CROWN GALL INCIDENCE: SEEDLING PARADOX WALNUT ROOTSTOCK VERSUS OWN-ROOTED ENGLISH WALNUT TREES

Abstract

Seedling 'Paradox' (Juglans hindsii × J. regia) has been the rootstock of choice for English walnut in California because of its vigor and greater tolerance of wet soil conditions. However, seedling 'Paradox' rootstock is highly susceptible to crown gall, a disease caused by the soil-borne bacterium Agrobacterium tumefaciens. In regions where tree death from walnut blackline disease (cherry leafroll virus) is prevalent, seedling English rootstock is used to avoid the hypersensitive response at the graft union associated with 'Paradox'. Own-rooted English walnut trees have replaced seedling English in the nursery trade and are now often used in counties where walnut blackline disease is severe. Early observations revealed no crown gall on own-rooted trees. In 2006-2007, we conducted a systematic statewide crown gall survey of own-rooted English walnut trees and 'Paradox' rooted walnut trees at five rootstock research sites and 14 commercial orchards planted in 2003 or earlier. Crown gall incidence was determined by visual inspection, and rated as present or absent at the crown level. Every own-rooted tree site was compared to 100 English trees on 'Paradox' rootstock growing within or near each site. Across the five research sites, seedling 'Paradox' had significantly more crown gall (20.5%) compared to the own-rooted trees (0.9%). The commercial orchards had a similar pattern with seedling 'Paradox' exhibiting 22.2% crown gall versus 0.3% in the own-rooted trees.