Paradox canker manifests as a bleeding bark canker that originates below the soil surface and spreads up and around the root crown and tree trunk. It produces profuse black viscous fluid from the dead bark. As the canker expands, shoot growth ceases, and tree defoliation (leaf drop) and dieback occur. An affected tree typically dies within 1 to 2 years of the appearance of the canker aboveground.
Paradox canker disease cankers superficially resemble cankers caused by Phytophthora and Cherry leafroll virus (blackline disease cause), but the cankers can be distinguished after removal of their outer bark as follows:
- Paradox canker disease cankers tend to be more rounded or lobed at their margins than those caused by Phytophthora, which tend to have relatively irregular or jagged advancing margins. Paradox canker disease cankers tend to generate light-brown, rounded lobes of dead tissue that spread out from dark-brown lobes. The lobes, especially the lighter colored ones, may exhibit concentric rings of color change.
- In contrast to cankers caused by Cherry leafroll virus on Paradox rootstock, Paradox canker disease cankers do not begin at the graft union and move toward the soil line.
Comments on the Disease
Paradox canker has been observed in walnut orchards throughout California's Central Valley, typically at a low incidence (less than 1%). It predominately affects 8- to 15-year-old trees on seedling Paradox rootstock and has not been observed conclusively on black walnut rootstock. Affected trees occur randomly in the orchard. The cause of Paradox canker, although unknown, is being investigated.
It is unknown whether some seedling Paradox rootstocks are more prone to Paradox canker than others; however, trees on clonal Paradox rootstock can be used to replace those killed by Paradox canker. Although it is unknown whether dragging symptomatic root systems through the orchard can spread Paradox canker disease, it is advisable to remove dead trees without spreading any of the dead tissue in the orchard.
There is no evidence that preplant soil fumigation reduces Paradox canker.