Effectiveness of resistance genes to the large raspberry aphid, Amphorophora idaei Borner, in different raspberry (Rubus idaeus L.) genotypes and under different environmental conditions

The introduction into commerce of raspberry cultivars with major gene resis tance to the large raspberry aphid, Amphorophora idaei, an important pest a nd virus vector on red raspberry in Europe, has been very effective both in decreasing pest numbers and greatly restricting infection with the viruses it transmits. However, biotypes of the aphid able to overcome these genes have developed in the field in recent years. Additionally, in field and lab oratory tests, the response to aphid biotypes and recognised aphid strains of certain raspberry cultivars, such as Glen Prosen and Delight, differ mar kedly despite the fact that they are reputed to contain the same A. idaei-r esistance gene, A(1). In attempts to understand the reasons for this differ ence in response, analysis was made of the segregation of progeny seedlings from crosses between A. idaei-resistant and -susceptible cultivars to two recognised strains of the aphid. These studies showed that, as expected, cv . Autumn Bliss contained the A. idaei-resistance gene, A(10), and cvs Delig ht and Glen Prosen each contained the A. idaei-resistance gene, A(1). When progeny seedlings were assayed in a heated glasshouse as young plants and i n an unheated Tygan house as 1 m tall plants, the segregation ratios for re sistance and susceptibility to A. idaei were largely unchanged. However, wh en the resistance of individual progeny plants was assessed, c. 37% of the putative gene A(1)-containing progeny and 9-23% of the putative gene A(10)- containing progeny, behaved differently in these two environments. Experime nts involving an A. idaei-resistant and -susceptible parent cultivar showed that shading plants increased their susceptibility to A. idaei colonisatio n. Whilst this shading effect has implications for experimentally detecting A. idaei-resistant progeny in segregating raspberry seedlings, it does not explain the difference in field resistance to A. idaei of cvs Delight and Glen Prosen. Such differences in the field seem best explained by the prese nce in these cultivars of 'minor' genes for A. idaei resistance and/or susc eptibility that influences the effectiveness of gene A(1).