours and lesions. Their
feeding reduces plant vigour and yields. Several genera of nematodes -
Meloidogyne
(knot nematode),
Ditylenchus, Tylenchus
and
Radopholus
- cause
plant diseases, and the last two are parasites of citrus roots.
Nematodes are more common in sandy soils and their control, till
now, has been by the use of fumigants. Fumigants are very poisonous and
the results obtained from their use have been mixed. Once you start on a
fumigation programme you have to continue. Post-plant nematicides are also
available. Other approaches are rotations that involve trap crops, organic
matter addition and soil solarization, which can be used with short-cycle crops
such as pineapple and banana.
