WELCOME

I would like to welcome you most cordially in the name of the Präsident der Biologischen Bundesanstalt fü Land-und Forstwirtschaft, Prof. Schuhmann. It is a special pleasure for me to welcome the Chairman of the working group 'virus deseases of small-fruits' of the International Society of Horticultural Science, Dr. Murant, in our house. Organizational circumstances did not allow to open this Symposium together with the Xth International Symmposium on Fruit Tree Viruses which will begin tomorrow. But as the majority of you is also going to paticipate in the Xth International Symposium, Ministerialrat Priew from the Ministerium fü Ernährung, Landwirtschaft und Forsten (the representative of the Ministry for Agriculture) will welcome all of you tomorrow and transmit to you the compliments of the Federal Minister for Agriculture.

Ladies and gentlemen, it is indeed a great pleasure for me that our institute had the honor to organize this Ist International Symposium on Small Fruit Virus Diseases. Please allow me now, at the beginning of your meeting, to make a few remarks on the situation of virus diseases that we are confronted with in our country.

The most important cultivar of small fruits in the Federal Republic is without any doubt strawberry. Until a few years ago we observed in Germany no strawberry viruses involving serious crop losses. Prevalent was a strain of strawberry mottle, however of little virulence. But in recent years we have observed with increasing pre-occupation the occurence of strawberry yellow edge and strawberry crincle in some southern German cultivation areas. It is possible that these viruses were imported through young plants from other countries. The situation of currant which, in production figures, is less important than strawberry may be called relatively fine. Black current is getting increasingly attacked by reversion which represents a serious threat to our black currant cultivation. Raspberry is to a large extent subject to virus infections. The crop decreases involved may be influencing the stagnation of raspberry cultivation observed during the last years.

Let me conclude my short words of introduction by wishing you all a fruitful exchange of experience on this meeting and by hoping that it will become the point of departure for intensive international cooperation of scientists working in that field. A cooperation that will also bear upon practice and will help to solve urgent problems of small fruit cultivation.


Opening Address

The Development of our Knowledge of Rubus Viruses

A.F. Murant

As Chairman of the ISHS Working Group on Small Fruit Virus Diseases it gives me great pleasure to welcome you to our 1st International Symposium and to express the hope that our deliberations will be sufficiently valuable to make this meeting the first of many.

Many of us have been fortunate in the past to have attended the Symposia on Fruit Tree Virus Diseases, which were originally conceived as meetings of a small number of specialists to discuss research on viruses and virus diseases of pome and stone fruits. Although there are undoubted advantages in keeping such conferences small, the number of people attending the Fruit Tree Virus Symposia has steadily increased over the years and because of this there has been understandable reluctance to broaden the scope of these meetings to include other kinds of perennial crops, such as grapevines, citrus or small fruits. Nevertheless, some of us have been able to present papers on small fruit viruses to these meetings, usually on the pre-text that the viruses in question also infect fruit tree species experimentally and might conceivably pose a problem in these crops in the field.

Increasingly, however, it became apparent that those virologists working with small fruit virus diseases needed a separate forum for their own discussions and in 1974 a Working Group on Small Fruit Virus Diseases was formed under the auspices of the International Society for Horticultural Science. In 1975 I was invited to become Chairman of this group and Dr Richard Stace-Smith agreed to become Secretary.

We decided that one of our first functions should be to convene a meeting which would provide an opportunity for the presentation of scientific papers and for planning the future progress and activities of the group. We agreed that, despite the formation of a separate Small Fruit Virus Group, many of the viruses infecting small fruits were likely to continue to ignore these man-made distinctions between different kinds of crops, so that there would remain close ties between us and the Fruit Tree Group. We therefore sought their agreement to hold our 1st Symposium immediately preceding their own 10th Symposium. We much appreciate their ready assent to this proposal. I also wish to express our gratitude to Dr Kunze and to Dr Krczal for accepting the heavy burden of organising this meeting.

In considering what topic would be appropriate for this opening address, it scemed to me that it might be useful to say a few words about the development and present status of our knowledge of Rubus viruses. I have chosen to confine myself to Rubus viruses, not because these are in any way more important than those of other small fruits, but because these are the ones I am most competent to talk about.

As in other fields of scientific endeavour, progress in the study of Rubus viruses has depended on the development of suitable techniques. Early workers, mainly in the United States and Canada, wrestled with a confusing mixture of viruses and tried to separate them for study by the only means available to them - transmission by graft inoculation and, where possible, by means of vectors. Despite the well-known limitations of these procedures remarkable progress was made, so that by 1927 Bennett was able to distinguish five virus diseases of Rubus. By the mid 1950s, largely as a result of the work of Cadman in Britain and Stace-Smith in Canada, most of the major virus diseases of Rubus had been recognised and described.

An important advance at this time was the demonstration by Chambers in 1954 that raspberry plants could be feed of many viruses by heat treatment. This made virus-free propagating material available to commerce for the first time, and also provided research workers with virus-free indicator plants. Those of us who grapple with graft-transmitted viruses today sometimes forget that the early workers had the additional problem of not being sure that their indicator plants wee virus-free, and this greatly complicated the interpretation of their results. In Britain, where the most commonly grown commercial cultivars even now are not resistant to the major aphid-borne viruses or their vectors, the steady improvement in the quality of raspberry stocks since the 1950s has depended in no small measure on the use of certified planting material derived from heat-treated stock.

It is difficult now to realise that despite the progress that had been made by the 1950s in unravelling the complex pattern of virus infection in Rubus, none of the causal viruses had been transmitted mechanically and none had been seen by electron microscopy. Probably the most important breakthrough in our study of Rubus and other small fruit viruses was the use by Cadman in 1956 of nicotine to prevent the inhibiting action of the tannins that are present in raspberry leaves and so enable the transmission of some viruses from raspberry to herbaceous host. Indeed this advance was important for the study of viruses in many other crops too, such as stone fruits and grapevines.

Whereas the studies had hitherto been largely of the diseases, the way was now open to the study of the causal viruses themselves, using laboratory techniques for virus purification, and serological and other techniques for determining relationships and affinities. Several of the viruses studied in this way proved to belong to a new group having nematode vectors, now called the nepoviruses, and this group proved to be of world-wide importance in a great variety of crop plants. Nevertheless, it was the need to resolve the problems created by nepoviruses in small fruits, principally raspberry and strawberry, that provided much of the stimulus for the investigation of the whole group. A good deal of this work was done in Britain, at Dundee by Cadman, Harrison and co-workers, and at East Malling by Posnette and co-workers. Progress with these studies was very rapid and it was soon possible to use nematicides and cultural methods to control them with the rsult they are now much less common in commercial raspberry and strawberry crops in Britain.

Much of the subsequent story is familiar to many of you but to illustrate how far our knowledge of the nepoviruses has progressed I would refer to recent work done in our laboratory at Dundee which shows how some of the biological propeties of raspberry ringspot virus are inherited. This virus has two functional pieces of RNA and, for example, RNA-2 determines nematode vector specificity, whereas RNA-1 determines virulence, seed transmissibility and the ability of some strains of the virus to infect raspberry cultivars such as Lloyd Geroge that are immune to the common strain. 'Pseudo-recombinant' isolates can be formed by re-assorting the RNA molecules from different parent strains and it can readily be seen that such reassortment can enable some biological characters to be exchanged, but not others. It is possible, for example, for a resistance-breaking strain to exchange one specific nematode vectro for another. In contrast, we now understand why all resistance-breaking strains so far isolated are also avirulent and poorly seed-transmitted; it is because genetic determinants for all three characters are carried on the same piece of RNA. Study of some quite basic properties of these viruses is thus leading us to an understanding of some of their important biological effects.

Unfortunately, not all Rubus viruses have yielded up their secrets to this kind of approach. Black raspberry necrosis virus, for example, although transmissible with difficulty by inoculation of sap, reach only low concentrations in herbaceous plants and this still impedes our progress in the study of this virus. Some further advance in technique is required hee, such as a better herbaceous host or a better environment in which to grow the virus. Black raspberry necrosis virus in nevertheless the only aphid-borne Rubus virus to have been transmitted by inoculationof sap and further progress with the aphid-borne viruses seems more likely to come from a differet approach. A technique that is currently proving useful is the electron microscopy of thin sections of infected Rubus. Using this technique, Puts and Meignoz in 1972 were the first to report the particles of an aphid-borne Rubus virus. They found large bacilliform (rhabdovirus) particles which are now known from work at Vancouver and Dundee to be those of raspberry vein chlorosis virus. Incidentally, analogy with other similar viruses would suggest that this virus is probably persistent (propagative) in its aphid vector (Aphis idaei) and not semi-persistent as previously thought. This is another example of how an apparently academic stydy can suggest conclusions of practical significance. Other studies using ultrathin sectioning at Dundee and at Vancouver are showing that rubus yellow net virus has small bacilliform particles.

Our relative ignorance of the nature of the aphid-borne viruses has not prevented us from achieving a substantial measure of control of these viruses in raspberry crops. This is being done by the use of tolerant or resistant cultivars. In Britain, the major commercial cultivar Malling Jewel is tolerant (i.e. shows no symptoms) to infection with black raspberry necrosis, leaf spot and leaf mottle viruses, although it is sensitive to rubus yellow net of 'virus-tested' planting material, derived from heat- treated stocks and raised in spawn beds subject to inspection for freedom from virus-like symptoms, has resulted in the establishment throughout the country of crops that are vigorous and largely although not entirely free from disease.

In the Pacific Northwest region of North America the major cultivar, willamette, is largely free from aphid- borne virus disease and this seems to be because it has some kind of innate resistance to infection, the nature of which is not understood. A similar kind of ability to escape infection is shown in Britain by the cultivar Norfolk Giant. This cultivar has a moderate degree of resistance to aphid colonisation, but this is possibly not the only reason for its ability to escape virus infection it does not explain, for example, why it only occasionaally becomes infected by the pollen-borne virus, raspberry bushy dwarf even when this is inoculated by grafting.

A more soundly-based approach to controlling aphid-borne viruses in raspberry is by breeding cultivars for a high level of resistance to aphid colonisation and in this connection it is perhaps fortunate that both in Britain and North America the most important viruses are transmitted by one aphid species, Amphorophora rubi in Britain, A. agathonica in North America. The existence of inheritable resistance to colonisation by A. agathonica was shown as long ago as 1938 by Huber and Schwartze in the USA, but breeders were at first slow to follow up this lead. It was not until 1959–60 that Knight, Briggs and Keep at East Malling reported the detection of genes from the cultivars Baumforth's seedling A and Chief, which confer resistance to the four British races of A. rubi. Subsequently additional resistance genes were found in R. occidentalis. Incorporation of resistance genes into commercially useful raspberry cultivars subsequently became an important part of the Rubus breeding programme at East Malling and at Dundee. The first aphid-resistant cultivars from the East Malling programme, Malling Orion, Malling Delight and Malling Leo, were made available to growers in the early 1970s. Jones recently found that cultivars with genes for aphid resistance remained largely free from infection with aphid-borne viruses over a period of 4 years. Even cultivars such as Norfolk Giant and Glen Clova, with rather low levels of resistance to the aphid A, agathonica, derived largely from European R, idaeus cultivars such as Lloyd George, is also being employed in breeding programmes. So far the cultivar Canby is the only important one that carries this form of resistance. However it seems certain that both in Europe and North America the next few years will see the introduction of many more aphid-resistant cultivars.

It is interesting to note that breeding for resistance towards virus vectors has few parallels in other crops and even in raspberry it seems that there has been no attempt to breed for resistance to vectors other than Amphorophora spp. In continental Europe, more attention should perhaps be paid to a search for resistance to Aphis idaei, the vector of raspberry vein chlorosis virus, or to the virus itself, which is more important there than the viruses transmitted by Amphorophora rubi.

In this brief and very incomplete survey I have tried to show how the application of a variety of techniques and experimental approaches has led to a progressively increasing understanding of the Rubus viruses and the diseases they cause. There is of course much more that we need to know about these and other small fruit viruses and I hope that our discussions today will serve to increase our understanding still further.