Unlike the situation with apples, until recently, there have been few commercially acceptable size-controlling rootstocks available for peach production. Nearly forty years ago a program was initiated to screen Prunus germplasm for ease of vegetative propagation, compatibility with peach and size-controlling potential. This initial research involved ~120 genotypes that were either various named Prunus species and cultivars or inter-specific Prunus hybrids. This collection was later narrowed down to nine selections that showed some promise for commercial production. After subsequent field trials two new rootstocks were released in 2004 (K146-43 and P30-135; Controller 5 and 9TM, respectively). Subsequently, a new breeding program was initiated based on the hypothesis that hybrids of two peach cultivars (Okinawa and Harrow Blood) would have the potential to reduce the scion vigor of peach and be resistant to root-knot nematodes. A seedling population of hybrids resulting for this cross were propagated and ‘O’Henry’ peach was grafted onto each individual genotype. Those that showed size-controlling potential were then put into a series of replicated field trials and evaluated for both size-controlling behavior and productivity characteristics. These trials resulted in the release of a series of new rootstocks (‘HBOK 27’, ‘HBOK 32’, ‘HBOK 10’, ‘HBOK 50’; Controller 6, 7, 8 and 9.5TM, respectively). Subsequent intensive research documented that a major mechanism involved in growth control of scions grafted on these rootstocks involves reduced hydraulic conductivity of these rootstocks during early spring growth caused by smaller sized xylem vessel diameters.
Keywords: dwarfing rootstocks, size-controlling, tree vigor, tree size, tree growth control