PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
DISEASE
CYCLE
HOST ONLY
Host plant. Because viruses can only multiply in living tissue, the host plant is of
primary importance in the disease cycle.
Weed hosts. Many viruses can persist in cultivated or weed hosts.
Vegetative propagation material. All vegetative propagules such as cuttings,
bulbs, stolons taken from infected plants will carry the virus.
Seed. Many of the known plant viruses may infect the seed of infected plants.
Sometimes a virus may be seedborne only on particular hosts.
Host plant Vegetative propagation material Seeds
HOST AND HOST DEBRIS
A few viruses, although unable to multiply on host plant debris, can survive for
varying periods of time in it, eg tobacco mosaic virus in tobacco leaves.
Host plant
Host debris
HOST, HOST DEBRIS AND SOIL
It is unusual for a virus disease to be soilborne, but some are known to be transmitted
by soilborne organisms, eg nematodes and fungi.
Host plant Host debris Soil
Persistent viruses
are acquired from an
infected plant during
feeding and circulated
internally. After passage
through insect tissues
the virus is introduced
into healthy plants again
during feeding. These
viruses persist in the
vector for long periods.
Some insects will
spread virus all their
lives and some will
persist through moults
and/or egg stages.
HOST AND VECTOR.
For viruses transmitted by a vector, part of the cycle may take place in the vector. In
insects, which are the most common and economically important vectors of virus
diseases, viruses may be:
Non-persistent. Virus is acquired by the insect (usually aphids) from an
infected plant in a few seconds or minutes and can be transmitted almost
immediately during feeding to a new host. Aphids only retain the virus for few
minutes and must acquire the virus again to transmit it again.
Insecticides can control aphids, but cannot prevent spread because transmission
occurs too quickly.
Persistent. Vector has a much longer feeding time, eg hours, followed by a
period, also many hours, during which it is unable to infect plants on which it feeds.
Persistent viruses may be:
– Circulative. The virus is retained for weeks or for the life of the insect (aphid,
leafhoppers and whiteflies). Insecticides can reduce spread of virus disease
within a crop. If only a few plants in the field are infected by early aphid flights
into the crop, it might be possible to control later generations of vectors before
they can acquire and transmit the disease.
– Propagative. Some viruses multiply in vectors (aphids, leafhoppers and
thrips) passing to the salivary glands. Insecticides are effective in reducing
virus spread, eg tomato spotted wilt virus.
Semi-persistent. These viruses have some characteristics of both non-persistent
and persistent viruses.
Host plant Aphids Leafhoppers Thrips
280 Virus and virus-like diseases