Blackline

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 (mainly black walnut)

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 on black walnut rootstock and will eventually completely girdle the tree, killing the scion in 2 to 6 years. In addition to the black line, however, Paradox rootstock typically develops a canker that extends into the rootstock. Blackline canker does not extend upward into the scion, as cankers produced by Phytophthora often will. Because of the extensive bark cankering, scions on Paradox decline faster than on black walnut rootstock.

Trees may become infected at any age once tree blossoms, but blackline is more common in trees 15 to 40 years old. All walnut cultivars are susceptible to the virus, but the rate of viral spread varies by cultivar.

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 on black walnut rootstock, or a downward canker on Paradox rootstock. You may need to check several places around the circumference of the trunk because the black line or canker does not extend all the way around the graft union until later in disease development.

Blackline was first a major problem in coastal areas of California but spread to the inland valleys decades ago. 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.

Blackline is caused by Cherry leafroll virus and occurs on English walnut cultivars grown on Northern California black walnut or Paradox rootstock. English walnut grown on English walnut rootstock or  own-rooted English walnut trees can be infected by the pathogen but remain asymptomatic (trees do not show disease symptoms).

The virus may be introduced into an orchard through graft wood or pollen from infected trees. 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 black walnut rootstock, a thin layer of rootstock cells, or in Paradox rootstock, the area that forms cankers, react 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 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.

• In areas known to be affected by blackline, consider using clonal own-rooted English walnut trees that will escape the disease. These trees can be productive when planted in good, loamy soils. Avoid planting in poor soils, or where there are known nematodes, Phytophthora, or salt problems.
• Interplants should always be the same as the main cultivar.
• Carefully select grafting material for top-working established trees from healthy virus-free English walnut trees.
• Monitor the orchard for blackline symptoms in late summer or fall. Look for declining trees with sucker shoots on rootstocks. To confirm disease presence, examine the graft union for the characteristic black line or canker by making small cuts and removing the bark. Cut at about 4-inch (10-cm) intervals around the circumference of the tree because the black line may not be continuous.