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Etiology

The Relationship of Cherry Leafroll Virus and Blackline Disease of English Walnut Trees. S. M. Mircetich, Research plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616; Adib Rowhani, postgraduate research plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 74:423-428. Accepted for publication 27 October 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1984. DOI: 10.1094/Phyto-74-423.

Two isolates of cherry leafroll virus (CLRV-W) recovered from walnut trees affected with walnut blackline disease (WBL) induced WBL in inoculated English walnut trees propagated either on rootstocks of Juglans hindsii or the natural hybrid Paradox (J. hindsii × J. regia). A virus isolate (CLRV-W7) from a single local lesion, propagated in cucumber, infected English walnut seedlings following transfer by mechanical inoculation. Thirteen of 17 English/J. hindsii and 10 of 10 English/Paradox walnut indicator plants (hereafter called indicators) graft-inoculated with bark patches from English walnut seedlings mechanically inoculated with CLRV-W7 became infected and developed the typical symptom of WBL (a blackline at the graft union) within 2 yr. Control trees similarly grafted with bark patches from healthy English walnut seedlings remained symptomless. Inoculum of a second single-lesion isolate (CLRV-W8) was purified from cucumber, suspended in a mixture of phosphate buffer and glycerin (10:1, v/v), and applied to the cambium under bark flaps in the scion portion of English/J. hindsii or English/Paradox walnut indicators. Within 2 yr, six of eight English/J. hindsii and five of five English/Paradox inoculated trees became infected and developed the blackline symptom. The blackline at the graft unions of indicators experimentally infected with CLRV-W8 was identical to the blackline symptom in walnut trees naturally infected with CLRV-W. Similar control applications of the phosphate buffer:glycerin mixture without the virus failed to produce the blackline symptom. Virus from the WBL-affected indicators was serologically identical to the original isolates used as the inoculum. Virus was not recovered from the scion of any indicator plant without the blackline at the graft union nor from the rootstock portion of any indicator either with or without a blackline at the graft union. WBL is caused by CLRV-W and the development of blackline at the graft union of English walnuts on rootstocks of J. hindsii and Paradox is apparently due to the hypersensitive reaction of the rootstocks to CLRV-W.