NAME OF THE ORGANISM: Nepovirus avii (cherry leaf roll virus) (CLRV00)

Evaluation continues

Candidate

CLRV has a wide natural host range, which includes a number of woody or herbaceous hosts (e.g. plants of genera: Betula, Celtis, Cornus, Fagus, Juglans, Ligustrum, Olea, Prunus, Populus, Ulmus, Rubus, Sambucus and Rheum. Its experimental host range includes plants from more than 36 families (Jones, 1985; EFSA PLH, 2014)).
Despite early reports suggesting that CLRV, like other nepoviruses, might be transmitted by soil-inhabiting nematodes, more recent studies have ruled out this possibility (EFSA PLH; 2014, Jones, 1985, Wang et al., 2002).
CLRV is transmitted through the vegetative multiplication of infected host plants.
In addition, CLRV is seed- and pollen-transmitted at variable rates in many of its natural hosts. In at least some natural hosts, pollen transmission is both vertical (resulting in infected seeds and, upon germination, seedlings) and horizontal (resulting in infection of the pollinated mother plant) (EFSA PLH, 2014). However, a population genetics study of CLRV isolates has shown that transfer between different host species is likely to be rare, possibly as a consequence of the need for pollen germination and ovule fertilization for transmission (Rebenstorf et al., 2006). As a consequence, the fact that CLRV may have high prevalence in some hosts in a given region does not necessarily imply that all potential host plants (e.g. olive plants) are at a high risk of infection in this region (EFSA PLH, 2014).
Cherry leaf roll virus may be introduced into a walnut orchard through graft wood or pollen from infected trees. Within the orchard, the pathogen is probably transmitted by infected pollen because it does not spread from diseased to healthy trees until the pistillate flowers are receptive for pollination. In addition, blackline spreads more rapidly in mixed-cultivar orchards, where pollen shedding overlaps with pistillate bloom for a longer time. Once a tree is infected, the virus spreads through the scion and eventually reaches the rootstock. Dependent on the rootstock the tree might die (black walnut or ‘Paradox’) or become a symptomless carrier (English walnut rootstock) (UCANR, 2020). In the latter case, pollen or graft wood from the tree will spread the disease (UCANR, 2020).
For walnut Quacquarelli & Savino (1977) showed that the frequency of CLRV transmission through walnut seeds varied with the storage temperature. The infection rate increased from 4% at 2 - 5 °C up to 32 % at room temperature.

Yes

CLRV is regarded as the economically most relevant virus in walnut. Cherry leaf roll virus can cause blackline in walnuts (Mircetich et al., 1980; Rowhani & Mircetich, 1988), which causes symptoms in the tree canopy that are similar to those caused by soilborne pathogens, nutrient deficiencies, and scion-rootstock incompatibilities. The first symptoms of the disease are poor terminal growth, yellowing and drooping leaves, and premature defoliation, particularly in the top branches. Later, diseased trees show dieback of terminal shoots and decline, often accompanied by profuse suckering from the rootstock (mainly black walnut). Blackline disease kills the tissues that transport nutrients and water between the rootstock and scion. The black line is usually exactly at the graft union on black walnut rootstock and will eventually completely girdle the tree, killing the scion in 2 to 6 years. In addition to the black line, however, Paradox rootstock typically develops a canker that extends into the rootstock. Blackline canker does not extend upward into the scion, as cankers produced by Phytophthora often will. Because of the extensive bark cankering, scions on Paradox decline faster than on black walnut rootstock. Trees may become infected at any age once tree blossoms, but blackline is more common in trees 15 to 40 years old. Tissue necrotization of the grafting point is the result of a hypersensitivity reaction of CLRV-resistant rootstocks when the scion is CLRV-infected. Necrosis at the graft union may lead to dieback and decline of entire trees (Langer et al. 2010). All walnut cultivars are susceptible to the virus, but the rate of viral spread varies by cultivar (UC ANR, 2020).
Using the same rootstock Paradox, symptoms of blackline disease were observed in Italy (Ferretti et al., 2017). This problem can be avoided by using non-susceptible rootstocks, such as J. regia, but these may be of lower agricultural interest (Grant & McGranahan, 2005).
In Hungary the spread of CLRV in walnut was surveyed within fruit bearing orchards in the 1980’s. An equal proportion of trees was on Juglans regia and J. nigra rootstocks. The initial infection in 1981 was 24,2 %, in 1982 48,4 %, and by 1989 it has increased to 96,8 %. On the one hand, there was no difference in the extent of infection between the trees of the two rootstocks, there was however a major difference in the ratio of the died trees. Trees on J. nigra died in 1981: 6,7 %, in 1982: 13,3 % and 1989: 50 %. Death of trees on J. regia rootstock started in 1982 with 3,3 %, and in 1989 only 10 % of the trees died in spite of the fact that the condition of the infected trees was very bad (Németh et al., 1990).

Medium

No

No

Candidate

Remark about impact in other hosts: Chlorotic or yellow ring patterns or ringspots, yellow vein netting and yellow spotting is reported in many of its hosts (Jones, 1985).
As examples, CLRV can cause decline in American elm; tree decline or death in cherry, in particular in situations of co-infection with Prunus necrotic ringspot virus and Prune dwarf virus. CLRV also causes severe disease symptoms in at least some circumstances in Rubus. The reported symptoms include leaf patterning and plant death in blackberry and leaf size reduction, deformation and chlorotic mottling or ringspotting and stunting of fruiting canes in raspberry. Affected plants produce less fruit. However, the precise contribution of CLRV to these severe symptoms remains to be precisely established. It is possible that these symptoms were caused by CLRV alone or in combination with co-infecting viruses, resulting in synergistic effects and increased symptomatology. It is also possible that these symptoms were caused by an unrelated agent, which was not identified during these early efforts. Overall, although there are elements to suggest that CLRV can cause severe symptoms in its regulated Rubus spp. hosts, this conclusion is affected by strong uncertainties (EFSA PHL, 2014).
The Fruit SEWG recommended not to consider indirect economic impact since interspecies infection is reported not to occur. Mechanical infection between different species is improbable since disinfection would prevent indirect economic impact.

Candidate

If non-susceptible rootstock is used the economic is impact can be reduced, but these may be of lower agricultural interest (Grant & McGranahan, 2005).
Certification scheme to avoid introduction of CLRV with graft wood.

Candidate

The pathogen causes relevant damage and can be controlled by plant certification.
The disease may develop slowly and remain symptomless over many years. Infected seedlings might therefore remain in the orchards for long periods, thereby contributing to virus spread via pollen. In the high yield phase after several years the disease might become apparent and lead to relevant economic losses (Langer et al. 2010).