California Rootstock Breeding Program
In 1987, David Ramming of USDA and Ted DeJong and Scott Johnson of UC collaborated on a rootstock evaluation project for peaches and plums. David had bred or collected over 100 items including many Prunus species and hybrids. Five trees each of O'Henry peach and Santa Rosa plum were grafted on most of these rootstocks and planted in a field at Kearney. Over the next 7 years these were extensively evaluated (Screening Rootstocks 1993).
By 1994, it was concluded that no rootstock was better than Citation for plums in terms of compatability, precocity, production, fruit size, suckering and tree size. No further evaluations were done for plum.
For peach, 8 selections were identified as potential dwarfing or semi-dwarfing rootstocks (see 1990s NC-140 reports) and were propagated for further evaluation. In 2004, two of these were patented as the slightly dwarfing rootstock, Controller 9 (P30-135) and the very dwarfing stock, Controller 5 (K146-43).
In the early 1990s, Fred Bliss from UC starting breeding new rootstocks using Harrow Blood and Okinawa as parents. This series of stocks was identified by the name HBOK. In 1998, Fred left the University of California and Ted DeJong took over the program. Three new rootstocks have now been released: Controller 7 (HBOK 32), Controller 8 (HBOK 10) and Controller 9.5 (HBOK 50). A fourth will be released soon as Controller 6 (HBOK 27).