gene for susceptibility to the fungicide azoxystrobin in apple and a tightly linked microsatellite marker

Various apple cultivars, including 'McIntosh' and 'Cox', are reported to be susceptible to the strobilurin fungicide azoxystrobin whereas others, including 'Delicious' and 'Golden Delicious', are resistant. To investigate the genetic control, progenies from various crosses between these four cultivars were raised and the seedlings tested for response by painting three expanded leaves with 200 mg/l of azoxystrobin solution and noting symptoms about a week later. From the segregations, it was concluded that susceptibility to azoxystrobin is due to a dominant gene, Azs, for which 'McIntosh' and 'Cox' are heterozygous. By scoring the segregation in the mapping progeny 'Fiesta' x 'Totem' and comparing it with the segregation of microsatellite [simple sequence repeat (SSR)] markers, Azs was found to co-segregate with SSR-GD127 on linkage group 12 and this tight linkage was confirmed in the progeny from the cross of 'Golden Delicious' x 'Cox' and 'McIntosh' x 'Golden Delicious'. The susceptibility of 'McIntosh', attributed to allele Azs-m, was greater than that of 'Cox', Azs-c, and in the 'McIntosh' x 'Cox' progeny the GD127 marker showed that Azs-c is dominant to Azs-m.