AbstractAttempts were evaluate the the difference in response to A. idaei biotypes and strains on certain raspberry cultivars reputed to contain the same A. idaei-resistance gene, A1. The response to two recognized strains of the aphid was assessed in segregating progeny seedlings from crosses between A. idaei-resistant and -susceptible cultivars. These studies showed that 'Autumn Bliss' contained the A. idaei-resistance gene, A10, and 'Delight' and 'Glen Prosen' each contained the A. idaei-resistance gene, A1. When progeny seedlings were assayed in a heated glasshouse as young plants and in an unheated Tygan house as 1-m tall plants, the segregation ratios for resistance and susceptibility to A. idaei were largely unchanged. However, when the resistance of individual progeny plants was assessed, ca. 37% of the gene A1-containing progeny and 9-23% of the gene A10-containing progeny, behaved differently in the two environments. Experiments involving parental cultivars showed that shading plants increased their susceptibility to A. idaei colonization. This shading effect has implications for experimentally detecting A. idaei-resistant progeny in segregating raspberry seedlings, but it does not explain the difference in field resistance to A. idaei of 'Delight' and 'Glen Prosen'. Such differences seem best explained by the presence in these cultivars of minor genes for A. idaei resistance and/or susceptibility that influences the effectiveness of gene A1.