Blackline of walnuts—Cherry leafroll virus
The viral disease blackline affects English walnut cultivars grown on rootstocks of Northern California black walnut and Paradox. English walnut grown on English walnut rootstock and own-rooted English walnut trees can be infected by the pathogen but do not develop blackline disease.
Identification
Blackline causes symptoms in the tree canopy that are similar to those caused by various other maladies including Armillaria root rot, drought stress, nutrient deficiencies, Phytophthora root and crown rot, and scion-rootstock incompatibilities.
The first symptoms of blackline are poor shoot growth, yellowing and drooping of leaves, and premature leaf drop. Trees develop dieback of shoots and decline overall, often accompanied by abundant suckering (shoot production) from the rootstock.
Positive diagnosis of blackline involves careful examination of the union between the rootstock and scion. Small holes and cracks develop in bark at the graft union. If you remove a piece of the bark, you can see a narrow, black, horizontal line or strip of dead tissue at the union. You may need to check several places around the circumference of the trunk because the black line may not extend all the way around the graft union until late in disease development. The dark line is usually exactly at the graft union.
In addition to the dark line, Paradox rootstock may develop a canker that extends downward into the rootstock. Blackline cankers do not extend upward into the scion, as do the cankers produced by Phytophthora root and crown rot. Because of the extensive bark cankering, scions on Paradox rootstock decline faster than scions on Northern California black walnut rootstock.
Life cycle
The virus can be introduced through graft wood or pollen from trees infected with Cherry leafroll virus. Once a tree is infected, the virus spreads through the scion and eventually reaches the rootstock. If the tree is on a susceptible rootstock, a thin layer of rootstock cells reacts to the presence of Cherry leafroll virus by dying. This results in the dark line symptom and interferes with nutrient and water transport, causing tree decline. If the tree is on English rootstock, no blackline reaction occurs. The virus spreads into the English rootstock and the tree becomes a symptomless carrier of the virus.
Trees may become infected at any age, but blackline is more common in trees 15 to 40 years old. After infection, trees decline over several years before dying.
Damage
Blackline kills the tissues that transport nutrients and water between the rootstock and scion. This causes leaves to turn yellow, wilt, and drop prematurely. In the early stages of the disease, the dark line is not continuous around the union, but it eventually spreads entirely around the graft union and completely girdles the tree. After a walnut tree on susceptible rootstock becomes infected with Cherry leafroll virus, the scion dies within 2 to 6 years.
Solutions
Trees with blackline cannot be cured. If a tree is not performing satisfactorily or has developed extensive dieback, remove and replace it. Replant using stock that is certified as disease free and obtained from a reputable, commercial supplier.
Adapted from Integrated Pest Management for Walnuts Third Edition and Pest Management Guidelines: Walnuts, University of California Statewide Integrated Pest Management Program (UC IPM). |
Pale, sparse leaves and suckers (shoots) from the rootstock due to blackline.
Bark removed from the graft union exposing a dark, horizontal line characteristic of blackline.
A canker spreading downward from the graft union due to blackline.
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