Abstract

The identification of major dominant genes for immunity to the Scottish type strains of four nepoviruses and to raspberry bushy dwarf virus has offered the possibility of genetic control through plant breeding. However, the occurrence of resistance-breaking (RB) strains of these viruses has limited their usefulness. Current approaches are therefore to engineer virus resistance by inserting DNA copies of genes of these viruses into plants. Although immunity to raspberry vein chlorosis virus, transmitted by Aphis idaei, is inherited in a complex manner, crosses of immune with infectible genotypes result in a significant number of immune progeny. In the absence of sources of immunity to the four viruses transmitted by Amphorophora idaei, the introduction of genes for resistance to the aphid vector has been very successful in preventing infection with these viruses. However, the occurrence of RB aphid biotypes necessitates a strategy for the continued usefulness of this approach to virus control. Little is known of the causal agent of black currant reversion disease or of its precise mode of transmission by the blackcurrant gall mite, Cecidophyopsis ribis. Sources of resistance both to the disease agent and to its vector are being used in breeding programmes. Determining the mode of inheritance of these forms of resistance and assessing their usefulness against different strains of the agent and possible biotypes/species of the mite vector are greatly hindered by the absence of rapid tests for the reversion agent, and to distinguish the biotype/species of the mite vector. Approaches to resolve these and other problems, and the relative merits of the different control strategies described, are discussed.