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The virus is believed to be the causal agent of blackcurrant reversion (Latvala et al., 1997; Lemmetty et al., 1997; Lemmetty and Lehto, 1999; Jones, 2000) the most damaging disease of black currants worldwide (Jones, 2000). The disease name reflects the change in plant habit, mostly in the leaf appearance, that is suggestive of 'reversion' to a wild plant type. Infected plants quickly become unproductive. In addition to blackcurrant, the virus infects red currant (Ribes rubrum), but causes much less severe symptoms. Gooseberry (Ribes uva-crispa) is immune (Adams and Thresh, 1987a; Latvala et al., 1997). The virus has also been reported in Ribes alpinum and Ribes spicatum (Latvala et al., 1997). After infection, symptoms in plants can commonly take 1-2 years to develop, and then only on one or two branches. Depending on the cultivar, it often takes several more years before infection extends to all branches of infected bushes (Adams and Thresh, 1987a).
Two forms (strains) of reversion are distinguished (Jones, 2000); the common European form (E), and the more severe R form found in Finland and countries of the former Soviet Union. Proliferation of shoot tips, once considered a possible third form (Jones, 1995), seems to be a feature of either of the two main forms of disease under certain circumstances. The two forms differ in the severity of symptoms expressed in black currant plants but the progression of the disease both in plants and crops is similar with each of these forms (Adams and Thresh, 1987a; Jones, 2000). Leaves of most black currant cultivars affected by either form of reversion are usually decreased in number and size and are characterized by a decrease in marginal serrations and the number of main veins, and they have a less clearly defined sinus at the petiole. In some of the recently produced black currant cultivars derived from interspecific crosses, diagnosis based on leaf symptoms alone may be difficult because the natural leaf morphology, especially in the juvenile stage of growth, is not always typical of that seen in older blackcurrant cultivars. The erratic and slow movement of the virus within the plant also affects symptom expression. In some black currant cultivars, and under some ill-defined conditions, leaves may develop a chlorotic line-pattern or veinal oak-leaf pattern (Adams and Thresh, 1987a; Atroshchenko, 1992; Lemmetty et al., 1997; Lemmetty and Lehto, 1999). Similar symptoms are reported in reverted plants of R. alpinum (Bremer, 1983). However, as infection with Cucumber mosaic virus, Alfalfa mosaic virus and some nepoviruses may induce similar symptoms (Adams and Thresh, 1987a, b; Jones and McGavin, 1996), and as the symptoms are erratic in occurrence in reverted plants, they are of no use diagnostically.
Nevertheless, these other viruses are distinguished from the agent of reversion by being readily transmitted by mechanical inoculation of sap extracts in 2% nicotine alkaloid to herbaceous test plants, whereas the probable agent of reversion is only transmitted very rarely in this way.
For diagnosis, in addition to the classic leaf symptoms, symptoms occur in flower buds as they open in early spring. The E form causes a marked decrease in the density of hairs on the flower buds and an increased intensity of colour in the buds. These symptoms are only noticeable just as the buds begin to open. The R form also induces division to form ten rather than five petals, and further increases their pigmentation. Stamens are usually absent and the style is elongated (Adams and Thresh, 1987a). When infected with either the E or R forms, affected flowers are sterile.
In red currant, leaf and flower symptoms are much less noticeable than those in black currant. This makes diagnosis difficult (Adams and Thresh, 1987a).
Heat Therapy of Infected Plants
In individual blackcurrant plants or plant parts, virus vector gall mites were eradicated from buds by immersing them in water at 46°C for 10-20 minutes (Savzdarg, 1957; Thresh, 1964; Adams and Thresh, 1987a). Reverted black currant plants were freed from infection by heat-treatment of plants followed by graft inoculation of the apical tip to healthy plants (Campbell, 1965). Some Ribes species are heat-sensitive and it is therefore best to heat-treat large, well established plants at a lower temperature (34°C) for a longer period (>20 days). Heat treatment at higher temperatures is possible if increased levels of carbon dioxide are used.
Maintenance of Healthy Crops
In the field, until very recently, the only satisfactory control measures were to plant healthy (certified) material away from sources of infection, to rogue out and burn as quickly as possible galled and/or reverted plants that develop in crops, together with adjacent neighbouring plants that are symptomless, and to apply an effective systemic acaricide during the main mite dispersal period. Successful chemical control of mites requires the accurate timing of applications to coincide with mite dispersal. Because of the difference in the opening time of buds down the length of the branches of a bush, chemical control of mites is at best only partially successful.
Because of difficulties in chemical control of vector mites, and of the susceptibility to reversion and to gall mite of almost all commercial black currant cultivars grown in Western Europe, breeding for resistance to mites and/or the reversion agent has been a major priority in black currant breeding programmes (Brennan, 1990; Brennan et al., 1993).
Vector-Resistant Mite Cultivars
Gene Ce from gooseberry, which is immune to the blackcurrant gall mite, Cecidophyopsis ribis (Knight et al., 1974), has been used extensively for C. ribis resistance in many breeding programmes in Western Europe. In addition, R. nigrum var. sibiricum, R. pauciflorum, R. petiolare and R. ussuriense have also been used as donors for gall mite resistance which, in these species, is determined by gene P (Anderson, 1971). The main distinction between these two main resistance sources is that the resistance conferred by gene Ce is much stronger than that of gene P, virtually preventing mite infestation altogether. In gene P-containing plants however, gall mites are initially able to infest buds but, once mite feeding begins, the bud tissue becomes necrotic inhibiting the further survival and reproduction of mites. In field trials, reversion has been found in plants containing gene P resistance (Pavlova, 1964; Jones et al., 1998), suggesting either, that gall mites are able to feed long enough to transmit the virus and/or, that other C. ribis biotypes or, other Cecidophyopsis species, are able to colonize and transmit the virus to these 'resistant' plants. By contrast, field experiments involving gene Ce-containing blackcurrant cultivars remained uninfected with reversion even under high inoculum pressure and from viruliferous mites (Jones et al., 1998).
Virus Resistant Cultivars
Resistance, possibly immunity, to the reversion agent occurs in several wild Ribes species (Pavlova, 1964; Brennan et al., 1993) but most work has been done using the resistance derived from R. dikuscha. In field trials under high inoculum pressure from the E and R forms of reversion, reversion-resistant black currant cultivars failed to become infected even though such plants contained many galls (Jones et al., 1998). Despite this, because of the serious damage caused by gall mites as pests, the use of gene Ce-containing cultivars that confer strong resistance to these mites with the added consequent protection from infection with BRAV, seems the better means of long-term control.
Blackcurrant reversion disease is the most destructive disease of black currants in Western and Eastern Europe, Russia and New Zealand (Jones, 2000). Once established in a crop with its vector mite, it is very difficult to control.