Clonal Walnut Rootstocks in Northern California

Clonal Walnut Rootstocks in Northern California
The California Walnut industry primarily utilizes two rootstocks for commercial production, Northern California Black (Juglans  hindsii) or Paradox hybrid (Juglans hindsii by Juglans regia). Both rootstocks are open pollinated resulting in genetic variability. 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 Paradox 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 rootstock experiments have been planted in Northern California. The Tehama County plot is a randomized complete block design with five rootstocks and five replicates. Rootstocks include VX211, Vlach, RX1, June budded Vlach and seedling Paradox as the control comparison. Rootstocks were micropropagated at Northern California Plant Lab and grown for one year in a commercial nursery. Ungrafted rootstocks were planted in March 2009 into fumigated class one Columbia loam soil at a tree spacing of14ft by 26ft. Trees were budded in September 2009 to the Howard (Juglans regia) variety.Microsprinklers are used for irrigation. The Solano County plot is a randomized complete block design with five rootstocks and six replicates. Rootstocks were propagated by the UC Davis walnut breeding program and were grown for one year in a commercial nursery. Rootstocks include Vlach, RX1, VX211, Burbank and seedling Paradox as the control comparison. Rootstocks were planted March 2009 at a tree spacing of 18.5ft by 24ft and budded in August to the Tulare (Juglans regia) variety. In both experiments, Vlach and VX211 demonstrated very good tree vigor. In the Tehama site seedling Paradox grew as well as the two clones while at the Solano site seedling Paradox was less vigorous. RX1 was the less vigorous in both sites when compared to VX211 and Valch. Burbank was the least vigorous rootstock in the Solano experiment.