Agriculture Reference
In-Depth Information
Bacteria infect plants through wounds or natural openings
such as stomata and hydathodes. Warm, wet weather
favours their development, whereas growth is often
arrested by hot, dry conditions.
various shapes, including spherical, ovoid and filamentous,
and lack a rigid cell wall. Phytoplasmas are spread by
sap-sucking leaf hoppers and planthoppers and infect only
the phloem tissue of plants. Typical diseases caused by
phytoplasmas are the 'big bud' and 'little leaf ' diseases of
many crop and weed plants.
Phytoplasmas, previously called mycoplasma-like
organisms, are similar to the true bacteria. They are of
Viruses and viroids
Viruses are extremely small, obligate parasites consisting of
a nucleic acid core, which contains the genetic information
necessary for replication, surrounded by a protective
protein or lipoprotein coat. Viruses cannot reproduce
outside a host cell and use the plants cell structures and
components to produce more virus particles, to the
detriment of the plant.
Plant viruses are spread by sap-sucking insects, in
particular, aphids, leaf hoppers, thrips and whitef lies.
Transmission is an intricate biological process, often
requiring the virus to form a close relationship with insect
tissues before transmission is possible. Particular viruses
are, almost always, spread by only one insect type. For
example, aphids can transmit
papaya ringspot virus
but not
tomato yellow leaf curl virus
, which, in turn, is transmitted
only by the silver leaf whitef ly.
Fig 1.9 Bacteria growing in culture. Right: a cell viewed through the
microscope. Left: bacterial ooze from plant tissue.
Viruses can be spread through vegetative propagation using
infected plant parts (e.g. bulbs, corms, cuttings and tissue-
cultured plantlets) and some are also transmitted through
seed, contact or infected pollen.
A summary of some important plant viruses occurring
in Australia is given in Table 1.3.
Symptoms caused by viruses are varied and several viruses
infecting one crop type may have similar symptoms,
requiring laboratory tests to determine which virus is
present. Symptoms of virus infection are sometimes
difficult to separate from those caused by chemical
damage, insect feeding and nutrient imbalances.
Viroids are smaller than viruses and are among the smallest
infectious agents known. A viroid consists of small, circular,
infectious nucleic acid and is entirely dependent on the host
for its reproduction. Viroids spread from plant to plant in
infected propagation material and in infected sap carried on
hands or on cutting and pruning instruments. Viroids
occurring in Australia include avocado sun blotch, citrus
exocortis, potato spindle tuber and pear blister canker.
Types of insect vector transmission
Non-persistent
: the virus can be acquired from an infected
plant, or transmitted to another plant in less than one
minute; the virus is usually retained on the insect's
mouthparts for only a few hours.
Fig 1.10 A papaya tree infected with yellow crinkle showing distorted,
new growth and phytoplasma in an infected cell.
