|
|
|
|
April 2000 This Month ONT will focus on: "Virus: The Invisible Enemy" Written By Mr. Frikkie Marais. Diseases caused by viruses are still a mystery to a lot of orchid growers' world-wide. The reason being, that they are extremely difficult to identify and impossible to get rid off after infection. In this article I will try to clarify some common issues pertaining to viruses, their identification and prevention.
Tomato spotted wilt virus - Photograph: Plant Research Institute of the ARC in Pretoria.
Virus diseases are a major problem all over the world, especially, in the ornamental and vegetable growing spheres. The first orchid virus was described as early as 1943; nearly a century after the cultivation of orchids became fashionable during the early 19th Century. Today there are more than 25 known viruses infecting orchids, with new ones being added every year.
Tobacco Mosaic virus - Photograph: Plant Research Institute of the ARC in Pretoria.
What is a virus?: Most of the viruses effecting orchids are extremely simple organisms consisting only of the genetic material necessary for multiplication, (ribonucleic acid RNA), covered by a protein coat for protection. The fact that viruses are constructed out of the basic building blocks of life places them amongst the smallest disease causing organisms known to science.
Orchid viruses vary in length and are between 450 and 300 nanometers long (one nanometre is one millionth of a millimetre). Because of their minute size they cannot be seen with the naked eye or ordinary light microscope but can only be seen with the aid of an electron microscope, and then only when magnified a couple of thousand times. (See picture of TSWV virus and TSWV infected plant.)
How do viruses live and multiply: Viruses are living organisms and can survive outside a host for a considerable time; however, they cannot multiply without a living host. Some viruses can spread by means of plant to plant contact, but this is the exception. Most viruses require a vector (a carrier organism) for their transmission and a variety of organisms can act as vectors. Nematodes (eelworms), beetles, aphids, white fly, thrips and humans are amongst the most common vectors. Some viruses can even be transmitted by fungus, but these are usually restricted to the soil.
As soon as a virus enters its host through wounds or damaged cells, it begins to multiply rapidly (Tobacco Mosaic Virus can multiply to 11 million particles within 20 hours) and soon spreads to neighbouring cells. These cells may die and leave a necrotic spot limiting further infection. However, most of the time the virus is carried throughout the whole plant by the sap, which results in a systemic infection. The virus can now easily be transferred to another host by feeding insects or by a growers tool or hands.
General effects of viruses in orchids: Viruses can cause a wide range of abnormalities in orchids of which the symptoms are highly variable among different genera infected with the same virus. The most noticeable symptoms are the yellowing (chlorosis) and death of plant tissues (necrosis) as well as stunting of plants. These symptoms might occur separately, in combination or not at all because of good growing conditions that suppress the development of symptoms. Plants under stress tend to show severe virus symptoms if infected.
The variability of virus symptoms are the result of various factors such as: the strain of virus, species of infected plant and environmental conditions such as light intensity, temperature and nutrition e.g. an overdose of urea has the same visual effect on Cymbidium leaves as Cymbidium Mosaic Virus (CyMV).
The three most important viruses for the orchid grower are undoubtedly Cymbidium Mosaic Virus (CyMV), Tobacco Mosaic Virus (TMV) and Tomato Spotted Wilt Virus (TSWV). To be able to recognise virus infections in orchids the grower should be aware of the symptoms most frequently associated with a virus infection.
(Left) Probably TSWV (Cattleya hybrid) - Photographer: Martiens Erasmus (Right) Probably TMV (Dendrobium kingianum) - Photographer: Martiens Erasmus.
Foliar symptoms:
Probably TMV (Cymbidium hybrid) - Photograph: Martiens Erasmus.
Flower symptoms:
How to control virus diseases:
The Plant Protection Unit of the Agricultural Research Council in Pretoria can be contacted for virus testing of suspicious plants. (at approximately R400.00 per test sample).
Sources: Lawson, R.H. Orchid pests and diseases, AOS, 1995 Spence, N. The Garden, Journal of the RHS, vol. 119 part 8, 1994 Trench, T.N. et al. South African disease control handbook, University of Natal, 1992.
National Department of Agriculture Directorate Genetic Resources: National Gene Bank P/Bag X973 Pretoria 0001
|
|
Please Note: The opinions & recommendations made in articles that appear on this website, are those of the individual authors, and not those of the Orchid Society of Northern Transvaal (ONT). The ONT neither adopts nor endorses such opinions & recommendations and disclaims all responsibility for them. This includes products advertised on this website. |
