Proc. EUCARPIA Symp. on Fruit Breed. and Genetics Eds M. Geibel, M. Fischer & C. Fischer Acta Hort. 538, ISHS 2000 719 NEW HARDY ROOTSTOCKS FROM THE QUEBEC APPLE BREEDING PROGRAM S. Khanizadeh, Y. Groleau, R. Granger, and G. L. Rousselle J. Cousineau Agriculture and Agri-Food Canada, Agriculture and Agri-Food Canada, 2560 Boul. Hochelaga, Research Station, 430 Boul. Gouin, Sainte-Foy, Quebec, Canada St-Jean-sur-Richelieu, QC, Canada G1V 2J3 J3B 3E6. Keywords : Malus, Quebec, Canada, selections Abstract Thirty-two hardy, woolly aphid resistant rootstocks were selected from a 908 rootstock seedling population obtained from three crosses: Malus robusta 5 (R.5) x M.27, R.5 x Budagovsky 57490 and open-pollinated Ottawa 3. A trial was conducted in Frelighsburg, Quebec, a sub-station of Agriculture and Agri-Food Canada, to evaluate the rootstock seedlings using ‘Spartan’ as a scion. Several of the evaluated rootstocks were found to be dwarfing and efficient under Quebec environmental conditions. 1. Introduction The harsh and cold winter climate is a limiting factor for tree survival in the apple- growing regions of Quebec which requires the use of hardy rootstocks to ensure survival of trees. Tree mortality caused by Quebec winters has been studied extensively and several factors have been found to be responsible for mortality (Khanizadeh et al., 1999, Granger 1982). Rootstock hardiness seems to be a crucial factor especially in areas that are not perfectly suited for apple growing. For example, higher mortality was observed in areas exposed to wind or among trees grown on dwarf rootstocks planted in soil which was in part made of sand or gravel (Khanizadeh et al., 1999). In spite of a large variation in mortality among the rootstocks in each region and site, M.9, EM.26 and M.26 showed little variation in percent mortality, indicating that these rootstocks are the most reliable in our growing conditions. Heeney (1981) reported that O.3 is hardier than M.26 and MM.106, and Czynczyk (1979) found the rootstock Budagovsky to be as hardy as M.26. Resistance to low temperatures is a primary concern in the development of new rootstocks for cold regions but the selected rootstocks must also be adapted to regional needs. The present study was conducted to evaluate hardiness, precocity, yield efficiency, dwarfing ability, woolly aphid resistance and ease of propagation of the most promising hardy rootstocks in the Quebec breeding program. 2. Materials and method Seeds were collected from crosses performed in 1975 between Malus robusta (R.5) and either M.27 (75-13 series) or Budagovsky 57490 (B.57490) (75-14 series). Seeds were also collected from open pollinated Ottawa 3 (O.3) trees (75-18 series). The seeds were stored in a seed storage room until 1979. They were then germinated under greenhouse conditions and planted in a nursery in 1980. Budding to ‘Spartan’ was carried out in 1982 and trees with bud failure were cleft-grafted in 1983. All trees were planted in 1984 at the experimental farm of Agriculture and Agri-Food Canada in Frelighsburg (Quebec) at a spacing of 5.5 x 3.0 m. Standard orchard management practices were applied each year (Anon., 1976). Of the 908 trees started in 1979, only 499 could be evaluated in 1990. The others had succumbed to winter injury or were eliminated for other