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Blueberry Scorch Virus

• IDENTITY
• HOST RANGE
• GEOGRAPHIC DISTRIBUTION
• BIOLOGY
• DETECTION AND IDENTIFICATION
• IMAGES
• MEANS OF MOVEMENT AND DISPERSAL
• PEST SIGNIFICANCE
• REFERENCES

IDENTITY

Blueberry Scorch Virus has been described as a heterogeneous collection of strains, including the ‘Northwest’ strains associated with Blueberry Scorch on the west coast of North America and others associated with Sheep Pen Hill Disease in New Jersey on the east coast (Cavileer et al., 1994). Sheep Pen Hill disease is caused by a carlavirus which should be considered to be the ‘New Jersey’ strain) of Blueberry Scorch Virus (BlScV-NJ) (Cavileer et al., 1994; Martin et al., 1992; Martin and Bristow, 1995). The ‘New Jersey’ strain (a.k.a. Sheep Pen Hill) of BlScV is considered more severe than the ‘Northwest strain’ and it infects all highbush blueberry varieties except for ‘Jersey’ (BC MAFF, 2002).

Sheep Pen Hill disease has been recognized in New Jersey since the late 1970's. Scorch was first identified in highbush blueberries in Washington in 1980. These diseases have similar but not identical symptoms that include leaf and flower blighting, advancing necrosis and eventual death of diseased plants.

HOST RANGE

Until recently highbush blueberry (Vaccinium corymbosum) was the only reported host of BlScV (Martin and Bristow, 1995). In 2000, Bristow et al. reported BlScV had been detected in several rabbiteye blueberry (V. ashei) cultivars at the National Clonal Germplasm Repository in Corvallis, OR, USA. In 2004 the virus was also reported from asymptomatic cranberry plants (V. macrocarpon L.) from bogs in British Columbia and Oregon and Washington where in some instances up to 70% of the tested uprights were positive for BlScV (Wegener & Punja, 2004). This suggests the need to investigate the potential for infection of other Vaccinium species that may act as an inoculum source for highbush blueberry crops.

Disease symptoms are more or less severe depending on the variety that is affected.  Diseased Berkeley plants may be killed within 3-6 years of infection, while Pemberton plants may live much longer, exhibiting only a slow progressive decline (Martin and Bristow, 1988). Atlantic, Berkeley, Bluejay, Blueray, Dixi, Herbert, Lateblue, Pemberton, Rubel, Spartan and Weymouth exhibit a flower and leaf necrosis in addition to marginal leaf chlorosis. On the other hand, cultivars such as Olympia and Stanley exhibit only the leaf chlorosis (Martin and Bristow, 1995).

The Sheep Pen Hill Strains of BlScV have been experimentally transmitted to the herbaceous indicator species Chenopodium amaranticolor Coste & Reyn. and C. quinoa using aphids or mechanical inoculation (Hillman et al., 1995). Symptoms of BlScV in the mechanically inoculated experimental host Nicotiana occidentalis (Wheeler) have recently been described as developing after 3 to 4 weeks, being mild and producing pronounced leaf twisting and vein swelling (Lowery et al., 2005). The use of N. occidentalis will greatly simplify the maintenance and study of BlScV isolates.

GEOGRAPHIC DISTRIBUTION

North America: Canada (British Columbia, Québec), United States (Connecticut, Massachusetts, New Jersey, Oregon, Washington)

Europe: Southern Piedmont, Italy (Costigliole Saluzzo, Cuneo Province) (Ciuffo et al., 2005)

The Italian find of BlScV, the first outside of North America, was based on electron microscopy, ELISA and RT-PCR and the cultivars Bluecrop, Blueray and Berkeley all tested positive for the virus. The sequence of the amino-terminal region of the coat protein of sequenced isolates was most similar to the NJ-1 strain of BlScV (Cuiffo et al. 2005).

It should be noted that New Jersery records refer to the Sheep Pen Hill disease, whereas western US reports are of Blueberry Scorch. Although the virus causing Sheep Pen Hill disease has been shown to be a strain of Blueberry Scorch Virus (Cavileer et al., 1994), the former reportedly causes a more severe disease in the east than that observed on the west coast.

A 1995 survey of commercial highbush blueberry crops in the Fraser River Valley of British Columbia did not detect the virus, although it was present in adjacent Oregon and Washington (Martin and Bristow, 1995).  However, a survey of commercial blueberry fields in lower mainland British Columbia conducted in 2000 confirmed that 20 fields were infected with the virus (Hudgins, 2000).  It has been detected in many locations in the Lower Mainland blueberry growing region, namely Abbotsford, Matsqui, Pitt Meadows, Port Coquitlam, Richmond, Cloverdale and Aldergrove (BC MAFF, 2001).

The cultivars 'Bluecrop' and 'Duke' are very important in British Columbia.  Neither shows symptoms when infected by the northwest strain of BlScV, although symptoms develop when these cultivars are infected with the New Jersey strain of the virus (Wegener et al., 2002).  Strain differentiation based on disease symptoms in cultivars testing positive in the field for BlScV revealed a 53% infection with the New Jersey strain and 3% with the northwest strain of BlScV in British Columbia (Wegener et al., 2002).  Further work will be necessary to confirm strain types present in British Columbia. The British Columbia Ministry of Agriculture, Food and Fisheries provides information on Blueberry Scorch Virus and Blueberry Scorch Virus - Questions to Ask The Propagator Before You Buy Plants....

One field in the MRC La Haute-Yamaska, Québec, was found positive by ELISA and PCR for BlScV as the result of a 2005 CFIA survey.

Fourteen samples (cultivars Blueray, Berkeley, Bluecrop, Colville and Elliot) from Connecticut and/or Massachusetts tested positive for BlScV by ELISA. Based on coat protein sequences five of these BlScV isolates were most similar to NJ-2 and NJ-1 strains (DeMarsay et al., 2004).

BIOLOGY

Blueberry Scorch Virus is transmitted by aphids relatively inefficiently, in the nonpersistent manner (Bristow et al., 2000; BC MAFF, 2001). The aphid Fimbriaphis fimbriata is regarded as the most important means by which bushes in commercial fields become infected (Bristow et al., 2000, Raworth, 2004). Transmission generally takes place between mid-May and mid-August when aphid numbers are high, and the virus spreads quickly in a radial pattern from a point source (Martin and Bristow, 1995). The number of infected bushes in a field approximately doubles each year, until about half of the plants in the field are infected. The rate of spread then slows but continues until all bushes are infected.

DETECTION AND IDENTIFICATION

Symptoms:

There is often a latent period of up to two years before the expression of symptoms (Bristow et al., 2000).  In some cultivars, scorch is marked by complete necrosis of the leaves and flowers and eventual death of the plant, whereas in other cultivars, infection remains symptomless and there is no significant yield loss report. Overall, 30 out of 59 cultivars tested exhibited severe symptoms which included severely blighted leaves and flowers and dieback of twigs (Bristow et al., 2000). Mildly affected cultivars displayed only chlorosis along leaf margins, and 26 cultivars showed no symptoms of infection at all following inoculation.

In cultivars demonstrating flower symptoms, blighting occurs just before the corolla opens, usually in late April to early May, with flowers turning first brown and bleaching to grey. Severe flower blighting may be accompanied by twig dieback of as much as 4-10 cm (Martin and Bristow, 1995). Blighted flowers remain on the plant, but fail to develop into fruit (Martin and Bristow, 1988). Some cultivars develop marginal chlorosis of leaves produced on older wood. Blighted leaves remain on the bushes for a short while, but usually drop within a few weeks of the onset of symptoms (Martin and Bristow, 1988). These symptoms usually develop on one or a few branches of a newly infected plant, and spread in subsequent years to affect the entire plant. The productivity of individual plants declines yearly as disease spreads. Apparently healthy shoots may develop as the season progresses, even though the plant continues to be infected (BC MAFF, 2001). The weight of fruit on healthy ‘Pemberton’ bushes bent branches almost to the ground whereas bushes that had been infected for several years were upright due to lower fruit production on plants exhibiting short twiggy growth (Bristow et al., 2000).

Symptoms of scorch may be confused with frost injury, bacterial blight, mummy berry blossom blight and Botrytis blossom blight (Martin and Bristow, 1988; Martin and Bristow, 1995). Potassium deficiency may also result in development of leaf scorch or marginal chlorosis in  affected blueberry plants (Hanson, 1995).

Blueberry scorch virus symptoms - Figures 1-4:

Figure 1:  'Pemberton' highbush blueberry with 2 years of blighted blossoms as a result of infection by blueberry scorch virus; the brown blossoms are newly blighted, while the gray blossoms were blighted the previous year. P.R. Bristow. American Phytopathological Society Press, 2000.

Figure 2:  Blossom necrosis of 'Weymouth' highbush blueberry infected with Sheep Pen Hill virus. A.W. Stretch, American Phytopathological Society Press, 2000.

Figure 3:  'Weymouth' highbush blueberry leaf with line pattern symptom in autumn, caused by Sheep Pen Hill virus. A.W. Stretch, American Phytopathological Society Press, 2000.

Figure 4:  Highbush blueberry leaves with symptoms resulting from double infection by blueberry scorch virus and blueberry shock virus. P.R. Bristow, American Phytopathological Society Press, 2000.

Other causes of scorch-like symptoms -Figures 5-8:

Figure 5. Potassium-deficient highbush blueberry leaves with marginal scorching. H.J. Amling, American Phytopathological Society Press, 2000.

Figure 6. Potassium-deficient lowbush blueberry leaves with marginal scorching and chlorosis. W.J. Kender, American Phytopathological Society Press, 2000.

Figure 7. Highbush blueberry blossoms blighted by Botrytis cinerea. R.S. Byther American Phytopathological Society Press, 2000.

Figure 8. Primary infection of blueberry leaves and shoots by Monilinia vaccinii-corymbosi, note the whitish gray spore masses. R.D. Milholland, American Phytopathological Society Press, 2000.

Morphology:

Particles of BlScV are rod-shaped and flexuous, with a monopartite, positive-sense, single-stranded RNA genome (Martin and Bristow, 1995).  Extracts from Sheep Pen Hill-infected plants contained particles that were rod-shaped, measuring 690 nm (+ or - 44 nm) X 14 nm (Podleckis & Davis, 1991). Virus inclusions are present in infected cells. They are viroplasms containing virions (Martin & Bristow, 1988).

Detection and inspection methods:

The virus may be detected by ELISA in leaf homogenate provided the buffer solution used is strong enough to maintain a neutral or near-neutral pH.  Detection is optimized by selecting mature leaves (Bristow and Martin, 1988). The virus has been isolated from blueberry leaves and flowers, although yield was found to be several times higher in flowers than leaves (Cavileer et al., 1994). Commercial BlScV ELISA kits are available from various suppliers including Agdia Inc. and Hydros Inc.

Scorched blueberry bushes were observed in all study areas during a survey of Washington, Oregon and British Columbia. However, Blueberry Scorch Virus was found to be restricted to a localized area of Washington when plants were tested using DAS-ELISA, suggesting that another agent may induce scorch-like symptoms in these areas (MacDonald et al., 1989).  Blueberry Shock Virus has since been isolated and identified in plants with scorch-like symptoms (MacDonald et al., 1991). Positive identification of Blueberry Scorch Virus cannot be accomplished by visual inspection.

Reverse-transcription polymerase chain reaction (RT-PCR) can also be used to test virus preparations from infected plants using specific primers designed to amplify a portion of the coat protein (DeMarsay et al., 2004) or the RNA-dependant RNA polymerase (Wegener & Punja, 2004) genes of BlScV. Group specific primers have also been developed to allow the detection of carlaviruses, including BlScV, using RT-PCR and oligo d(T) 16 primer (Maroon & Zavriev, 2002).

Recently the genomes of both a British Columbia isolate resembling that of BlScV-NJ (BC-1) and the Northwest strain (BC-2) were fully sequenced (Bernardy et al., 2005). This sequence information will facilitate the development of improved RT-PCR diagnostic methods.

MEANS OF MOVEMENT AND DISPERSAL

The virus may be expected to move with infected planting stock. A virus certification program for blueberry planting stock would help prevent further distribution of the virus (MacDonald and Martin, 1990). It is quite likely that BlScV found in the Pacific Northwest was introduced via infected planting stock from New Jersey (Martin et al., 1992). The virus appeared in several blueberry fields in Oregon and Washington at about the same time, and has not been found in any native vegetation or weeds in or around infected sites in those states.

Since the virus spreads quickly in a radial pattern from a point source in an infested field, and since most members of the carlavirus group are aphid-vectored, it is suspected that natural spread within an infested areas occurs by means of aphids (Martin and Bristow, 1995). Aphid-transmission, however, has not been demonstrated experimentally. Field-to-field spread does not occur over long distances, but is limited to a range of less than 1 kilometre. Spread between fields separated by 5 to 10 metres occurs readily (Martin and Bristow, 1995). The use of mechanical harvesters may indirectly spread the virus by transporting aphids between fields more quickly and over greater distances than they would move naturally. It is improbable that the virus is spread mechanically from plant to plant via machinery as all reported attempts to transmit the virus from blueberry to blueberry by mechanical means have failed (Martin & Bristow, 1988).

The movement of infected asymptomatic or tolerant plants and their use in commercial plantings can act as inoculum sources for aphid mediated spread of the virus within or between back gardens and commercial operations (Bristow et al., 2000).

PEST SIGNIFICANCE

Economic impact:

No figures are available for economic losses incurred as a result of infestation by Blueberry Scorch Virus, but the seriousness of the disease in some cultivars suggests that significant losses may result from premature death of plants, loss of productivity and vigour and loss of yield. In some cultivars, infection leads to complete necrosis and eventual death of the bush, whereas in other (tolerant) cultivars infection is symptomless and no significant yield loss is experienced (Martin & Bristow, 1995). Yield was reduced by more than 85% in the third year of symptom expression in severely affected cultivars (Bristow et al., 2000). BlScV had no effect on the germination of pollen from several cultivars and 6 of 59 cultivars tested (presumably with northwest strains of the virus) displayed no symptoms and no significant reduction in yield (Bristow et al., 2000).

Control:

Early detection along with removal of diseased bushes should be an effective means of control (MacDonald and Martin, 1990). Certification programs should be in place to slow the spread of the virus within infested areas and prevent its further introduction to new areas (Martin and Bristow, 1995).  Heat therapy and meristem culture has been used successfully to eliminate BlScV from infected plants now maintained in the Vaccinium collection at the National Clonal Germplasm Repository in Corvallis, USA (Postman, 1997).

If aphid-transmission is causing spread of the virus, then aphid control programs in conjunction with roguing diseased plants may effectively contain the virus (Martin and Bristow, 1995). The BC MAFF recommendations for management include:
1.  Accurate diagnosis by virus testing of young leaves.
2.  Early removal and disposal of infected plants in fields where the disease is limited.
3.  Aphid control based on monitoring for aphid populations and application of approved pesticides.
4.  Propagation from virus-tested stock only.

Imidacloprid is available in British Columbia, under an emergency registration, to control Ericaphis fimbriata, the predominant resident aphid feeding on blueberry, and a confirmed vector of Blueberry scorch virus (Wegener et al., 2002). Dormant oil is ineffective in preventing aphid emergence. Aphids on blueberry need to be controlled before bloom and the appearance of winged forms and large population increases. The effectiveness of this approach in reducing BlScV still need to be tested (Raworth, 2004).

REFERENCES

BC MAFF, 2001. Blueberry Scorch Virus. British Columbia Ministry of Agriculture, Food and Fisheries, Crop Protection Factsheet.  February 2002.  4 pp.

Bernardy, M.G., Dubeau, C.R., Braun, A., Harlton, C.E., Bunckle, A., Wegener, L.A., Lowery, D.T., French, C.J. 2005. Molecular characterization any phylogenetic analysis of two distinct strains of Blueberry scorch virus from western Canada. Can. J. Plant Pathol. 27: 581-591.

Bristow, P.R., R. R. Martin and G.E. Windom, 2000.  Transmission, field spread, cultivar response, and impact on yield in highbush blueberry infected with Blueberry scorch virus. Phytopathology 90 (5): 474 - 479.

Cavileer, T.D., B.T. Halpern, D.M. Lawrence, E.V. Podleckis, R.R. Martin and B.I. Hillman, 1994. Nucleotide sequence of the carlavirus associated with blueberry scorch and similar diseases.  J. General Virology 75: 711 - 720.

Caruso and D.C. Ramsdell. American Phytopathological Society Press, St. Paul, Minnesota, USA. 87 pp.

Ciuffo, M., Pettiti, D., Gallo, S., Masenga, V., Turina, M. 2005. First report of Blueberry scorch virus in Europe. Plant Pathology 54: 565.

DeMarsay, A., Hillman, B.I., Petersen, F.P., Oudemans, P.V., Schloemann, S. 2004. First report of blueberry scorch virus on highbush blueberry in Connecticut and Massachusetts. Plant-Disease 88: 572.

Hanson, E.J., 1995. Nutritional Disorders. pp. 65 - 67 IN Compendium of Blueberry and Cranberry Diseases, edited by F.L.

Hilman, B.I., Lawrence, D.M., Halpern, B.T. 1995 Characterization and detection of blueberry scorch carlavirus and red ringspot caulimovirus. J. Small Fruit Vitic. 3:83-93. IN: Bristow, P.R., R. R. Martin and G.E. Windom, 2000. Transmission, field spread, cultivar response, and impact on yield in highbush blueberry infected with Blueberry scorch virus. Phytopathology 90 (5): 474 - 479.

Hudgins, E., 2000. Survey of Blueberry Scorch Virus in Highbush Blueberries in British Columbia, 2000. Canadian Plant Disease Survey 81: 144 - 147.

Lowery, D.T., French, C.J., Bernardy, M. 2005. Nicotiana occidentalis: a new herbaceous host for Blueberry scorch virus. Plant-Disease 89: 205.

MacDonald, S.G. and R.R. Martin, 1990. Survey of Highbush Blueberries for Scorch Viruses. Canadian Plant Disease Survey 70:1.

MacDonald, S.G., R.R. Martin and P.R. Bristow, 1989. Viruses capable of causing a scorch disease of highbush blueberry. Acta Horticulturae 241: 295-300.

MacDonald, S.G., R.R. Martin and P.R. Bristow, 1991. Characterization of an Ilarvirus Associated with a Necrotic Shock Reaction in Blueberry.  Phytopathology 81: 210-214.

Maroon, C.J.M., Zavriev,-S. 2002. PCR-based tests for the detection of tobamoviruses and carlaviruses. ED: Hammond, J. Acta-Horticulturae. 568: 117-122.

Martin, R.R. and P.R. Bristow, 1988. A Carlavirus Associated with Blueberry Scorch Disease. Phytopathology 78: 1636 - 1640.

Martin, R.R. and P.R. Bristow, 1995. Scorch.  pp. 51 - 53 IN Compendium of Blueberry and Cranberry Diseases, edited by F.L. Caruso and D.C. Ramsdell.  American Phytopathological Society Press, St. Paul, Minnesota, USA. 87 pp.

Martin, R.R., S.G. MacDonald and E.V. Podleckis, 1992. Relationships between Blueberry Scorch and Sheep Pen Hill Viruses of Highbush Blueberry. Acta Horticulturae 308: 131 -139.

Podleckis, E.V. and R.F. Davis, 1989. Infection of highbush blueberries with a putative carlarvirus. Acta Horticulturae 241: 338-343.

Postman, J.D., 1997.  Blueberry scorch carlavirus eliminated from infected blueberry (Vaccinium corymbosum) by heat therapy and apical meristem culture.  Plant Disease 81: 111.

Raworth, D.A. 2004. Ecology and management of Ericaphis fimbriata (Hemiptera: Aphididae) in relation to the potential for spread of Blueberry scorch virus. Canadian-Entomologist. 136:711-718. [abstract].

Wegener, L., L. MacDonald, M. Sweeney, V. Joshi, E. Hudgins, R. Costello, C. French, D. Raworth and R.R. Martin, 2002.  Blueberry Scorch - a new disease in British Columbia.  Canadian J. Plant Pathology 24: 91.  [abstract].