By Christine Engelbrecht
Extension Plant Pathologist
Iowa State University
Willows and poplars are common trees for use in windbreaks or privacy screens because they grow very fast, providing effective screening within a few years. One of the most popular willows is the Austree, a hybrid willow (a cross of white willow and corkscrew willow) developed in New Zealand. However, many people are dismayed when these beautiful screens start declining and dying after a dozen or years, or even sooner. These fast-growing trees exemplify the saying, “live fast, die young” because, despite their fast growth, they are highly susceptible to several lethal diseases.
Perhaps the most important diseases on willows and poplars are fungal canker diseases. A canker is an area of dead bark, and may appear as a sunken or discolored area on the bark of the trunk or limbs. When a canker grows all the way around the limb it causes the branch to wilt and die. A tree affected by a canker disease is usually first noticed for its wilting leaves on scattered limbs.
Many fungi can cause cankers on willows and poplars, the most common of which are Cytospora and Cryptosphaeria. Cytospora causes brown, round, slightly sunken lesions on the bark. As the lesions expand, they may turn black or gray. Fruiting structures of the fungus may appear in the canker as small black dots or warts, sometimes oozing orange goo filled with spores. This fungus attacks both willows and poplars. Cryptosphaeria does not always cause a distinctive canker, but may grow inside the tree and cause wood rot. Occasionally it causes long, narrow lesions to appear on the bark. This fungus attacks only poplars, including aspens.
Cytospora and many other canker diseases are most common on trees that are stressed. Willows and poplars are adapted to wet sites such as near streams, and the most common stress they undergo is drought stress. Willows and poplars growing in dry sites are especially prone to canker diseases. Cryptosphaeria appears to attack trees whether they are stressed or not. Wounds are common entry points for the canker fungi. Cankers typically appear after the trees are about 12-15 years old, although they may also appear much earlier.
Prevention of canker diseases begins with choosing an appropriate tree for the site. In dry sites, a tree other than willow may be more appropriate. Reducing stress to existing trees and avoiding unnecessary wounding can also help to prevent cankers. When cankers occur, infected limbs should be pruned out of the tree well below the cankered area.
In addition to suffering from canker diseases, willows and poplars have very weak wood that breaks easily in storms. They also are prone to fungal decay of their wood, which further weakens it.
Willows and poplars do provide reliably fast growth, and are a good choice when very fast growth for a temporary screen is desired. They are adapted to wet conditions, and so are appropriate for very moist sites, such as in stream bank restoration. However, because of their short lifespan, they should not be relied on exclusively. Other, slower-growing but more permanent species should be planted in combination with willows and poplars to provide a long-lasting planting.