As increasingly more young Chandler walnut blocks come into production, we have been noticing more off type or “bulls”. These are trees that will periodically have light or virtually no crop. This past growing season seems to be a year when symptom expression was particularly severe.
The bearing pattern of these trees is erratic with on and off years occurring at unpredictable intervals. These trees will often be larger than non-bull trees presumably because of greater nutrients available for vegetative growth due to lighter cropping.
The cause of this problem is unknown. We do know that it is not a virus and that it is spread by graft wood, using wood from affected trees for propagation.
Over the life of the orchard, the production of these trees will be less than that of non-affected trees. The sooner these trees are discovered and something is done, the better. Usually these trees will not be noticed until the orchard begins to bear a significant crop. It has been observed this year in some 5th leaf orchards.
Time to topwork
Trees from the 5th to 9th year would be good candidates for topworking with wood from Chandler trees which are known to be free of the problem.
Topworking would probably be preferable to removing trees and replanting due to poor growth often associated with replants. With older trees the benefit of doing something will be lessened because there will be less time in the life of the orchard to recoup costs.
Additionally, topworking older trees will be more difficult and expensive and replant problems can be worse.