PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
OVERWINTERING,
OVERSUMMERING
"Overwintering"
may in reality be
oversummering,
perhaps a better
term might by
overseasoning
IN THE HOST PLANT
Once a plant is infected with virus it remains infected for the rest of its life. Viruses
with a wide host range, eg tomato spotted wilt, may ‘overwinter’ in weeds or
perennial hosts which are then a potential source of infection for many future
ornamental, vegetable and other crops.
Host plant
IN SEED
Probably more than 20% of virus diseases are seedborne, some on certain hosts only.
Not all seed from an infected plant may carry the virus. Crops grown from virus-free
seeds may escape later infection if they are kept away from insect vectors that have
access to infected plants.
Seed
IN VEGETATIVE PROPAGATION MATERIAL
All virus diseases are carried over into new plantings if the new bulbs, corms, tubers,
stolons, cuttings and nursery stock are taken from parent plants which are already
infected with virus.
Grafting Cuttings Runners Bulb, tubers
IN VECTORS
Persistent circulatory viruses can ‘overwinter’ in certain insect vectors, eg
leafhoppers. Perennial parasitic flowering plants such as Devil's twine (Cassytha
spp.) could also carry virus.
Aphids Leaf Thrips
hoppers
IN PLANT DEBRIS & SOIL (not common?)
Tobacco mosaic virus is spread by mechanical inoculation, by grafting, by seed and
by contact between plants. It can survive in dead infected tobacco leaves in
cigarettes for years and may be passed from them to healthy plants during smoking.
It is considered that some viruses, eg tobacco mosaic virus, which infect orchids,
could survive in infected leaves in the soil for limited periods of time.
Plant debris
Soil
Virus and virus-like diseases 281