Maataloustieteellinen A ikakauskirja Voi 59: 161—168, 1987 Virus diseases of berry plants in Finland KATRI BREMER Department of Plant Pathology, University of Helsinki SF-00710 Helsinki, Finland Abstract. Virus diseases of berry plants became more common and harmful in the 19605, when berry cultivation expanded in Finland. Virus diseases seldom occur in strawberry because the main vector, Chaetosiphonfragae- folii, does not thrive in Finland. However NEPO-viruses are found in Finland in plant nur- series and in berry cultivations, and they may become a danger for strawberry as well as for raspberry growing. Both wild and cultivated raspberries are commonly infected by viruses. The vector aphids also occur in Finland. Reversion disease infects black currants. The vein- banding virus disease is common in red currants and gooseberries. Virus diseases of berries are poorely investigated in Finland. The healthy plant propaga- tion and certification scheme was established in the 19705. More research is needed in order to understand our virus problems, to developproper test methods, and to prevent virus spread. Index words; berry plants, virus diseases Introduction Virus diseases of berry plants were nearly unknown in Finland until the 19705. Only an epidemic-like occurrence and first spread over the whole country of a vector, the gall mite (Eriophyes ribis) had been described earlier (Hukkinen 1923). As a result of a change in agricultural policy in the 19605, farmers began to specialize and cultivate unconventional plants. Particularly farmers in central and eastern Finland had to find crops suitable for their small hill-side fields that would also help them avoid competition with the cereal growers in southern Finland. At the same time, the general market situation was favour- able for berry cultivation because dairy in- dustry began to use more berries and fruit in their products, e.g. yoghurts. Strawberry cultivation area increased from 953 ha in 1973 to 2825 ha in 1983 (Yearbook Of Farm Statistics 1983). The area of black currant, the second important berry, has not increased recently, but many black currant fields were replanted in the 19705, partly as a attempt to control mildew (Sphaerotheca ribis) by using a more resistant cultivar than before. 161 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=bs3fE_0K7iazwhrL.HJfVybcgWobsVCP7TNs9oQ.BcBCUg-AP0DA2kcYYLrfwYpkMCaKF4st69UmMM1I6Q_dh8uLLXzsUVJ30Y7tZ_nLkMVaQEcWVhWd4f3xhMCFZVryYxLkUCATPJM5gT9bLbw6rvGlmvuwmTHsT0izreX9tyXnKV__I3hir6dHKq682B6Q1UGLH-lUV6o 162 Originally berries were cultivated in small home gardens scattered all over the country, which naturally hindered outbreaks of epidem- ics. Thus many diseases, particularly virus diseases, remained unnoticed and unrecognized. Fungal diseases were known earlier, and their control by fungicides was adviced to farmers. The rapid increase in commercial berry growing also increased the demand for prop- agation material. In this situation plant nurs- eries failed to keep the quality of planting material high enough, and lots of ungenuine and diseased plants were produced and sold. At the beginning of the 19705, a committee was called to plan the production of healthy and genuine propagation material of berry plants, and tests to index their virus diseases were carried out. In 1976, the healthy plant propagation programme was confirmed and settled by law, and a station to maintain and propagate healthy and genuine planting ma- terial was founded (Bremer and Ylimäki 1978). Virus diseases of strawberry 54 virus diseases and 8 mycoplasma diseases are known to infect strawberry in the world (Aerts 1974). Harmful virus diseases can be divided into two groups according to their vectors: aphid-transmitted and nematode- transmitted diseases (Tables 1 and 2). Straw- berry aphid, Chaetosiphon fragaefolii and other Chaetosiphon sp., transmit several viruses of strawberry, the most common in Europe being strawberry crinkle, strawberry mottle, strawberry veinbanding, and straw- berry mild yellow edge. Only the first two have been described in detail (Maas 1984). Aphid- borne virus diseases have caused severe yield losses in central and southern Europe (Aerts 1974, Babovic 1976). However, none of the Chaetosiphon aphids occurs in Finland be- cause of the cold climate. Even in northern Germany, cold winters restrict the propaga- tion and spread of C. fragaefolii, the aphids often die before the latent period of the virus is completed (Krcal 1980). Some other aphids capable of transmitting strawberry viruses occur in Finland, but they are inef- ficient vectors (Maas 1984). Thus aphid transmissible viruses spread in Finland mainly by the help of man through vegetative pro- pagation material. Further, sources of infec- tion also seem to be scarce. The author tested wild strawberries (Fragaria vesca) collected from field borders and forests, and none of them was virus infected. According to simi- lar tests, about 12 % of the plants in straw- berry fields were virus infected before tested healthy plants were used (Bremer and Peth- man 1978). Part of the planting material was Table 1. Strawberry virus diseases. Disease Cryptogram Virus group Transmission by Distribution Vector Sap In Finland 1) Strawberry mottle Chaetosiphon sp.p. +? Worldwide + 2) Strawberry crinkle */*;*/*:u/E:S,I/Ap Rhabdovirus » In Europe, + USA 3) Strawberry veinbanding */*;*/*:S/S:S/Ve/Ap Caulimovirus » + » +? + Amphomorpha and Myzus sp.p. Strawberry mild Luteovirus Chaetosiphon World wide + ? yellow edge sp.p. 1) Sylvester et al. (1976) 2) Frazier & Converse (1980) 3) Maas (1984) 2 Table 2. NEPO-viruses in berry plants. Virus Cryptogram Host Vector Other means Distribution of the vector the virus in Finland Arabis mosaic R/I:*/41:S/S:S/Ne Strawberry, Xiphihema Sap, seed In Europe + virus raspberry, diversicaudatum, pollen currants X. index Raspberry ring R/1:(2,4/43)+ Strawberry, Longidorus Sap, seed » + spot (1,4/30) raspberry, elongatus, pollen + OR 2x 1,4/(46:5) currants L. macrosoma S/S:S/C,Ve/Ne Tomato black R/*:*/38:S/S:S/Ne Strawberry, L. elongatus Sap, seed, » + + ring R/1:2,5+1,5 currants L. attenuatus pollen ? Strawberry latent R/1:2,6 Strawberry, X. diversi- Sap, seed, » + ring spot 2x 1,6/38:S/S:S/Ne black & red caudatum pollen currant, X. coxi raspberry Tobacco ring R/1:2,2/40: Strawberry, X. americanum Sap, seed USA, some spot S/S:S/Ne raspberry, European currants countries Tomato ring R/1:2,3/40: Raspberry X. americanum Sap, seed USA, in im- spot S/S:S/Ne ported plants in Europe imported. It seems that aphid transmissible virus diseases spread to Finland only in im- ported planting material, and the use of tested plants controls them effectively. NEPO-virus diseases NEPO-viruses, Arabis mosaic, raspberry romg spot, tomato black ring, and strawberry latent ring spot, cause severe damage to straw- berries in many countries, the first three being most destructive (Table 3). All these viruses have been found in Finland in plant nurseries and in the field (Tapio 1972, 1985, Bremer 1985), but it is not known how common they are or whether they are a potential danger to strawberry cultivation in Finland. NEPO- viruses also infect raspberry and currants, and they are especially harmful to raspberry. All NEPO-viruses have similar properties, but they are distinctly different viruses. They contain bipartite single-strand RNA genomes, which are encapsidated separately but have identical protein coats (Harrison and Mu- rant 1977). Both parts are necessary for in- fection. NEPO-viruses are transmitted by nematodes Xiphinema and Longidorus sp. via sap, seed, and pollen (Harrison and Murant 1977). Xiphinema nematodes have not been found in Finland, though Arabis mosaic and strawberry latent ring spot viruses, which are transmitted by them (Taylor and Thomas 1968), occur in Finnish plant nurseries (Tapio 1972, 1985). They have probably been im- ported to Finland in foreign propagation material. The only means they can spread here in nature is via seed or pollen. Arabis mosaic is transmitted via seed of several weeds, straw- berry, and raspberry. Pollen transmission by weeds and strawberry is also known (Lister and Murant 1967). Berry plants are not propagated by seed ex- cept in breeding. Thus seed transmission of the viruses that have no vector in Finland, is not important. Pollen transmission, if it causes infection in the mother plant, can be very destructive in the production of virus tested plants. In one experiment, healthy 163 strawberry mother plants were pollinated by pollen containing raspberry ring spot virus, but no plants became infected. Infected pol- len competed poorly with virus-free pollen (Lister and Murant 1967). However, rasp- berry bushy dwarf virus is naturally transmit- ted via pollen, and plants get infected (Mu- rant et al. 1974). At present several tested strawberry culti- vars are available in Finland. On average, they have yielded 59 % more than untested plants in experimental fields (Kallio et ai. 1980). At the same time yields in farmers’ fields have increased, on average 1000 kg/ha (Associa- tion Of Berry Growers, oral comm.). However, most of this increase may be due to the eradication of mites and leaf nema- todes. Virus diseases of raspberry Red raspberry (Rubus idaeus) is not largely grown in Finland, and its area has decreased from about 168 ha in 1973 to 92 ha in 1983 (Yearbook Of Farm Statistics 1983). Black raspberry (R. occidentalis) is only grown in home gardens. Recently, however, raspberry cultivation has aroused more interest, and some fields have been planted. Experiments on suitable cultivationand harvesting methods, including the control of pests and cane dis- eases, are in progress. Wild raspberries are common in Finland. They often show symptoms of virus diseases (Jamalainen 1957) as well as virus-like yellow fleck symptoms caused by the mite Eriophyes gracilis (Lindroth (Tiro) 1902). According to Tapio (1961), several virus diseases occur in Finland, e.g. red raspberry mosaic, raspberry vein chlorosis, raspberry vein banding, rasp- berry yellows, raspberry leaf mottle, and rasp- berry leaf curl, but they have not been de- scribed in more detail. A mycoplasma disease, Rubus stunt, has also been found in Finland. More than twenty viruses infecting rasp- berry have been found in Europe, but not all of them are significant, and some are still poorly described. Some virus diseases trans- mitted by aphids or nematodes have caused severe yield losses (Freeman and Stace-Smith 1970, Baumann 1984, 1986). Raspberry viruses often occur together in the same plant. Thus Table 3. Virus diseases of raspberry (Rubus idaeus) occurring in Europe. Disease and virus Cryptogram Virus group Distribution Transmission by Vector sap seed pollen 1) Raspberry bushy R/l:(2.0 + 0.8 + 0.3) Possible ilar In some cvs. + + + dwarf /(24):5/S:S/0 virus world wide + Heat labile viruses: Raspberry leaf Not enough charac- ? Common in Amphoropha mottle terised Europe rubi Raspberry leaf » ? » » spot Black raspberry » ? » » + necrosis Heat stabile viruses: 2) Raspberry vein */:*/*:Ue/E:S Rhabdovirus » Aphis idaei chlorosis /Ve/As + in USA, Canada 3) Rubus yellow */*:*/*:U/*:S A possible » Amphoropha net /Ve/Ap cacao swollen rubi shoot virus 1) Murant 1976 2) Jones et al. 1977 3) R. Stace-Smith & Jones 1978 164 raspberry mosaic is caused by several viruses (Table 3), and symptoms vary according to the combination of infecting viruses. The testing and production of virus-free raspberries by heat treatment was started in the early 1960 s on the initiative of Tapio. Virus tested raspberries have performed well. For example, the yield of cultivar Preussen in a field experiment was 8000—6000 kg/ha during five years compared to 530—1200 kg/ha of untested, normal plants of the same age (Bremer 1980). At present virus tested planting material of three cultivars is avail- able. Virus diseases of currants and gooseberries Several virus diseases of currants have been found in Finland (Table 4) but none of them is well characterized, and they have been iden- tified mainly on the basis of their symptoms in host and test plants. The reversion disease, the causal agent of which is unknown, can be very harmful to black currants, and several strains are known. The severe strain, which causes deformations and sterility of flowers, is rare in Finland. The weaker strains have also become rare after virus tested plants became commonly used. The veinbanding virus disease and its aphid vectors are very common in red currant and gooseberry. Infectious variegation and some viruslike diseases have been found in some currant fields (Bremer 1983). Same NEPO-viruses infect both currants and strawberry. In Finland, raspberry ring spot and tomato black ring are found in black currants (Bremer 1983). They do not seem to be harmful to the plants but may be spread and preserved with them. Other berries Nordic bramble, Rubus arcticus, and its hybrids with R. stellatus are grown on a small scale for berry industry. A sap transmissible, unidentified virus causing yellow mosaic on the leaves of Nordic brambles has been found (Bremer 1985). Aronia melanocarpa cv. Viking is cultivated for berry production. A virus, causing ring spot symptoms on the leaves and having iso- metric particles, has been found. The disease does not seem to spread (Bremer 1984). A mycoplasma disease causing withces’ broom symptoms occurs in blueberries (Vac- cinium myrtillus) and red bilberries (V. vitis- idaea) in forests, and it has probably been in Finland for a long time (Bremer 1981). Table 4. Virus diseases of currants and gooseberry. Disease Causal agent Vector Sap Distribution Importance transmission abroad and in Finland Reversion of Unknown Mite, Erio- Common + + + black currant phyes ribis Veinbanding of Probably Several » + + gooseberry virus aphid sp.p. Veinbanding of » » » + + red currant Infectious varie- » Unknown Rare gation of currants Cucumber mosaic R/ 1:1/1 8:S Aphids + Seldom in virus /S:S/Ap Ribes, com- mon in other plants 165 Discussion Virus diseases of berry plants are not a serious problem in Finland, mainly because the cold climate restricts vectors, and there are probably not many virus sources. Wild strawberries seem to be free of viruses, and about 90 °7o of commercial strawberry farmers now use tested healthy plants for founding their fields. In contrast to strawberry, wild raspberries and raspberries in home gardens are often virus-infected. Aphid vectors Aphis idaei and Amphorophora rubi, which transmit several viruses, are common in Finland (Heikinheimo 1956). Reversion disease occurs in black currants and occasionally in wild Ribes alpinum. The vector, E. ribis, occurs all over Finland, though to a lesser extent in the north. The use of tested planting material has diminished the occurrence of the vector and disease. NEPO-viruses occur in berry plants and in plant nurseries, where they are harmful to many perennial ornamental plants. NEPO- and other soil-borne viruses might become a problem in the future because they persist in soil and infect nursery plants. Thus there is a consistent danger of their spreading into new areas. More research should be done on soil-borne viruses, on collecting and comparing isolates and characterizing them. To get quick and re- liable test methods, suitable immunological tests and practical applications of nucleic acid analysis and DNA hybridization for large scale tests should be developed. Further, be- cause of our short summer, there is an in- creasing interest amont nurseries in prop- agating plants through tissue culture. The first commercial laboratory already exists. New cultivars are propagated through tissue culture and should also be tested. Tissue culture pro- pagation is a very effective way of spreading plant diseases ifcareful inspection and testing are neglected. Another subject worth while examining would be virus and virus-like diseases of cur- rants and gooseberry. They are as a whole poorly investigated. The etiology of the rever- sion disease is not known. A potyvirus, a bacterium (Jacob 1976), and a mycoplasma (Silvere 1970) have been suggested as causes of this disease. The agent of the veinbanding disease of red currant and gooseberry is also unknown, though the disease causes yield losses (Thresh 1970). A good knowledge of our viruses is neces- sary for developing proper test methods and for preventing virus spread into fields. It is vitally important for resistance breeding, too. Resistance against some viruses has been found in red raspberry. Inserting viral DNA copies into plants has opened new possibilities for getting virus tolerant and resistant plants. (Harrison et al. 1987). The necessity of a quarantine station (jointed into an existing plant protection institute) for nontested peren- nial plants, such as bushes and trees, should be taken into consideration. Plant quarantine inspection should include plant nurseries and new cultivars of perennial plants bred in Fin- land. References Aerts, J. 1974. Survey of viruses and mycoplasmas in strawberry. Neth. J. Plant Path. 80: 215—217. Babovic, M. V. 1976. Changes in the yield and quality of strawberry fruits infected by strawberry crinkle virus. Acta Horticulturae. 66: 25—28. Baumann, G. 1984. Virusbefall in Entrags- und Vermeh- rungsbeständen von Himbeere und Brombeere. I Blatt- laus iibertragbare Viren. Erwerbsobstblau 26: 188 192. 1986. The influence of heat-labile viruses of raspberry common mosaic on growth and cropping ofred rasp- berry. Acta Florticulturae 186: 39—46. Bremer, K. & Pethman, M. 1978, Occurrance of virus diseases in Finnish strawberries. Ann. Agric. Fenn. 17: 38—41. Bremer, K. & Ylimäki, A. 1978. A certificate system to produce and distribute virus tested propagation material 166 from berry plants in Finland. Ann. Agric. Fenn. 17: 42—44. Bremer, K. 1980. Hyviä satoja vadelman terveillä taimil- la. Puutarha. 83: 526—527. 1981. Witches’ broom disease of Arctostaphylos and Vaccinium species in Finland, Ann. Agric. Fenn. 20: 188—191. 1983. Viral diseases occurring in Ribes species in Fin- land. Ann. Agric. Fenn. 22: 104—109. 1984. Ring spot of Aronia melanocarpa, a disease caused by an isometric virus transmissible via sap and seed. Ann. Agric. Fenn. 23: 176—182. 1985. Strawberry latent ring spot virus in ornamental plants in Finland. Ann. Agric. Fenn. 24: 101—lO2. Frazier, N. W. & Converse, R. H. 1980. Strawberry vein banding virus. Commonw, Mycol. Inst. Assoc. Appi. Biol. Descript, of Plant Viruses No 219. Com- monw. Mycol. Inst. Kew, Surrey 4p. Harrison, B, D,, Mayo, A. A. & Baulcombe, D. C. 1987. Virus resistance in transgenec plants that express that express Cucumber mosaic virus satellite R.N.A. Nature 328: 799—802. Harrison, B. B. & Murrant, A. F. 1977. Nepovirus group. Commonw. Mycol. Inst. Assoc. Appi. Biol. Descript, of Plant Viruses No 185. C.M.I, Kew, Surrey U.K. Heikinheimo, O. 1956. Kasviviruksia siirrostavien lehti- kirvojen esiintymisestä maassamme. Valt. maatal. koe- toim. julk. 178: 20—40. Kukkinen, Y. 1923. Havaintoja herukan äkämäpunkin (Eriophyides ribis Nai) esiintymisestä Suomessa. (fiber das Auftreten des Johannisbeeren-Gallmilbe (Eriophyi- des ribes Nai) in Finland) Agr. Fors. Fin. No 23. Jacob, H. 1976. Investigation on symptomatology trans- mission, etiology and host specifity of black currant reversion virus. Acta Horticulturae 66: 99—104. Jamalainen, E. A, 1957. Virus and virus-like diseases in Finland. Valt. maatal. koetoim. julk. 178: 20—40. Jones, A. T., Murant, A. F. & Stace-Smith, R. 1977. Raspberry vein chlorosis virus. Commonw. Mycol. Inst. Assoc. Appi. Biol. Descript, of Plant Viruses No 219. Commonw. Mycol. Inst. Kew, Surrey 4pp. Kallio, T. K., Heikinheimo, O. & Ryynänen, A, 1980. Terveiden taimien vaikutus mansikan satoon. Koetoi- minta ja käytäntö 18.3. 1980. Krcal, H. 1980. Transmission of the strawberry crinkle virus by the strawberry aphid, Chaetosiphon fragae- folii. Acta Phytopathologica Acad. Scient. Hungaricae 15: 97—102. Lister, R. M. & Murant, A. F. 1967. Sap transmission of nematode-borne viruses. Ann. appi. Biol. 54: 167—176. Lindroth (Tiro) 1902. Tre för den finska faunen nya Eriophyiden. Acta Soc. Fauna et Flora Fenn. 28: B,l: 3—7. Maas, J. L. (Edit.) 1984. Compendium of strawberry diseases. St. Paul, Minnesota 138 p. Murant, A. F., Chambers, J. & Jones, A. T. 1974. Spread of raspberry bushy dwarf virus by pollination, its association with crumbly fruit and problems of con- trol. Ann. Appi. Biol. 77: 221—281. Murant, A. F. 1976. Raspberry bushy dwarf. Com- monw. Mycol. Inst. Appi. Biol. Descript, of plant Viruses No 165. Commonw. Mycol. Inst. Kew, Sur- rey 4p. Silvere, A. P. 1970. Mycoplasma-like organisms in as- sociation with black currant revision. Resum. X. Cong- rese Internac. de Microbiol. Mexico 1970. Stace-Smith, R. & Jones, 1978. Rubus yellow net virus. Commonw. Mycol. Instit. Assoc. Appi. Biol. Descript, of Plant Viruses No 219. Commonw. Mycol. Instit. Kew, Surrey 4p. Sylvester, E. S., Frazier, N. W. & Richardson, J. 1976. Strawberry crinkle virus. Commonw. mycol. Instit. As- soc. Appi. Biol. Descript, of Plant Viruses No 163. Commonw. Mycol. Instit. Kew, Surrey 4p. Tapio, E. 1961. Vadelman virustaudit (Virus diseases of raspberry). Valt. maatal. koetoim. julk. No 184. 1972. The appearence of soil-borne viruses in Finnish plant nurseries. Maatal. tiet. Aikak. 44: 83—92. 1985. The appearance of soil-borne viruses in Finnish plant nurseries 11. Maatal. tiet. Aikak. 59: 167—181. Taylor, C. E. & Thomas, P, R. 1968. The association Xiphinema diversicaudatum (Micoletsky) with straw- berry latent ringspot and Arabis mosaic viruses in rasp- berry plantation. Ann. Appi, Biol. 62: 147—157. Tresh, J. M. 1970. Virus and viruslike diseases of gooseberry and currant. In »Virus diseases of small fruit and grapevines». 290 p. Berkley. Yearbook Of Farm Statistics 1983. 167 SELOSTUS Suomessa esiintyvät marjakasvien virustaudit Katri Bremer Kasvipatologian laitos, Helsingin yliopisto. 00710 Helsinki Marjan viljelyn laajetessa kahtena viime vuosikymme- nenä lisääntyi myös taimien kysyntä. Tällöin markkinoille pääsi myös heikkolaatuista aineistoa ja marjakasvien tau- tisuus lisääntyi. Virustaudit osoittautuivat hankalimmiksi, koska viljelijällä ei ole mahdollisuuksia niiden torjumi- seksi. Tervetaimitoiminnan alettua 1970-luvulla tilanne on parantunut. Mansikan virustauteja on meillä tutkittu vain vähän. Testaamattomilla taimilla perustetuilla mansikkaviljelmil- lä noin 12 % kasveista oli viroottisia. Virustaudit man- sikassa lienevät tuontitavaraa, sillä pahimmat saastunnat on todettu ulkomailta tuoduissa kasveissa. Lisäksi man- sikan virustautien pahimmat levittäjät, Chaetosiphon- suvun kirvat eivät menesty kylmässä ilmastossamme. Meillä on tavattu taimistoissa monissa kasvilajeissa usei- ta ankeroislevintäisiä, ns. NEPO-viruksia, jotka ovat hai- tallisia myös mansikalle. Meillä ei ole tietoa niiden esiin- tymisestä mansikalla. Vadelman virustautitutkimus on myös rajoittunut vi- rustautitestauksiin ja terveiden taimien luottamiseen. Kui- tenkin on todettu, että virustaudit ovat yleisiä sekä vil- jellyissä että luonnonvaraisissa vadelmissa ja niiden le- vittäjät iso- (Aphis idaei) japieni vattukirva (Amphoro- phora rubi) esiintyvät yleisinä meillä. Vadelmalle ovat hai- tallisia myös monet NEPO-virukset. Niiden yleisyyttä va- delmissa ei tunneta. Virustaudeista puhdistaminen on kannattanut, sillä ter- veet vadelmat ovat tuottaneet moninkertaisen sadon tes- taamattomiin taimiin verrattuina usean vuoden ajan. Herukoissa on meillä todettu useita virustauteja. Kaikki eivät ole haitallisia. Satoa antavat reversion- eli suonen- katotauti mustaherukalla ja suonikloroosi punaherukal- la ja karviaisella. Suonenkatotaudin aiheuttajaa ei tun- neta. Tauti leviää äkämäpunkkien (Cecidophyopsis (Erio- phyes) ribis) välityksellä. Tämä äkämäpunkkilaji esiin- tyy meillä yleisenä. Suonikloroosi leviää useiden kirvalajien välityksellä. Nämä kirvalajit ovat meillä yleisiä punaherukassa. NEPO-viruksia on tavattu meillä sekä musta- että pu- naherukassa, mutta ne eivät ole olleet haitallisia. Kuiten- kin ne saattavat levitä herukoiden mukana uusille vilje- lyksille. Marjakasvien virustaudit, etenkin herukan ja mansi- kan, mutta myös vadelman ovat meillä samoinkuin muuallakin liian vähän tutkittuja. Monien ns. virustau- tien aiheuttajaa ei tunneta, eräiden leviämistapaa ei tun- neta ja kaikille tarvittaisiin tehokkaat testausmenetelmät. Alkanut tervetaimitoiminta tarvitsee sopivia testausme- netelmiä sekä perustietoa taudinaiheuttajista, jonka avulla tautien levinnän rajoittaminen voidaan toteuttaa. Meillä olisi syytä kiinnittää erityisesti huomiota maalevintäisten etenkin NEPO-virusten rotulajiston selvittelyyn tehokkai- den testausmenetelmien saamiseksi. 168