The 2012 ASHS Annual Conference

Apple breeders in the United States and other countries have focused on developing disease resistant cultivars as a way to improve the sustainability of apple (Malus pumila Mill.) production.  A leading concern for many apple growers and consumers is the extensive use of fungicides in controlling apple scab [Venturia inaequalis (Cke.)Wint]. Genetic resistance has been introgressed into many breeding programs from related apple species (M. floribunda, M. bacatta, M. bacatta jackii, etc.).  The genetic drag of poor fruit quality traits linked to resistance has hindered selection of  superior cultivars with wide commercially viability. The cultivar Honeycrisp has shown resistance to apple scab in organic field conditions but was not known to have any resistant parent.  This has led to an investigation to explore the genetic resistance in populations derived from  ‘Honeycrisp’ and its ancestors.  A 2011 greenhouse seedling screening of two subpopulations of  ‘Honeycrisp’ x ‘Gala Twin Bee’ (a susceptible genotype) progeny show a segregation ratio of 1 resistant : 1 susceptible  [Χ²  (N = 316) = 0.114, P = 0.736; Χ² ( N = 114) = 0.561,  P = 0.454]. Three other seedling populations of ‘Honeycrisp’ x susceptible parent exhibited a 3 resistant : 1 susceptible ratio suggesting the presence of two genes in ‘Honeycrisp’ (Χ² (N = 203) = 0.199, P = 0.656;  Χ² (N = 150) = 0.009, P =0.925; Χ² (N = 106) = 0.013, P = 0.911).  The putative ‘Honeycrisp’ parent, ‘Keepsake’, and grandparent, ‘Frostbite’, each were crossed to a susceptible cultivar and the progeny populations exhibited a 1:1 segregation ratio [Χ²  (N = 101) = 0.802, P = 0.371; Χ² (N = 31) = 0.209, P = 0.209, respectively].  It appears that at least one resistance gene present in ‘Honeycrisp’ can be traced through these ancestors.  Results from screenings in Spring 2012 to test possible escapes and evaluate different pathogen isolates should provide additional support for either the 1 or 2 gene model of genetic resistance in ‘Honeycrisp’.  Molecular markers will be utilized to map the resistance gene(s) and elucidate the inheritance of the resistance trait.