Natural Spread, Graft-transmission, and Possible Etiology of Walnut Blackline Disease

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Abstract
Walnut blackline (WBL), a disease characterized by necrosis at the rootstock-scion junction, is widely distributed and commonly associated with declining English walnuts (Juglans regia) propagated on Northern California Black walnut (J. hindsii) or Paradox (J. hindsii .times. J. regia) seedling rootstocks in California [USA]. Natural spread of WBL from infected to healthy trees was demonstrated by the results of annual surveys of commercial orchards. Typically, the disease spreads from infected to adjacent healthy orchard trees. A virus, identified as a walnut isolate of cherry leafroll virus (CLRV-W) on the basis of its serological reaction with antisera to several strains of CLRV, was consistently isolated from English walnut scions but never from J. hindsii and ''Paradox'' rootstocks of naturally infected orchard trees. The causal agent of WBL was readily graft transmitted by bark patches from English walnut scions of naturally infected trees to healthy English walnuts on J. hindsii or ''Paradox'' rootstocks only when the inoculum was applied to the English walnut scion; indicator trees developed characteristic blackline at the union within 1 yr. No transmission of CLRV-W or blackline occurred when bark patches from J. hindsii or ''Paradox'' rootstock of WBL-affected trees were applied to English walnut scion or J. hindsii or ''Paradox'' rootstock of the indicator trees. The WBL agent induced chlorotic spots, rings and line patterns in leaves of graft-inoculated open-pollinated seedlings of English walnut cultivar Ashley, but English walnut cultivar Trinta remained symptomless. Leaf symptoms similar to those in graft-inoculated ''Ashley'' seedlings occurred in ''Eureka'' English walnut seedlings that had been mechanically inoculated with CLRV-W, but no infection occurred in J. hindsii and ''Paradox'' walnut rootstock seedlings. Apparently, CLRV-W is present only in the English walnut scions of naturally WBL-affected trees. The development of blackline at the union of English walnut on J. hindsii or ''Paradox'' rootstocks apparently is due to the hypersensitive reaction of the rootstocks to the WBL agent.