PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
BIOLOGY & IDENTIFICATION
Viruses, viroids, phytoplasmas
NO. DISEASES
IN AUSTRALIA
SOME
DISTINCTIVE
FEATURES
One nanometer (nm)
= one billionth of a metre
= 0.000,000,001 metres
= 0.000,001 millimetre (mm)
Viruses are a significant and growing threat to crop production worldwide.
There are many hundred virus and virus-like diseases of plants. Most plants are
susceptible to viruses although trees and Australian native plants seem to have
fewer recorded virus disease problems. This may be due to lack of research.
New viruses and phytoplasmas are constantly being detected and identified.
The Australasian Plant Pathology Society (APPSnet)
www.australasianplantpathologysociety.org.au/
The American Phytopathology Society (APSnet) www.apsnet.org/
VIRUSES, VIROIDS, PHYTOPLASMAS
1. Can only multiply in living cells, however, a few phytoplasmas have
been cultured on complex media.
2. Are infectious and can spread from one plant to another.
3. Can only be seen with the aid of an electron microscope. They are too
small to be seen with a light microscope.
4. Vary in structure and size.
VIROIDS 1. About 10-50 times smaller than viruses (require a
magnification of x 100,000 or more to be seen).
2. Consist of free ribonucleic acid (RNA) with no protein
coat.
VIRUSES 1. Require a magnification of x 10,000 or more to be seen.
Some are rigid rods about 15 by 300 nm; many appear as
thin threads usually about 10-13 nm wide and range in
length from 480 to 2,000 nm long; most spherical viruses
range from 17-60 nm in diameter (Agrios 2005).
2. Consist of RNA or deoxyribonucleic acid (DNA) with a
protein coat.
PHYTOPLASMAS 1. Are larger than viruses but smaller than bacteria (require
a magnification of x 5,000 or more to be seen).
2. Have a cell membrane, but no cell wall, cytoplasm and
strands of nuclear material. They colonise plant phloem.
3. Phytoplasmas are a group of organisms that cause
symptoms similar to viruses and may be spread by insects,
but they are structurally different and are more closely
related to bacteria. They have been included here because
they are spread by insects and symptoms generally are
more virus-like than bacterial-like.
Viroids Viruses Phytoplasmas and
(x 100,000) (x 10,000) spiroplasmas (x 5,000)
(various shapes)
‘LIFE CYCLE’
They are obligate
parasites because
they can only
multiply in
living plants
Unlike fungi and bacteria, viruses cannot reproduce by themselves but can only
multiply inside a living plant or animal.
Once inside the plant cell, viruses multiply by inducing the host cells to
make more virus (Agrios 2005).
This interferes with photosynthesis and respiration in plant cells so the plant
cannot grow properly resulting in stunting, reduced yield, etc.
An analogy can be made between virus multiplication in a cell and the
photocopying of a written page by a photocopier. The virus, like the
written page, contains information, and just as the reproduction of the
written page is done entirely by the photocopier so the multiplication of the
virus is the work of the infected plant cell.
274 Virus and virus-like diseases