Rootstocks are bred to grow in different soil types and conditions, and provide the best anchorage, vigor, and resistance or tolerance to soil borne pests and diseases. However, no individual rootstock is tolerant of all factors that impact walnut production. The strengths and weaknesses of each rootstock should be considered in the context of a specific orchard location.
Traditionally two rootstock choices have been available to the walnut industry in California, seedling Northern California black walnut (J. hindsii) and seedling Paradox. Paradox rootstock is a hybrid, produced from a Northern California black walnut tree pollinated by an English walnut. In recent years, three clonal Paradox rootstocks have become available; Vlach, RX1 and VX211. A few growers also use own-rooted English walnuts in areas where blackline disease is prevalent.
Northern California black walnut seedlings provide a less vigorous rootstock that is less susceptible to crown gall (Agrobacterium tumefaciens) than Paradox, but more susceptible to Phytophthora crown and root rot. Paradox seedlings are more vigorous and less susceptible to Phytophthora root rot but very susceptible to crown gall. Both of these seedling rootstocks grow poorly in the presence of pathogenic nematodes. The clonal Paradox rootstocks have specific beneficial characteristics. For example, VX211 is very vigorous and shows some tolerance to nematodes. RX1 is a less vigorous rootstock but has shown moderate to high resistance to Phytophthora citricola and high resistance to P. cinnamomi. RX1 may also be moderately resistant to crown gall disease. Vlach is a vigorous rootstock. None of the Paradox or California black rootstocks are resistant to blackline disease. If blackline is prevalent in an area it can be avoided entirely by growing scion varieties on their own roots but tree size, resistance to root problems, and yield may be reduced.
For a description of the biology of scion-rootstock interactions and reasons for using specific scion and rootstock cultivars see the “General Propagation” page in the Orchard Management section of the website.
There are more than 30 cultivars of walnut grown in California. The most popular cultivars, accounting for 80% of production, include Chandler, Howard, Hartley and Tulare (CDFA, 2012). Several factors are important to consider when selecting a walnut cultivar including local climate and pest conditions. Walnut requires a period of winter chill to break dormancy and initiate leaf and flower production (see Fruit Development & Thinning for description of “chill hours” and “chill requirement”), but there is very little experimental data available to verify specific requirements for individual cultivars. In the absence of better data, leaf-out dates provide a relative estimate of chill requirement because early leaf dates generally correspond to a lower chill requirement.
The prevalence of blackline disease in an area will also affect the choice of scion and rootstock to plant in an orchard. Blackline disease causes the death of the graft union after infection by a pollen-transmitted virus when trees are grafted on either black or Paradox rootstock. It is most common in cooler areas along the coast and near the Sacramento Delta but can occur elsewhere in California. When replanting an orchard in a blackline infected area, consider planting on English walnut or other virus-tolerant rootstock or select a new scion cultivar with female bloom time differing from the pollen shed of any nearby infected trees. In addition to considering how a potential scion cultivar would fare in local climate and pest conditions, growers should consider other characteristics of each cultivar including leaf and bloom date, timing of harvest, nut quality and bearing habit (Table 1, Van Steenwyk and Bernett 1998).
Because pollination is required to set a crop, growers should select a cultivar with overlapping male and female flower maturity or, if a suitable pollen source is not nearby, plant a few trees of a pollinizer variety. Early leafing and flowering varieties are more exposed to spring rains that contribute to bacterial blight and are therefore more suitable to the southern part of the Central Valley where there is less spring rainfall. Growers should choose cultivars with harvest dates that suit their individual location including climate, expected availability of harvest equipment and drier capacity. Harvesting earlier can help growers avoid early fall rains, and planting several cultivars with a range of harvest dates may aid in optimizing equipment use.