2011 WALNUT CLONAL PARADOX ROOTSTOCK TRIALS IN NORTHERN CALIFORNIA

The California walnut industry utilizes two seedling rootstocks for commercial production, Northern California Black (Juglans hindsii) and Paradox hybrid seedling (Juglans hindsii x Juglans regia). Both rootstocks are open pollinated resulting in genetic variability. This genetic variation leads to non-uniformity in the field related to size, vigor, compatibility, and disease susceptibility. Due to superior vigor, better adaptability to marginal soils and greater tolerance to Phytophthora crown and root rot, Paradox is the preferred rootstock for Northern California. Recent technology has resulted in micropagation and commercial availability of three new clonal walnut rootstocks, RX1, VX211 and Vlach. Clonal rootstocks have several horticultural advantages. First, they can be selected for desirable attributes such as disease resistance, nematode tolerance and vigor giving farmers the opportunity to match rootstock selection with planting sites. Second, they will impart less genetic variability and be more predictable in the orchard. Disadvantages include the loss of genetic diversity in orchard plantings and additional expertise is required to micropropagate, nursery culture and graft to produce a commercially viable product. Two long term trials were planted in 2009 in the Sacramento Valley to evaluate the potential of the new clonal paradox rootstocks. One in Tehama County is evaluating VX211, Vlach, RX1, and June budded Vlach. The second in Solano County is evaluating VX211, Vlach, RX1, and Burbank. Both trials included Paradox seedling as the control comparison. Results in 2011 found no significant differences between the clonal rootstocks for scion circumferences at either location. There was also no difference in tree height measured at the Solano trial. There was significant difference found between the clonal rootstock circumference measurements in the Solano trial with VX211 being largest and RX1 smallest though these differences did not translate to overall larger or smaller tree size.