VIEW ARTICLE

Etiology

Natural Spread, Graft-transmission, and Possible Etiology of Walnut Blackline Disease. Srecko M. Mircetich, Research plant pathologist, Agricultural Research, Science and Education Administration, U.S. Department of Agriculture, University of California, Davis 95616; R. R. Sanborn(2), and D. E. Ramos(3). (2)(3)Farm advisor and extension pomologist, Cooperative Extension Service, respectively, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 70:962-968. Accepted for publication 28 March 1980. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-962.

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 × J. regia) seedling rootstocks in California. 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 root-stocks 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 symtomless. 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 appears to be due to the hypersensitive reaction of the rootstocks to the WBL agent.