Leucostoma canker is probably the most common and damaging infectious disease of spruce in Illinois. Colorado blue and Norway spruces are very susceptible, especially 10- to 20-year-old trees. This 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. The needles may drop early from affected branches or hang on for several months, leaving dry, brittle twigs. An important diagnostic feature to notice 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. Sometimes this disease is confused with Rhizosphaera needle cast, which affects older needles first, while needles at the tips of branches are apparently unaffected. (Refer to issue no. 3 of this newsletter for details about Rhizosphaera.) Leucostoma can continue to spread until all the branches on the tree are dead. Conspicuous patches of white resin commonly form on the bark in cankered areas at the base of dead branches. The diseased tissue is brown under the thin layer of outer bark. Black, pinheadsized fruiting bodies of the fungus (pycnidia) form in the inner bark, often embedded in the resin. They can be hard to find without the aid of a dissecting microscope.

Don’t be fooled when diagnosing Leucostoma canker. 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 it is possible that injury has 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. You can find this disease in most Illinois communities with older spruce trees.

There are no chemical controls to prevent or eradicate this disease. Remove dead branches as they occur, but be certain to wait for dry weather for this pruning. Try to improve tree vitality by watering in drought-stress periods. It may be helpful to apply an organic mulch under the full spread of the branches but not up against the trunk. Mulch helps retain moisture and maintain a more even temperature and moisture environment for the roots. For more information on Cytospora or Leucostoma canker of spruce, consult Report on Plant Disease (RPD), no. 604, available on the VISTA Web site or through your Extension office.