English walnut rootstocks help avoid blackline disease, but produce less than ‘Paradox’ hybrid

C first walnut trees and orchards were planted during the Spanish mission period (around 1800), using seedlings of early varieties of Persian or English walnut (Juglans regia) chosen for their superior growth and nut quality. Since the 1890s, walnut trees in California have been propagated by grafting or budding desired cultivars onto rootstocks chosen for their adaptation to different physical, chemical or biological soil conditions at individual orchard sites. From the early to mid20th century, seedlings of Northern California black walnut (Juglans hindsii) were used as rootstocks because they grew vigorously and were more tolerant of saline and saturated soil and more resistant to soil-borne pests than English walnut seedlings. Since the 1950s, ‘Paradox’ hybrid seedlings have supplanted black and English walnut as rootstocks of choice for California orchards, though black walnut (J. hindsii and others) is still RESEARCH ARTICLE