California Agriculture

While walnut trees on Juglans hindsii x J. regia ‘Paradox’ rootstocks are highly susceptible to crown gall, it is unknown whether this bacterial disease is acquired in the nursery or the orchard. We selected two groups of gall-free trees in nurseries, those adjacent to trees with and without galls. Two years after being transplanted in the orchard, trees in the group adjacent to those with galls had significantly greater — more than four times more — crown gall incidence than those adjacent to trees without galls (14% versus 3%). In addition, trees in prolonged (17-day), bare-root, unrefrigerated storage before transplanting were associated with higher crown-gall incidence. We also found that crown gall can decrease walnut tree productivity. For every quarter of trunk circumference that was galled, there was a 12% decrease in cumulative nut yield over the first 4 years of production.