PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
SPREAD
Aphids are
the most
important
insect
vectors
Knowing how a
virus is spread is
essential for
effective
management
INSECTS AND OTHER VECTORS
Insects, especially sucking species such as aphids, leafhoppers, whiteflies and
thrips, are common vectors of virus diseases. Viruses may be persistent or nonvirus
diseases.
persistent in insects (page 280). Other insects and mites may occasionally transmit
Nematodes. Viruses spread by nematodes are called nepoviruses, eg grapevine
fanleaf virus is spread by the dagger nematode. This is presently an uncommon
method of spread in Australia but these viruses cause probably the most serious
viral diseases of horticultural crops, particularly perennial woody and bulb crops in
many plant families, and are of serious concern to quarantine authorities
worldwide. Attempts are being made develop a suitable generic test for the whole
nepovirus group. www.daff.gov.au/ba/publications/nepoviruses
Fungi. This is a rare method of spread, eg lettuce big vein virus is spread by a soilinhabiting
fungus (Olpidium sp.).
Protozoa. This is also an uncommon method of spread, eg Polymyxa graminis
can transmit virus diseases in cereal crops.
Flowering plants. Plants such as dodder (Cuscuta spp.) which parasitize stems
of plants may transfer virus diseases from one plant to another.
Leaf Thrips Nematodes Dodder
hopper
VEGETATIVE PROPAGATION
Budding, grafting, cuttings, rootstock, scions, tubers etc. For crops
propagated in this manner, this is the most important method of spread.
Tissue cultures may also transfer virus particles.
Grafting Cuttings Runners Bulb, tubers Tissue
culture
Infected
pollen
Healthy
plant
Some virus
diseases
are spread by only
one method,
others may be
spread by several
means
SEED AND POLLEN
More than 100 viruses are transmitted by
seed. Usually only 1-30% of seed may be
infected but 100% of seed can carry virus.
Some are only seedborne on some hosts, eg
tomato spotted wilt virus is seedborne in
beans.
Prunus necrotic ringspot virus is spread by
pollen.
Infected Seed carrying
plant virus
MECHANICAL TRANSMISSION IN SAP
A few plant viruses and viroids are spread
in plant sap adhering to fingers, secateurs.
budding knives, cigarettes, etc, eg tobacco
mosaic virus. Some orchid viruses spread
when healthy plants contact diseased ones.
NATURAL ROOT GRAFTS
Natural root grafts may occur in orchards
and other tree plantings.
Infected Healthy plant
EPIDEMIOLOGY.
Vegetative plant parts and seed primarily spread viruses between
generations resulting in primary infection of plants.
Insects not only bring the virus into the crop but also spread it from infected to
healthy plants and during the same growing season (secondary infections). Such virus
diseases may have many disease cycles per season (10-20 cycles for aphid
transmitted viruses).
If spread by vegetative parts, seed and also by insects there may be an
early and total infection of the crop with subsequent severe damage.
282 Virus and virus-like diseases