NAME OF THE ORGANISM: Carlavirus vaccinii (blueberry scorch virus) (BLSCV0)

Candidate

In natural setting the host-plant range of blueberry scorch virus is limited to Vaccinium spp., like highbush blueberry - Vaccinium corymbosum, cranberry - V. macrocarpon, black huckleberry - V. membranaceum, (Wegener et al., 2006) and rabbit-eye blueberry - V. virgatum (V. ashei, Moretti et al., 2011).
BlScV is transmitted non-persistently by the common blueberry aphid, Ericaphis fimbriata (Richards) (Bristow et al., 2000). In Italy Ericaphis scammelli transmitted BlScV, but again not very efficiently (Pansa & Tavella., 2008). Illinoia pepperi (eastern blueberry aphid) also transmits BlScV (Polashock & Hillman, 2017). Within a field, aphids are responsible for spread between plants.
Ericaphis species are relatively inefficient vectors (Raworth et al., 2008), but high aphid populations increase the risk of virus transmission. The rate of spread is about 4% per year in the absence of aphid control. The virus is not transmitted by contact between plants or mechanical means (Schilder, 2019). Under laboratory circumstances also other aphid species were able to transmit BlScV (Martin, 2006).
BlScV can be spread over large distances and enter new areas by import and trade of infected plants for planting, other than seeds, of Vaccinium spp. Findings of BlScV in Italy, the Netherlands and Poland during the last 10 years indicate BlScV has been introduced with import of plants for planting (Anonymous, 2012).
Plants for planting is a pathway. Since BlScV is not yet widespread in the EU, plant material is also an important pathway for long distance spread.
No evidence for seed transmission has ever been found (Martin, 2006).

Yes

Symptoms caused by BlScV in Vaccinium corymbosum range from complete necrosis – blighting – of flowers and young leaves and twig dieback in some cultivars (sensitive) to no visible damage in other cultivars (tolerant). Blighted flowers are initially brown and then tan and bleach to gray over time. They are retained through the summer and may be present the following spring if not removed by pruning during the dormant season. Severely blighted bushes bare little fruit. In cultivars with severe blossom blight, the twigs often die back 4 to 10 cm from the shoot apex. Twig dieback causes lateral buds below the point of necrosis to grow and produce branches later in the season. Over a period of several years, infected bushes become very twiggy and no longer resemble healthy plants of the same cultivar. The difference is especially striking just before harvest. Branches of healthy ‘Pemberton’ bushes droop under the weight of ripe fruit, whereas infected bushes have an upright habit because the branches are shorter and the fruit load is markedly reduced. The productivity of cultivars that exhibit symptoms declines each year, and plants of some cultivars, such as Berkeley, eventually die (Bristow et al., 2006).
Most field bushes showed symptoms during the year following infection, but a few did not show symptoms until the second or third year. Thirty out of 59 cultivars or selections infected with BlScV exhibited severe blighting of flowers and young leaves and dieback of twigs. Three cultivars showed only chlorosis of leaf margins. The virus was also detected in numerous cultivars (26 out of 59) that exhibited no symptoms, and they were considered tolerant of BlScV. BlScV reduced yield in ‘Pemberton’, with the loss being related to the number of years bushes displayed symptoms. Yield was reduced by more than 85% in the third year of symptom expression. The virus did not significantly reduce the yield of six tolerant cultivars that were infected with the virus but displayed no symptoms (Bristow et al., 2006).
In cranberry (V. macrocarpon) BlScV was symptomless (Wegener et al., 2006). However, infected cranberry plants could act as a source for BlScV in blueberry in its vicinity or act as an important pathway for long distance movement of BlScV into new areas (Martin, 2006).

Major

No

Candidate

BlScV can have a major impact on blueberry harvest in sensitive cultivars. Infected tolerant blueberry cultivars and cranberry may act as a source. Remark: In the EU the occurrence of the virus is still limited, virus tested propagation material will also reduce the spread of BlScV.

Candidate

Control of BlScV is primarily through the production of virus-tested nursery plants and should not be based on virus symptoms, since many cultivars do not express symptoms in nursery stock (Martin, 2006).
Once a field has become infected a grower has two options to control BlScV.
• The first is removal of all plants in the field, which is recommended if the percentage of infected plants is high. If replanting with blueberries, it is important that the root system of the plants have been removed to prevent suckers growing from the infected roots system, which would provide a source of inoculum of BlScV (Martin, 2006).
• The second course of action is for cases where the incidence of infection is low, where a grower can combine aphid control with virus testing and removal. Aphid control should be implemented prior to removal of plants in order that aphids are not dispersed during the removal process. It is important to maintain aphid control for at least two years after the last infected plants have been removed to allow any recently infected plants to develop symptoms (Martin, 2006)
The existence of tolerant cultivars creates a dilemma in efforts to control the scorch disease. While tolerant cultivars remain productive once infected, they are a source of inoculum for further disease spread (Bristow et al., 2000).

Candidate