EVALUATION OF DWARFING ROOTSTOCKS IN WASHINGTON APPLE REPLANT SITES

Abstract

Apple replant disorder is an increasingly significant factor challenging apple (Malus × domestica) orchard establishment and consistent production. Standard fumigation practices using Telone or Vapam offer effective control, but must be applied correctly and safely, are expensive, subject to restrictive regulation, and not acceptable in organic systems. Dwarfing rootstocks with resistance or tolerance to apple replant disorder offer a clear alternative to producers in tree fruit production areas of the Pacific Northwest of the USA, as long as their genotypes are suitably adapted to the diverse production conditions of the region. In collaboration with the USDA-ARS/Cornell University National Apple Rootstock Breeding Program in Geneva, NY, the Washington Tree Fruit Research Commission (WTFRC) has conducted eight rootstock trials in commercial Washington orchards since 2003. These trials collectively included 65 rootstocks and three scion cultivars (‘Gala’, ‘Fuji’ and ‘Honeycrisp’) planted in modern, high density systems in known replant sites. They featured 46 Geneva (G.) series selections, as well as 19 other genotypes. Trials were designed as split block (fumigated vs. non-fumigated) randomized complete blocks with a minimum of four replications and typically five or more trees per plot. Most Geneva rootstocks outperformed the industry standards of Budagovski (B.) 9, Malling (M.) 9 Pajam 2, and M.26 for yield, trunk cross sectional area, and fruit size. Supporter 1, Supporter 2, and Supporter 3 have performed similarly to M.9 clones and have shown no advantage over standard commercial rootstocks. Several Geneva selections G.41 (tested as CG.3041), G.935 (tested as CG.5935), CG.4214, CG.4814 have performed well in non-fumigated treatments across sites. Trees with G.41 and G.11 rootstocks have shown less vigor than trees on G.935, CG.4214, G.202, and CG.4814. Many of the Geneva series also show significant improvement over currently available rootstocks for other traits including wooly apple aphid resistance, fire blight resistance, ease of propagation. Our collaborative trials under commercial conditions clearly indicate the potential of improved genotypes for apple production in the Pacific Northwest of the USA.