Vol. 6 (1997): 323-336. Review Viruses and their significance in agricultural and horticultural crops in Finland Eeva Tapio Department ofPlant Biology, Plant Pathology Section, PO Box 28, FIN-00014 University ofHelsinki, Finland Katri Bremer (formerly Ikäheimo) AgriculturalResearch Centre ofFinland. Institute ofPlant Protection, FIN-31600 Jokioinen, and Department of Plant Biology, Plant Pathology Section, PO. Box 28, FIN-00014 University ofHelsinki, Finland Jari RT.Valkonen Institute ofBiotechnology, PO Box 56, FIN-00014 University ofHelsinki, and Department ofPlant Biology, Plant Pathology Section, PO Box 28. FIN-00014 University ofHelsinki, Finland. Current address: Swedish University ofAgricultural Sciences, Genetic Centre. PO Box 7080, S-750 07 Uppsala, Sweden. e-mail: jari.Valkonen @ vbiol.slu.se This paper reviews the plant viruses and virus vectors that have been detected in agricultural and horticultural crop plants and some weeds in Finland. The historical and current importance of virus diseases and the methods used for controlling them in cereals, potato, berry plants, fruit trees, orna- mental plants and vegetables are discussed. Plant viruses have been intensely studied inFinland over 40 years. Up to date, 44 plant virus species have been detected, and many tentatively identified virus- es are also reported. Control of many virus diseases has been significantly improved. This has been achieved mainly through changes in cropping systems, production ofhealthy seed potatoes and healthy stocks of berry plants, fruit trees and ornamental plants in the institutes set up for such production, and improved hygiene. At the present, barley yellow dwarf luteovirus, potato Y potyvirus and potato mop-top furovirus are considred to be economically the most harmful plant viruses in Finland. Key words: berry plants, cereals, healthy plant production, onion, potato, vegetables, virus control. virus transmission, virus vector, yield losses ntroduction Plant viruses cause huge economic losses in many crop species worldwide (Matthews 1991). In Finland, too, severe outbreaks of virus infec- tions frequently affect many crops, but, fortu- nately, the climate restricts occurrence of insect species that can transmit viruses in the field (Roi- vainen 1947, Heikinheimo 1959, Vappula 1962, Raatikainen 1967,Kurppa and Rajala 1986). The very long days of summer reduce the timeneed- © Agricultural and Food Science in Finland Manuscript received August 1997 323 AGRICULTURAL AND FOOD SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=1hAhogjQBPSBsdn-.ggR-dNqD3bqUe2lBEjW9yg.hPI4zSIsWziSjM0P1-fNwBILQHwav_KyGPE9QkzmKHeDyxDBM8DMeUJW9xv7QpgW9krl5MP0syZS5VwjkzZQNmZE_MsL5jMlGJQikMsijbLZuqh4iKeJWky1b61VJz4ptE8p4VdW4RLTmLRt-cQ9AWd9Syqhk0Sy3Z-YDzj6eZ0tz5zJXOnTsMQkvO0ZHCjki9zOK2N-0_kGOWYx0Oe8mOrOt44fmDngAxSiCYdIs0rJWedBS-vQG2Ke4sgtyhLCU-ysNQ-ly17MrdIPrm20SGMBoEJHy4kLr9tVtanOaPQO ed for the maturation of crops (Pohjakallio et al. 1961b) and thus also the duration ofgrowth stag- es when plants are susceptible to virus infections. However, the climate restricts the number of agricultural and horticultural crops that can be cultivated outdoors (Broekhuizen 1969, Mukula and Rantanen 1987), and so many horticultural and ornamental crops are grown in the green- house in Finland (Laurila 1995). Viruses can be transmitted to new crops via virus-infected seeds, tubers, seedlings and cut- tings. Many viruses also persist in the agricul- tural environment in perennial crops and wild plants, from which they are transmitted to new crops by vectors (Thresh 1981). Efforts to pre- vent viruses from spreading by controlling vi- rus vectors in the field tend to be ineffective and are useless against viruses transmitted mechani- cally, i.e., in the sap of virus-infected plants via wounds by contact. Thus, plant viruses are best controlled by using virus-free planting materi- als and by growing virus-resistant cultivars. Choice of the appropriate control method or vi- rus-resistant cultivar requires knowledge of the locally prevalent viruses. New forms of virus resistance have recently been developed in crop plants using genetic en- gineering. In Finland, these applications have mainly been used on the potato (Mehto 1991, Truve et al. 1993, Pehu et al. 1995, Seppänen et al. 1997). Assessing risks of the experimental and commercial release of transgenic crop species that express viral sequences has therefore be- come an important task for researchers and pub- lic authorities alike (OECD 1996, Robinson 1996). Further, the use of plant viruses as gene vectors in plants (Scholthof et al. 1996) will re- quire risk assessment. The potential risks caused by each transgenic crop and gene vector must be assessed in the specific environment and crop- ping system in which they will be used. Such assessments require knowledge not only of the properties of the transgenic plant or gene vector but also of the plant viruses that naturally occur in the crop and the environment. This review lists the viruses and their vec- tors detected in agricultural and horticultural crop plants in Finland. No complete list similar to that given here is available, and previous re- views are either no longer up to date (lantalai- nen 1957) or cover only a limited range of crop plants (Bremer 1987)or groups of viruses (Lind- sten and Tapio 1986). Furthermore, some of the data we provide have not been published in ref- ereed journals and so are difficult to obtain. The Finnish names of the viruses have been given elsewhere (Valkonen 1993) as have those of the vectors (Vappula 1962, Markkula 1993). Virus- es occurring in forest trees in Finland were re- cently reviewed by Bremer et al. (1991). The present data may be useful for the authorities responsible for assessing the risks associated with the release of transgenic crops expressing viral sequences and also for those involved in plant protection and quarantine. We also sum- marize a few important milestones in plant vi- rus research in Finland, and examine the histor- ical and current significance of viruses and their control in crops grown in this country. Viruses detected in crop plants in Finland The viruses listed in Table 1 have been identi- fied on the basis of the symptoms they cause in experimentally inoculated test plant species. The majority have also been identified by serologi- cal tests and the morphology of particles ob- served under the electron microscope. Many, but not all, of the viruses in Table 1 have been test- ed for transmission by specific vector species. However, it is possible that many of the viruses for which no vector has been identified in Fin- land are transmitted here by a vector related to those identified elsewhere (Table 1). At this point, it is appropriate to note that Myzus persi- cae, one of the most efficient and important aphid vectors of viruses elsewhere, is not known to overwinter in the wild in Finland. It does, how- ever, in the greenhouses and can spread to near- 324 Tapio, E. et al. Viruses in agricultural and horticultural crops AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 6 (1997): 323-336. Table 1, Viruses identified in agricultural and horticultural crops in Finland, and their vectors. Transmission By vectors’ Through Virus Host species Finland Elsewhere seeds'* Reference' Agropyron mosaic virus Agropyron repens, wheat eriophyid mites Bremer 1964, 1974a, [llB]nr Alfalfa mosaic virus Applechlorotic leaf spot virus luzeme aphids E unknown E. Tapio (unpublished)* 1, [46, 229] Lemmetty 1988, [3o] nr nrMains sylveslris Aronia ring spot virus Arabis mosaic virus Barley yellow dwarf virus Amnia melanocarpa rhubarb unknown Bremer 1984nr nr Xiphinema sp. Heinonen 1978,116] Bremer 1965,cereals and grasses Rhopalosiphumpadi, Silobion avenue, Korhonen 1981, [32] Melapolophiumdirhodum nrBean common mosaic virus french bean aphids F, E Jamalainen 1957, Tapio 1970, [73, 337] Bean yellow mosaic virus red and alsike clover, pea, fababean Acyrthosiphonpisum, aphids Aphisfabae Jamalainen 1957, Raininko 1964, Tapio 1964, 1970, [4o] Bremer et al. 1990Beet soilbome virus Black currant reversion associated virus sugarbeet Ribes spp. nr Polymyxa betae Cecidophyopsis ribis Bremer and Heikinheimo 1980, Lemmetty et al. 1997 Brome mosaicvirus cereals, timothy grass, wheat, Agrostis tenuis, Agmpyron repens nr no vector Bremer 1973, 1974a, (3,180] Carnation etched ring virus carnation Myzuspersicae Bremer and Lahdenperä 1981, [lB2] Carnation mottle virus Carnation necrotic fleck virus carnation carnation unknown Bremer 1978, [7]nr M. persicae Bremer and Lahdenperä 1981, [l36] Carnation ringspot virus Carnation vein mottle virus carnation carnation nr nr nematodes aphids Bremer and Lahdenperä 1981, [2l] Bremer and Lahdenperä 1981, [7B] Cherry leaf roll virus Chrysanthemum Sambucus spp. chrysanthemum, tomato nr Xiphinema sp.E Cooper and Edwards 1980, [Bo] Tapio 1963b, Bremer andM. persicae Lahdenperä 1981aspermy virus Chrysanthemum virus B chrysanthemum nr aphids Tapio 1963b, Bremer and Lahdenperä 1981,1110] Linnasalmi 1966, [ls4]Cucumber green mottle mosaic virus cucumber nr unknown F,E Cucumber mosaic virus cucumber, tomato, black currant, aphids F(?)b, E Rainio 1941, Linnasalmi 1966, Linnasalmi jaMurtomaa Slellaria media, Mentha gentilis, Phlox paniculata garlic 1966, Bremer 1983, Lemmetty 1985, Tegel 1987,(1,213] Kokkola 1992Garlic latent virus aphids aphids nr nr nr Leek yellow stripe virus Lettuce mosaic virus Oat sterile dwarf virus shallot, garlic lettuce Bremer 1990, Kokkola 1992, [24o] Lahdenperä 1981,[9]aphids F, E oat Javesellapellucida, J. obscurella, Kanervo et al. 1957, Ikäheimo 1961, Ikäheimo and Dicranotropsis hamata Raatikainen 1961,1963, [2l7] continues 325 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Transmission By vectors' Through Virus Host species Finland Elsewhere seeds' 1 Reference 1 Onion yellow dwarf virus shallot, garlic nr aphids Jamalainen 1957, Bremer 1990, Kokkola 1992,1158] Potato leaf roll virus potato M. persicae, Kurppa 1983, Uusitalo 1985, Aphis frcmgulae-nasturtii, [36, 291 ] Aulacorlhum solani, Macrosiphum euphorbiae Potato mop-top virus potato Spongospora subterranea Kurppa A. 1989, [ 138) Potato vimsA potato nr aphids Kurtto 1969, Kurppa 1983, [s4| Potato viras M potato Myzus persicae, Tapio 1980, Kurppa 1983, [B7] A. frangulae-nasturtii Potato vims S potato M. persicae Aura 1957, Kurppa 1983, [6o] Potato virus X potato, tomato nr no vector Aura 1957,Linnasalmi 1964, (4,354] Potato vimsY potato Myzus persicae, A. solani, Pohjakallio et al. 1961a, Tapio A. frangulae-nasturtii 1980,Kurppa 1983, [37,242] Raspberry bushy dwarf Rubus arcticus nr unknown E Kokko et al. 1996, [l6s] vims Raspberry ringspot vims Rihes spp, plants and Longidorus elongatus E Bremer 1983,Tapio 1985, [6, 198] soil in plant nurseries' Raspberry vein chlorosis Rubus spp. Aphis idaeus Tapio 1961, [l74] vims Shallot latent vims shallot, garlic nr aphids Bremer 1990, Kokkola 1992, [2so] Strawberry latent Aslible \ arendsii, nr nematodes Bremer 1985, [l26] ringspot vims Peonia officinalis, Phlox spp. Tobacco mosaic vims' tomato, rhubarb, nr no vector Linnasalmi 1964, Heinonen plants and soil in 1978, Tapio 1985, [lsl] plant nurseries 1 Tobacconecrosis vims cucumber, tulip, plants Olpidium brassicae Tapio 1972b, Bremer and Lahden- and soil in plant nurseries' perä 1980,Lemmetty 1991a, [l4] Tobacco rattle vims potato, plants and soil Trichodorus sp. Tapio 1972b, Kurppa 1983, in plant nurseries' Keskinen 1991,112,346] Paeonia Lactiflora hybrids Tomato black ring vims black currant, plants and Longidorus sp. Bremer 1983, Tapio 1985, [3B] soil in plant nurseries' Tomato spotted wilt tomato, Dahlia, cineraria, Thrips lahaci, Lemmetty 1991b, Lemmetty vims chrysanthemum Frankliniellaoccidentalis and Lindqvist 1993, [39] Turnip mosaic vims rhubarb nr aphids Heinonen 1978, [B] Wheat striate mosaic oat Javesellapellucida, Ikäheimo 1960, Ikäheimo viras J. ohscurella and Raatikainen 1961, [99] •Vectors of Finnish origin experimentally shown to transmit Finnish virus isolates are indicated. The group of vectors known to transmit the virus elsewhere is mentioned if the vectorin Finland is not identified. “No vector” means that the virus is mechanically transmitted only. “Unknown” means that the virus probably has a vector but it has not been identified, nr, not reported. bSeed-transmissibility is indicated only if it is considered to be important for dispersal of the virus in the field. F, in Finland; E, elsewhere; (?), transmission of cucumber mosaic virus probably occurs via seeds of Stellaria media (Lemmetty 1985). 'References to reports in which isolation, host range and vectors of Finnish isolates of the virus were described first. Numbers in parentheses correspond to the numbers of viruses in the series of C.M.1./A.A.B. Descriptions of Plant Viruses (Kew, UK). ‘‘The Finnish isolates of alfalfa mosaic virus were identified in 1971 as previously described for isolates from other Scandinavian countries (Tapio 1970). 'A few strains isolated from tomato plants and originally identified as tobacco mosaic virus are currently considered to be strains of tomato mosaic virus (Linnasalmi 1972). 'Many plant species were infected (Tapio 1972b, 1985), but for brevity they are not listed here. 326 Tapio, E. et al. Viruses in agricultural and horticultural crops AGRICULTURAL AND FOOD SCIENCE IN FINLAND Voi 6 (1997): 323-336. Table 2. Tentatively identified viruses and virus-like diseases observed in crops grown in Finland. Virus or disease Crop Reference Apple flat limb disease apple Jumalainen 1964 Apple green crinckle disease apple Jumalainen 1964 Apple rubbery wood disease apple Jumalainen 1964 Apple star crack disease apple Jumalainen 1964 Apple stem grooving disease apple Jumalainen 1964 Beet mild yellowing virus sugarbeet E. Tapio, unpublished Beet mosaic virus sugarbeet, E. Tapio, unpublished Chenopodium spp. Beet yellows virus sugarbeet E. Tapio, unpublished Fuchsia latent virus Fuchsia spp. K. Bremer, unpublished Garlic mosaic virus garlic Kokkola 1992 Infectious variegation disease Ribes spp. Bremer 1983 Pelargonium leaf curl virus Pelargonium spp. E. Tapio, unpublished Phleum green stripe disease timothy grass Nuorteva 1962,Heikinheimo and Raatikainen 1976 Potato aucuba mosaic virus potato Kurtto 1969 Raspberry vein banding disease Rubus spp, Tapio 1961 Raspberry yellows/yellow mosaic disease Rubus spp. Tapio 1961 Red raspberry mosaic/raspberry leaf raspberry Tapio 1961 mottle disease Red clover mottle virus red clover E. Tapio, unpublished Soil-borne wheat mosaic virus rye Bremer and Vestberg 1986 Strawberry crinkle virus strawberry E. Tapio and K. Bremer, unpublished Strawberry mild yellow edge virus strawberry E. Tapio, unpublished Tomato ringspot virus Phlox paniculata Tegel 1987 Vein banding disease Ribes spp. K. Bremer, unpublished by fields during the summer (Heikinheimo 1959). Aphids of the genus Chaetosiphon (Bre- mer and Pethman 1978) and nematodes of the genus Xiphinema (Tapio 1972b, 1985, Kari Tii- likkala, pers. comm.) are important virus vec- tors elsewhere but have not been found in Fin- land. Seed-transmitted viruses (Table 1)may also be transmitted by pollen (Matthews 1991), but adequate tests are not always carried out. Table 2 lists viruses that have been only ten- tatively identified, mainly on the basis of symp- toms observed in crop plants. In some cases, test plant responses or a positive reaction in sero- logical tests with virus-specific antisera have been reported. Some of the viruses could proba- bly be included in either Table 2 or Table I. In- clusion in Table 2, however, is usually based on the original author’s suspicions concerning the identity of the virus; identity of the viruses and even confirmation of the viral nature of some of the diseases listed in Table 2 awaits more def- inite determination. Nevertheless, we felt that we should include these data in this review, because in so doing we could point to crops and viruses that may deserve more detailed investigation by virologists in Finland. Significance of plant virus diseases in Finland The occurrence of plant virus diseases in Fin- land has been known for over 60 years (Liro 327 AGRICULTURAL AND FOOD SCIENCE IN FINLAND 1930, Rainio 1941,Brummer 1946, Jumalainen 1943, 1946). However, it was only 40 years ago that the economic damage caused by viruses in crop plants was more fully recognized and ef- forts to identify and control plant viruses were launched on a broader front, first at the Agricul- tural Research Centre of Finland, Tikkurila (Jamalainen 1952, 1957), and then at the Uni- versity of Helsinki, Viikki. For the next 30 years, plant virologists were mainly concerned with virus identification and the development of con- trol methods. This work involved tight collabo- ration with the entomologists. During that time, a few foreign scientists spent a few months in Finland participating in the identification of vi- ruses in potato (A.B.R. Beemster, The Nether- lands, 1955), cereals and grasses (E. Banttari, USA, 1966-67) and woody plants (J.I. Cooper, UK, in the 1970 s and 1980s). During this dec- ade, M. Saarma (Estonia) and his research group have contributed to research mainly of a theo- retical nature on plant viruses in Finland. The significance of virus diseases has de- clined in many crops in Finland during the past 40 years due to the development of schemes for producing virus-freeplanting materials, the bet- ter standards of hygiene in the greenhouse pro- duction, changes in cropping systems to reduce vector populations, introduction of procedures to predict and control the outbreak of virus ep- idemics, and improvements in cultivar resist- ance to viruses. Inspection of imported plants for viruses by the plant quarantine authorities has significantly decreased the occurrence of viruses in many horticultural and ornamental plants, the natural transmission of plant virus- es to Finland being largely limited due to the country’s isolation from other agricultural are- as in Europe by the Baltic Sea. Only a few per- sistently transmitted viruses are known to be occasionally carried to Finland over the sea by wind-borne aphids (Kurppa 1983, Kurppa S. 1989a). Virus transmission from the east is probably limited because the land there is main- ly occupied by forests and the prevailing winds during the growing season are from the south and west. Viruses in cereals and grasses The severe disease thataffected oats growing at the coast of the Gulf of Bothnia in the 1950s (Jamalainen 1957) was shown to be caused by oat sterile dwarf virus (OSDV) transmitted by leafhoppers in a persistent manner. Wheat stri- ate mosaic virus, also detected in the diseased oat crops, was transmitted by the same vectors as OSDV but occurred in fewer plants and was less damaging (Ikäheimo 1960, 1961, Ikäheimo and Raatikainen 1961, 1963). The epidemic in oats drew the attention of agriculturalists to vi- ruses in cereals and grasses in Finland. Although the disease caused by OSDV was reported in many parts of Finland (Jamalainen 1957), the epidemic was most severe at the western coast, possibly due to the higher vectoring capacity of local leafhopper populations (Bremer 1974b). At the same time, epidemics caused by OSDV were reported on the other side of the GulfofBothnia in Sweden (Lindsten 1961), and also in the So- viet Union and the US (Slykhuis 1967). In Fin- land, OSDV was brought under control by chang- ing the cropping system. Grass was abandoned as an undercrop of oat, which reduced the num- bers of leafhopper nymphs that could overwin- ter and transmit OSDV to new oat crops during the next growing season (Ikäheimo 1962, Jama- lainen and Murtomaa 1966, Raatikainen 1967). Since then, OSDV has not caused any signifi- cant problems in Finland. Other viruses have also been detected in ce- reals and grasses (Table 1), of which a few await more definite identification (Table 2). Barley yellow dwarf virus (BYDV) causes economic losses in oats, barley and wheat (Bremer 1965, Korhonen 1981, Peltonen 1988). Epidemics in cereals thought to be caused by BYDV occurred in 1926, 1947 and 1954 (Jamalainen 1957), and epidemics known to be caused by BYDV in 1959 (Ikäheimo 1960), 1973, 1975 and 1988 (Kurppa S. 1989b). Many perennial grasses are natural hosts of BYDV (Korhonen 1981, Kurppa A. et al. 1989) and the aphids (Heikinheimo 1959, Rautapää 1970) that persistently transmit BYDV 328 Tapio, E. et al. Viruses in agricultural and horticultural crops AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol 6 (1997): 323-336. (Ikäheimo 1960, Bremer 1965). Epidemics of BYDV therefore coincide with outbreaks of aphids, notably Rhopalosiphum padi (Kurppa S. 1989a, 1989b). Long-distance wind-borne mi- gration of viruliferuous R. padi from the south and southeast was a significant factor in the build-up of the epidemic caused by BYDV in 1988 (Kurppa S. 1989a). Epidemics caused by BYDV can be forecast. The numbers of living eggs of R. padi are re- corded in the winter host Prunus padus in the spring, and the development of aphid populations in grasses is monitoredfollowing migration from the winter host early in the summer. To prevent economic losses due to BYDV infection, migra- tion of R. padi from grasses to cereals is moni- tored and chemical sprays are applied to kill the aphids in the cereal crop when a threshold value (one aphid found in one seedling out of five) is reached (Kurppa S. 1989a, 1989b). Long-dis- tance migration of aphids can be observed with radar (Puhakka et al. 1986). Viruses in potato The occurrence of viruses in potato was recog- nized in the 19405, and viruses were tentatively identified based on symptoms (Brummer 1946). The first viruses to be detected using serologi- cal tests were the potato viruses X and S (Aura 1957). Since then, several viruses have been de- tected and studied in the potato in Finland (Ta- ble 1). As potatoes are vegetatively propagated, many cultivars were found to be 100% infected by viruses (Aura 1957, Seppänen 1972). The yields ofsome cultivars declined faster than those of others following virus infection (Pohjakallio et al. 1961a, Kurtto 1969, Seppänen 1972) and a few susceptible cultivars were abandoned from use. The deleterious effects of viruses on potato quality were also recognized, and breeding for virus resistance was emphasized (Varis 1970). Professional farmers control potato viruses mainly by using virus-free seed potatoes (Tapio 1972a), of which the highest quality classes are produced by the Seed Potato Center established in Tyrnävä in 1976 (Pietarinen and Seppänen 1981) and by contract farmers in the protected seed potato production zone in the same area. Many potato cultivars currently grown in Fin- land are susceptible to aphid-transmissible vi- ruses such as potato virus Y (PVY) (Valkonen and Palohuhta 1996)and potatoes grown in home gardens are often heavily infected with viruses. Potato crops are therefore frequently affected in the field with viruses non-persistently transmit- ted by aphids, particularly in southern Finland. In field experiments carried out on a farm in Renko (southern Finland), 26% and 38% of the initially healthy crop of potato cv. Rekord was infected with PVY after the first and second year, respectively (Tiilikkala 1987). The yield losses per hectare were equivalent to 48 000 Fmk (cur- rent value) at the second year (Tiilikkala 1987). The capacity of different aphid species to trans- mit potato viruses in the field has not been stud- ied in Finland, but the abundance of R. padi and Aphisfrangulae-nasturtii in potato fields (Kurp- pa and Rajala 1986) and their ability to transmit the potato viruses Y and M under experimental conditions (Tapio 1980) suggest that they may be important vector species. Large numbers of Aphis fabae and Cavariella theobaldihave also been reproted in potato fields (Kurppa and Ra- jala 1986, Tiilikkala 1987) but their capacity to transmit potato viruses in Finland is not yet known. Sprays with mineral oils can diminish the transmission of PVY by aphids in potato crops, whereas sprays with insecticides reduce the number of aphids but not the transmission of PVY (Tiilikkala 1987). Potato leaf roll virus (PLRV) is the most im- portant potato virus in many countries, but it has been detected only intermittently in Finland and is not economically damaging (Kurppa 1983). It is probably transmitted over the Baltic sea by wind-borne aphids (Kurppa 1983). Although a few aphid species that occur in potato fields in Finland can transmit PLRV (Uusitalo 1985), the most efficient vector species, M. persicae, is not known to overwinter outdoors here (Heikinhei- 329 AGRICULTURAL AND FOOD SCIENCE IN FINLAND mo 1959). This may explain why no significant dispersal of PLRV from initially infected potato plants has been observed in the field in Finland. Unlike other potato viruses that occur in this country, PLRV is not transmitted mechanically (Kurppa 1983, Uusitalo 1985). Potato mop-top virus (PMTV) was first de- tected in Finland only 10 years ago (Kurppa A. 1989) but it has become increasingly prevalent and is now considered a serious problem (Aarne Kurppa, pers. comm.). Control of PMTV is dif- ficult because it can persist in the resting spores of its fungal vector, Spongospora subterranea, in the soil for many years, and only a few potato cultivars currently grown in Finland are resist- ant to PMTV in the field (Hassi 1991). Viruses in berry plants, fruit trees and ornamental plants The viruses detected in berry plants in Finland are listed in Table 1 and have been reviewed else- where (Tapio 1963a, Bremer 1987). Recently, raspberry bushy dwarf virus was shown to be prevalent in arctic bramble (Rubus arcticus ) in eastern and southeastern Finland (Kokko et al. 1996). A virus resembling nepoviruses has been isolated from the reversion-diseased black cur- rants (Lemmetty et al. 1997). Back-inoculation tests to healthy plants (Anne Lemmetty, pers. comm.) suggest that this virus may be the pri- mary causal agent of the reversion disease (Bremer and Heikinheimo 1980). Apple is the only fruit tree that is widely grown and therefore economically significant in Finland. Virus disease-like symptoms have been reported in apple trees and may be attributable to infection by several different viruses (Table 2; Jamalainen 1964); only apple chlorotic leaf spot virus has, however, been identified (Lem- metty 1988). In the past, virus infections were common in imported chrysanthemum (Tapio 1963b) and car- nation (Bremer 1978, Bremer and Lahdenperä 1981) grown in the greenhouse (Table 1).As the diagnostic tools used in plant quarantine im- proved, these viruses became less common in the cultivations. Many ornamental plant species grown outdoors are infected by soil-borne virus- es transmitted by nematodes or fungi (Tapio 1972b. 1985. Bremer 1985,Keskinen 1991) (Ta- ble 1,2).The Phlox spp. seem to be infected with the largest number of viruses (Tapio 1972b, 1985, Bremer 1985, Tegel 1987). Major achievements in efforts to improve the quality and yield of berries and fruit in Finland were the schemes set up for producing healthy stocks of berry plants, fruit trees and ornamen- tal plants at the Agricultural Research Centre of Finland (Bremer and Ylimäki 1978) and the foun- dation of the Healthy Plant Center in 1976, now located in Laukaa (Uosukainen and Kurppa 1988). Before the healthy plant production scheme for raspberries was introduced, 95% of the raspberry plants in the 20 plant nurseries in- spected by Tapio (1961) were virus-infected. Later, it was shown that the yields of raspber- ries produced using virus-free plants were six times bigger than those of plants naturally in- fected with viruses (Bremer 1980). A few viruses that infect berry and ornamen- tal plants in Finland are difficult to eradicate from farms and gardens because sources of the viruses exist in the wild. Almost anywhere in Finland cultivated raspberries can be infected with viruses transmitted by aphids from virus- infected wild raspberries (Tapio 1961, 1964). Strawberries are frequently infected with aphid- transmitted viruses in the field, possibly because aphids of the genus Chaetosiphon, which are the most important vectors of strawberry viruses elsewhere, do not occur in Finland (Bremer and Pethman 1978). Viruliferous nematodes (Tricho- dorus spp., Longidorus spp.) and fungi (Olpid- ium brassicae) may exist in plant nurseries, gar- dens and parks where ornamental or other per- ennial plants have been grown for a long time (Tapio 1972b, 1985). Therefore, transport of soil from nurseries and gardens may present a risk of virus dissemination. 330 Tapio, E. et al. Viruses in agricultural and horticultural crops AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 6 (1997): 323-336. Viruses in vegetable crops The aphid-transmitted cucumber mosaic virus (CMV) was early associated with a mosaic dis- ease of cucumbers grown in the greenhouse (Rainio 1941). However, neither CMV nor the seed-transmittedcucumber green mottle mosaic virus (CGMV), which were found in ten and two, respectively, of the 263 crops inspected by Lin- nasalmi (1966), have become economically dam- aging to any crop in Finland. The studies ofLinnasalmi (1964) showed that 62% of the 387 tomato crops inspected were in- fected with viruses in 1961-63. Most of the dis- eased plants had mottle symptoms and were in- fected with tobacco mosaic virus (TMV), where- as 10% had streak symptoms and were infected with TMV (6-8%) or mixedly infected with TMV and potato virus X (PVX) (2-4%) (Linna- salmi 1964, Linnasalmi and Murtomaa 1966). TMV and PVX are readily transmitted mechan- ically but no insect vectors are known. There- fore, once the two viruses had been identified as the cause of the tomato mosaic and streak dis- eases, they could be controlled by improved hy- giene. TMV and PVX are no longer significant in tomato crops. Tomato spotted wilt virus (TSWV) was re- cently introduced to a few greenhouses, proba- bly in infected, imported ornamental plants (Lemmetty 1991 b). TSWV has a very broad host range, causes severe yield losses in many orna- mental plants and horticultural crops, and is transmitted by two species of thrips (Franklin- iella occidentalis and Thrips tabaci) that occur as pests in greenhouses in Finland (Lemmetty and Lindqvist 1993). TSWV is subject to spe- cial quarantine and control measures in Finland. If the virus is detected, the infected crop is de- stroyed and the greenhouse cleaned according to special instructions. Therefore, TSWV is not established in Finland, but the risk of reintro- duction from other countries in imported plants remains and is continuously monitoredby Finn- ish plant quarantine authorities (Anne Lemmet- ty, pers. comm.). Until recently, vegetatively propagated on- ions of the aggregatum group (Allium cepa) and garlic were heavily virus-infected in Finland (Jamalainen 1952, 1957,Bremer 1990,Kokkola 1992). Many viruses have been identified in both crops (Table 1) and several unidentified viruses have also been observed (Bremer 1990,Kokko- la 1992,Table 2). The viruses were recently erad- icated from a few local clones of onion and gar- lic (Bremer 1990, Kokkola 1992) and the virus- free clones are maintained at and available from the Seed Potato Center. Viruses continue to be detected time to time in vegetable crops in Finland (Tables 1,2), but with exception of the viruses occurring in vege- tatively propagated crops of potato, onion and garlic, as discussed above, economically signif- icant losses no longer occur. Virus in other crops Viruses in legumes were extensively studied in Finland and other Scandinavian countries 30 years ago (Tapio 1970). Many viruses and virus strains were detected (Table 1), particularly in the experimental fields ofplant breeders. A large number of these viruses caused a severe disease in infected legume plants, but were not preva- lent in the legume crops in the farmers’ fields and therefore not economically damaging. Foliar symptoms resembling those caused by aphid-transmissible viruses have occasionally been observed in sugarbeets and fodderbeets (Table 2). Wind-borne aphids may sometimes carry the semi-persistently transmitted beet yel- lows virus (BYV) over the sea to southwestern Finland. This assumption is supported by the occurrence of symptoms resembling those caused by BYV in beets in Finland in years when epidemics caused by BYV occur in southern Sweden (E. Tapio, unpublished). Beet soil-borne virus has been detected in the roots of sugarbeets collected from several farms in Finland (Bremer et al. 1990). 331 AGRICULTURAL AND FOOD SCIENCE IN FINLAND No virus has been reported in any brassicas in Finland. Turnip mosaic virus that occurs in brassicas elsewhere has been detected only in rhubarb in Finland (Heinonen 1978). Some of the viruses detected many years ago (Table 1,2) have not been restudied recently in Finland. If the host range of a virus is restricted to the crop plant in which it has been detected, the virus may eventually be eradicated when new, virus-resistant cultivars will be introduced to cultivation. Further, if the virus has no vector or other means of dispersal in the environment in which it is introduced, it may be eradicated when the originally infected plant will be harvested or will die. However, no virus listed here is known to have ceased to exist in Finland. A few viruses may not have been detected in Finland because only the main crops and/or crops with severe disease symptoms have been inspected, the mi- nor crops, crops with no obvious disease symp- toms and wild plants having remained largely uninspected. Therefore, when new crop species and cropping systems are introduced in the fu- ture, new virus diseases caused by viruses and vectors that are currently unknown or considered non-important may appear in Finland. Acknowledgements. We are grateful to A. Kurppa, K. Lehto, A. Lemmetty and K. Tiilikkala for making unpublished data available. Financial support to J. Valkonen from the Academy of Finland (grant #36256) is gratefully acknowledged. References Aura, K. 1957. Of potato virus diseases in Finland. The Journal of the Scientific Agricultural Society of Fin- land 29: 103-110. Baulcombe, D. 1994. Novel strategies for engineering virus resistance in plants. Current Opinion in Biotech- nology5; 117-124. Bremer, K. 1964. Agropyron mosaic virus in Finland. An- nales AgriculturaeFenniae 3: 324-333. - 1965. Characteristics of the barley yellow dwarf vi- rus in Finland. Annales AgriculturaeFenniae 4: 105- 120. - 1973. The brome grass mosaic virus as a cause of a cereal disease in Finland. Annales Agriculturae Fen- niae 12: 207-214. -1974a. Occurrence of the brome grass mosaic and agropyron mosaic viruses in Finland in 1971-1973. Annales Agriculturae Fenniae 13: 1-4. -1974b. Virus diseases of Graminea-plants in Finland and Turkey. Annales AgriculturaeFenniae 13: 125- 148. - 1978. The carnation mottle virus in Finnish carna- tions. Annales AgriculturaeFenniae 17: 36-44. - 1980.Hyviä satoja vadelman terveillä taimilla (Good yields obtained using healthy raspberries). Puutarha 83: 526-527, 578. (In Finnish). - 1983. Viral diseases occurring on Ribes species in Finland. Annales AgriculturaeFenniae 22: 104-109. - 1984. Ring spot of Aronia melanocarpa, a disease caused by an isometric virus transmissible via sap and seed. Annales Agriculturae Fenniae23:l76-182. - 1985.Strawberry latent ring spot virus in ornamental plants in Finland. Annales Agriculturae Fenniae 24: 101-102. - 1987. Virus diseases of berry plants in Finland. 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Agricultural and Food Science in Finland 5: 57-62. Vappula, N.A. 1962. Suomen viljelykasvien tuhoeläinla- jisto (Animal pests in crop plants in Finland). Annat- es Agricutturae Fenniae 1, Supplement 1.275 p. (In Finnish). Varis, E. 1970. Variation in the quality of table potato and the factors influencing it in Finland. Acta Agralia Fen- nica 118: 1-99. 335 AGRICULTURAL AND FOOD SCIENCE IN FINLAND SELOSTUS Maatalous- ja puutarhakasveissa havaitut virukset ja niiden merkitys Suomessa Eeva Tapio, Katri Bremer ja Jari P.T. Valkonen Helsingin yliopisto ja Maatalouden tutkimuskeskus Intensiivinen kasvivirustutkimus alkoi Suomessa vas- ta n. 40 vuotta sitten, vaikka virusten tartuttamista viljelykasveista on dokumentoituja havaintoja yli 60 vuoden ajalta. Suomen maanviljelyssä tapahtui suu- ria muutoksia toisen maailmansodan jälkeen. Maati- loilla ja puutarhaviljelmillä erikoistuttiin tiettyihin viljelykasveihin, minkä seurauksena virustautiongel- mat pahenivat. Tiedot viruksista ja niiden torjunnas- ta olivat puutteellisia eikä viruksetonta lisäysaineis- toa ollut käytettävissä suvuttomasti lisättävistä lajeis- ta. Siten 1950- ja 1960-luvuilla tutkimus keskittyi virusten ja niitä siirtävien vektorien (hyönteiset, an- keroiset, sienet) kartoittamiseen ja torjuntamenetel- mien kehittämiseen. Viljojen ja heinien virustutkimus sai merkittävän sysäyksen Pohjanmaalla 1950-luvulla esiintyneestä kaurantuhosta. Tauti todettiin kaskaslevintäisen kau- ran kääpiökasvuviruksen aiheuttamaksi ja torjuttiin lopettamalla heinän viljely kauran aluskasvina, jol- loin virusta kantavien ja heinissä talvehtivien kas- kaantoukkien määrä väheni. Ohran kääpiökasvuvirus (BYDV) on nykyisin tärkein vilja- ja heinäkasvien viruksista ja aiheuttaa epidemioita viljoissa niinä vuo- sina, jolloin kirvoja (erityisesti tuomikirvoja) on run- saasti. Epidemioita pystytään kuitenkin ennustamaan kirvapopulaatioiden kehittymistä seuraamalla. Perunanvirusten merkitys väheni, kun terveen sie- menperunan tuotanto käynnistyi v. 1976 perustetus- sa Siemenperunakeskuksessa. Kirvalevintäinen peru- nan Y-virus (PVY) sekä kuorirokkosienen levittämä perunan maltokaarivirus (PMTV) ovat kuitenkin yhä merkittäviä taudinaiheuttajia perunaviljelmillä. Virus- ten aiheuttamia tappioita marjantuotannossa ja koris- tekasvien viljelyssä on merkittävästi vähentänyt ter- ve lisäysaineisto, jota tuotetaan v. 1976 toimintansa aloittaneella Laukaan tutkimus-ja valiotaimiasemal- la. Vihanneskasveista tomaatti ja sipulit ovat olleet virusten pahoin vaivaamia. Tomaatin virustautien merkitys väheni hygienian parantuessa kasvihuone- viljelyssä. Ryväs-ja valkosipuleista on tuotettu muu- tamia viruksettomia klooneja, mutta useimmilla tiloil- la kasvustot ovat edelleen virusten saastuttamia. Te- hostunut virusten tarkastus maahantuoduista kasveis- ta vähensi monien kasvihuoneissa viljeltyjen koris- tekasvien, kuten krysanteemin ja neilikan, virustau- tisuutta 1980-luvulla. Kasvintarkastustoiminnalla on edelleen suuri merkitys uusien virustautien maahan- tulon estäjänä, sillä virusten luonnollinen levintä Suo- meen Euroopan muilta viljelyalueilta on vähäistä Itä- meren eristäessä maamme näistä alueista. Sen sijaan muutamat, kirvojen mukana pysyvästi leviävät viruk- set kulkeutuvat Suomeen ajoittain kirvojen kaukole- vinnän yhteydessä. 336 Tapio , E. et al. Viruses in agriculturaland horticultural crops AGRICULTURAL AND FOOD SCIENCE IN FINLAND