Cause A virus (walnut isolate of Cherry leafroll virus) was consistently isolated from affected English scions but not from black walnut or Paradox rootstocks in California. Because spread in California is not natural in young orchards until walnut trees flower, the virus is spread by pollen. Bark patch grafts from English walnut scions on black walnut (Juglans hindsii), or Paradox rootstocks, readily transmit the causal agent of blackline but only if the inoculum was applied to the English walnut scion. Indicator trees developed characteristic blackline at the union within 1 year. The virus is only in the English walnut scions of naturally infected trees. Blackline occurs at the union of English walnut on J. hindsii or Paradox rootstocks due to the hypersensitive reaction of the rootstocks.
Symptoms The disease is in Oregon only in the 'Franquette' variety when grafted on black walnut rootstocks. It has not been found in Persian walnuts or seedling Persian walnut stocks. Mature trees are affected. Symptoms include poor shoot growth, and leaves yellow early and drop from part or all of the tree. Often the tree dies 3 to 5 years after the first symptoms. At the graft line, a water-soaked brownish area constricts vascular tissues.
- Use only propagating material from pathogen-free trees.
- Rogue out affected trees.
- For high-value trees, inarch grafting with J. regia may be effective.
Reference Mircetich, S.M., Sanborn, S.A., and Ramos, D.E. 1980. Natural spread, graft-transmission and possible etiology of walnut blackline disease. Phytopathology 70:962-968.