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Issue No. 121 - JUL 1995
Volume XIX No. 2.

CAMELLIA VIRUSES

by Michael B. Thomas

Plant Science Dept. Lincoln University

Overseas comment on camellia virus
There has been a whole series of articles in American camellia literature about the whole issue of "how we should consider camellia viruses". This centres around the popularity of virus mottled blooms and the practice of deliberately infecting camellias. One or two writers stated their appreciation of virused camellias but there are many articles condemning all association with the disease of our plants! Pat Greutert, past editor of Southern Camellia Review, warns of the lethal heritage that we may be bequeathing our favourite flowers. This is strongly backed-up by Dr William Ackerman, a leading researcher on camellia breeding, who speaks of the slippery slope that exists if we encourage viruses because it can end up in disaster. Dr Rod Bieleski, horticultural scientist and husband of our editor, has also been very outspoken in condemning any tolerance of viruses saying that no obviously virus infected plants are acceptable in an orchid show and the same should apply for camellias.

How do you know if plants are virus infected?
The most conclusive symptoms are mottling of leaves and flowers. Leaves appear with yellow spotting, while flowers can have varying levels of pale or white blotching. These forms of variegation tend to be quite irregular while that caused by genetic change or sports will often be more uniform such as regular bands that occur on flowers or leaves. It should also be recognised that viruses can vary greatly in their expression. It has been shown from virus studies in woody and herbaceous plants that the plants around us in horticulture can be 'loaded' with many viruses but with no symptoms. One apple tree can be infected with several viruses and in fact the plant may show no symptoms. If that plant were infected with Apple Mosaic virus it could then show the leaf mottling that is very similar to Camellia Mosaic virus. The point is that many factors influence the level of symptoms such as the interaction between the genetic make- up of the host and the type of virus. In strawberries it is found that severity varies according to whether there is a complex of viruses or not. These complexes often result in a very short life span of the host plant.

A further aspect of the symptoms is that viruses can be hidden within the plant and may only cause a slight depression of vigour. Viruses with little or no obvious effect are referred to as latent or sleeping but the same virus in another variety could be quite devastating.

This was illustrated to the writer one time when grafting a Camellia sasanqua variety onto C. japonica (see photo 1). The graft had taken and the rootstock later sent up a sucker shoot (on left of photo) which showed strong viral variegation, yet the scion variety showed no such symptoms. It was concluded that the sasanqua, which may have been the original carrier of the disease, would not show any symptoms. In fact sasanquas often appear to be free of viral variegation.

How are these viruses spread?
Viruses are living organisms that need to live within living tissues and will not live for long outside a host. Plant viruses can be transmitted by insects which can act as an intermediary carrier or by any means of vegetative propagation where the sap is transferred. The main virus of Camellias is Camellia Mosaic and is not spread by insects. But every time camellias are propagated by cuttings or grafting it is highly likely that any virus present will continue on in the offspring. Grafting in the wild where shoots or roots may graft naturally due to' close contact in the garden, is another possibility for viral transfer.

Why should we be concerned about viruses'?
Viruses tend to have a negative effect on the vigour and general health of plants. This can particularly be noticed in propagation. Cuttings and grafts of virus infected varieties can often fail or be very slow to establish. This was noticed by the writer when grafting infected Camellia reticulata plants. Scion growth would often be very weak. These plants would tend to be harder to establish in the garden and remain more prone to succumb to unfavourable conditions or attack by other diseases or pests. However if you are fond of the mottled and variegated plants you may well say that the virus helps to keep them from getting too big and that your variegated C. japonica 'Ville De Nantes' has lasted many years in the garden. Camellias do tend to cope quite well with viral problems, and usually better than roses. For example rose varieties like 'Whisky' have often been hard to 'keep going' for more than 2 or 3 years. Virus infected plants will always be made more vulnerable in their environment and never have more vigour than high health species. Adverse effects can be made worse by mutation, as shown with the stream of new influenza strains that have harassed the human population over the years.

Can viruses be eradicated?

There is no simple or practical way that virus can be successfully eradicated from camellia varieties and still retain the same cultivar. Laboratory procedures would be necessary but have not been fully established. Propagation by seed is a means of eliminating Camellia Mosaic but unfortunately the variety that you grow by this means, will normally be lost as well. It is possible to successfully 'clean-up' plants like apples. This is done by growing plants in a special heat chamber at a steady temperature of 38þC for 35 days. Tip cuttings from these plants are then grafted onto seedlings. It is therefore a long and expensive process to eradicate viruses from clones and is usually only carried-out by skilled staff in large institutions like universities and research institutes.

Conclusions
Camellia growers probably have mixed feelings about viruses with most preferring plants and flowers free of variegation. It is good to remember the hidden and debilitating nature of viruses and that eradication of the 'blotch' is no easy matter. We are fortunate that viruses tend to be less severe in camellias than in other plants but I don't feel that is an excuse for tolerating them.

In conclusion I will quote from an article by C. C. Bush in the Southern California Camellia Review where viruses are defended. This article concludes with the statement that our Creator blessed us with virus. I am sure this is a wrong interpretation for the Bible clearly states that after the 'fall of man' and banishment of Adam from the Garden of Eden, the Creator said that the ground would be cursed and he would need to sweat and toil to cope with the weeds and all the hard work. Viruses are not a blessing but a curse and as Dr Rod Bieleski wrote, we need to seek to eliminate virus from our show benches by 2005 A.D., and our gardens by 2010.