A NEW SERIES OF PEST- AND DISEASE-RESISTANT VIGOUR-CONTROLLING APPLE ROOTSTOCKS

Abstract

Contemporary dwarfing and semi-dwarfing root stocks cultivated extensively worldwide lack durable resistance to economically damaging pests and diseases. In recognition of these limitations, a series of hybrid families was created in 1986-87 by crossing resistance sources Malus ‘Robusta 5’ and ‘Aotea’ (Malus seiboldii) to maternal parent ‘Malling 9’ (M.9) dwarf rootstock. More than 10,000 genotypes were screened as young seedlings for Phytophthora cactorum tolerance and the survivors established as individual stoolbeds. After three years, those individuals exhibiting field resistance to natural infestation by woolly apple aphid (WAA) (Eriosoma lanigerum) and freedom from spines/branches were grafted with the scion ‘Gala’ and planted in a replicated agronomic performance trial. The evaluation site was chosen to have a high natural disease pressure for Phytophthora cactorum and periods of water logging. After five years, about 15 genotypes were identified for their reduced tree stature, freedom from suckering, continued WAA resistance, freedom from Phytophthora symptoms and high floral and fruiting precocity. Currently, seven selections have been retained in advanced evaluation orchard productivity and adaptability trials, compared with standards M.9 and Geneva® 210. Five of the seven selections show immunity or resistance to fire blight (Erwinia amylovora) in shoot inoculation bioassays. Tree size, productivity and fruit quality comparisons with M.9 and Geneva 210 are presented. Three of the selections are similar in stature to M.9, three selections are semi-dwarfing, similar or slightly smaller than Geneva 210 and one is more vigorous than Geneva 210 but judged to be less vigorous than MM.106. International development of these rootstock selections is being conducted in collaboration with International Fruit Obtention (IFO).