Report on the 12th Meeting of the International Council for the Study of Viruses and Virus-like Diseases of the Grapevine {ICVG)
2 The virus diseases which affect Australian grapevines have been imported from overseas together with their host plant. Whilst the effect of viral diseases varies depending on conditions, the Australian viticultural industry can benefit from knowledge of research done in other viticultural areas of the world. The forum of the ICVG was founded for the purpose of communicating achievements in the study of viruses and virus-like diseases of the grapevine and gives the opportunity for participants to discuss these findings. The 12th meeting of the ICVG was held in Lisbon, Portugal, during the period 28th September to 2nd October, 1997. The excellent conference facilities were located within the Calouste Gulbenkian Foundation Complex. The meeting was regarded as the largest held to date, with 143 registered participants from countries including Italy (31), Portugal (26), France (18), USA (11), Australia (8), Spain (7), Germany (5), South Africa (5), Slovenia (5), Switzerland (4), Hungary (4), Canada (3), Israel (3), Austria (3), Greece (2), Chile (2), Croatia (1), Cyprus (1 ), Japan (1), Tunisia (1), Turkey (1), and Uruguay (1). There were 45 oral presentations and 52 posters spanning the following topics: studies on known viruses, diseases of unclear aetiology, grapevine viroids, phytoplasmas and phytoplasma diseases, advances in diagnosis, epidemiology, new strategies fo r introducing resistance to viruses, selection, control, performance of virus-tested clones, health preservation and economic importance. An analysis of the topics presented, shows that viruses associated with leafroll disease were the subject of 23% of the presentations, followed by phytoplasmas ( 19%), fanleaf virus (15%), rugose wood associated viruses (14%), molecular stratagies fo r induced resistance to viruses (12%), sanitary procedures and certification (8%), fleck virus (3%), enation disease (2%), viroids (2%), and shiraz disease from South Af ri ca (1%).
3 The highlights of grapevine virology during the period 1994-97 were reviewed by the President of the ICVG Scientific Committee (Prof. G. P.Martelli). It was claimed that approximately 400 papers had been published on various aspects of grapevine virology since the 11th Meeting of the ICVG. It was shown that to date, some 44 different viruses have been detected in grapevines (Table 1). Table 1. Viruses isolated from grapevine and their vectors Virus species Known vectors Nepoviruses (Nematode-transmitted viruses) Artichoke Italian latent Arabis mosaic Blueberry leaf mottle Bulgarian latent Grapevine chrome mosaic Grapevine fanleaf Grapevine Tunisian ringspot Raspberry ringspot Tobacco ringspot Tomato ringspot Tomato black ring Strawberry latent ringspot Peach rosette mosaic X. diversicaudatum Xiphinema index, X. italiae Paralongidorus maximus X. americanum X. californicum, X. rivesi Longidorus attenuatus X. americanum, X. rivesi, L. diadecturus Viruses (closteroviruses) associated with the leafroll disease complex Grapevine leafroll-associated virus 1 Grapevine leafroll-associated virus 2 Grapevine leafroll-associated virus 3 Grapevine leafroll-associated virus 4 Grapevine leafroll-associated virus 5 Grapevine leafroll-associated virus 6 Grapevine leafroll-associated virus 7 Scale (2 spp.) Mealy bug (2 spp.) Mealy bug (4 spp.), Scale (1 sp.) Viruses (Vitiviruses) associated with the rugose wood complex Grapevine A Grapevine B Grapevine C Grapevine D Mealy bug (4 spp.), Scale (1 sp) Mealy bug (3 spp.) -
4 Table 1. (cont.) Viruses isolated from grapevine Virus species Known vectors Viruses isolated, which directly cause a grapevine disease Grapevine fleck Grapevine asteroid mosaic Grapevine ajinashika Grapevine stunt Grapevine berry inner necrosis ( Trichovirus) Alfalfa mosaic (Aifamovirus) Grapevine line pattern (1/arvirus) Other viruses isolated from grapevines Cucumber mosaic (Cucumovirus)? Tomato spotted wilt (Tospovirus) Broadbean wilt (Fabavirus) Petunia asteroid mosaic (Tombusvirus) Grapevine Algerian latent (Tombusvirus) Carnation mottle ( Carmovirus) Tobacco necrosis 0 (Necrovirus) Sowbane mosaic ( Sobemovirus) Potato X (Potexvirus) Tobacco mosaic (Tobamovirus) Tomato mosaic (Tobamovirus) Grapevine labile rod-shaped Un-named virus (?Capillovirus) The un-named capillo-like virus (particles 600-?00nm long) was observed in vines affected by Rupestris stem pitting (RSP) and LN33 stem grooving diseases, from Canada. The relationship of this virus to the previously named diseases remains unknown. This virus was serologically unrelated to a number of filamentous viruses. Leafroll disease complex It was widely recognised that leafroll-affected vines are commonly infected with variable mixtures of different viruses, and the vines are seldom infected with one virus. Currently, there are seven distinct long filamentous viruses (closteroviruses) recognized to be associated with leafroll disease, and there was informal talk of the possibility of an
5 eighth type. It was suggested that at least three different closteroviruses (GLRaV-1, GLRaV-3, and GLRaV-7) are genuine causal agents of leafroll disease. The GLRaV-7 was first found in an Albanian grapevine accession, but recently found in grapevines from several countries including Italy, Greece, Turkey, Palestine and Armenia. Some renaming of disease-associated viruses has occurred. The virus known previously as corky bark-associated virus was shown to be the same as the French source of GLRaV-2, which has been designated as the authentic reference species. This source has been mechanically-transmitted to a herbaceous plant (Nicotiana benthamiana) in two different laboratories, and shown to possess two biological variants which induce different symptoms in the inoculated herbaceous host. The source of GLRaV-2 has also been implicated in the cause of a graft-incompatibily which occurs with the Kober 588 rootstock. In 1995, it was suggested that the virus previously known as GLRaV-IIa should be renamed as GLRaV-6. This proposal had been accepted. Information was provided on the molecular characterization of GLRaV-1 (Australia), GLRaV-2 (Italy, and USA), and GLRaV-3 (USA). The genome organization of these viruses have largely been determined, and all have been shown to be typical closteroviruses. Further information was provided on advances in the diagnosis of leafroll-associated viruses, using tests including enzyme-linked immunosorbent assays (ELISA) and the polymerase chain reaction (PCR). Details on the relative merits and limitations of these techniques were provided. Generally, the PCR techniques were reported to detect a given virus earlier than ELISA-based techniques, while both techniques had their advantages, neither were 100% reliable for all situations. It was generally agreed that basal leaf petioles sampled from the flowering period to autumn, and one year old bark were regarded the best sources of tissues for detection of viruses by ELISA. The young leaves gave erratic results.
6 Several papers have been previously published on the spread of leafroll disease in the field. These reports have come from Yugoslavia {1973), South Africa (1985, 1990), Mexico (1989), Italy (1993), Cyprus (1993), New Zealand (1993), Australia (1993), and France (1997). Experimental transmission of GLRaV-3 from grapevine to grapevine had been demonstrated for scale insects (Pu/vinaria vitis L.) in Italy, and by at least the first instars of mealybug (Pseudococcus /ongispinus, Ps. ca/ceolariae) in New Zealand, by Ps. citri in Spain, and by Pseudococcus affinis and Ps. /ongispinus in the USA Also, a low rate of transmission of GLRaV-2 was also obtained with mealybug (Pseudococcus longispinus, and Ps. affinis) in the USA Previous published reports from Spain and Italy, indicated the probablity of a virul iferous mealybug transmitting the virus to another grapevine was in the order of 1-11%. The interval between mealybug-mediated viral transmission and detection of the viral infection by ELISA, was influenced by the number of viruliferous mealybugs. When the population of viruliferous mealybug was reduced to 5, the interval between feeding and detection of the virus in the grapevine was 13 months. If the population of viruliferous mealybugs was increased to 20-30 and 50, the period between feeding and detection of the virus was 13 and 8 months respectively. In this meeting of the ICVG, information from Cyprus showed the relationship between spread of leafroll disease in Cyprus, the incidence of GLRaV-3 and the occurrence of mealybugs (Pianococcus ficus and P. citn) in the field. The natural spread of GLRaV-3 was monitored on four sites including the old and new propagation station. The first indication of natural spread of leafroll was observed in the late 1980's, when previously clean stocks introduced from California or those selected locally, started to become infected. The monitoring results showed significant differences among the sites. The highest rate of spread increased from 20% in 1991 to 70% in 1996. By contrast, the virus incidence at another site has remained at about 5% since 1991. The incidence of GLRaV-
7 3 was not found at the new station until 1996, when the incidence was found to be 1%, contrasting with the incidence of GLRaV-3 at the old station, which increased from 44% in 1991 to 70% in 1996. The viral incidence varied between Vitis vinifera cultivars (73-94%) and V. rupestris (7%) rootstocks. Results also indicated the importance of microclimate in the development of mealybug infestation and its subsequent spreading of the virus. Transmission tests were done with both field-collected and laboratory-reared mealybugs. It was found that 75 larvae of Planococcus ficus gave 100% transmission of GLRaV-3 to Cabernet Franc plants, whereas the same number of insects of P. citri larvae gave only a 45% transmission rate. Research in Italy provided new information on the field spread of leafroll disease caused by GLRaV-1, and on its transmission in Italian vineyards by scale insects (Partheno/ecanium corni and by Neopulvinaria innumerabilis, when mixed with the GVA virus). A survey of viruses associated with leafroll in Gamay and Pinot Noir vines in the different localities within France was reported. The results showed the variable incidence of GLRaV-1 and GLRaV-3 in these cultivars. The incidence of GLRaV-1 ranged from 3-17% in Gamay vines in the Beaujolais region and 1-30% in the Bourgogne region, while the incidence of GLRaV-3 ranged from 0-80% in Pinot Noir vines in the Beaujolais region and 0.5-11% in the Bourgogne region. A poster was displayed giving further information on the graft-transmissible vein yellowing leafroll symptoms from France, which was described in 1983. Previously it was noted that the symptoms were similar to leafroll disease, appeared two years after planting, and appeared to spread quickly through the vineyard. New information was provided showing that the disease did not spread. The apparent spread was due to the milder expression of symptoms in young vines, which were not observed in the previous year. The disease was thought to be contributed by the rootstock used, and the affected plants were found infected with NY-1 strain of closterovirus (?GLRaV-3). In addition,
8 other closterovirus-like particles were observed in symptomatic tissues. It is not known whether the symptoms are caused by a new virus or due to synergy between different viruses. Table 2. Relative percentage of Closteroviruses found in samples of grapevine Country Cultivar GLRaV-1 GLRaV-2 GLRaV-3 GLRaV-7 New Zealand V. vinifera 3.5% 97% France Gamay 3-17% 0-80% France Pi not Noir 1-30% 1-11% South Africa Table cvs. 10% 73% 70% Palestine V. vinifera 46% 8% 22% 0.2% Palestine Rootstocks 0% 1% 10% 0% Turkey V. vinifera 0-100% 0-50% 0-25% Spain (Guadiana) V. vinifera 2-12% 14-37% Spain (NW) V. vinifera 40% Greece V. vinifera 42% 48% Southern Italy Table cv 0% 0-76% 0-55% Italy (Sardinia) V. vinifera 33% 16% 83% Slovenia Refosk 5% 10% Slovenia Laski rizling 76% Portugal V. vinifera 45% Australia V. vinifera 27% 8% 10%
9 Several papers were presented on the effects of GLRaV-3 on the performance of grapevines. A Spanish group compared the field measurements for net photosynthesis on diseased and healthy vines. The leaves from GLRaV-3 infected vines when showing the leaf rolling symptoms gave significant lower rates of photosynthesis than similar aged leaves on healthy plants. The net photosynthesis of symptom-free leaves of infected plants were also lower than similar leaves on healthy plants, but the difference was only statistically significant in one out of three tests. These findings indicate the reduced photosynthetic rate of symptomatic leaves is one of the factors contributing to the differences between healthy and GLRaV-3 diseased plants. It seems likely that this deleterious affect would be greater in cultivars in which the leafroll symptoms develop earlier. It should be recognized that other viruses and combinations of viruses can induce earlier leafroll symptoms than GLRaV-3 in the same cultivar. These mixtures of viruses may also induce a lower photosynthetic rate in leaves of diseased vines. Another paper was presented showing the effects of a combined GLRaV-3 and GVA infection in Nebbiolo (clone 415) grapevines in north-east Italy. In this case, the Nebbiolo clone was compared before and after heat-therapy. The vines derived from the heattherapy treatment were free of the two viruses. Leaf measurements showed a reduction in the rate of photosynthesis of leaves from diseased plants compared with the healthy vines. The healthy vines produced a larger and greener canopy which resulted in better grape maturity. No significant differences in the yield per vine were obtained between the diseased and healthy vines. The heat-treated vines produced a signifiantly higher level of prunings (1058g verses 905g), larger bunches (212g verses 199g), more soluble solids (21.3 Brix verses 20.8 ), essentially the same ph (3.01 verses 3.00), lower malic acid (3.3g/l verses 3.55g/l), essentially the same tartaric acid (7.08g/l verses 6.91 g/1) and potassium content (1.35% verses 1.36%).
10 In Portugal, the occurrence of leafroll disease was common in many different cultivars, with GLRaV-3 the predominant virus. The incidence of GLRaV-3 was very high in some cultivars, 90% infection in the cultivar Tinta Francisca, 93% infection in Alvarinho, and 98% infection in Esgana Cao. Studies have evaluated the effect of GLRaV-3 on yield of 27 Portugese cultivars. The yield influenced by this virus was reported to range from -17% to +14%, with a mean of about -5%. In the Apulia region of Italy, studies were reported on the effect of filamentous viruses on the performance of Red Globe and King's Ruby table grape cultivars. Leafroll symptoms were common in these cultivars and affected vines had delayed bud-burst by a couple of weeks and the vines exhibited extensive bud-break failure. Four different viruses were found in these cultivars. A survey of Red Globe vines showed that 85% of the vines showed leafroll symptoms. Of these, 76% were infected with GLRaV-2, 54% infected with GLRaV-3, 51% infected with GVA, and 10% infected with fleck virus. These vines yielded a mean of 19 kg per vine compared with 24 kg from symptomless vines, produced a mean of 23 bunches compared with 30 bunches on symptomless vines, and developed 9% sugar concentration compared with 14% sugar in the symptomless vines. In addition, the fruit from the vines showing leafroll symptoms had pale coloured berries, and many of the bunches were unmarketable. Decline and death of symptomatic Red Globe vines was not uncommon, for it was recorded that 28% of affected vines died. None of the symptomless vines died, but they were not virus-free. It was found that 12% of them were infected with GLRaV-2, while 2% was additionally infected with fleck virus. It seems possible that GVA may also be involved in the decline of Red Globe, because this virus was present with closteroviruses in 51% of the symptomatic vines and was detected in more than 67% of those which died. Symptomless and leafroll-infected vines of the King's Ruby cultivar were also compared. The results showed that 78% of the vines surveyed had leafroll symptoms, of these, 24% were infected with GLRaV-2, 55% infected with GLRaV-3, 24% infected with GVA,
11 and 53% infected with fleck virus. Some virus infections were found in the symptomless vines, these included 5% infection with GLRaV-3, 11% infection rate of GVA, and 53% infection with fleck virus. The lower infection rate of GLRaV-3 and GVA in symptomless vines may indicate natural spread of viruses to previously healthy vines. The origin of the heavy viral inoculum was thought to have come from budwood and/or rootstocks, rather than from viruliferous mealybugs. Another study of the effect of viruses on the performance of grapevines was reported in Sardinia. A survey of clones of cultivars Cannonau, Malvasia di Bosa, Pascale di Cagliari, Vermintino, and Vernaccia, showed that some clones of most cultivars were free of GLRaV-1, -2, -3, GVA and fleck virus. The exception was Malvasia di Bosa, which was totally infected with either GLRaV-2 plus fleck or GLRaV-3 and fleck. Following the survey, 25 virus tested-free clones of four cultivars (Cannonau, Pascale di Cagliari, Vermentino and Vernaccia) and 56 virus-infected clones of the same cultivars plus Malvasia di Bosa, were observed for 9 years. Results showed that the presence of viruses did not significantly reduce the yield of most cultivars, except for Cannonau, where an average reduction of 25% in the yield was recorded. Likewise, the viruses _did not seem to affect the sugar concentration and titratable acidity of the juice. Phytoplasmas Several papers were presented on methods for detecting and identifying the associated phytoplasma organisms, phytoplasma-vector relationships, and epidemiology of recent outbreaks of phytoplasma diseases. In Europe, flavescence dore~e disease is known to be transmitted from infected vine to vine by the leafhopper Scaphoideus titanus. The disease is known to occur in France and northern Italy. The causal agent is widely believed to be a phytoplasma organism classified as a member of the elm yellows group. The disease can be graft-transmitted provided that the budwood has been taken from shoots that are symptomatic for the disease.
12 Also in Europe, other yellows diseases are recognised that are not vectored by Scaphoideus titanus but are vectored by another leafhopper, Hyalesthes obsoletus. This type of yellows disease is known as Bois Noir in France, Vergibungskrankheit in Germany, and also known as Mediterranean grapevine yellows. It is generally agreed that the causal agent of the regionally-named diseases are the same and are caused by a phytoplasma classified as a member of stolbur group of phytoplasmas. The principal mode of infection in the vineyard is believed to be from weeds via the leafhopper to grapevine, and not from grapevine to grapevine. Experimental graft-transmission has been achieved using newly-infected shoots. The subject of one presentation at the conference was on the transmission of Bois Noir by bench grafting. The results presented indicated that the rate of disease transmission by bench grafting was low, and was in the range of0.1-0.14% probability. In Spain, the incidence of Bois Noir infection in vineyards during 1994 was 3%, increasing to 7% in 1995, and to 14% in 1996. The survey showed that the incidence of the disease increased along the rows, spanning 1-25 vines. A survey of Vergilbungskrankheit-infected vineyards in Germany showed that the infected vines increased from 18% in 1990 to 26% in 1995. The disease incidence varied between individual vineyards from 5-65%. It has increased in 7 of 12 vineyards and decreased in 3 vineyards. The increase in symptomatic vines was the highest in young plantings which often provide favorable conditions for the phytoplasma vector Hyalesthes obsoletus and its weed host known as bindweed (Convolvulus arvensis). Clusters of infected vines were frequently found associated with clumps of bindweed. It was found that the incidence of Vergilbungskrankheit depended on the interactions between new infections and recovery from the disease. Another group of vines seemed to recover, but symptoms re-occurred after one or two years. Although some of the vines may have been re-infected, it was concluded that some vines remained latently infected. Almost 1/3 of the infected vines recovered from the disease and stayed free
13 from symptoms for at least two years, even in vineyards where the disease incidence was increasing. The recovery may have resulted from pruning. In one vineyard where the symptomatic shoots were removed during two years, consequently 61 % of the symptomatic vines recovered. The epidemiology of a grapevine die-back disease in the Liguria region of northern Italy was also presented. It was shown that the disease spread rapidly from 1992-1995, causing dramatic losses. The vineyards, which were most affected were located higher on the hill-side, had inter-row crops of vegatables, and the vineyards were surrounded by shrub species of the Linaria sp., Plantago media, Prunus spinosa, Rubus fruticosus and Spartium junceum. These shrubs were all infected with a phytoplasma of the aster yellows group. Several leafhopper species were found including the known vector of aster yellows phytoplasma. Samples of symptomatic vine material were also found to be infected with aster yellows phytoplasma. The recommendations given to reduce the incidence of the disease included keeping the vineyard and surrounding fields free from weeds and shrubs during the vegatative season, the removal and destruction of all pruning cuttings, to avoid growing a mixture with herbaceous plants among the vines, monitor the leafhopper population and spray to control the leafhopper infestation. A new type of yellows disease was reported in the cultivar Scheurebe growing in the Palatine area of Germany. The disease closely resembled flavescence doree, but was shown not to be identical. The vector of flavescence doree has not been found in Germany. The plants grown around the affected vineyards were alder, blackberry and almond. Phytoplasma organisms related to the elm yellows phytoplasmas were found in the symptomatic vine tissues and also from tissues of most alder plants. Phyllid insects were found feeding on the alder leaves, and a considerable proportion of Psylla alni insects were found carrying the same phytoplasma organism as found in the alders and affected vines. Transmission experiments with Psylla alni and the phytoplasma-infected plants are in progress.
14 Further information was given supporting a previous published result that an organism associated with Australian grapevine yellows was a member a different subgroup of the stolbur group and thus a different organism to those phytoplasmas associated with yellows diseases in Europe. Nepoviruses Presentations were centered on interactions between nepoviruses and their nematode vectors, methods to study the molecular determinants of vector specificity, and surveys for fanleaf virus in Palestine (1% incidence), Turkey (0-35% incidence), Azorean Islands (high incidence) and Spain (7% incidence). Rugose wood complex This complex consists of the stem-pitting, stem-grooving and corky-bark disorders. They are widely regarded as important graft-transmitted diseases of the grapevine. The ICVG currently recognises four distinct diseases (corky-bark, LN33 stem grooving, Rupestris stem pitting, and Kober stem grooving) within the complex. These diseases commonly occur in mixtures, in different combinations and commonly occur with leafroll disease. To date, the causal viruses of the rugose wood diseases are unknown, but the vitiviruses (GV A, GVB, GVC, GVD) are commonly found in diseased vines. In Hungary, the percentage of European vines affected with rugose wood diseases was moderately high: the frequency of Rupestris stem-pitting was found to be 22%, Kober stem-grooving was 20%, and LN33 stem-grooving disease was 13%. Samples of 566 European vines growing in Palestine showed a 66% infection with GVA and a 4% infection with GVB, whereas 69 samples of rootstock vines showed only a 6% infection with GVA. Data was presented suggesting that GVA may be the causal agent of the Kober stem grooving disease. This conclusion was based on the common association of GVA occurring with diseased vines. No direct proof has been obtained. In Italy, another study showed the association of GVA with Kober stem grooving to be only 62%, also no
15 relationship was found between GVA and Rupestris stem pitting and no relationship of GVA with either corky-bark or LN33 stem grooving diseases. The GVA and GVB viruses have been shown to be transmitted by mealybugs (Pseudococcus spp. and Planococcus spp.) and GVA was additionally reported to be transmitted by the scale insect, Neopulvinaria innumerabilis. The genome of both GVA and GVB have been completely sequenced and GVD is partially completed. This information has allowed the development of molecular detection tests. New stratagies for introducing resistance to viruses Research groups in France, USA, Italy, Switzerland, Israel and Austria are attempting to genetically engineer grapevines to produce resistance to some of the common viruses. The general approach used is based on the reported viral resistance resulting from the insertion of either a viral gene or part of a viral gene into the genome of plants. Some rootstocks and V. vinifera cultivars which have been transformed with the viral genes from fanleaf viru s, arabis mosaic and grapevine chrome yellow mosaic virus, are currently being evaluated. Most of the research groups have used the strategy of testing their gene inserts in tobacco plants before attempting to transform grapevines. Some of these gene inserts have delayed the replication of viruses in the experimental inoculated tobacco plants. The French group has been testing the usefulness of viral coat protein-mediated protection against fan leaf virus in the rootstock varieties, 110 Richter and V. rupestris. Several transformants were obtained exhibiting a significant level of coat protein production. The protection tests against fanleaf virus included green-graft inoculation of transformants, micro-graft inoculation of the in vitro plantlets, and transmission te sts using fanleaf-viruliferous nematodes. All green-graft inoculated transformants became infected after I month. The micro-graft inoculated plants also became infected, although 2 plants showed apparent recovery. None of the plants exposed to the viruliferous
16 nematodes were immune to fanleaf virus infection. Several alternative versions of viral gene constructs have been made with the view of further testing pathogen-derived resistance in the 11 OR rootstock. The Americans have produced several putative transformed lines of Riparia Gloire, 3309 C, 110 Richter, and MGT 101-14 containing a range of molecular constructions derived from genes of fanleaf virus, GLRaV-2, and GLRaV-3. The process of characterizing the plants is in progress and there are plans to test resistance to fanleaf virus infection by approach-grafting to infected Chenopodium quinoa, micro-grafting onto infected vines, green-graft inoculation of plants. In Austria, attempts are being made to transform V. vinifera cv. Russalka and 110 Richter with gene inserts based on the coat protein gene from fanleaf virus, arabis mosaic virus and GVA and GVB. In Italy and Israel, the transformation of grapevines has been attempted with gene inserts based on the coat protein. Also in Italy, gene inserts based on the putative movement protein genes from GVA and GVB are being used. In Switzerland, the rootstocks V. rupestris du Lot and 3309 Couderc have been transformed with the coat protein genes of fanleaf virus and arabis mosaic virus The plants tranformed with the fanleaf virus coat protein gene were challenge-inoculated using micro-graft and green-graft inoculation techniques, but all were found to be susceptible to infection with the fanleaf virus. Infection tests with viruliferous nematodes have been initiated. Selection and certification of grapevines Recently an expert group of the O.I.V. in France discussed clonal selection of the grapevine. The issue of virus infection was considered as well as the recommended tests which should be done to achieve sanitary certification. It was agreed that there was plenty of evidence showing that fanleaf, leafroll, rugose wood, and other viruses can be very detrimental to grapevines, and to their products. It was suggested that vines
17 that underwent field selection, prior to planting in performance plots for clonal evaluation, should be hot-water treated to eliminate phytoplasmas, and must be checked for disease freedom with the following tests: indexing for leafroll (Pi not Noir or Cabernet Franc) indexing on V. rupestris (for fanleaf, stem-pitting, fleck) indexing on Kober 588 (stem-grooving, graft-incompatibility) indexing on LN33 (corky-bark, stem-grooving) ELISA for nepoviruses which may occur in the grapevine source region ELISA for mealybug-transmitted closteroviruses ELISA for mealybug-transmitted vitiviruses Foundation blocks for the production of basic and certified material must be kept under continuous surveillance, especially for contamination by vector-transmitted viruses and phytoplasmas. Concluding comments It should be recognized that the effect of viruses on the performance of the grapevine is not only influenced by cultivar and environmental conditions but also by the vine management. Generally, a lower fruiting demand on the vine results in a smaller difference between the normal and virus-infected vines. As the fruiting potential is increased, the difference between normal and virus-infected vines become greater. The threat to the viticultural industry from viral diseases is real and on-going. The interest in importing new clones and cultivars into Australia from overseas carries both benefits and the risk of introducing new diseases. The common practice of top-working unpopular varieties to premium cultivars needed today, gives further opportunity to create new combinations of viruses in the premium vine material. Today's relatively harmless virus may be tomorrows viticultural problem, when combined with other harmless viruses or other unknown disease agents.
18 I wish to thank the Dried Fruits Research and Development Council and the Grape and Wine Research and Development Corporation for some financial support, to attend the 12'h Meeting of the International Council for the study of virus and virus-like diseases of the grapevine.