Abstract

Levels and components of resistance to Amphorophora idaei in raspberry cultivars containing different resistance genes to A. idaei were studied under infestation tunnel, glasshouse and laboratory conditions. Each test consistently ranked raspberry cultivars, with increasing levels of resistance in the order, non-resistant (Malling Jewel), resistant cultivars containing minor genes, major gene A₁, gene A₁₀ (red raspberry) and gene A₁₀ (black raspberry). Resistance was expressed in 3 different ways: a decrease in alatae settling on plants; a decrease in apterae settling and feeding on leaves; and a decrease in aphid fecundity and rate of nymphal development. Following exposure to a large population of alatae, significantly fewer aphids settled on A₁, A₁₀ and to a lesser extent, minor gene-containing cultivars, than on Malling Jewel. More alatae settled on the top than the bottom zone of non-resistant, minor gene resistant and 2 of 4 A₁-containing cultivars; alatae settling was low on all zones of A₁₀-containing raspberries. Aphid fecundity and nymphal development patterns on different cultivars and resistance classes were similar to those found for alatae settling. After 7 days reproduction more than 30% of nymphs developed to the 3rd or 4th instar on Malling Jewel, while on minor gene and major gene-containing cultivars the total number of nymphs and the proportion of later instars decreased. On resistant cultivars the nymphs were found mainly on the middle and bottom leaf zones, compared to Malling Jewel. A rapid (48 h) screening test using floating leaflets was developed, and, on the basis of apterae settling, ranked the cultivars reliably; it was particularly effective in distinguishing moderate and strong resistance to strain 1 of A. idaei. Gene A₁-containing cultivars bred in England were much more resistant to strain 1 of A. idaei than those bred in Scotland. Possible reasons for this difference are discussed in relation to modified screening procedures and to the control of viruses transmitted by this aphid vector.