The Relationship of Cherry Leafroll Virus and Blackline Disease of English Walnut Trees

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Abstract
Two isolates of cherry leafroll virus (CLRV-W) recovered from walnut trees affected on walnut blackline disease (WBL) induced WBL in inoculated English walnut trees propagated either on rootstocks of J. hindsii or the natural Paradox (J. hindsii .times. J. regia). A virus isolate (CLRV-W7) froma 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 2nd single-lesion isolate (CLRV-W8) was purified from cucumber, suspended in a mixture of phosphate buffer and glycerin (10:1, vol/vol), and applied to the cambium under bark flaps in the scion portion of English/J. hindsii or English/Paradox walnut indicators. Within 2 yr, 6 of 8 English/J. hindsii and 5 of 5 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 symptoms 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.