PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
HOW VIRUSES
INFECT HOST
PLANTS
DISTRIBUTION
WITHIN A PLANT
It may be
assumed that
for all practical
purposes,
even though
symptoms only
appear on parts
of the plant, all
living cells within
the plant are
infected
THROUGH WOUNDS MADE
By vectors which are the most common and economically serious method of
spread. The most important vectors are insects.
Mechanically by plants rubbing against each other during handling or pruning.
This is not so common or important (exceptions).
DEPOSITION IN HOST PLANT MATERIAL
Viruses may be deposited in plants via pollen.
Scions may become infected when grafted onto infected rootstocks.
MOVEMENT THROUGH PLANTS
Viruses may move through plants in many ways including:
Direct cell-to-cell invasion, eg in leaves viruses may move through
8-10 cells (about 1 mm) per day.
Transportation through the phloem may occur rapidly, eg 15 cm in
6 minutes. Most viruses take 2-5 days to move from inoculated leaves.
DISTRIBUTION WITHIN A PLANT
Some viruses are fully systemic while others leave some tissues virus-free, eg the
growing points.
Infected plants usually remain infected for a lifetime (page 283). Plants propagated
from such material are usually infected.
Direction and rate of translocation of a virus in a plant (adapted from Agrios 1997).
DETECTION AND
IDENTIFICATION
Symptoms of
rose mosaic
Virus particles
Diseased
bud to
healthy
plant
New
growth
develops
symptoms
Virus protein
injected into rabbit
ELISA Testing
DNA tests
SYMPTOMS EXHIBITED BY THE HOST PLANT
Some viruses cause distinctive symptoms in their hosts and so the disease and the
virus can be identified from symptoms. However, frequently this is not possible.
Some virus symptoms can be confused with nutrient deficiencies or
excesses, herbicide or insect injury (page 275, Table 54).
DETECTION AND IDENTIFICATION BY EXPERTS
Experts test for the presence of virus in plants, parent stock and certification
schemes, eg strawberry, cut flowers, potato, pome and stone fruit, grape. Testing is
difficult, slow and expensive.
Electron microscopy identifies the shape of viruses particles (rods, bullets or
spheres), in plant sap or ultra-thin plant segments. For some viruses, though, the
shape of particles is not a reliable means of identification.
Indicator plants. Some herbaceous plants, eg tobacco, petunia, readily show
symptoms when infected with many different plant viruses. A virus can be
transferred by budding, grafting, mechanically rubbing the plant with sap, or by a
vector, from a diseased host plant which does not show obvious symptoms, to a
healthy indicator species, which does show characteristic symptoms.
Serology. Virus protein is injected into a mammal, eg a rabbit, resulting in
antibodies in the blood system which react specifically with the virus antigen
injected.
– ELISA (enzyme-linked immunosorbent assay) is a serological test in which one
antibody carries with it an enzyme that releases a colored compound if virus is
present. Kits identify some viruses in some hosts, and are a quick, sensitive and
specific method of testing large numbers of plant samples.
– More sensitive tests are being developed for viruses that accumulate in low
amounts in their natural hosts and escape detection, eg carnation.
DNA technology.
– DNA can be used to detect unknown viruses for which there is no antiserum or
information available. Can also be used for detecting woody plant viruses.
– PCR (polymerase chain reaction) multiplies over a million times, a short
segment of DNA, so that can be seen as a gel.
A quick, simple, inexpensive generic test is being developed for the
whole nepovirus (nematode-transmitted) group of viruses. They are a group of
about 46 viruses that infect many plant families that cause probably the most
serious viral diseases of horticultural crops, particularly perennial woody and bulb
crops. Many have not been recorded in Australia so quarantine tests are important.
276 Virus and virus-like diseases