Abstract

Resistance to the large raspberry aphid (Amphorophora idaei) in red raspberry, using single major genes or polygenic minor genes, has proved successful in controlling this virus vector aphid in Europe for the last 30 years. At present, ~90% of raspberry UK plantations, valued at more than 28 million, contain cultivars with these A. idaei resistance genes. Surveys done in the early 1990s found that >75% of the UK A. idaei population consisted of biotypes with the ability to break the most widely used resistance gene, A₁. Since then, growers in England (but not yet in Scotland) have reported intermittent breakdown of the formerly strongest resistance gene, A₁₀. In this current growing season (2001), the incidence of A. idaei infestation on A₁₀-containing cultivars is increasing. Genetic analysis of raspberry aphid populations, based on rDNA IGS DNA patterns, has shown that A. idaei populations in the UK are genetically very variable within and between the 5 known A. idaei biotypes. This is attributed to alate migrations of parthogenetic asexual females in summer and sexual males in autumn. This means that resistance-breaking genes are readily exchanged between raspberry aphid populations. It is therefore predicted that the A₁₀ gene will be overcome throughout the UK within the next few years. Virus incidence is increasing in parallel with breakdown of the aphid resistance genes in UK raspberry plantations. Other aphid resistance genes are not readily available within the genus Rubus, although search has continued. As an alternative source of resistance, anti-aphid genes from other plants (coding for plant lectins) have been genetically engineered into experimental plants. Initial risk:benefit assessment of one candidate aphid resistance transgene was presented, together with future prospects for introducing other sources of aphid resistance. The challenge to the fruit breeders and biotechnologists is initially to find durable resistance genes from other Rubus sources, either wild raspberry or closely related species. If acceptable to the general public, existing raspberry cultivars could later be modified with novel resistance genes from other sources, through genetic engineering. Aphid resistance has proved a valuable method of reducing insecticide use in red raspberry for >30 years, but the loss of pest resistance will result in increased pesticide use and make any Integrated Crop Management (ICM) more difficult. An important aim of ICM is to ensure that future pest resistance genes are compatible with beneficial natural enemies of raspberry aphids and other pests of Rubus.