WSU Extension


Caption: Walnut blackline
Photo by: J. W. Pscheidt
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Walnut : Blackline
(revision date: 5/20/2014)

Blackline is a viral disease of English walnut (Juglans regia). It causes death of water-conducting tissues at the graft union. Initially, infected trees show poor growth, yellowing and drooping leaves, early leaf loss, and other signs of inhibited water and nutrient uptake. As the disease progresses, branches die back and trees decline and die over a period of years. Bark at the graft union may have small cracks or pits. Cuts made through the bark at the graft site show an intermittent or continuous thin black line of dead tissue between the rootstock and the scion wood. Black cankers may also be present beneath the bark on the rootstock 'Paradox'. The virus can be transmitted to healthy trees via infected pollen and can also be spread by grafting and seed transmission. The disease is primarily found in mature trees (15-40 years old).
Management Options

Non-Chemical Management
  • Plant disease-resistant varieties of English walnut. The variety 'Franquette' is known to be affected by blackline in Oregon. Black walnut rootstocks and Paradox rootstocks are resistant. Persian walnuts are resistant.
  • Do not graft using infected wood.
  • Remove and destroy infected trees and grafts.
Select non-chemical management options as your first choice!

Chemical Management

None recommended

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Caption: Walnut blackline
Photo by: J. W. Pscheidt