Blackline is caused by cherry leaf roll virus. First symptoms of blackline are poor terminal growth,
yellowing and drooping leaves, and premature defoliation. Trees may show dieback of terminal shoots and
decline, often accompanied by profuse suckering of the rootstock. Small holes and cracks in the bark
at the graft union can be seen. If you remove a piece of the bark on black rootstocks, you can see a
narrow black line or strip of dead tissue at the union. On Paradox rootstocks, blackline may spread into
the rootstock as a canker. Trees die.
The blackline virus is transmitted mostly by pollination; very little transmission is made by grafting.
There is no known cure. English walnuts on English rootstocks will get the virus, but will not die. Black
walnut rootstocks are not susceptible to blackline.
of dead tissue at graft union
of disease into a canker