Luther Burbank’s Contributions to Walnuts

Luther Burbank began making controlled crosses between walnut species in the late 19th century after hearing about a “supposed natural European hybrid walnut.” He crossed Juglans hindsii (northern California black walnut) × J. regia (Persian walnut) and produced progeny that he named ‘Paradox’ because of its extremely fast growth and other “anomalies.” He also crossed two American species, J. hindsii × J. nigra (eastern black walnut), producing ‘Royal’ walnut progeny that were fast-growing and prolific nut producers. A third interspecific hybrid was a cross between J. ailantifolia (Japanese walnut) × J. regia that resulted in extremely vigorous progeny but was not named. He observed segregation in the F2 populations and described giants and dwarfs as reversions to ancestral forms. Luther Burbank also made selections for walnut scion cultivars and was especially interested in thin-shelled nuts. He collected seeds from a J. regia growing in San Francisco because it produced regularly and had very high-quality nuts with relatively thin but poorly sealed shells. He selected one of its seedlings as ‘Santa Rosa Soft-Shell’ and described it as bearing large crops of nuts that were nearly white with thin shells and delicious white meat. Burbank’s contributions to the walnut industry endure to this day, especially through the widespread use of seedling and clonal ‘Paradox’ walnut rootstocks.

Incidence of Agrobacterium tumefaciens Biovar 1 in and on ‘Paradox’ (Juglans hindsii × Juglans regia) Walnut Seed Collected from Commercial Nurseries

The walnut rootstock ‘Paradox’ (Juglans hindsii × J. regia) is susceptible to Agrobacterium tumefaciens, which often results in a high incidence of crown gall in nursery or walnut production orchards. Though A. tumefaciens is susceptible to the commonly used preplant soil fumigants, crown gall incidence can rise above acceptable levels. We examined the ability of Paradox seed to acquire A. tumefaciens as a function of harvest method used prior to planting. Over a 2-year period at two participating commercial nurseries, Paradox seed were collected directly from the mother tree without contacting the soil or gathered after sitting on the orchard floor for up to 28 days. A. tumefaciens was never detected in or on the 2,650 seeds collected directly from the mother tree. Both virulent and avirulent A. tumefaciens strains were detected in and on the husk of nuts incubated on the orchard floor at a frequency directly proportional to the time spent on the orchard floor. Regardless of A. tumefaciens contamination in or on the husk, A. tumefaciens was never detected in the seed interior. Avoiding soilborne populations of A. tumefaciens at the time of seed collection will play an important role in managing crown gall.

Role of Systemic Agrobacterium tumefaciens Populations in Crown Gall Incidence on the Walnut Hybrid Rootstock ‘Paradox’

Greater than 75% of English walnut production in the United States occurs on the walnut rootstock Juglans hindsii × J. regia ‘Paradox’, which is highly susceptible to infection by Agrobacterium tumefaciens. When seed were germinated and grown in the presence of A. tumefaciens, in the absence of wounding, 94% of the seedlings exhibited tumors while 89% contained systemic A. tumefaciens populations. When seedlings were wound inoculated, A. tumefaciens established endophytic populations in stem tissue and often migrated from the site of infection. Distribution of A. tumefaciens in the stem was random and may exhibit seasonal variation. A. tumefaciens populations in root tissue were more readily detected than in stem tissue and may serve as a reservoir for subsequent infection of the aerial portions of the tree. Importantly, 7% of inoculated, asymptomatic seedlings contained endophytic populations of A. tumefaciens. In all, 17% of seedlings inoculated as seeds developed galls at secondary stem-wound sites. These results provide an ecological and epidemiological foundation upon which to modify existing tree-handling practices in both nursery and orchard production environments to manage crown gall incidence.