Dead, sunken areas on tree trunks and branches are sometimes caused by cytospora canker. On deciduous trees (trees that lose their leaves in the fall), sunken, elongated areas form on the trunk or branches. The bark in the elongated area is often cracked or discolored, with a ridge around the margin. Small, black "pimples" can appear on twigs and branches or on the area of the canker. Small branches and twigs are rapidly girdled, while larger branches and the trunk may take several years to become girdled. Girdled portions of the tree eventually die.
On conifers, the lowest branches are killed, then branch death progresses up the tree. Dying branches first exhibit yellow-green, then purple, foliage and later drop their needles. A large amount of resin flows from infected branches, coats the entire surface of the bark around the cankers and drips onto lower branches.
The cytospora fungus overwinters on the bark and in the cankers. During wet weather in spring, spores are formed and are washed to other branches by rain. These spores can also be transported by insects, pruning tools, and clothing. If the spores land on a wound, they can cause an infection. A canker will eventually develop as the fungus invades healthy bark.
To control cytospora canker, remove and burn all cankered limbs preferably in late fall or in winter, although removal is acceptable any time of year. Prune sprouts and avoid wounds such as lawnmower or vehicle damage to prevent infection. Maintain plant vigor through proper watering and fertilizing. Water trees deeply over the entire extended root zone area; for deciduous trees this is 2-3 times farther than the branches extend, for conifers, 1½ farther than the branches. Allow the water to soak to a depth of about three feet. Deep water all trees, both deciduous and conifer, in late fall, after leaves of deciduous trees have turned autumn color but before the ground freezes. Water again in spring as soon as the ground thaws. For more information on good watering practices, see the topic sheet in this series "Watering Trees and Lawns: The Pest Connection". To determine if your trees need fertilizing, have the soil tested or look for symptoms of nutrient deficiency such as yellowing, stunted growth, or twisted, deformed foliage. In many parts of Montana, tree fertilizers are simply not needed. Avoid unnecessary fertilization because it can lead to other pest problems, winter damage, contamination of groundwater, overly rapid growth, and unnecessary expenses. For additional information, see the topic sheet in this series, "Fertilizing Trees & Shrubs: Good Practices Can Help Prevent Pest Problems".
Tree injections, in which fungicides are injected from pressurized capsules directly into the tree through small holes drilled in the bark have shown variable results. If the tree is particularly important or valuable you may wish to try tree injections. Tree injections should be done only by professional arborists who can show evidence they have received formal training in the technique. Improper injections can cause irreparable damage. Avoid annual injections because pockets of dead wood result each time injections are done, even when done correctly, and over time, the dead wood can coalesce. For more information about the method, see the fact sheet in this series, "Tree Injections and Implants".
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Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997