How to Manage Pests
UC Pest Management Guidelines
Blackline causes symptoms in the tree canopy that are similar to those caused by soilborne pathogens such as Phytophthora and Armillaria, nutrient deficiencies, and scion-rootstock incompatibilities. The first symptoms of the disease are poor terminal growth, yellowing and drooping leaves, and premature defoliation, particularly in the top branches. Later, diseased trees show dieback of terminal shoots and decline, often accompanied by profuse suckering from the rootstock.
Blackline disease kills the tissues that transport nutrients and water between the rootstock and scion. The black line is usually exactly at the graft union and will eventually completely girdle the tree, killing the scion in 2 to 6 years. Trees may become infected at any age, but blackline is more common in trees 15 to 40 years old. All walnut cultivars are susceptible to the virus, but the virus spreads more rapidly in some.
Positive diagnosis of blackline requires careful examination of the union between scion and rootstock. Trees with blackline usually have small holes and cracks in the bark at the graft union. If you remove a piece of the bark, you can see a narrow black 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 does not extend all the way around the graft union until later in disease development.
In addition to the black line, however, Paradox rootstock may develop a canker that extends into the rootstock. Blackline canker does not extend upward into the scion, as does the canker produced by Phytophthora. Because of the extensive bark cankering, scions on Paradox decline faster than on black walnut rootstock.
Blackline was first a major problem in coastal areas of California but has since spread to the inland valleys and has become a threat to the walnut industry in many areas. Blackline is caused by Cherry leafroll virus and occurs on English walnut cultivars grown on northern California black walnut or Paradox rootstock. Although it occurs in all walnut-growing areas of California, it is most prevalent in coastal orchards and orchards in the northern San Joaquin Valley and southern Sacramento Valley. English walnut grown on English walnut rootstock or an own-rooted English walnut tree can be infected by the pathogen but does not develop disease symptoms.
The virus may be introduced into an orchard through graft wood or pollen from trees infected with blackline. Within the orchard, the pathogen is probably transmitted by infected pollen because it does not spread from diseased to healthy trees until the pistillate flowers are receptive for pollination. In addition, blackline spreads more rapidly in mixed-cultivar orchards, where pollen shedding overlaps with pistillate bloom for a longer time. Spread of the pathogen through the soil or by nematodes is unlikely because black walnut and Paradox rootstocks are immune to the virus.
Once a tree is infected, the virus spreads through the scion and eventually reaches the rootstock. If the tree is on Paradox or black walnut rootstock, a thin layer of rootstock cells reacts to the presence of Cherry leafroll virus by dying. If the tree is on English rootstock, no reaction occurs. The virus spreads into the rootstock and the tree becomes a symptomless carrier of the virus. Pollen or graft wood from the tree will spread the disease.
At present, trees infected with blackline cannot be cured. No practical method is available to detect blackline before symptoms appear. A few management practices can help reduce the spread of the disease.
UC IPM Pest Management Guidelines: Walnut