Here is another disease that has been appearing frequently in the stress of 2005. Most landscape professionals probably know this disease as Cytospora canker. Although that is still a valid name, the current name for the causal fungus is Leucostoma, and thus the disease name is Leucostoma canker. The perfect stage of the fungus is Valsa kunzei. You may have heard of Valsa cankers on other woody hosts as well. |
Leucostoma canker is probably the most common and damaging infectious disease of spruce in Illinois. Colorado blue and Norway spruces are very susceptible, especially trees 10 to 20 years old. The disease appears on spruces as dead or dying branches, usually starting at the base of the tree and moving upward. Occasionally the affected branches are scattered throughout the tree. Needles may drop early from affected branches or hang on for several months, leaving dry, brittle twigs. An important diagnostic feature is that Leucostoma causes entire branches to die, including branch tips. A girdling canker forms at the base of the branch, and symptoms show first as a branch tip death. Leucostoma can continue to spread until all the branches on the tree are dead, but this process may take many years. Conspicuous patches of white resin commonly form on the bark in cankered areas at the base of dead branches, usually next to the trunk. The diseased tissue is brown under the thin layer of outer bark. Black pinhead-sized fruiting bodies of the fungus (pycnidia) form in the inner bark, often embedded in the resin. They can be hard to find, even with the aid of a dissecting microscope.
I think Leucostoma canker gets blamed for many site problems; donít be fooled when diagnosing it. The mere presence of dead branches does not confirm the disease. Look for resin areas at the base of the dead branches, then look more closely for the black pycnidia. We have seen so much damage to spruce from environmental stress over the past several years that injury may have nothing to do with an infectious agent. On the other hand, stressed spruce trees are more susceptible to Leucostoma canker, and it is highly likely that the disease will eventually invade the stressed tree as a secondary pathogen. This disease is found in most Illinois communities with older spruce trees.
There are no chemical controls to prevent or eradicate Leucostoma canker. Remove dead branches as they occur. Try to improve tree vitality by watering in drought stress periods. It may be helpful to apply organic mulch, such as shredded bark, under the full spread of the branches, but not up against the trunk. Mulch will help retain moisture and maintain a more even temperature and moisture environment for the roots. For more information on Leucostoma (cytospora) canker of spruce, consult RPD No.604, available at http://www.ag.uiuc.edu/%7Evista/horticul.htm or through a University of Illinois Extension office.